ANSYS Fluent Theory Guide

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Table of Contents Using This Manual .................................................................................................................................... xxix 1. The Contents of This Manual ............................................................................................................ xxix 2. The Contents of the Fluent Manuals .................................................................................................. xxx 3. Typographical Conventions ............................................................................................................ xxxii 4. Mathematical Conventions ............................................................................................................ xxxiv 5.Technical Support ........................................................................................................................... xxxv 1. Basic Fluid Flow ....................................................................................................................................... 1 1.1. Overview of Physical Models in ANSYS Fluent .................................................................................... 1 1.2. Continuity and Momentum Equations ............................................................................................... 2 1.2.1. The Mass Conservation Equation .............................................................................................. 2 1.2.2. Momentum Conservation Equations ........................................................................................ 3 1.3. User-Defined Scalar (UDS) Transport Equations .................................................................................. 3 1.3.1. Single Phase Flow .................................................................................................................... 4 1.3.2. Multiphase Flow ....................................................................................................................... 5 1.4. Periodic Flows .................................................................................................................................. 5 1.4.1. Overview ................................................................................................................................. 6 1.4.2. Limitations ............................................................................................................................... 7 1.4.3. Physics of Periodic Flows .......................................................................................................... 7 1.4.3.1. Definition of the Periodic Velocity .................................................................................... 7 1.4.3.2. Definition of the Streamwise-Periodic Pressure ................................................................ 7 1.5. Swirling and Rotating Flows .............................................................................................................. 8 1.5.1. Overview of Swirling and Rotating Flows .................................................................................. 9 1.5.1.1. Axisymmetric Flows with Swirl or Rotation ....................................................................... 9 1.5.1.1.1. Momentum Conservation Equation for Swirl Velocity ............................................. 10 1.5.1.2. Three-Dimensional Swirling Flows .................................................................................. 10 1.5.1.3. Flows Requiring a Moving Reference Frame ................................................................... 10 1.5.2. Physics of Swirling and Rotating Flows .................................................................................... 10 1.6. Compressible Flows ........................................................................................................................ 11 1.6.1. When to Use the Compressible Flow Model ............................................................................ 13 1.6.2. Physics of Compressible Flows ................................................................................................ 13 1.6.2.1. Basic Equations for Compressible Flows ......................................................................... 13 1.6.2.2. The Compressible Form of the Gas Law .......................................................................... 14 1.7. Inviscid Flows ................................................................................................................................. 14 1.7.1. Euler Equations ...................................................................................................................... 14 1.7.1.1. The Mass Conservation Equation .................................................................................... 15 1.7.1.2. Momentum Conservation Equations .............................................................................. 15 1.7.1.3. Energy Conservation Equation ....................................................................................... 15 2. Flows with Moving Reference Frames ................................................................................................... 17 2.1. Introduction ................................................................................................................................... 17 2.2. Flow in a Moving Reference Frame .................................................................................................. 18 2.2.1. Equations for a Moving Reference Frame ................................................................................ 19 2.2.1.1. Relative Velocity Formulation ......................................................................................... 20 2.2.1.2. Absolute Velocity Formulation ....................................................................................... 21 2.2.1.3. Relative Specification of the Reference Frame Motion ..................................................... 21 2.3. Flow in Multiple Reference Frames .................................................................................................. 22 2.3.1. The Multiple Reference Frame Model ...................................................................................... 22 2.3.1.1. Overview ....................................................................................................................... 22 2.3.1.2. Examples ....................................................................................................................... 23 2.3.1.3. The MRF Interface Formulation ...................................................................................... 24 2.3.1.3.1. Interface Treatment: Relative Velocity Formulation ................................................. 24 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 2.3.1.3.2. Interface Treatment: Absolute Velocity Formulation ............................................... 25 2.3.2. The Mixing Plane Model ......................................................................................................... 25 2.3.2.1. Overview ....................................................................................................................... 26 2.3.2.2. Rotor and Stator Domains .............................................................................................. 26 2.3.2.3. The Mixing Plane Concept ............................................................................................. 27 2.3.2.4. Choosing an Averaging Method ..................................................................................... 28 2.3.2.4.1. Area Averaging ..................................................................................................... 28 2.3.2.4.2. Mass Averaging .................................................................................................... 28 2.3.2.4.3. Mixed-Out Averaging ............................................................................................ 29 2.3.2.5. Mixing Plane Algorithm of ANSYS Fluent ........................................................................ 29 2.3.2.6. Mass Conservation ........................................................................................................ 30 2.3.2.7. Swirl Conservation ......................................................................................................... 30 2.3.2.8. Total Enthalpy Conservation .......................................................................................... 31 3. Flows Using Sliding and Dynamic Meshes ............................................................................................ 33 3.1. Introduction ................................................................................................................................... 33 3.2. Dynamic Mesh Theory .................................................................................................................... 34 3.2.1. Conservation Equations ......................................................................................................... 35 3.2.2. Six DOF Solver Theory ............................................................................................................ 36 3.3. Sliding Mesh Theory ....................................................................................................................... 37 4. Turbulence ............................................................................................................................................. 39 4.1. Underlying Principles of Turbulence Modeling ................................................................................. 39 4.1.1. Reynolds (Ensemble) Averaging .............................................................................................. 39 4.1.2. Filtered Navier-Stokes Equations ............................................................................................. 40 4.1.3. Hybrid RANS-LES Formulations ............................................................................................... 41 4.1.4. Boussinesq Approach vs. Reynolds Stress Transport Models ..................................................... 41 4.2. Spalart-Allmaras Model ................................................................................................................... 42 4.2.1. Overview ............................................................................................................................... 42 4.2.2. Transport Equation for the Spalart-Allmaras Model ................................................................. 43 4.2.3. Modeling the Turbulent Viscosity ............................................................................................ 43 4.2.4. Modeling the Turbulent Production ........................................................................................ 43 4.2.5. Modeling the Turbulent Destruction ....................................................................................... 44 4.2.6. Model Constants .................................................................................................................... 45 4.2.7. Wall Boundary Conditions ...................................................................................................... 45 4.2.7.1. Treatment of the Spalart-Allmaras Model for Icing Simulations ....................................... 45 4.2.8. Convective Heat and Mass Transfer Modeling .......................................................................... 46 4.3. Standard, RNG, and Realizable k-ε Models ........................................................................................ 46 4.3.1. Standard k-ε Model ................................................................................................................ 47 4.3.1.1. Overview ....................................................................................................................... 47 4.3.1.2. Transport Equations for the Standard k-ε Model ............................................................. 47 4.3.1.3. Modeling the Turbulent Viscosity ................................................................................... 47 4.3.1.4. Model Constants ........................................................................................................... 48 4.3.2. RNG k-ε Model ....................................................................................................................... 48 4.3.2.1. Overview ....................................................................................................................... 48 4.3.2.2. Transport Equations for the RNG k-ε Model ..................................................................... 48 4.3.2.3. Modeling the Effective Viscosity ..................................................................................... 49 4.3.2.4. RNG Swirl Modification .................................................................................................. 49 4.3.2.5. Calculating the Inverse Effective Prandtl Numbers .......................................................... 50 4.3.2.6. The R-ε Term in the ε Equation ........................................................................................ 50 4.3.2.7. Model Constants ........................................................................................................... 51 4.3.3. Realizable k-ε Model ............................................................................................................... 51 4.3.3.1. Overview ....................................................................................................................... 51 4.3.3.2. Transport Equations for the Realizable k-ε Model ............................................................ 52

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Theory Guide 4.3.3.3. Modeling the Turbulent Viscosity ................................................................................... 53 4.3.3.4. Model Constants ........................................................................................................... 54 4.3.4. Modeling Turbulent Production in the k-ε Models ................................................................... 54 4.3.5. Effects of Buoyancy on Turbulence in the k-ε Models ............................................................... 54 4.3.6. Effects of Compressibility on Turbulence in the k-ε Models ...................................................... 55 4.3.7. Convective Heat and Mass Transfer Modeling in the k-ε Models ............................................... 56 4.4. Standard, BSL, and SST k-ω Models ................................................................................................... 57 4.4.1. Standard k-ω Model ............................................................................................................... 58 4.4.1.1. Overview ....................................................................................................................... 58 4.4.1.2. Transport Equations for the Standard k-ω Model ............................................................. 58 4.4.1.3. Modeling the Effective Diffusivity ................................................................................... 58 4.4.1.3.1. Low-Reynolds Number Correction ......................................................................... 58 4.4.1.4. Modeling the Turbulence Production ............................................................................. 59 4.4.1.4.1. Production of k ..................................................................................................... 59 4.4.1.4.2. Production of ω ..................................................................................................... 59 4.4.1.5. Modeling the Turbulence Dissipation ............................................................................. 59 4.4.1.5.1. Dissipation of k ..................................................................................................... 59 4.4.1.5.2. Dissipation of ω ..................................................................................................... 60 4.4.1.5.3. Compressibility Effects .......................................................................................... 60 4.4.1.6. Model Constants ........................................................................................................... 61 4.4.2. Baseline (BSL) k-ω Model ........................................................................................................ 61 4.4.2.1. Overview ....................................................................................................................... 61 4.4.2.2. Transport Equations for the BSL k-ω Model ..................................................................... 61 4.4.2.3. Modeling the Effective Diffusivity ................................................................................... 62 4.4.2.4. Modeling the Turbulence Production ............................................................................. 62 4.4.2.4.1. Production of k ..................................................................................................... 62 4.4.2.4.2. Production of ω ..................................................................................................... 62 4.4.2.5. Modeling the Turbulence Dissipation ............................................................................. 63 4.4.2.5.1. Dissipation of k ..................................................................................................... 63 4.4.2.5.2. Dissipation of ω ..................................................................................................... 63 4.4.2.6. Cross-Diffusion Modification .......................................................................................... 63 4.4.2.7. Model Constants ........................................................................................................... 63 4.4.3. Shear-Stress Transport (SST) k-ω Model ................................................................................... 64 4.4.3.1. Overview ....................................................................................................................... 64 4.4.3.2. Modeling the Turbulent Viscosity ................................................................................... 64 4.4.3.3. Model Constants ........................................................................................................... 64 4.4.3.4. Treatment of the SST Model for Icing Simulations ........................................................... 64 4.4.4. Turbulence Damping .............................................................................................................. 65 4.4.5. Wall Boundary Conditions ...................................................................................................... 66 4.5. k-kl-ω Transition Model ................................................................................................................... 66 4.5.1. Overview ............................................................................................................................... 67 4.5.2. Transport Equations for the k-kl-ω Model ................................................................................ 67 4.5.2.1. Model Constants ........................................................................................................... 69 4.6. Transition SST Model ....................................................................................................................... 70 4.6.1. Overview ............................................................................................................................... 70 4.6.2. Transport Equations for the Transition SST Model .................................................................... 71 4.6.2.1. Separation-Induced Transition Correction ...................................................................... 73 4.6.2.2. Coupling the Transition Model and SST Transport Equations ........................................... 73 4.6.2.3. Transition SST and Rough Walls ...................................................................................... 73 4.6.3. Mesh Requirements ............................................................................................................... 74 4.6.4. Specifying Inlet Turbulence Levels .......................................................................................... 77 4.7. Intermittency Transition Model ....................................................................................................... 78 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 4.7.1. Overview ............................................................................................................................... 78 4.7.2.Transport Equations for the Intermittency Transition Model ..................................................... 79 4.7.3. Coupling with the Other Models ............................................................................................. 81 4.7.4. Intermittency Transition Model and Rough Walls ..................................................................... 81 4.8. The V2F Model ................................................................................................................................ 81 4.9. Reynolds Stress Model (RSM) ........................................................................................................... 82 4.9.1. Overview ............................................................................................................................... 82 4.9.2. Reynolds Stress Transport Equations ....................................................................................... 83 4.9.3. Modeling Turbulent Diffusive Transport .................................................................................. 84 4.9.4. Modeling the Pressure-Strain Term ......................................................................................... 84 4.9.4.1. Linear Pressure-Strain Model .......................................................................................... 84 4.9.4.2. Low-Re Modifications to the Linear Pressure-Strain Model .............................................. 85 4.9.4.3. Quadratic Pressure-Strain Model .................................................................................... 86 4.9.4.4. Stress-Omega Model ..................................................................................................... 86 4.9.4.5. Stress-BSL Model ........................................................................................................... 88 4.9.5. Effects of Buoyancy on Turbulence ......................................................................................... 88 4.9.6. Modeling the Turbulence Kinetic Energy ................................................................................. 88 4.9.7. Modeling the Dissipation Rate ................................................................................................ 89 4.9.8. Modeling the Turbulent Viscosity ............................................................................................ 89 4.9.9. Wall Boundary Conditions ...................................................................................................... 90 4.9.10. Convective Heat and Mass Transfer Modeling ........................................................................ 90 4.10. Scale-Adaptive Simulation (SAS) Model ......................................................................................... 91 4.10.1. Overview ............................................................................................................................. 91 4.10.2. Transport Equations for the SST-SAS Model ........................................................................... 92 4.10.3. SAS with Other ω-Based Turbulence Models .......................................................................... 94 4.11. Detached Eddy Simulation (DES) ................................................................................................... 94 4.11.1. Overview ............................................................................................................................. 94 4.11.2. DES with the Spalart-Allmaras Model .................................................................................... 95 4.11.3. DES with the Realizable k-ε Model ......................................................................................... 95 4.11.4. DES with the BSL or SST k-ω Model ....................................................................................... 96 4.11.5. DES with the Transition SST Model ........................................................................................ 97 4.11.6. Improved Delayed Detached Eddy Simulation (IDDES) .......................................................... 97 4.11.6.1. Overview of IDDES ....................................................................................................... 97 4.11.6.2. IDDES Model Formulation ............................................................................................ 98 4.12. Shielded Detached Eddy Simulation (SDES) ................................................................................... 98 4.12.1. Shielding Function ............................................................................................................... 99 4.12.2. LES Mode of SDES .............................................................................................................. 100 4.13. Stress-Blended Eddy Simulation (SBES) ........................................................................................ 101 4.13.1. Stress Blending ................................................................................................................... 102 4.13.2. SDES and SBES Example ..................................................................................................... 102 4.14. Large Eddy Simulation (LES) Model .............................................................................................. 103 4.14.1. Overview ........................................................................................................................... 103 4.14.2. Subgrid-Scale Models ......................................................................................................... 104 4.14.2.1. Smagorinsky-Lilly Model ............................................................................................ 105 4.14.2.2. Dynamic Smagorinsky-Lilly Model .............................................................................. 106 4.14.2.3. Wall-Adapting Local Eddy-Viscosity (WALE) Model ...................................................... 107 4.14.2.4. Algebraic Wall-Modeled LES Model (WMLES) .............................................................. 107 4.14.2.4.1. Algebraic WMLES Model Formulation ................................................................ 108 4.14.2.4.1.1. Reynolds Number Scaling ......................................................................... 108 4.14.2.4.2. Algebraic WMLES S-Omega Model Formulation ................................................. 109 4.14.2.5. Dynamic Kinetic Energy Subgrid-Scale Model ............................................................. 110 4.14.3. Inlet Boundary Conditions for the LES Model ....................................................................... 110

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Theory Guide 4.14.3.1. Vortex Method ........................................................................................................... 111 4.14.3.2. Spectral Synthesizer ................................................................................................... 112 4.15. Embedded Large Eddy Simulation (ELES) ..................................................................................... 113 4.15.1. Overview ........................................................................................................................... 113 4.15.2. Selecting a Model ............................................................................................................... 113 4.15.3. Interfaces Treatment ........................................................................................................... 114 4.15.3.1. RANS-LES Interface .................................................................................................... 114 4.15.3.2. LES-RANS Interface .................................................................................................... 114 4.15.3.3. Internal Interface Without LES Zone ........................................................................... 115 4.15.3.4. Grid Generation Guidelines ........................................................................................ 115 4.16. Near-Wall Treatments for Wall-Bounded Turbulent Flows .............................................................. 116 4.16.1. Overview ........................................................................................................................... 116 4.16.1.1. Wall Functions vs. Near-Wall Model ............................................................................. 117 4.16.1.2. Wall Functions ........................................................................................................... 119 4.16.2. Standard Wall Functions ..................................................................................................... 119 4.16.2.1. Momentum ............................................................................................................... 119 4.16.2.2. Energy ....................................................................................................................... 120 4.16.2.3. Species ...................................................................................................................... 122 4.16.2.4. Turbulence ................................................................................................................ 122 4.16.3. Scalable Wall Functions ....................................................................................................... 123 4.16.4. Non-Equilibrium Wall Functions .......................................................................................... 123 4.16.4.1. Standard Wall Functions vs. Non-Equilibrium Wall Functions ....................................... 125 4.16.4.2. Limitations of the Wall Function Approach ................................................................. 125 4.16.5. Enhanced Wall Treatment ε-Equation (EWT-ε) ...................................................................... 125 4.16.5.1. Two-Layer Model for Enhanced Wall Treatment ........................................................... 126 4.16.5.2. Enhanced Wall Treatment for Momentum and Energy Equations ................................. 127 4.16.6. Menter-Lechner ε-Equation (ML-ε) ...................................................................................... 129 4.16.6.1. Momentum Equations ............................................................................................... 131 4.16.6.2. k-ε Turbulence Models ............................................................................................... 131 4.16.6.3. Iteration Improvements ............................................................................................. 131 4.16.7. y+-Insensitive Wall Treatment ω-Equation ........................................................................... 131 4.16.8. User-Defined Wall Functions ............................................................................................... 132 4.16.9. LES Near-Wall Treatment ..................................................................................................... 132 4.17. Curvature Correction for the Spalart-Allmaras and Two-Equation Models ..................................... 132 4.18. Production Limiters for Two-Equation Models .............................................................................. 134 4.19. Definition of Turbulence Scales .................................................................................................... 136 4.19.1. RANS and Hybrid (SAS, DES, and SDES) Turbulence Models .................................................. 136 4.19.2. Large Eddy Simulation (LES) Models .................................................................................... 136 4.19.3. Stress-Blended Eddy Simulation (SBES) Model ..................................................................... 137 5. Heat Transfer ....................................................................................................................................... 139 5.1. Introduction ................................................................................................................................. 139 5.2. Modeling Conductive and Convective Heat Transfer ...................................................................... 139 5.2.1. Heat Transfer Theory ............................................................................................................. 139 5.2.1.1. The Energy Equation .................................................................................................... 139 5.2.1.2. The Energy Equation in Moving Reference Frames ........................................................ 140 5.2.1.3. The Energy Equation for the Non-Premixed Combustion Model .................................... 140 5.2.1.4. Inclusion of Pressure Work and Kinetic Energy Terms .................................................... 141 5.2.1.5. Inclusion of the Viscous Dissipation Terms .................................................................... 141 5.2.1.6. Inclusion of the Species Diffusion Term ........................................................................ 141 5.2.1.7. Energy Sources Due to Reaction ................................................................................... 142 5.2.1.8. Energy Sources Due To Radiation ................................................................................. 142 5.2.1.9. Energy Source Due To Joule Heating ............................................................................ 142 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 5.2.1.10. Interphase Energy Sources ......................................................................................... 142 5.2.1.11. Energy Equation in Solid Regions ............................................................................... 142 5.2.1.12. Anisotropic Conductivity in Solids .............................................................................. 143 5.2.1.13. Diffusion at Inlets ....................................................................................................... 143 5.2.2. Natural Convection and Buoyancy-Driven Flows Theory ........................................................ 143 5.3. Modeling Radiation ...................................................................................................................... 144 5.3.1. Overview and Limitations ..................................................................................................... 144 5.3.1.1. Advantages and Limitations of the DTRM ..................................................................... 145 5.3.1.2. Advantages and Limitations of the P-1 Model ............................................................... 145 5.3.1.3. Advantages and Limitations of the Rosseland Model .................................................... 146 5.3.1.4. Advantages and Limitations of the DO Model ............................................................... 146 5.3.1.5. Advantages and Limitations of the S2S Model .............................................................. 146 5.3.1.6. Advantages and Limitations of the MC Model ............................................................... 147 5.3.2. Radiative Transfer Equation .................................................................................................. 148 5.3.3. P-1 Radiation Model Theory .................................................................................................. 149 5.3.3.1. The P-1 Model Equations ............................................................................................. 150 5.3.3.2. Anisotropic Scattering ................................................................................................. 151 5.3.3.3. Particulate Effects in the P-1 Model .............................................................................. 151 5.3.3.4. Boundary Condition Treatment for the P-1 Model at Walls ............................................. 152 5.3.3.5. Boundary Condition Treatment for the P-1 Model at Flow Inlets and Exits ...................... 153 5.3.4. Rosseland Radiation Model Theory ....................................................................................... 153 5.3.4.1. The Rosseland Model Equations ................................................................................... 153 5.3.4.2. Anisotropic Scattering ................................................................................................. 154 5.3.4.3. Boundary Condition Treatment at Walls ........................................................................ 154 5.3.4.4. Boundary Condition Treatment at Flow Inlets and Exits ................................................. 154 5.3.5. Discrete Transfer Radiation Model (DTRM) Theory ................................................................. 154 5.3.5.1. The DTRM Equations .................................................................................................... 154 5.3.5.2. Ray Tracing .................................................................................................................. 155 5.3.5.3. Clustering .................................................................................................................... 155 5.3.5.4. Boundary Condition Treatment for the DTRM at Walls ................................................... 156 5.3.5.5. Boundary Condition Treatment for the DTRM at Flow Inlets and Exits ............................ 156 5.3.6. Discrete Ordinates (DO) Radiation Model Theory ................................................................... 157 5.3.6.1. The DO Model Equations ............................................................................................. 157 5.3.6.2. Energy Coupling and the DO Model ............................................................................. 158 5.3.6.2.1. Limitations of DO/Energy Coupling ..................................................................... 159 5.3.6.3. Angular Discretization and Pixelation ........................................................................... 159 5.3.6.4. Anisotropic Scattering ................................................................................................. 162 5.3.6.5. Particulate Effects in the DO Model .............................................................................. 163 5.3.6.6. Boundary and Cell Zone Condition Treatment at Opaque Walls ..................................... 163 5.3.6.6.1. Gray Diffuse Walls ............................................................................................... 165 5.3.6.6.2. Non-Gray Diffuse Walls ........................................................................................ 165 5.3.6.7. Cell Zone and Boundary Condition Treatment at Semi-Transparent Walls ...................... 165 5.3.6.7.1. Semi-Transparent Interior Walls ........................................................................... 166 5.3.6.7.2. Specular Semi-Transparent Walls ......................................................................... 167 5.3.6.7.3. Diffuse Semi-Transparent Walls ............................................................................ 169 5.3.6.7.4. Partially Diffuse Semi-Transparent Walls ............................................................... 170 5.3.6.7.5. Semi-Transparent Exterior Walls ........................................................................... 170 5.3.6.7.6. Limitations .......................................................................................................... 172 5.3.6.7.7. Solid Semi-Transparent Media ............................................................................. 173 5.3.6.8. Boundary Condition Treatment at Specular Walls and Symmetry Boundaries ................. 173 5.3.6.9. Boundary Condition Treatment at Periodic Boundaries ................................................. 173 5.3.6.10. Boundary Condition Treatment at Flow Inlets and Exits ............................................... 173

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Theory Guide 5.3.7. Surface-to-Surface (S2S) Radiation Model Theory .................................................................. 173 5.3.7.1. Gray-Diffuse Radiation ................................................................................................. 173 5.3.7.2. The S2S Model Equations ............................................................................................. 174 5.3.7.3. Clustering .................................................................................................................... 175 5.3.7.3.1. Clustering and View Factors ................................................................................ 175 5.3.7.3.2. Clustering and Radiosity ...................................................................................... 175 5.3.8. Monte Carlo (MC) Radiation Model Theory ............................................................................ 176 5.3.8.1. The MC Model Equations ............................................................................................. 176 5.3.8.1.1. Monte Carlo Solution Accuracy ............................................................................ 176 5.3.8.2. Boundary Condition Treatment for the MC Model ......................................................... 177 5.3.9. Radiation in Combusting Flows ............................................................................................ 177 5.3.9.1. The Weighted-Sum-of-Gray-Gases Model ..................................................................... 177 5.3.9.1.1. When the Total (Static) Gas Pressure is Not Equal to 1 atm .................................... 178 5.3.9.2. The Effect of Soot on the Absorption Coefficient ........................................................... 179 5.3.9.3. The Effect of Particles on the Absorption Coefficient ..................................................... 179 5.3.10. Choosing a Radiation Model ............................................................................................... 179 5.3.10.1. External Radiation ...................................................................................................... 180 6. Heat Exchangers .................................................................................................................................. 181 6.1. The Macro Heat Exchanger Models ................................................................................................ 181 6.1.1. Overview of the Macro Heat Exchanger Models .................................................................... 181 6.1.2. Restrictions of the Macro Heat Exchanger Models ................................................................. 183 6.1.3. Macro Heat Exchanger Model Theory .................................................................................... 184 6.1.3.1. Streamwise Pressure Drop ........................................................................................... 185 6.1.3.2. Heat Transfer Effectiveness ........................................................................................... 186 6.1.3.3. Heat Rejection ............................................................................................................. 187 6.1.3.4. Macro Heat Exchanger Group Connectivity .................................................................. 188 6.2. The Dual Cell Model ...................................................................................................................... 189 6.2.1. Overview of the Dual Cell Model ........................................................................................... 189 6.2.2. Restrictions of the Dual Cell Model ........................................................................................ 190 6.2.3. Dual Cell Model Theory ......................................................................................................... 190 6.2.3.1. NTU Relations .............................................................................................................. 191 6.2.3.2. Heat Rejection ............................................................................................................. 191 7. Species Transport and Finite-Rate Chemistry ..................................................................................... 193 7.1. Volumetric Reactions .................................................................................................................... 193 7.1.1. Species Transport Equations ................................................................................................. 193 7.1.1.1. Mass Diffusion in Laminar Flows ................................................................................... 194 7.1.1.2. Mass Diffusion in Turbulent Flows ................................................................................ 194 7.1.1.3. Treatment of Species Transport in the Energy Equation ................................................. 194 7.1.1.4. Diffusion at Inlets ......................................................................................................... 194 7.1.2. The Generalized Finite-Rate Formulation for Reaction Modeling ............................................ 195 7.1.2.1. Direct Use of Finite-Rate Kinetics (no TCI) ...................................................................... 195 7.1.2.2. Pressure-Dependent Reactions .................................................................................... 197 7.1.2.3. The Eddy-Dissipation Model ......................................................................................... 199 7.1.2.4. The Eddy-Dissipation Model for LES ............................................................................. 200 7.1.2.5. The Eddy-Dissipation-Concept (EDC) Model ................................................................. 200 7.1.2.6. The Thickened Flame Model ......................................................................................... 202 7.1.2.7. The Relaxation to Chemical Equilibrium Model ............................................................. 203 7.2. Wall Surface Reactions and Chemical Vapor Deposition .................................................................. 205 7.2.1. Surface Coverage Reaction Rate Modification ....................................................................... 206 7.2.2. Reaction-Diffusion Balance for Surface Chemistry ................................................................. 207 7.2.3. Slip Boundary Formulation for Low-Pressure Gas Systems ..................................................... 207 7.3. Particle Surface Reactions ............................................................................................................. 209 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 7.3.1. General Description .............................................................................................................. 209 7.3.2. ANSYS Fluent Model Formulation ......................................................................................... 210 7.3.3. Extension for Stoichiometries with Multiple Gas Phase Reactants .......................................... 211 7.3.4. Solid-Solid Reactions ............................................................................................................ 212 7.3.5. Solid Decomposition Reactions ............................................................................................ 212 7.3.6. Solid Deposition Reactions ................................................................................................... 212 7.3.7. Gaseous Solid Catalyzed Reactions on the Particle Surface .................................................... 212 7.4. Electrochemical Reactions ............................................................................................................. 213 7.4.1. Overview and Limitations ..................................................................................................... 213 7.4.2. Electrochemical Reaction Model Theory ................................................................................ 213 7.5. Reacting Channel Model ............................................................................................................... 216 7.5.1. Overview and Limitations ..................................................................................................... 216 7.5.2. Reacting Channel Model Theory ........................................................................................... 217 7.5.2.1. Flow Inside the Reacting Channel ................................................................................. 217 7.5.2.2. Surface Reactions in the Reacting Channel ................................................................... 218 7.5.2.3. Porous Medium Inside Reacting Channel ...................................................................... 219 7.5.2.4. Outer Flow in the Shell ................................................................................................. 219 7.6. Reactor Network Model ................................................................................................................ 220 7.6.1. Reactor Network Model Theory ............................................................................................ 220 7.6.1.1. Reactor network temperature solution ......................................................................... 221 8. Non-Premixed Combustion ................................................................................................................. 223 8.1. Introduction ................................................................................................................................. 223 8.2. Non-Premixed Combustion and Mixture Fraction Theory ............................................................... 223 8.2.1. Mixture Fraction Theory ....................................................................................................... 224 8.2.1.1. Definition of the Mixture Fraction ................................................................................ 224 8.2.1.2. Transport Equations for the Mixture Fraction ................................................................ 226 8.2.1.3. The Non-Premixed Model for LES ................................................................................. 227 8.2.1.4. Mixture Fraction vs. Equivalence Ratio .......................................................................... 227 8.2.1.5. Relationship of Mixture Fraction to Species Mass Fraction, Density, and Temperature ..... 228 8.2.2. Modeling of Turbulence-Chemistry Interaction ..................................................................... 229 8.2.2.1. Description of the Probability Density Function ............................................................ 229 8.2.2.2. Derivation of Mean Scalar Values from the Instantaneous Mixture Fraction ................... 229 8.2.2.3. The Assumed-Shape PDF ............................................................................................. 230 8.2.2.3.1. The Double Delta Function PDF ........................................................................... 230 8.2.2.3.2. The β-Function PDF ............................................................................................. 231 8.2.3. Non-Adiabatic Extensions of the Non-Premixed Model .......................................................... 232 8.2.4. Chemistry Tabulation ........................................................................................................... 234 8.2.4.1. Look-Up Tables for Adiabatic Systems ........................................................................... 234 8.2.4.2. 3D Look-Up Tables for Non-Adiabatic Systems .............................................................. 236 8.2.4.3. Generating Lookup Tables Through Automated Grid Refinement .................................. 238 8.3. Restrictions and Special Cases for Using the Non-Premixed Model ................................................. 240 8.3.1. Restrictions on the Mixture Fraction Approach ...................................................................... 240 8.3.2. Using the Non-Premixed Model for Liquid Fuel or Coal Combustion ...................................... 243 8.3.3. Using the Non-Premixed Model with Flue Gas Recycle .......................................................... 244 8.3.4. Using the Non-Premixed Model with the Inert Model ............................................................ 244 8.3.4.1. Mixture Composition ................................................................................................... 245 8.3.4.1.1. Property Evaluation ............................................................................................. 246 8.4. The Diffusion Flamelet Models Theory ........................................................................................... 246 8.4.1. Restrictions and Assumptions ............................................................................................... 246 8.4.2. The Flamelet Concept ........................................................................................................... 246 8.4.2.1. Overview ..................................................................................................................... 246 8.4.2.2. Strain Rate and Scalar Dissipation ................................................................................. 248

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Theory Guide 8.4.2.3. Embedding Diffusion Flamelets in Turbulent Flames ..................................................... 248 8.4.3. Flamelet Generation ............................................................................................................. 249 8.4.4. Flamelet Import ................................................................................................................... 250 8.5. The Steady Diffusion Flamelet Model Theory ................................................................................. 251 8.5.1. Overview ............................................................................................................................. 252 8.5.2. Multiple Steady Flamelet Libraries ........................................................................................ 252 8.5.3. Steady Diffusion Flamelet Automated Grid Refinement ......................................................... 253 8.5.4. Non-Adiabatic Steady Diffusion Flamelets ............................................................................. 253 8.6. The Unsteady Diffusion Flamelet Model Theory ............................................................................. 254 8.6.1. The Eulerian Unsteady Laminar Flamelet Model .................................................................... 254 8.6.1.1. Liquid Reactions .......................................................................................................... 256 8.6.2. The Diesel Unsteady Laminar Flamelet Model ....................................................................... 257 8.6.3. Multiple Diesel Unsteady Flamelets ....................................................................................... 257 8.6.4. Multiple Diesel Unsteady Flamelets with Flamelet Reset ........................................................ 258 8.6.4.1. Resetting the Flamelets ................................................................................................ 258 9. Premixed Combustion ......................................................................................................................... 261 9.1. Overview and Limitations ............................................................................................................. 261 9.1.1. Overview ............................................................................................................................. 261 9.1.2. Limitations ........................................................................................................................... 262 9.2. C-Equation Model Theory .............................................................................................................. 262 9.2.1. Propagation of the Flame Front ............................................................................................ 262 9.3. G-Equation Model Theory ............................................................................................................. 264 9.3.1. Numerical Solution of the G-equation ................................................................................... 265 9.4. Turbulent Flame Speed Models ..................................................................................................... 265 9.4.1. Zimont Turbulent Flame Speed Closure Model ...................................................................... 265 9.4.1.1. Zimont Turbulent Flame Speed Closure for LES ............................................................. 266 9.4.1.2. Flame Stretch Effect ..................................................................................................... 267 9.4.1.3. Gradient Diffusion ....................................................................................................... 267 9.4.1.4. Wall Damping .............................................................................................................. 268 9.4.2. Peters Flame Speed Model .................................................................................................... 268 9.4.2.1. Peters Flame Speed Model for LES ................................................................................ 269 9.5. Extended Coherent Flamelet Model Theory ................................................................................... 270 9.5.1. Closure for ECFM Source Terms ............................................................................................. 272 9.5.2.Turbulent Flame Speed in ECFM ............................................................................................ 274 9.5.3. LES and ECFM ...................................................................................................................... 274 9.6. Calculation of Properties ............................................................................................................... 276 9.6.1. Calculation of Temperature ................................................................................................... 277 9.6.1.1. Adiabatic Temperature Calculation ............................................................................... 277 9.6.1.2. Non-Adiabatic Temperature Calculation ....................................................................... 277 9.6.2. Calculation of Density .......................................................................................................... 277 9.6.3. Laminar Flame Speed ........................................................................................................... 278 9.6.4. Unburnt Density and Thermal Diffusivity ............................................................................... 278 10. Partially Premixed Combustion ........................................................................................................ 279 10.1. Overview .................................................................................................................................... 279 10.2. Limitations .................................................................................................................................. 279 10.3. Partially Premixed Combustion Theory ........................................................................................ 280 10.3.1. Chemical Equilibrium and Steady Diffusion Flamelet Models ............................................... 280 10.3.2. Flamelet Generated Manifold (FGM) model ......................................................................... 281 10.3.2.1. Premixed FGMs .......................................................................................................... 281 10.3.2.2. Diffusion FGMs .......................................................................................................... 283 10.3.3. FGM Turbulent Closure ....................................................................................................... 283 10.3.4. Calculation of Mixture Properties ........................................................................................ 285 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 10.3.5. Calculation of Unburnt Properties ....................................................................................... 286 10.3.6. Laminar Flame Speed ......................................................................................................... 286 11. Composition PDF Transport .............................................................................................................. 289 11.1. Overview and Limitations ............................................................................................................ 289 11.2. Composition PDF Transport Theory ............................................................................................. 289 11.3. The Lagrangian Solution Method ................................................................................................. 290 11.3.1. Particle Convection ............................................................................................................ 291 11.3.2. Particle Mixing ................................................................................................................... 292 11.3.2.1. The Modified Curl Model ............................................................................................ 292 11.3.2.2. The IEM Model ........................................................................................................... 292 11.3.2.3. The EMST Model ........................................................................................................ 293 11.3.2.4. Liquid Reactions ........................................................................................................ 293 11.3.3. Particle Reaction ................................................................................................................. 293 11.4. The Eulerian Solution Method ..................................................................................................... 294 11.4.1. Reaction ............................................................................................................................. 295 11.4.2. Mixing ................................................................................................................................ 295 11.4.3. Correction .......................................................................................................................... 295 11.4.4. Calculation of Composition Mean and Variance ................................................................... 296 12. Chemistry Acceleration ..................................................................................................................... 297 12.1. Overview and Limitations ............................................................................................................ 297 12.2. In-Situ Adaptive Tabulation (ISAT) ................................................................................................ 297 12.3. Dynamic Mechanism Reduction .................................................................................................. 299 12.3.1. Directed Relation Graph (DRG) Method for Mechanism Reduction ....................................... 300 12.4. Chemistry Agglomeration ........................................................................................................... 301 12.4.1. Binning Algorithm .............................................................................................................. 302 12.5. Chemical Mechanism Dimension Reduction ................................................................................ 304 12.5.1. Selecting the Represented Species ...................................................................................... 304 12.6. Dynamic Cell Clustering with ANSYS CHEMKIN-CFD Solver ........................................................... 305 13. Engine Ignition .................................................................................................................................. 307 13.1. Spark Model ................................................................................................................................ 307 13.1.1. Overview and Limitations ................................................................................................... 307 13.1.2. Spark Model Theory ............................................................................................................ 307 13.1.3. ECFM Spark Model Variants ................................................................................................. 310 13.2. Autoignition Models ................................................................................................................... 311 13.2.1. Model Overview ................................................................................................................. 311 13.2.2. Model Limitations .............................................................................................................. 311 13.2.3. Ignition Model Theory ........................................................................................................ 312 13.2.3.1. Transport of Ignition Species ...................................................................................... 312 13.2.3.2. Knock Modeling ........................................................................................................ 312 13.2.3.2.1. Modeling of the Source Term ............................................................................. 313 13.2.3.2.2. Correlations ...................................................................................................... 313 13.2.3.2.3. Energy Release .................................................................................................. 314 13.2.3.3. Ignition Delay Modeling ............................................................................................. 314 13.2.3.3.1. Modeling of the Source Term ............................................................................. 314 13.2.3.3.2. Correlations ...................................................................................................... 315 13.2.3.3.3. Energy Release .................................................................................................. 315 13.3. Crevice Model ............................................................................................................................. 315 13.3.1. Overview ........................................................................................................................... 315 13.3.1.1. Model Parameters ...................................................................................................... 316 13.3.2. Limitations ......................................................................................................................... 317 13.3.3. Crevice Model Theory ......................................................................................................... 318 14. Pollutant Formation .......................................................................................................................... 319

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Theory Guide 14.1. NOx Formation ........................................................................................................................... 319 14.1.1. Overview ........................................................................................................................... 319 14.1.1.1. NOx Modeling in ANSYS Fluent .................................................................................. 319 14.1.1.2. NOx Formation and Reduction in Flames .................................................................... 320 14.1.2. Governing Equations for NOx Transport .............................................................................. 320 14.1.3. Thermal NOx Formation ...................................................................................................... 321 14.1.3.1. Thermal NOx Reaction Rates ...................................................................................... 321 14.1.3.2. The Quasi-Steady Assumption for [N] ......................................................................... 321 14.1.3.3. Thermal NOx Temperature Sensitivity ......................................................................... 322 14.1.3.4. Decoupled Thermal NOx Calculations ......................................................................... 322 14.1.3.5. Approaches for Determining O Radical Concentration ................................................ 322 14.1.3.5.1. Method 1: Equilibrium Approach ....................................................................... 322 14.1.3.5.2. Method 2: Partial Equilibrium Approach ............................................................. 323 14.1.3.5.3. Method 3: Predicted O Approach ....................................................................... 323 14.1.3.6. Approaches for Determining OH Radical Concentration .............................................. 323 14.1.3.6.1. Method 1: Exclusion of OH Approach ................................................................. 323 14.1.3.6.2. Method 2: Partial Equilibrium Approach ............................................................. 323 14.1.3.6.3. Method 3: Predicted OH Approach ..................................................................... 324 14.1.3.7. Summary ................................................................................................................... 324 14.1.4. Prompt NOx Formation ....................................................................................................... 324 14.1.4.1. Prompt NOx Combustion Environments ..................................................................... 324 14.1.4.2. Prompt NOx Mechanism ............................................................................................ 324 14.1.4.3. Prompt NOx Formation Factors .................................................................................. 325 14.1.4.4. Primary Reaction ....................................................................................................... 325 14.1.4.5. Modeling Strategy ..................................................................................................... 325 14.1.4.6. Rate for Most Hydrocarbon Fuels ................................................................................ 326 14.1.4.7. Oxygen Reaction Order .............................................................................................. 326 14.1.5. Fuel NOx Formation ............................................................................................................ 327 14.1.5.1. Fuel-Bound Nitrogen ................................................................................................. 327 14.1.5.2. Reaction Pathways ..................................................................................................... 327 14.1.5.3. Fuel NOx from Gaseous and Liquid Fuels .................................................................... 327 14.1.5.3.1. Fuel NOx from Intermediate Hydrogen Cyanide (HCN) ....................................... 328 14.1.5.3.1.1. HCN Production in a Gaseous Fuel ............................................................ 328 14.1.5.3.1.2. HCN Production in a Liquid Fuel ................................................................ 328 14.1.5.3.1.3. HCN Consumption .................................................................................... 329 14.1.5.3.1.4. HCN Sources in the Transport Equation ..................................................... 329 14.1.5.3.1.5. NOx Sources in the Transport Equation ..................................................... 329 14.1.5.3.2. Fuel NOx from Intermediate Ammonia (NH3) ..................................................... 330 14.1.5.3.2.1. NH3 Production in a Gaseous Fuel ............................................................. 330 14.1.5.3.2.2. NH3 Production in a Liquid Fuel ................................................................ 330 14.1.5.3.2.3. NH3 Consumption .................................................................................... 331 14.1.5.3.2.4. NH3 Sources in the Transport Equation ..................................................... 331 14.1.5.3.2.5. NOx Sources in the Transport Equation ..................................................... 331 14.1.5.3.3. Fuel NOx from Coal ........................................................................................... 332 14.1.5.3.3.1. Nitrogen in Char and in Volatiles ............................................................... 332 14.1.5.3.3.2. Coal Fuel NOx Scheme A ........................................................................... 332 14.1.5.3.3.3. Coal Fuel NOx Scheme B ........................................................................... 332 14.1.5.3.3.4. HCN Scheme Selection ............................................................................. 333 14.1.5.3.3.5. NOx Reduction on Char Surface ................................................................ 333 14.1.5.3.3.5.1. BET Surface Area .............................................................................. 334 14.1.5.3.3.5.2. HCN from Volatiles ........................................................................... 334 14.1.5.3.3.6. Coal Fuel NOx Scheme C ........................................................................... 334 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 14.1.5.3.3.7. Coal Fuel NOx Scheme D ........................................................................... 335 14.1.5.3.3.8. NH3 Scheme Selection ............................................................................. 336 14.1.5.3.3.8.1. NH3 from Volatiles ........................................................................... 336 14.1.5.3.4. Fuel Nitrogen Partitioning for HCN and NH3 Intermediates ................................ 336 14.1.6. NOx Formation from Intermediate N2O ............................................................................... 337 14.1.6.1. N2O - Intermediate NOx Mechanism .......................................................................... 337 14.1.7. NOx Reduction by Reburning ............................................................................................. 338 14.1.7.1. Instantaneous Approach ............................................................................................ 338 14.1.7.2. Partial Equilibrium Approach ..................................................................................... 339 14.1.7.2.1. NOx Reduction Mechanism ............................................................................... 339 14.1.8. NOx Reduction by SNCR ..................................................................................................... 341 14.1.8.1. Ammonia Injection .................................................................................................... 341 14.1.8.2. Urea Injection ............................................................................................................ 342 14.1.8.3. Transport Equations for Urea, HNCO, and NCO ............................................................ 343 14.1.8.4. Urea Production due to Reagent Injection .................................................................. 344 14.1.8.5. NH3 Production due to Reagent Injection ................................................................... 344 14.1.8.6. HNCO Production due to Reagent Injection ................................................................ 344 14.1.9. NOx Formation in Turbulent Flows ...................................................................................... 345 14.1.9.1. The Turbulence-Chemistry Interaction Model ............................................................. 345 14.1.9.2. The PDF Approach ..................................................................................................... 346 14.1.9.3. The General Expression for the Mean Reaction Rate .................................................... 346 14.1.9.4. The Mean Reaction Rate Used in ANSYS Fluent ........................................................... 346 14.1.9.5. Statistical Independence ............................................................................................ 346 14.1.9.6. The Beta PDF Option .................................................................................................. 347 14.1.9.7. The Gaussian PDF Option ........................................................................................... 347 14.1.9.8. The Calculation Method for the Variance .................................................................... 347 14.2. SOx Formation ............................................................................................................................ 348 14.2.1. Overview ........................................................................................................................... 348 14.2.1.1. The Formation of SOx ................................................................................................. 348 14.2.2. Governing Equations for SOx Transport ............................................................................... 349 14.2.3. Reaction Mechanisms for Sulfur Oxidation .......................................................................... 350 14.2.4. SO2 and H2S Production in a Gaseous Fuel ......................................................................... 351 14.2.5. SO2 and H2S Production in a Liquid Fuel ............................................................................. 352 14.2.6. SO2 and H2S Production from Coal ..................................................................................... 352 14.2.6.1. SO2 and H2S from Char .............................................................................................. 352 14.2.6.2. SO2 and H2S from Volatiles ........................................................................................ 352 14.2.7. SOx Formation in Turbulent Flows ....................................................................................... 353 14.2.7.1. The Turbulence-Chemistry Interaction Model ............................................................. 353 14.2.7.2. The PDF Approach ..................................................................................................... 353 14.2.7.3. The Mean Reaction Rate ............................................................................................. 353 14.2.7.4. The PDF Options ........................................................................................................ 353 14.3. Soot Formation ........................................................................................................................... 353 14.3.1. Overview and Limitations ................................................................................................... 354 14.3.1.1. Predicting Soot Formation ......................................................................................... 354 14.3.1.2. Restrictions on Soot Modeling ................................................................................... 354 14.3.2. Soot Model Theory ............................................................................................................. 355 14.3.2.1. The One-Step Soot Formation Model .......................................................................... 355 14.3.2.2. The Two-Step Soot Formation Model .......................................................................... 356 14.3.2.2.1. Soot Generation Rate ........................................................................................ 356 14.3.2.2.2. Nuclei Generation Rate ...................................................................................... 357 14.3.2.3. The Moss-Brookes Model ........................................................................................... 357 14.3.2.3.1. The Moss-Brookes-Hall Model ............................................................................ 359

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Theory Guide 14.3.2.3.2. Soot Formation in Turbulent Flows .................................................................... 360 14.3.2.3.2.1. The Turbulence-Chemistry Interaction Model ............................................ 360 14.3.2.3.2.2. The PDF Approach .................................................................................... 361 14.3.2.3.2.3. The Mean Reaction Rate ........................................................................... 361 14.3.2.3.2.4. The PDF Options ....................................................................................... 361 14.3.2.3.3. The Effect of Soot on the Radiation Absorption Coefficient ................................. 361 14.3.2.4. The Method of Moments Model ................................................................................. 361 14.3.2.4.1. Soot Particle Population Balance ....................................................................... 361 14.3.2.4.2. Moment Transport Equations ............................................................................ 363 14.3.2.4.3. Nucleation ........................................................................................................ 363 14.3.2.4.4. Coagulation ...................................................................................................... 365 14.3.2.4.5. Surface Growth and Oxidation ........................................................................... 368 14.3.2.4.6. Soot Aggregation .............................................................................................. 370 14.4. Decoupled Detailed Chemistry Model ......................................................................................... 374 14.4.1. Overview ........................................................................................................................... 374 14.4.1.1. Limitations ................................................................................................................ 375 14.4.2. Decoupled Detailed Chemistry Model Theory ..................................................................... 375 15. Aerodynamically Generated Noise ................................................................................................... 377 15.1. Overview .................................................................................................................................... 377 15.1.1. Direct Method .................................................................................................................... 377 15.1.2. Integral Method Based on Acoustic Analogy ....................................................................... 378 15.1.3. Broadband Noise Source Models ........................................................................................ 379 15.2. Acoustics Model Theory .............................................................................................................. 379 15.2.1. The Ffowcs-Williams and Hawkings Model .......................................................................... 379 15.2.2. Broadband Noise Source Models ........................................................................................ 382 15.2.2.1. Proudman’s Formula .................................................................................................. 382 15.2.2.2.The Jet Noise Source Model ........................................................................................ 383 15.2.2.3. The Boundary Layer Noise Source Model .................................................................... 384 15.2.2.4. Source Terms in the Linearized Euler Equations ........................................................... 385 15.2.2.5. Source Terms in Lilley’s Equation ................................................................................ 385 16. Discrete Phase ................................................................................................................................... 387 16.1. Introduction ............................................................................................................................... 387 16.1.1. The Euler-Lagrange Approach ............................................................................................. 387 16.2. Particle Motion Theory ................................................................................................................ 388 16.2.1. Equations of Motion for Particles ........................................................................................ 388 16.2.1.1. Particle Force Balance ................................................................................................ 388 16.2.1.2. Particle Torque Balance .............................................................................................. 388 16.2.1.3. Inclusion of the Gravity Term ...................................................................................... 389 16.2.1.4. Other Forces .............................................................................................................. 389 16.2.1.5. Forces in Moving Reference Frames ............................................................................ 389 16.2.1.6. Thermophoretic Force ................................................................................................ 390 16.2.1.7. Brownian Force .......................................................................................................... 390 16.2.1.8. Saffman’s Lift Force .................................................................................................... 391 16.2.1.9. Magnus Lift Force ...................................................................................................... 391 16.2.2. Turbulent Dispersion of Particles ......................................................................................... 392 16.2.2.1. Stochastic Tracking .................................................................................................... 392 16.2.2.1.1. The Integral Time .............................................................................................. 393 16.2.2.1.2. The Discrete Random Walk Model ...................................................................... 393 16.2.2.1.3. Using the DRW Model ....................................................................................... 394 16.2.2.2. Particle Cloud Tracking ............................................................................................... 395 16.2.2.2.1. Using the Cloud Model ...................................................................................... 397 16.2.3. Integration of Particle Equation of Motion ........................................................................... 397 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 16.3. Laws for Drag Coefficients ........................................................................................................... 399 16.3.1. Spherical Drag Law ............................................................................................................. 400 16.3.2. Non-spherical Drag Law ..................................................................................................... 400 16.3.3. Stokes-Cunningham Drag Law ............................................................................................ 400 16.3.4. High-Mach-Number Drag Law ............................................................................................ 401 16.3.5. Dynamic Drag Model Theory .............................................................................................. 401 16.3.6. Dense Discrete Phase Model Drag Laws .............................................................................. 401 16.3.7. Bubbly Flow Drag Laws ...................................................................................................... 402 16.3.7.1. Ishii-Zuber Drag Model .............................................................................................. 402 16.3.7.2. Grace Drag Model ...................................................................................................... 403 16.3.8. Rotational Drag Law ........................................................................................................... 403 16.4. Laws for Heat and Mass Exchange ............................................................................................... 404 16.4.1. Inert Heating or Cooling (Law 1/Law 6) ............................................................................... 404 16.4.2. Droplet Vaporization (Law 2) ............................................................................................... 406 16.4.2.1. Mass Transfer During Law 2—Diffusion Controlled Model ........................................... 407 16.4.2.2. Mass Transfer During Law 2—Convection/Diffusion Controlled Model ........................ 408 16.4.2.3. Defining the Saturation Vapor Pressure and Diffusion Coefficient (or Binary Diffusivity) ......................................................................................................................................... 408 16.4.2.4. Defining the Boiling Point and Latent Heat ................................................................. 409 16.4.2.5. Heat Transfer to the Droplet ....................................................................................... 410 16.4.3. Droplet Boiling (Law 3) ....................................................................................................... 412 16.4.4. Devolatilization (Law 4) ...................................................................................................... 413 16.4.4.1. Choosing the Devolatilization Model .......................................................................... 413 16.4.4.2.The Constant Rate Devolatilization Model ................................................................... 413 16.4.4.3. The Single Kinetic Rate Model .................................................................................... 414 16.4.4.4. The Two Competing Rates (Kobayashi) Model ............................................................. 415 16.4.4.5. The CPD Model .......................................................................................................... 415 16.4.4.5.1. General Description .......................................................................................... 415 16.4.4.5.2. Reaction Rates .................................................................................................. 416 16.4.4.5.3. Mass Conservation ............................................................................................ 417 16.4.4.5.4. Fractional Change in the Coal Mass .................................................................... 417 16.4.4.5.5. CPD Inputs ........................................................................................................ 418 16.4.4.5.6. Particle Swelling During Devolatilization ............................................................ 419 16.4.4.5.7. Heat Transfer to the Particle During Devolatilization ........................................... 420 16.4.5. Surface Combustion (Law 5) ............................................................................................... 420 16.4.5.1. The Diffusion-Limited Surface Reaction Rate Model .................................................... 421 16.4.5.2. The Kinetic/Diffusion Surface Reaction Rate Model ..................................................... 421 16.4.5.3. The Intrinsic Model .................................................................................................... 422 16.4.5.4. The Multiple Surface Reactions Model ........................................................................ 423 16.4.5.4.1. Limitations ........................................................................................................ 424 16.4.5.5. Heat and Mass Transfer During Char Combustion ....................................................... 424 16.4.6. Multicomponent Particle Definition (Law 7) ........................................................................ 424 16.4.6.1. Raoult’s Law .............................................................................................................. 426 16.4.6.2. Peng-Robinson Real Gas Model .................................................................................. 426 16.5. Vapor Liquid Equilibrium Theory .................................................................................................. 426 16.6. Physical Property Averaging ........................................................................................................ 428 16.7. Wall-Particle Reflection Model Theory .......................................................................................... 430 16.7.1. Rough Wall Model .............................................................................................................. 432 16.8. Wall-Jet Model Theory ................................................................................................................. 434 16.9. Wall-Film Model Theory ............................................................................................................... 435 16.9.1. Introduction ....................................................................................................................... 435 16.9.2. Interaction During Impact with a Boundary ......................................................................... 436

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Theory Guide 16.9.2.1. The Stanton-Rutland Model ....................................................................................... 437 16.9.2.1.1. Regime Definition ............................................................................................. 437 16.9.2.1.2. Rebound ........................................................................................................... 438 16.9.2.1.3. Splashing .......................................................................................................... 438 16.9.2.2. The Kuhnke Model ..................................................................................................... 443 16.9.2.2.1. Regime definition ............................................................................................. 443 16.9.2.2.2. Rebound ........................................................................................................... 446 16.9.2.2.3. Splashing .......................................................................................................... 446 16.9.3. Separation Criteria .............................................................................................................. 449 16.9.4. Conservation Equations for Wall-Film Particles .................................................................... 449 16.9.4.1. Momentum ............................................................................................................... 449 16.9.4.2. Mass Transfer from the Film ........................................................................................ 450 16.9.4.2.1. Film Vaporization and Boiling ............................................................................ 450 16.9.4.2.2. Film Condensation ............................................................................................ 453 16.9.4.3. Energy Transfer from the Film ..................................................................................... 454 16.10. Wall Erosion .............................................................................................................................. 456 16.10.1. Finnie Erosion Model ........................................................................................................ 457 16.10.2. Oka Erosion Model ........................................................................................................... 457 16.10.3. McLaury Erosion Model .................................................................................................... 458 16.10.4. Accretion ......................................................................................................................... 459 16.11. Particle–Wall Impingement Heat Transfer Theory ....................................................................... 460 16.12. Atomizer Model Theory ............................................................................................................. 462 16.12.1. The Plain-Orifice Atomizer Model ...................................................................................... 462 16.12.1.1. Internal Nozzle State ................................................................................................ 464 16.12.1.2. Coefficient of Discharge ........................................................................................... 465 16.12.1.3. Exit Velocity ............................................................................................................. 466 16.12.1.4. Spray Angle ............................................................................................................. 467 16.12.1.5. Droplet Diameter Distribution .................................................................................. 467 16.12.2. The Pressure-Swirl Atomizer Model ................................................................................... 468 16.12.2.1. Film Formation ........................................................................................................ 469 16.12.2.2. Sheet Breakup and Atomization ............................................................................... 470 16.12.3. The Air-Blast/Air-Assist Atomizer Model ............................................................................. 472 16.12.4.The Flat-Fan Atomizer Model ............................................................................................. 473 16.12.5. The Effervescent Atomizer Model ...................................................................................... 474 16.13. Secondary Breakup Model Theory ............................................................................................. 475 16.13.1. Taylor Analogy Breakup (TAB) Model ................................................................................. 475 16.13.1.1. Introduction ............................................................................................................ 475 16.13.1.2. Use and Limitations ................................................................................................. 476 16.13.1.3. Droplet Distortion .................................................................................................... 476 16.13.1.4. Size of Child Droplets ............................................................................................... 477 16.13.1.5. Velocity of Child Droplets ......................................................................................... 478 16.13.1.6. Droplet Breakup ...................................................................................................... 478 16.13.2. Wave Breakup Model ........................................................................................................ 479 16.13.2.1. Introduction ............................................................................................................ 479 16.13.2.2. Use and Limitations ................................................................................................. 480 16.13.2.3. Jet Stability Analysis ................................................................................................. 480 16.13.2.4. Droplet Breakup ...................................................................................................... 481 16.13.3. KHRT Breakup Model ........................................................................................................ 482 16.13.3.1. Introduction ............................................................................................................ 482 16.13.3.2. Use and Limitations ................................................................................................. 482 16.13.3.3. Liquid Core Length .................................................................................................. 482 16.13.3.4. Rayleigh-Taylor Breakup ........................................................................................... 483 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 16.13.3.5. Droplet Breakup Within the Liquid Core .................................................................... 484 16.13.3.6. Droplet Breakup Outside the Liquid Core .................................................................. 484 16.13.4. Stochastic Secondary Droplet (SSD) Model ........................................................................ 484 16.13.4.1. Theory ..................................................................................................................... 484 16.14. Collision and Droplet Coalescence Model Theory ....................................................................... 485 16.14.1. Introduction ..................................................................................................................... 485 16.14.2. Use and Limitations .......................................................................................................... 486 16.14.3. Theory .............................................................................................................................. 486 16.14.3.1. Probability of Collision ............................................................................................. 486 16.14.3.2. Collision Outcomes .................................................................................................. 487 16.15. Discrete Element Method Collision Model .................................................................................. 488 16.15.1. Theory .............................................................................................................................. 488 16.15.1.1. The Spring Collision Law .......................................................................................... 489 16.15.1.2. The Spring-Dashpot Collision Law ............................................................................ 490 16.15.1.3. The Hertzian Collision Law ....................................................................................... 490 16.15.1.4. The Hertzian-Dashpot Collision Law ......................................................................... 491 16.15.1.5. The Friction Collision Law ......................................................................................... 491 16.15.1.6. Rolling Friction Collision Law for DEM ....................................................................... 492 16.15.1.7. DEM Parcels ............................................................................................................. 493 16.15.1.8. Cartesian Collision Mesh .......................................................................................... 493 16.16. One-Way and Two-Way Coupling ............................................................................................... 494 16.16.1. Coupling Between the Discrete and Continuous Phases .................................................... 494 16.16.2. Momentum Exchange ...................................................................................................... 495 16.16.3. Heat Exchange ................................................................................................................. 495 16.16.4. Mass Exchange ................................................................................................................. 496 16.16.5. Under-Relaxation of the Interphase Exchange Terms ......................................................... 496 16.16.6. Interphase Exchange During Stochastic Tracking ............................................................... 498 16.16.7. Interphase Exchange During Cloud Tracking ..................................................................... 498 16.17. Node Based Averaging .............................................................................................................. 498 17. Multiphase Flows .............................................................................................................................. 501 17.1. Introduction ............................................................................................................................... 501 17.1.1. Multiphase Flow Regimes ................................................................................................... 501 17.1.1.1. Gas-Liquid or Liquid-Liquid Flows .............................................................................. 501 17.1.1.2. Gas-Solid Flows .......................................................................................................... 502 17.1.1.3. Liquid-Solid Flows ...................................................................................................... 502 17.1.1.4. Three-Phase Flows ..................................................................................................... 502 17.1.2. Examples of Multiphase Systems ........................................................................................ 503 17.2. Choosing a General Multiphase Model ........................................................................................ 504 17.2.1. Approaches to Multiphase Modeling .................................................................................. 504 17.2.1.1. The Euler-Euler Approach ........................................................................................... 504 17.2.1.1.1. The VOF Model .................................................................................................. 504 17.2.1.1.2. The Mixture Model ............................................................................................ 505 17.2.1.1.3.The Eulerian Model ............................................................................................ 505 17.2.2. Model Comparisons ........................................................................................................... 505 17.2.2.1. Detailed Guidelines ................................................................................................... 506 17.2.2.1.1. The Effect of Particulate Loading ........................................................................ 506 17.2.2.1.2. The Significance of the Stokes Number .............................................................. 507 17.2.2.1.2.1. Examples .................................................................................................. 507 17.2.2.1.3. Other Considerations ........................................................................................ 508 17.2.3.Time Schemes in Multiphase Flow ....................................................................................... 508 17.2.4. Stability and Convergence .................................................................................................. 509 17.3. Volume of Fluid (VOF) Model Theory ............................................................................................ 510

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Theory Guide 17.3.1. Overview of the VOF Model ................................................................................................ 510 17.3.2. Limitations of the VOF Model .............................................................................................. 510 17.3.3. Steady-State and Transient VOF Calculations ....................................................................... 510 17.3.4. Volume Fraction Equation ................................................................................................... 511 17.3.4.1. The Implicit Formulation ............................................................................................ 511 17.3.4.2.The Explicit Formulation ............................................................................................. 512 17.3.4.3. Interpolation Near the Interface ................................................................................. 513 17.3.4.3.1. The Geometric Reconstruction Scheme ............................................................. 514 17.3.4.3.2. The Donor-Acceptor Scheme ............................................................................. 515 17.3.4.3.3. The Compressive Interface Capturing Scheme for Arbitrary Meshes (CICSAM) ..... 515 17.3.4.3.4. The Compressive Scheme and Interface-Model-based Variants ........................... 516 17.3.4.3.5. Bounded Gradient Maximization (BGM) ............................................................. 516 17.3.5. Material Properties ............................................................................................................. 517 17.3.6. Momentum Equation ......................................................................................................... 517 17.3.7. Energy Equation ................................................................................................................. 517 17.3.8. Additional Scalar Equations ................................................................................................ 518 17.3.9. Surface Tension and Adhesion ............................................................................................ 518 17.3.9.1. Surface Tension ......................................................................................................... 518 17.3.9.1.1. The Continuum Surface Force Model ................................................................. 518 17.3.9.1.2. The Continuum Surface Stress Model ................................................................. 519 17.3.9.1.3. Comparing the CSS and CSF Methods ................................................................ 520 17.3.9.1.4. When Surface Tension Effects Are Important ...................................................... 520 17.3.9.2. Wall Adhesion ............................................................................................................ 520 17.3.9.3. Jump Adhesion .......................................................................................................... 521 17.3.10. Open Channel Flow .......................................................................................................... 522 17.3.10.1. Upstream Boundary Conditions ............................................................................... 522 17.3.10.1.1. Pressure Inlet .................................................................................................. 522 17.3.10.1.2. Mass Flow Rate ................................................................................................ 523 17.3.10.1.3. Volume Fraction Specification .......................................................................... 523 17.3.10.2. Downstream Boundary Conditions ........................................................................... 523 17.3.10.2.1. Pressure Outlet ................................................................................................ 523 17.3.10.2.2. Outflow Boundary ........................................................................................... 523 17.3.10.2.3. Backflow Volume Fraction Specification ........................................................... 524 17.3.10.3. Numerical Beach Treatment ..................................................................................... 524 17.3.11. Open Channel Wave Boundary Conditions ........................................................................ 525 17.3.11.1. Airy Wave Theory ..................................................................................................... 527 17.3.11.2. Stokes Wave Theories ............................................................................................... 527 17.3.11.3. Cnoidal/Solitary Wave Theory ................................................................................... 528 17.3.11.4. Choosing a Wave Theory .......................................................................................... 530 17.3.11.5. Superposition of Waves ............................................................................................ 532 17.3.11.6. Modeling of Random Waves Using Wave Spectrum ................................................... 533 17.3.11.6.1. Definitions ...................................................................................................... 533 17.3.11.6.2. Wave Spectrum Implementation Theory .......................................................... 533 17.3.11.6.2.1. Long-Crested Random Waves (Unidirectional) ......................................... 533 17.3.11.6.2.1.1. Pierson-Moskowitz Spectrum ......................................................... 533 17.3.11.6.2.1.2. JONSWAP Spectrum ....................................................................... 534 17.3.11.6.2.1.3. TMA Spectrum ............................................................................... 534 17.3.11.6.2.2. Short-Crested Random Waves (Multi-Directional) .................................... 534 17.3.11.6.2.2.1. Cosine-2s Power Function (Frequency Independent) ....................... 535 17.3.11.6.2.2.2. Hyperbolic Function (Frequency Dependent) ................................. 535 17.3.11.6.2.3. Superposition of Individual Wave Components Using the Wave Spectrum ........................................................................................................................... 536 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 17.3.11.6.3. Choosing a Wave Spectrum and Inputs ............................................................ 537 17.3.11.7. Nomenclature for Open Channel Waves .................................................................... 539 17.3.12. Coupled Level-Set and VOF Model .................................................................................... 540 17.3.12.1. Theory ..................................................................................................................... 540 17.3.12.1.1. Surface Tension Force ...................................................................................... 541 17.3.12.1.2. Re-initialization of the Level-set Function via the Geometrical Method ............. 542 17.3.12.2. Limitations .............................................................................................................. 543 17.4. Mixture Model Theory ................................................................................................................. 543 17.4.1. Overview ........................................................................................................................... 544 17.4.2. Limitations of the Mixture Model ........................................................................................ 544 17.4.3. Continuity Equation ........................................................................................................... 545 17.4.4. Momentum Equation ......................................................................................................... 545 17.4.5. Energy Equation ................................................................................................................. 546 17.4.6. Relative (Slip) Velocity and the Drift Velocity ........................................................................ 546 17.4.7. Volume Fraction Equation for the Secondary Phases ............................................................ 548 17.4.8. Granular Properties ............................................................................................................ 548 17.4.8.1. Collisional Viscosity .................................................................................................... 548 17.4.8.2. Kinetic Viscosity ......................................................................................................... 548 17.4.9. Granular Temperature ......................................................................................................... 549 17.4.10. Solids Pressure ................................................................................................................. 549 17.4.11. Interfacial Area Concentration .......................................................................................... 550 17.4.11.1. Hibiki-Ishii Model ..................................................................................................... 550 17.4.11.2. Ishii-Kim Model ........................................................................................................ 551 17.4.11.3. Yao-Morel Model ...................................................................................................... 552 17.5. Eulerian Model Theory ................................................................................................................ 553 17.5.1. Overview of the Eulerian Model .......................................................................................... 554 17.5.2. Limitations of the Eulerian Model ........................................................................................ 554 17.5.3. Volume Fraction Equation ................................................................................................... 555 17.5.4. Conservation Equations ...................................................................................................... 556 17.5.4.1. Equations in General Form ......................................................................................... 556 17.5.4.1.1. Conservation of Mass ........................................................................................ 556 17.5.4.1.2. Conservation of Momentum .............................................................................. 556 17.5.4.1.3. Conservation of Energy ..................................................................................... 557 17.5.4.2. Equations Solved by ANSYS Fluent ............................................................................. 557 17.5.4.2.1. Continuity Equation .......................................................................................... 557 17.5.4.2.2. Fluid-Fluid Momentum Equations ...................................................................... 557 17.5.4.2.3. Fluid-Solid Momentum Equations ...................................................................... 558 17.5.4.2.4. Conservation of Energy ..................................................................................... 558 17.5.5. Interfacial Area Concentration ............................................................................................ 558 17.5.6. Interphase Exchange Coefficients ....................................................................................... 559 17.5.6.1. Fluid-Fluid Exchange Coefficient ................................................................................ 560 17.5.6.1.1. Schiller and Naumann Model ............................................................................. 560 17.5.6.1.2. Morsi and Alexander Model ............................................................................... 561 17.5.6.1.3. Symmetric Model .............................................................................................. 561 17.5.6.1.4. Grace et al. Model .............................................................................................. 562 17.5.6.1.5. Tomiyama et al. Model ....................................................................................... 563 17.5.6.1.6. Ishii Model ........................................................................................................ 564 17.5.6.1.7. Universal Drag Laws for Bubble-Liquid and Droplet-Gas Flows ........................... 564 17.5.6.1.7.1. Bubble-Liquid Flow .................................................................................. 565 17.5.6.1.7.2. Droplet-Gas Flow ...................................................................................... 565 17.5.6.2. Fluid-Solid Exchange Coefficient ................................................................................ 566 17.5.6.3. Solid-Solid Exchange Coefficient ................................................................................ 569

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Theory Guide 17.5.6.4. Drag Modification ...................................................................................................... 569 17.5.6.4.1. Brucato et al. Correlation ................................................................................... 569 17.5.7. Lift Force ............................................................................................................................ 570 17.5.7.1. Lift Coefficient Models ............................................................................................... 570 17.5.7.1.1. Moraga Lift Force Model .................................................................................... 571 17.5.7.1.2. Saffman-Mei Lift Force Model ............................................................................ 571 17.5.7.1.3. Legendre-Magnaudet Lift Force Model .............................................................. 572 17.5.7.1.4. Tomiyama Lift Force Model ................................................................................ 572 17.5.8. Wall Lubrication Force ........................................................................................................ 573 17.5.8.1. Wall Lubrication Models ............................................................................................. 573 17.5.8.1.1. Antal et al. Model .............................................................................................. 573 17.5.8.1.2.Tomiyama Model ............................................................................................... 574 17.5.8.1.3. Frank Model ...................................................................................................... 574 17.5.8.1.4. Hosokawa Model .............................................................................................. 574 17.5.9. Turbulent Dispersion Force ................................................................................................. 575 17.5.9.1. Models for Turbulent Dispersion Force ....................................................................... 575 17.5.9.1.1. Lopez de Bertodano Model ............................................................................... 576 17.5.9.1.2. Simonin Model .................................................................................................. 576 17.5.9.1.3. Burns et al. Model .............................................................................................. 576 17.5.9.1.4. Diffusion in VOF Model ...................................................................................... 576 17.5.9.2. Limiting Functions for the Turbulent Dispersion Force ................................................ 577 17.5.10. Virtual Mass Force ............................................................................................................. 578 17.5.11. Solids Pressure ................................................................................................................. 578 17.5.11.1. Radial Distribution Function ..................................................................................... 580 17.5.12. Maximum Packing Limit in Binary Mixtures ....................................................................... 581 17.5.13. Solids Shear Stresses ......................................................................................................... 581 17.5.13.1. Collisional Viscosity .................................................................................................. 581 17.5.13.2. Kinetic Viscosity ....................................................................................................... 582 17.5.13.3. Bulk Viscosity ........................................................................................................... 582 17.5.13.4. Frictional Viscosity ................................................................................................... 582 17.5.14. Granular Temperature ....................................................................................................... 583 17.5.15. Description of Heat Transfer .............................................................................................. 585 17.5.15.1. The Heat Exchange Coefficient ................................................................................. 585 17.5.15.1.1. Constant ......................................................................................................... 586 17.5.15.1.2. Nusselt Number .............................................................................................. 586 17.5.15.1.3. Ranz-Marshall Model ....................................................................................... 586 17.5.15.1.4. Tomiyama Model ............................................................................................. 586 17.5.15.1.5. Hughmark Model ............................................................................................ 586 17.5.15.1.6. Gunn Model .................................................................................................... 587 17.5.15.1.7. Two-Resistance Model ..................................................................................... 587 17.5.15.1.8. Fixed To Saturation Temperature ...................................................................... 588 17.5.15.1.9. User Defined ................................................................................................... 588 17.5.16. Turbulence Models ........................................................................................................... 588 17.5.16.1. k- ε Turbulence Models ............................................................................................. 589 17.5.16.1.1. k- ε Mixture Turbulence Model ......................................................................... 589 17.5.16.1.2. k- ε Dispersed Turbulence Model ..................................................................... 590 17.5.16.1.2.1. Assumptions .......................................................................................... 590 17.5.16.1.2.2. Turbulence in the Continuous Phase ....................................................... 591 17.5.16.1.2.3. Turbulence in the Dispersed Phase .......................................................... 592 17.5.16.1.3. k- ε Turbulence Model for Each Phase ............................................................... 592 17.5.16.1.3.1. Transport Equations ................................................................................ 592 17.5.16.2. RSM Turbulence Models ........................................................................................... 593 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 17.5.16.2.1. RSM Dispersed Turbulence Model .................................................................... 594 17.5.16.2.2. RSM Mixture Turbulence Model ....................................................................... 594 17.5.16.3. Turbulence Interaction Models ................................................................................. 595 17.5.16.3.1. Simonin et al. .................................................................................................. 595 17.5.16.3.1.1. Formulation in Dispersed Turbulence Models .......................................... 595 17.5.16.3.1.1.1. Continuous Phase .......................................................................... 595 17.5.16.3.1.1.2. Dispersed Phases ........................................................................... 596 17.5.16.3.1.2. Formulation in Per Phase Turbulence Models ........................................... 597 17.5.16.3.2. Troshko-Hassan ............................................................................................... 597 17.5.16.3.2.1. Troshko-Hassan Formulation in Mixture Turbulence Models ..................... 597 17.5.16.3.2.2. Troshko-Hassan Formulation in Dispersed Turbulence Models ................. 598 17.5.16.3.2.2.1. Continuous Phase .......................................................................... 598 17.5.16.3.2.2.2. Dispersed Phases ........................................................................... 598 17.5.16.3.2.3.Troshko-Hassan Formulation in Per-Phase Turbulence Models .................. 598 17.5.16.3.2.3.1. Continuous Phase .......................................................................... 598 17.5.16.3.2.3.2. Dispersed Phases ........................................................................... 599 17.5.16.3.3. Sato ................................................................................................................ 599 17.5.16.3.4. None ............................................................................................................... 599 17.5.17. Solution Method in ANSYS Fluent ..................................................................................... 599 17.5.17.1. The Pressure-Correction Equation ............................................................................. 599 17.5.17.2. Volume Fractions ..................................................................................................... 600 17.5.18. Dense Discrete Phase Model ............................................................................................. 600 17.5.18.1. Limitations .............................................................................................................. 601 17.5.18.2. Granular Temperature .............................................................................................. 601 17.5.19. Multi-Fluid VOF Model ...................................................................................................... 602 17.5.20. Wall Boiling Models .......................................................................................................... 603 17.5.20.1. Overview ................................................................................................................. 603 17.5.20.2. RPI Model ................................................................................................................ 604 17.5.20.3. Non-equilibrium Subcooled Boiling .......................................................................... 606 17.5.20.4. Critical Heat Flux ...................................................................................................... 607 17.5.20.4.1. Wall Heat Flux Partition .................................................................................... 607 17.5.20.4.2. Flow Regime Transition ................................................................................... 608 17.5.20.5. Interfacial Momentum Transfer ................................................................................. 609 17.5.20.5.1. Interfacial Area ................................................................................................ 609 17.5.20.5.2. Bubble and Droplet Diameters ........................................................................ 609 17.5.20.5.2.1. Bubble Diameter .................................................................................... 609 17.5.20.5.2.2. Droplet Diameter .................................................................................... 610 17.5.20.5.3. Interfacial Drag Force ...................................................................................... 610 17.5.20.5.4. Interfacial Lift Force ......................................................................................... 610 17.5.20.5.5. Turbulent Dispersion Force .............................................................................. 610 17.5.20.5.6. Wall Lubrication Force ..................................................................................... 610 17.5.20.5.7. Virtual Mass Force ........................................................................................... 610 17.5.20.6. Interfacial Heat Transfer ............................................................................................ 611 17.5.20.6.1. Interface to Liquid Heat Transfer ...................................................................... 611 17.5.20.6.2. Interface to Vapor Heat Transfer ....................................................................... 611 17.5.20.7. Mass Transfer ........................................................................................................... 611 17.5.20.7.1. Mass Transfer From the Wall to Vapor ............................................................... 611 17.5.20.7.2. Interfacial Mass Transfer .................................................................................. 611 17.5.20.8. Turbulence Interactions ............................................................................................ 611 17.6. Wet Steam Model Theory ............................................................................................................ 611 17.6.1. Overview of the Wet Steam Model ...................................................................................... 612 17.6.2. Limitations of the Wet Steam Model .................................................................................... 612

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Theory Guide 17.6.3. Wet Steam Flow Equations .................................................................................................. 612 17.6.4. Phase Change Model .......................................................................................................... 613 17.6.5. Built-in Thermodynamic Wet Steam Properties .................................................................... 615 17.6.5.1. Equation of State ....................................................................................................... 615 17.6.5.2. Saturated Vapor Line .................................................................................................. 616 17.6.5.3. Saturated Liquid Line ................................................................................................. 616 17.6.5.4. Mixture Properties ..................................................................................................... 616 17.7. Modeling Mass Transfer in Multiphase Flows ................................................................................ 616 17.7.1. Source Terms due to Mass Transfer ...................................................................................... 617 17.7.1.1. Mass Equation ........................................................................................................... 617 17.7.1.2. Momentum Equation ................................................................................................. 617 17.7.1.3. Energy Equation ........................................................................................................ 617 17.7.1.4. Species Equation ....................................................................................................... 618 17.7.1.5. Other Scalar Equations ............................................................................................... 618 17.7.2. Unidirectional Constant Rate Mass Transfer ......................................................................... 618 17.7.3. UDF-Prescribed Mass Transfer ............................................................................................. 618 17.7.4. Cavitation Models .............................................................................................................. 618 17.7.4.1. Limitations of the Cavitation Models .......................................................................... 619 17.7.4.1.1. Limitations of Cavitation with the VOF Model ..................................................... 620 17.7.4.2. Vapor Transport Equation ........................................................................................... 620 17.7.4.3. Bubble Dynamics Consideration ................................................................................ 621 17.7.4.4. Singhal et al. Model .................................................................................................... 621 17.7.4.5. Zwart-Gerber-Belamri Model ..................................................................................... 623 17.7.4.6. Schnerr and Sauer Model ........................................................................................... 624 17.7.4.7. Turbulence Factor ...................................................................................................... 625 17.7.4.8. Additional Guidelines for the Cavitation Models ......................................................... 626 17.7.4.9. Extended Cavitation Model Capabilities ..................................................................... 628 17.7.4.9.1. Multiphase Cavitation Models ........................................................................... 628 17.7.4.9.2. Multiphase Species Transport Cavitation Model ................................................. 628 17.7.5. Evaporation-Condensation Model ....................................................................................... 629 17.7.5.1. Lee Model ................................................................................................................. 629 17.7.5.2. Thermal Phase Change Model .................................................................................... 631 17.7.6. Interphase Species Mass Transfer ........................................................................................ 632 17.7.6.1. Modeling Approach ................................................................................................... 633 17.7.6.1.1. Equilibrium Model ............................................................................................. 633 17.7.6.1.2. Two-Resistance Model ....................................................................................... 634 17.7.6.2. Species Mass Transfer Models ..................................................................................... 636 17.7.6.2.1. Raoult’s Law ...................................................................................................... 636 17.7.6.2.2. Henry’s Law ...................................................................................................... 637 17.7.6.2.3. Equilibrium Ratio .............................................................................................. 638 17.7.6.3. Mass Transfer Coefficient Models ................................................................................ 638 17.7.6.3.1. Constant ........................................................................................................... 638 17.7.6.3.2. Sherwood Number ............................................................................................ 638 17.7.6.3.3. Ranz-Marshall Model ......................................................................................... 639 17.7.6.3.4. Hughmark Model .............................................................................................. 639 17.7.6.3.5. User-Defined ..................................................................................................... 639 17.8. Modeling Species Transport in Multiphase Flows ......................................................................... 639 17.8.1. Limitations ......................................................................................................................... 640 17.8.2. Mass and Momentum Transfer with Multiphase Species Transport ....................................... 641 17.8.2.1. Source Terms Due to Heterogeneous Reactions .......................................................... 641 17.8.2.1.1. Mass Transfer .................................................................................................... 641 17.8.2.1.2. Momentum Transfer .......................................................................................... 641 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 17.8.2.1.3. Species Transfer ................................................................................................ 642 17.8.2.1.4. Heat Transfer ..................................................................................................... 642 17.8.3. The Stiff Chemistry Solver ................................................................................................... 643 17.8.4. Heterogeneous Phase Interaction ....................................................................................... 643 18. Solidification and Melting ................................................................................................................. 645 18.1. Overview .................................................................................................................................... 645 18.2. Limitations .................................................................................................................................. 646 18.3. Introduction ............................................................................................................................... 646 18.4. Energy Equation ......................................................................................................................... 646 18.5. Momentum Equations ................................................................................................................ 647 18.6. Turbulence Equations .................................................................................................................. 648 18.7. Species Equations ....................................................................................................................... 648 18.8. Back Diffusion ............................................................................................................................. 650 18.9. Pull Velocity for Continuous Casting ............................................................................................ 650 18.10. Contact Resistance at Walls ........................................................................................................ 652 18.11. Thermal and Solutal Buoyancy ................................................................................................... 652 19. Eulerian Wall Films ............................................................................................................................ 655 19.1. Introduction ............................................................................................................................... 655 19.2. Mass, Momentum, and Energy Conservation Equations for Wall Film ............................................. 656 19.2.1. Film Sub-Models ................................................................................................................. 657 19.2.1.1. DPM Collection .......................................................................................................... 657 19.2.1.2. Particle-Wall Interaction ............................................................................................. 657 19.2.1.3. Film Separation .......................................................................................................... 657 19.2.1.3.1. Separation Criteria ............................................................................................ 657 19.2.1.3.1.1. Foucart Separation ................................................................................... 658 19.2.1.3.1.2. O’Rourke Separation ................................................................................. 658 19.2.1.3.1.3. Friedrich Separation ................................................................................. 658 19.2.1.4. Film Stripping ............................................................................................................ 659 19.2.1.5. Secondary Phase Accretion ........................................................................................ 660 19.2.1.6. Coupling of Wall Film with Mixture Species Transport ................................................. 661 19.2.2. Boundary Conditions .......................................................................................................... 661 19.2.3. Obtaining Film Velocity Without Solving the Momentum Equations .................................... 662 19.2.3.1. Shear-Driven Film Velocity ......................................................................................... 662 19.2.3.2. Gravity-Driven Film Velocity ....................................................................................... 662 19.3. Passive Scalar Equation for Wall Film ............................................................................................ 663 19.4. Numerical Schemes and Solution Algorithm ................................................................................ 664 19.4.1. Temporal Differencing Schemes .......................................................................................... 664 19.4.1.1. First-Order Explicit Method ........................................................................................ 664 19.4.1.2. First-Order Implicit Method ........................................................................................ 665 19.4.1.3. Second-Order Implicit Method ................................................................................... 665 19.4.2. Spatial Differencing Schemes .............................................................................................. 666 19.4.3. Solution Algorithm ............................................................................................................. 667 19.4.3.1. Steady Flow ............................................................................................................... 667 19.4.3.2. Transient Flow ........................................................................................................... 667 20. Electric Potential ............................................................................................................................... 669 20.1. Overview and Limitations ............................................................................................................ 669 20.2. Electric Potential Equation ........................................................................................................... 669 20.3. Energy Equation Source Term ...................................................................................................... 670 21. Solver Theory .................................................................................................................................... 671 21.1. Overview of Flow Solvers ............................................................................................................ 671 21.1.1. Pressure-Based Solver ......................................................................................................... 672 21.1.1.1. The Pressure-Based Segregated Algorithm ................................................................. 672

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Theory Guide 21.1.1.2. The Pressure-Based Coupled Algorithm ...................................................................... 673 21.1.2. Density-Based Solver .......................................................................................................... 674 21.2. General Scalar Transport Equation: Discretization and Solution ..................................................... 676 21.2.1. Solving the Linear System ................................................................................................... 678 21.3. Discretization .............................................................................................................................. 678 21.3.1. Spatial Discretization .......................................................................................................... 678 21.3.1.1. First-Order Upwind Scheme ....................................................................................... 679 21.3.1.2. Power-Law Scheme .................................................................................................... 679 21.3.1.3. Second-Order Upwind Scheme .................................................................................. 680 21.3.1.4. First-to-Higher Order Blending ................................................................................... 681 21.3.1.5. Central-Differencing Scheme ..................................................................................... 681 21.3.1.6. Bounded Central Differencing Scheme ....................................................................... 682 21.3.1.7. QUICK Scheme .......................................................................................................... 682 21.3.1.8. Third-Order MUSCL Scheme ....................................................................................... 683 21.3.1.9. Modified HRIC Scheme .............................................................................................. 683 21.3.1.10. High Order Term Relaxation ..................................................................................... 685 21.3.2. Temporal Discretization ...................................................................................................... 685 21.3.2.1. Implicit Time Integration ............................................................................................ 686 21.3.2.2. Bounded Second Order Implicit Time Integration ....................................................... 686 21.3.2.2.1. Limitations ........................................................................................................ 686 21.3.2.3. Explicit Time Integration ............................................................................................ 687 21.3.3. Evaluation of Gradients and Derivatives .............................................................................. 687 21.3.3.1. Green-Gauss Theorem ............................................................................................... 687 21.3.3.2. Green-Gauss Cell-Based Gradient Evaluation .............................................................. 688 21.3.3.3. Green-Gauss Node-Based Gradient Evaluation ............................................................ 688 21.3.3.4. Least Squares Cell-Based Gradient Evaluation ............................................................. 688 21.3.4. Gradient Limiters ................................................................................................................ 690 21.3.4.1. Standard Limiter ........................................................................................................ 690 21.3.4.2. Multidimensional Limiter ........................................................................................... 691 21.3.4.3. Differentiable Limiter ................................................................................................. 691 21.4. Pressure-Based Solver ................................................................................................................. 691 21.4.1. Discretization of the Momentum Equation .......................................................................... 692 21.4.1.1. Pressure Interpolation Schemes ................................................................................. 692 21.4.2. Discretization of the Continuity Equation ............................................................................ 693 21.4.2.1. Density Interpolation Schemes ................................................................................... 694 21.4.3. Pressure-Velocity Coupling ................................................................................................. 694 21.4.3.1. Segregated Algorithms .............................................................................................. 695 21.4.3.1.1. SIMPLE .............................................................................................................. 695 21.4.3.1.2. SIMPLEC ........................................................................................................... 696 21.4.3.1.2.1. Skewness Correction ................................................................................ 696 21.4.3.1.3. PISO .................................................................................................................. 696 21.4.3.1.3.1. Neighbor Correction ................................................................................. 696 21.4.3.1.3.2. Skewness Correction ................................................................................ 697 21.4.3.1.3.3. Skewness - Neighbor Coupling ................................................................. 697 21.4.3.2. Fractional-Step Method (FSM) .................................................................................... 697 21.4.3.3. Coupled Algorithm .................................................................................................... 697 21.4.3.3.1. Limitations ........................................................................................................ 698 21.4.4. Steady-State Iterative Algorithm ......................................................................................... 699 21.4.4.1. Under-Relaxation of Variables .................................................................................... 699 21.4.4.2. Under-Relaxation of Equations ................................................................................... 699 21.4.5. Time-Advancement Algorithm ............................................................................................ 699 21.4.5.1. Iterative Time-Advancement Scheme ......................................................................... 700 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 21.4.5.1.1. The Frozen Flux Formulation .............................................................................. 701 21.4.5.2. Non-Iterative Time-Advancement Scheme .................................................................. 702 21.5. Density-Based Solver ................................................................................................................... 704 21.5.1. Governing Equations in Vector Form ................................................................................... 704 21.5.2. Preconditioning ................................................................................................................. 705 21.5.3. Convective Fluxes ............................................................................................................... 707 21.5.3.1. Roe Flux-Difference Splitting Scheme ......................................................................... 707 21.5.3.2. AUSM+ Scheme ......................................................................................................... 707 21.5.3.3. Low Diffusion Roe Flux Difference Splitting Scheme ................................................... 708 21.5.4. Steady-State Flow Solution Methods ................................................................................... 708 21.5.4.1. Explicit Formulation ................................................................................................... 709 21.5.4.1.1. Implicit Residual Smoothing .............................................................................. 709 21.5.4.2. Implicit Formulation .................................................................................................. 710 21.5.4.2.1. Convergence Acceleration for Stretched Meshes ................................................ 710 21.5.5. Unsteady Flows Solution Methods ...................................................................................... 711 21.5.5.1. Explicit Time Stepping ............................................................................................... 711 21.5.5.2. Implicit Time Stepping (Dual-Time Formulation) ......................................................... 711 21.6. Pseudo Transient Under-Relaxation ............................................................................................. 713 21.6.1. Automatic Pseudo Transient Time Step ............................................................................... 713 21.7. Multigrid Method ........................................................................................................................ 715 21.7.1. Approach ........................................................................................................................... 715 21.7.1.1. The Need for Multigrid ............................................................................................... 715 21.7.1.2. The Basic Concept in Multigrid ................................................................................... 716 21.7.1.3. Restriction and Prolongation ...................................................................................... 716 21.7.1.4. Unstructured Multigrid .............................................................................................. 717 21.7.2. Multigrid Cycles .................................................................................................................. 717 21.7.2.1. The V and W Cycles .................................................................................................... 717 21.7.3. Algebraic Multigrid (AMG) .................................................................................................. 721 21.7.3.1. AMG Restriction and Prolongation Operators ............................................................. 721 21.7.3.2. AMG Coarse Level Operator ....................................................................................... 722 21.7.3.3. The F Cycle ................................................................................................................ 722 21.7.3.4. The Flexible Cycle ...................................................................................................... 722 21.7.3.4.1. The Residual Reduction Rate Criteria .................................................................. 723 21.7.3.4.2. The Termination Criteria .................................................................................... 724 21.7.3.5. The Coupled and Scalar AMG Solvers .......................................................................... 724 21.7.3.5.1. Gauss-Seidel ..................................................................................................... 725 21.7.3.5.2. Incomplete Lower Upper (ILU) ........................................................................... 725 21.7.4. Full-Approximation Storage (FAS) Multigrid ......................................................................... 726 21.7.4.1. FAS Restriction and Prolongation Operators ............................................................... 727 21.7.4.2. FAS Coarse Level Operator ......................................................................................... 727 21.8. Hybrid Initialization ..................................................................................................................... 727 21.9. Full Multigrid (FMG) Initialization ................................................................................................. 729 21.9.1. Overview of FMG Initialization ............................................................................................ 729 21.9.2. Limitations of FMG Initialization .......................................................................................... 730 22. Adapting the Mesh ............................................................................................................................ 733 22.1. Static Adaption Process ............................................................................................................... 733 22.1.1. Hanging Node Adaption ..................................................................................................... 733 22.1.1.1. Hanging Node Refinement ......................................................................................... 734 22.1.1.2. Hanging Node Coarsening ......................................................................................... 735 22.2. Boundary Adaption ..................................................................................................................... 735 22.3. Gradient Adaption ...................................................................................................................... 737 22.3.1. Gradient Adaption Approach .............................................................................................. 737

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Theory Guide 22.3.2. Example of Steady Gradient Adaption ................................................................................. 739 22.4. Isovalue Adaption ....................................................................................................................... 741 22.5. Region Adaption ......................................................................................................................... 743 22.5.1. Defining a Region ............................................................................................................... 743 22.5.2. Region Adaption Example .................................................................................................. 744 22.6. Volume Adaption ........................................................................................................................ 745 22.6.1. Volume Adaption Approach ................................................................................................ 745 22.6.2. Volume Adaption Example .................................................................................................. 746 22.7.Yplus/Ystar Adaption ................................................................................................................... 747 22.7.1. Yplus/Ystar Adaption Approach .......................................................................................... 747 22.8. Anisotropic Adaption .................................................................................................................. 749 22.9. Geometry-Based Adaption .......................................................................................................... 749 22.9.1. Geometry-Based Adaption Approach .................................................................................. 750 22.9.1.1. Node Projection ......................................................................................................... 750 22.9.1.2. Example of Geometry-Based Adaption ....................................................................... 752 22.10. Registers ................................................................................................................................... 755 22.10.1. Adaption Registers ........................................................................................................... 755 22.10.2. Mask Registers .................................................................................................................. 756 23. Reporting Alphanumeric Data .......................................................................................................... 759 23.1. Fluxes Through Boundaries ......................................................................................................... 759 23.2. Forces on Boundaries .................................................................................................................. 760 23.2.1. Computing Forces, Moments, and the Center of Pressure ..................................................... 760 23.3. Surface Integration ..................................................................................................................... 762 23.3.1. Computing Surface Integrals .............................................................................................. 763 23.3.1.1. Area .......................................................................................................................... 763 23.3.1.2. Integral ...................................................................................................................... 764 23.3.1.3. Area-Weighted Average ............................................................................................. 764 23.3.1.4. Custom Vector Based Flux .......................................................................................... 764 23.3.1.5. Custom Vector Flux .................................................................................................... 764 23.3.1.6. Custom Vector Weighted Average .............................................................................. 764 23.3.1.7. Flow Rate ................................................................................................................... 764 23.3.1.8. Mass Flow Rate .......................................................................................................... 765 23.3.1.9. Mass-Weighted Average ............................................................................................ 765 23.3.1.10. Sum of Field Variable ................................................................................................ 765 23.3.1.11. Facet Average .......................................................................................................... 766 23.3.1.12. Facet Minimum ........................................................................................................ 766 23.3.1.13. Facet Maximum ....................................................................................................... 766 23.3.1.14. Vertex Average ......................................................................................................... 766 23.3.1.15. Vertex Minimum ...................................................................................................... 766 23.3.1.16. Vertex Maximum ...................................................................................................... 766 23.3.1.17. Standard-Deviation .................................................................................................. 766 23.3.1.18. Uniformity Index ...................................................................................................... 767 23.3.1.19. Volume Flow Rate .................................................................................................... 767 23.4. Volume Integration ..................................................................................................................... 768 23.4.1. Computing Volume Integrals .............................................................................................. 768 23.4.1.1. Volume ...................................................................................................................... 768 23.4.1.2. Sum .......................................................................................................................... 769 23.4.1.3. Sum*2Pi .................................................................................................................... 769 23.4.1.4. Volume Integral ......................................................................................................... 769 23.4.1.5. Volume-Weighted Average ......................................................................................... 769 23.4.1.6. Mass-Weighted Integral ............................................................................................. 769 23.4.1.7. Mass .......................................................................................................................... 770 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Theory Guide 23.4.1.8. Mass-Weighted Average ............................................................................................ A. Nomenclature ....................................................................................................................................... Bibliography ............................................................................................................................................. Index ........................................................................................................................................................

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770 771 775 807

Using This Manual This preface is divided into the following sections: 1.The Contents of This Manual 2.The Contents of the Fluent Manuals 3.Typographical Conventions 4. Mathematical Conventions 5.Technical Support

1. The Contents of This Manual The ANSYS Fluent Theory Guide provides you with theoretical information about the models used in ANSYS Fluent.

Important Under U.S. and international copyright law, ANSYS, Inc. is unable to distribute copies of the papers listed in the bibliography, other than those published internally by ANSYS, Inc. Use your library or a document delivery service to obtain copies of copyrighted papers. A brief description of what is in each chapter follows: • Basic Fluid Flow (p. 1), describes the governing equations and physical models used by ANSYS Fluent to compute fluid flow (including periodic flow, swirling and rotating flows, compressible flows, and inviscid flows). • Flows with Moving Reference Frames (p. 17), describes single moving reference frames, multiple moving reference frames, and mixing planes. • Flows Using Sliding and Dynamic Meshes (p. 33), describes sliding and deforming meshes. • Turbulence (p. 39), describes various turbulent flow models. • Heat Transfer (p. 139), describes the physical models used to compute heat transfer (including convective and conductive heat transfer, natural convection, radiative heat transfer, and periodic heat transfer). • Heat Exchangers (p. 181), describes the physical models used to simulate the performance of heat exchangers. • Species Transport and Finite-Rate Chemistry (p. 193), describes the finite-rate chemistry models. This chapter also provides information about modeling species transport in non-reacting flows. • Non-Premixed Combustion (p. 223), describes the non-premixed combustion model. • Premixed Combustion (p. 261), describes the premixed combustion model. • Partially Premixed Combustion (p. 279), describes the partially premixed combustion model. • Composition PDF Transport (p. 289), describes the composition PDF transport model. • Chemistry Acceleration (p. 297), describes the methods used to accelerate computations for detailed chemical mechanisms involving laminar and turbulent flames. • Engine Ignition (p. 307), describes the engine ignition models. Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Using This Manual • Pollutant Formation (p. 319), describes the models for the formation of NOx, SOx, and soot. • Aerodynamically Generated Noise (p. 377), describes the acoustics model. • Discrete Phase (p. 387), describes the discrete phase models. • Multiphase Flows (p. 501), describes the general multiphase models (VOF, mixture, and Eulerian). • Solidification and Melting (p. 645), describes the solidification and melting model. • Eulerian Wall Films (p. 655), describes the Eulerian wall film model. • Solver Theory (p. 671), describes the Fluent solvers. • Adapting the Mesh (p. 733), describes the solution-adaptive mesh refinement feature. • Reporting Alphanumeric Data (p. 759), describes how to obtain reports of fluxes, forces, surface integrals, and other solution data.

2. The Contents of the Fluent Manuals The manuals listed below form the Fluent product documentation set. They include descriptions of the procedures, commands, and theoretical details needed to use Fluent products. • Fluent Getting Started Guide contains general information about getting started with using Fluent and provides details about starting, running, and exiting the program. • Fluent Migration Manual contains information about transitioning from the previous release of Fluent, including details about new features, output changes, and text command list changes. • Fluent User's Guide contains detailed information about running a simulation using the solution mode of Fluent, including information about the user interface, reading and writing files, defining boundary conditions, setting up physical models, calculating a solution, analyzing your results, and revising the design. • ANSYS Fluent Meshing Migration Manual contains information about transitioning from the previous release of Fluent Meshing, including descriptions of new features and text command list changes. • ANSYS Fluent Meshing User's Guide contains detailed information about creating 3D meshes using the meshing mode of Fluent. Related video help on meshing can be found on the ANSYS How To Videos page. • Fluent in Workbench User's Guide contains information about getting started with and using Fluent within the Workbench environment. • Fluent Theory Guide contains reference information for how the physical models are implemented in Fluent. • Fluent Customization Manual contains information about writing / using user-defined functions (UDFs) and customizing the Fluent graphical user interface (GUI) for easy management of the data used by UDFs. • Fluent Tutorial Guide contains a number of examples of various flow problems with detailed instructions, commentary, and postprocessing of results.

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The Contents of the Fluent Manuals The latest updates of the ANSYS Fluent tutorials are available on the ANSYS Customer Portal. To access tutorials and their input files on the ANSYS Customer Portal, go to http://support.ansys.com/ training. • The latest updates of the ANSYS Fluent Meshing tutorials are available on the ANSYS Customer Portal. To access tutorials and their input files on the ANSYS Customer Portal, go to http://support.ansys.com/training. • Fluent Text Command List contains a brief description of each of the commands in Fluent’s solution mode text interface. • ANSYS Fluent Meshing Text Command List contains a brief description of each of the commands in Fluent’s meshing mode text interface. • ANSYS Fluent Advanced Add-On Modules contains information about the usage of the different advanced Fluent add-on modules, which are applicable for specific modeling needs. – Part I: ANSYS Fluent Battery Module contains information about the background and usage of Fluent's Battery Module that allows you to analyze the behavior of electric batteries. – Part II: ANSYS Fluent Continuous Fiber Module contains information about the background and usage of Fluent's Continuous Fiber Module that allows you to analyze the behavior of fiber flow, fiber properties, and coupling between fibers and the surrounding fluid due to the strong interaction that exists between the fibers and the surrounding gas. – Part III: ANSYS Fluent Macroscopic Particle Module contains information about the background and usage of Fluent's Macroscopic Particle Model (MPM) that predicts the behavior of large (macroscopic) particles and their interaction with the fluid flow, walls, and other particles. – Part IV: ANSYS Fluent Fuel Cell Modules contains information about the background and the usage of two separate add-on fuel cell models for Fluent that allow you to model polymer electrolyte membrane fuel cells (PEMFC), solid oxide fuel cells (SOFC), and electrolysis with Fluent. – Part V: ANSYS Fluent Magnetohydrodynamics (MHD) Module contains information about the background and usage of Fluent's Magnetohydrodynamics (MHD) Module that allows you to analyze the behavior of electrically conducting fluid flow under the influence of constant (DC) or oscillating (AC) electromagnetic fields. – Part VI: ANSYS Fluent Population Balance Module contains information about the background and usage of Fluent's Population Balance Module that allows you to analyze multiphase flows involving size distributions where particle population (as well as momentum, mass, and energy) require a balance equation. • Fluent as a Server User's Guide contains information about the usage of Fluent as a Server which allows you to connect to a Fluent session and issue commands from a remote client application. • Running ANSYS Fluent Using a Load Manager contains information about using third-party load managers with ANSYS Fluent. – Part I: Running ANSYS Fluent Under LSF contains information about using Fluent with Platform Computing’s LSF software, a distributed computing resource management tool. – Part II: Running ANSYS Fluent Under PBS Professional contains information about using Fluent with Altair PBS Professional, an open workload management tool for local and distributed environments.

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Using This Manual – Part III: Running ANSYS Fluent Under SGE contains information about using Fluent with Univa Grid Engine (formerly Sun Grid Engine) software, a distributed computing resource management tool. Related video help for Fluent can be found on the ANSYS How To Videos page.

3. Typographical Conventions Several typographical conventions are used in this manual’s text to help you find commands in the user interface. • Different type styles are used to indicate graphical user interface items and text interface items. For example: Iso-Surface dialog box surface/iso-surface text command • The text interface type style is also used when illustrating exactly what appears on the screen to distinguish it from the narrative text. In this context, user inputs are typically shown in boldface. For example, solve/initialize/set-fmg-initialization Customize your FMG initialization: set the number of multigrid levels [5] set FMG parameters on levels .. residual reduction on level 1 is: [0.001] number of cycles on level 1 is: [10] 100 residual reduction on level 2 is: [0.001] number of cycles on level 2 is: [50] 100

• Mini flow charts are used to guide you through the ribbon or the tree, leading you to a specific option, dialog box, or task page. The following tables list the meaning of each symbol in the mini flow charts. Table 1: Mini Flow Chart Symbol Descriptions Symbol

Indicated Action Look at the ribbon Look at the tree Double-click to open task page Select from task page Right-click the preceding item

For example, Setting Up Domain → Mesh → Transform → Translate... indicates selecting the Setting Up Domain ribbon tab, clicking Transform (in the Mesh group box) and selecting Translate..., as indicated in the figure below:

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Typographical Conventions

And Setup → Models → Viscous

Model → Realizable k-epsilon

indicates expanding the Setup and Models branches, right-clicking Viscous, and selecting Realizable k-epsilon from the Model sub-menu, as shown in the following figure:

And Setup →

Boundary Conditions →

porous-in

indicates opening the task page as shown below:

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Using This Manual

In this manual, mini flow charts usually accompany a description of a dialog box or command, or a screen illustration showing how to use the dialog box or command. They show you how to quickly access a command or dialog box without having to search the surrounding material. • In-text references to File ribbon tab selections can be indicated using a “/”. For example File/Write/Case... indicates clicking the File ribbon tab and selecting Case... from the Write submenu (which opens the Select File dialog box).

4. Mathematical Conventions • Where possible, vector quantities are displayed with a raised arrow (e.g., , ). Boldfaced characters are reserved for vectors and matrices as they apply to linear algebra (e.g., the identity matrix, ). • The operator , referred to as grad, nabla, or del, represents the partial derivative of a quantity with respect to all directions in the chosen coordinate system. In Cartesian coordinates, is defined to be (1) appears in several ways:

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Technical Support – The gradient of a scalar quantity is the vector whose components are the partial derivatives; for example, (2) – The gradient of a vector quantity is a second-order tensor; for example, in Cartesian coordinates, (3) This tensor is usually written as

(4)

– The divergence of a vector quantity, which is the inner product between

and a vector; for example, (5)

– The operator

, which is usually written as

and is known as the Laplacian; for example, (6)

is different from the expression

, which is defined as (7)

• An exception to the use of is found in the discussion of Reynolds stresses in Turbulence in the Fluent Theory Guide, where convention dictates the use of Cartesian tensor notation. In this chapter, you will also find that some velocity vector components are written as , , and instead of the conventional with directional subscripts.

5. Technical Support If you encounter difficulties while using ANSYS Fluent, please first refer to the section(s) of the manual containing information on the commands you are trying to use or the type of problem you are trying to solve. The product documentation is available from the online help, or from the ANSYS Customer Portal. To access documentation files on the ANSYS Customer Portal, go to http://support.ansys.com/ documentation. If you encounter an error, please write down the exact error message that appeared and note as much information as you can about what you were doing in ANSYS Fluent. Technical Support for ANSYS, Inc. products is provided either by ANSYS, Inc. directly or by one of our certified ANSYS Support Providers. Please check with the ANSYS Support Coordinator (ASC) at your Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Using This Manual company to determine who provides support for your company, or go to www.ansys.com and select Contacts> Contacts and Locations. If your support is provided by ANSYS, Inc. directly, Technical Support can be accessed quickly and efficiently from the ANSYS Customer Portal, which is available from the ANSYS Website (www.ansys.com) under Support > Customer Portal. The direct URL is: support.ansys.com. One of the many useful features of the Customer Portal is the Knowledge Resources Search, which can be found on the Home page of the Customer Portal. To use this feature, enter relevant text (error message, etc.) in the Knowledge Resources Search box and click the magnifying glass icon. These Knowledge Resources provide solutions and guidance on how to resolve installation and licensing issues quickly. NORTH AMERICA All ANSYS Products except Esterel, Apache and Reaction Design products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. Toll-Free Telephone: 1.800.711.7199 (Please have your Customer or Contact ID ready.) Support for University customers is provided only through the ANSYS Customer Portal. GERMANY ANSYS Mechanical Products Telephone: +49 (0) 8092 7005-55 (CADFEM) Email: [emailprotected] All ANSYS Products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. National Toll-Free Telephone: (Please have your Customer or Contact ID ready.) German language: 0800 181 8499 English language: 0800 181 1565 Austria: 0800 297 835 Switzerland: 0800 564 318 International Telephone: (Please have your Customer or Contact ID ready.) German language: +49 6151 152 9981 English language: +49 6151 152 9982 Email: [emailprotected] UNITED KINGDOM All ANSYS Products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. Telephone: Please have your Customer or Contact ID ready. UK: 0800 048 0462 Republic of Ireland: 1800 065 6642 Outside UK: +44 1235 420130 Email: [emailprotected] Support for University customers is provided only through the ANSYS Customer Portal. xxxvi

Technical Support

JAPAN CFX and Mechanical Products Telephone: +81-3-5324-7305 Email: Mechanical: [emailprotected] Fluent: [emailprotected]; CFX: [emailprotected]; Polyflow: [emailprotected]; Icepak Telephone: +81-3-5324-7444 Email: [emailprotected] Licensing and Installation Email: [emailprotected] INDIA All ANSYS Products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. Telephone: +91 1 800 209 3475 (toll free) or +91 20 6654 3000 (toll) (Please have your Customer or Contact ID ready.) FRANCE All ANSYS Products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. Toll-Free Telephone: +33 (0) 800 919 225 Toll Number: +33 (0) 170 489 087 (Please have your Customer or Contact ID ready.) Email: [emailprotected] Support for University customers is provided only through the ANSYS Customer Portal. BELGIUM All ANSYS Products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. Toll-Free Telephone: (0) 800 777 83 Toll Number: +32 2 620 0152 (Please have your Customer or Contact ID ready.) Email: [emailprotected] Support for University customers is provided only through the ANSYS Customer Portal. SWEDEN All ANSYS Products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. Telephone: +46 (0) 10 516 49 00 Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Using This Manual Email: [emailprotected] Support for University customers is provided only through the ANSYS Customer Portal. SPAIN and PORTUGAL All ANSYS Products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. Spain: Toll-Free Telephone: 900 933 407 Toll Number: +34 9178 78350 (Please have your Customer or Contact ID ready.) Portugal: Toll-Free Telephone: 800 880 513 (Portugal) Email: [emailprotected], [emailprotected] Support for University customers is provided only through the ANSYS Customer Portal. ITALY All ANSYS Products Web: Go to the ANSYS Customer Portal (http://support.ansys.com) and select the appropriate option. Toll-Free Telephone: 800 789 531 Toll Number: +39 02 00621386 (Please have your Customer or Contact ID ready.) Email: [emailprotected] Support for University customers is provided only through the ANSYS Customer Portal. TAIWAN, REPUBLIC OF CHINA Telephone: 866 22725 5828 K0REA Telephone: 82-2-3441-5000 CHINA Toll-Free Telephone: 400 819 8999 Toll Number: +86 10 82861715

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Chapter 1: Basic Fluid Flow This chapter describes the theoretical background for some of the basic physical models that ANSYS Fluent provides for fluid flow. The information in this chapter is presented in the following sections: 1.1. Overview of Physical Models in ANSYS Fluent 1.2. Continuity and Momentum Equations 1.3. User-Defined Scalar (UDS) Transport Equations 1.4. Periodic Flows 1.5. Swirling and Rotating Flows 1.6. Compressible Flows 1.7. Inviscid Flows For more information about:

See

Models for flows in moving zones (including sliding and dynamic meshes)

Flows with Moving Reference Frames (p. 17) and Flows Using Sliding and Dynamic Meshes (p. 33)

Models for turbulence

Turbulence (p. 39)

Models for heat transfer (including radiation)

Heat Transfer (p. 139)

Models for species transport and reacting flows

Species Transport and Finite-Rate Chemistry (p. 193) – Composition PDF Transport (p. 289)

Models for pollutant formation

Pollutant Formation (p. 319)

Models for discrete phase

Discrete Phase (p. 387)

Models for general multiphase

Multiphase Flows (p. 501)

Models for melting and solidification

Solidification and Melting (p. 645)

Models for porous media, porous jumps, and lumped parameter fans and radiators

Cell Zone and Boundary Conditions in the User’s Guide.

1.1. Overview of Physical Models in ANSYS Fluent ANSYS Fluent provides comprehensive modeling capabilities for a wide range of incompressible and compressible, laminar and turbulent fluid flow problems. Steady-state or transient analyses can be performed. In ANSYS Fluent, a broad range of mathematical models for transport phenomena (like heat transfer and chemical reactions) is combined with the ability to model complex geometries. Examples of ANSYS Fluent applications include laminar non-Newtonian flows in process equipment; conjugate heat transfer in turbomachinery and automotive engine components; pulverized coal combustion in utility boilers; external aerodynamics; flow through compressors, pumps, and fans; and multiphase flows in bubble columns and fluidized beds. To permit modeling of fluid flow and related transport phenomena in industrial equipment and processes, various useful features are provided. These include porous media, lumped parameter (fan and heat exchanger), streamwise-periodic flow and heat transfer, swirl, and moving reference frame models. The moving reference frame family of models includes the ability to model single or multiple reference Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Basic Fluid Flow frames. A time-accurate sliding mesh method, useful for modeling multiple stages in turbomachinery applications, for example, is also provided, along with the mixing plane model for computing time-averaged flow fields. Another very useful group of models in ANSYS Fluent is the set of free surface and multiphase flow models. These can be used for analysis of gas-liquid, gas-solid, liquid-solid, and gas-liquid-solid flows. For these types of problems, ANSYS Fluent provides the volume-of-fluid (VOF), mixture, and Eulerian models, as well as the discrete phase model (DPM). The DPM performs Lagrangian trajectory calculations for dispersed phases (particles, droplets, or bubbles), including coupling with the continuous phase. Examples of multiphase flows include channel flows, sprays, sedimentation, separation, and cavitation. Robust and accurate turbulence models are a vital component of the ANSYS Fluent suite of models. The turbulence models provided have a broad range of applicability, and they include the effects of other physical phenomena, such as buoyancy and compressibility. Particular care has been devoted to addressing issues of near-wall accuracy via the use of extended wall functions and zonal models. Various modes of heat transfer can be modeled, including natural, forced, and mixed convection with or without conjugate heat transfer, porous media, and so on. The set of radiation models and related submodels for modeling participating media are general and can take into account the complications of combustion. A particular strength of ANSYS Fluent is its ability to model combustion phenomena using a variety of models, including eddy dissipation and probability density function models. A host of other models that are very useful for reacting flow applications are also available, including coal and droplet combustion, surface reaction, and pollutant formation models.

1.2. Continuity and Momentum Equations For all flows, ANSYS Fluent solves conservation equations for mass and momentum. For flows involving heat transfer or compressibility, an additional equation for energy conservation is solved. For flows involving species mixing or reactions, a species conservation equation is solved or, if the non-premixed combustion model is used, conservation equations for the mixture fraction and its variance are solved. Additional transport equations are also solved when the flow is turbulent. In this section, the conservation equations for laminar flow in an inertial (non-accelerating) reference frame are presented. The equations that are applicable to moving reference frames are presented in Flows with Moving Reference Frames (p. 17). The conservation equations relevant to heat transfer, turbulence modeling, and species transport will be discussed in the chapters where those models are described. The Euler equations solved for inviscid flow are presented in Inviscid Flows (p. 14). For more information, see the following sections: 1.2.1.The Mass Conservation Equation 1.2.2. Momentum Conservation Equations

1.2.1. The Mass Conservation Equation The equation for conservation of mass, or continuity equation, can be written as follows: (1.1) Equation 1.1 (p. 2) is the general form of the mass conservation equation and is valid for incompressible as well as compressible flows. The source is the mass added to the continuous phase from the dispersed second phase (for example, due to vaporization of liquid droplets) and any user-defined sources.

2

User-Defined Scalar (UDS) Transport Equations For 2D axisymmetric geometries, the continuity equation is given by (1.2) where is the axial coordinate, velocity.

is the radial coordinate,

is the axial velocity, and

is the radial

1.2.2. Momentum Conservation Equations Conservation of momentum in an inertial (non-accelerating) reference frame is described by [30] (p. 776) (1.3) where is the static pressure, is the stress tensor (described below), and and are the gravitational body force and external body forces (for example, that arise from interaction with the dispersed phase), respectively. also contains other model-dependent source terms such as porous-media and user-defined sources. The stress tensor

is given by (1.4)

where is the molecular viscosity, is the unit tensor, and the second term on the right hand side is the effect of volume dilation. For 2D axisymmetric geometries, the axial and radial momentum conservation equations are given by

(1.5)

and

(1.6)

where (1.7) and is the swirl velocity. (See Swirling and Rotating Flows (p. 8) for information about modeling axisymmetric swirl.)

1.3. User-Defined Scalar (UDS) Transport Equations ANSYS Fluent can solve the transport equation for an arbitrary, user-defined scalar (UDS) in the same way that it solves the transport equation for a scalar such as species mass fraction. Extra scalar transport

3

Basic Fluid Flow equations may be needed in certain types of combustion applications or for example in plasma-enhanced surface reaction modeling. This section provides information on how you can specify user-defined scalar (UDS) transport equations to enhance the standard features of ANSYS Fluent. ANSYS Fluent allows you to define additional scalar transport equations in your model in the User-Defined Scalars Dialog Box. For more information about setting up user-defined scalar transport equations in ANSYS Fluent, see User-Defined Scalar (UDS) Transport Equations in the User's Guide. Information in this section is organized in the following subsections: 1.3.1. Single Phase Flow 1.3.2. Multiphase Flow

1.3.1. Single Phase Flow For an arbitrary scalar

, ANSYS Fluent solves the equation (1.8)

where

and

are the diffusion coefficient and source term you supplied for each of the

equations. Note that is therefore

scalar

is defined as a tensor in the case of anisotropic diffusivity. The diffusion term

For isotropic diffusivity,

could be written as

where I is the identity matrix.

For the steady-state case, ANSYS Fluent will solve one of the three following equations, depending on the method used to compute the convective flux: • If convective flux is not to be computed, ANSYS Fluent will solve the equation (1.9) where and equations.

are the diffusion coefficient and source term you supplied for each of the

scalar

• If convective flux is to be computed with mass flow rate, ANSYS Fluent will solve the equation (1.10) • It is also possible to specify a user-defined function to be used in the computation of convective flux. In this case, the user-defined mass flux is assumed to be of the form (1.11)

where

4

is the face vector area.

Periodic Flows

1.3.2. Multiphase Flow For multiphase flows, ANSYS Fluent solves transport equations for two types of scalars: per phase and mixture. For an arbitrary

scalar in phase-1, denoted by

, ANSYS Fluent solves the transport equation

inside the volume occupied by phase-l (1.12)

where and

,

, and

are the volume fraction, physical density, and velocity of phase-l, respectively.

are the diffusion coefficient and source term, respectively, which you will need to specify. In

this case, scalar

is associated only with one phase (phase-l) and is considered an individual field

variable of phase-l. The mass flux for phase-l is defined as (1.13)

If the transport variable described by scalar

represents the physical field that is shared between

phases, or is considered the same for each phase, then you should consider this scalar as being associated with a mixture of phases,

. In this case, the generic transport equation for the scalar is (1.14)

where mixture density according to

, mixture velocity

, and mixture diffusivity for the scalar

are calculated

(1.15) (1.16) (1.17) (1.18) (1.19) To calculate mixture diffusivity, you will need to specify individual diffusivities for each material associated with individual phases. Note that if the user-defined mass flux option is activated, then mass fluxes shown in Equation 1.13 (p. 5) and Equation 1.17 (p. 5) will need to be replaced in the corresponding scalar transport equations.

1.4. Periodic Flows Periodic flow occurs when the physical geometry of interest and the expected pattern of the flow/thermal solution have a periodically repeating nature. Two types of periodic flow can be modeled in ANSYS Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Basic Fluid Flow Fluent. In the first type, no pressure drop occurs across the periodic planes. In the second type, a pressure drop occurs across translationally periodic boundaries, resulting in “fully-developed” or “streamwiseperiodic” flow. This section discusses streamwise-periodic flow. A description of no-pressure-drop periodic flow is provided in Periodic Boundary Conditions in the Fluent User's Guide, and a description of streamwiseperiodic heat transfer is provided in Modeling Periodic Heat Transfer in the Fluent User's Guide. For more information about setting up periodic flows in ANSYS Fluent, see Periodic Flows in the Fluent User's Guide. Information about streamwise-periodic flow is presented in the following sections: 1.4.1. Overview 1.4.2. Limitations 1.4.3. Physics of Periodic Flows

1.4.1. Overview ANSYS Fluent provides the ability to calculate streamwise-periodic — or “fully-developed” — fluid flow. These flows are encountered in a variety of applications, including flows in compact heat exchanger channels and flows across tube banks. In such flow configurations, the geometry varies in a repeating manner along the direction of the flow, leading to a periodic fully-developed flow regime in which the flow pattern repeats in successive cycles. Other examples of streamwise-periodic flows include fullydeveloped flow in pipes and ducts. These periodic conditions are achieved after a sufficient entrance length, which depends on the flow Reynolds number and geometric configuration. Streamwise-periodic flow conditions exist when the flow pattern repeats over some length , with a constant pressure drop across each repeating module along the streamwise direction. Figure 1.1: Example of Periodic Flow in a 2D Heat Exchanger Geometry (p. 6) depicts one example of a periodically repeating flow of this type that has been modeled by including a single representative module. Figure 1.1: Example of Periodic Flow in a 2D Heat Exchanger Geometry

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Periodic Flows

1.4.2. Limitations The following limitations apply to modeling streamwise-periodic flow: • The flow must be incompressible. • When performing unsteady-state simulations with translational periodic boundary conditions, the specified pressure gradient is recommended. • If one of the density-based solvers is used, you can specify only the pressure jump; for the pressure-based solver, you can specify either the pressure jump or the mass flow rate. • No net mass addition through inlets/exits or extra source terms is allowed. • Species can be modeled only if inlets/exits (without net mass addition) are included in the problem. Reacting flows are not permitted. • Discrete phase and multiphase modeling are not allowed. • When you specify a periodic mass-flow rate, Fluent will assume that the entire flow rate passes through one periodic continuous face zone only.

1.4.3. Physics of Periodic Flows 1.4.3.1. Definition of the Periodic Velocity The assumption of periodicity implies that the velocity components repeat themselves in space as follows:

(1.20)

where is the position vector and is the periodic length vector of the domain considered (see Figure 1.2: Example of a Periodic Geometry (p. 7)). Figure 1.2: Example of a Periodic Geometry

1.4.3.2. Definition of the Streamwise-Periodic Pressure For viscous flows, the pressure is not periodic in the sense of Equation 1.20 (p. 7). Instead, the pressure drop between modules is periodic: Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Basic Fluid Flow (1.21) If one of the density-based solvers is used, is specified as a constant value. For the pressure-based solver, the local pressure gradient can be decomposed into two parts: the gradient of a periodic component,

, and the gradient of a linearly-varying component,

: (1.22)

where is the periodic pressure and is the linearly-varying component of the pressure. The periodic pressure is the pressure left over after subtracting out the linearly-varying pressure. The linearlyvarying component of the pressure results in a force acting on the fluid in the momentum equations. Because the value of is not known a priori, it must be iterated on until the mass flow rate that you have defined is achieved in the computational model. This correction of occurs in the pressure correction step of the SIMPLE, SIMPLEC, or PISO algorithm where the value of is updated based on the difference between the desired mass flow rate and the actual one. You have some control over the number of sub-iterations used to update . For more information about setting up parameters for in ANSYS Fluent, see Setting Parameters for the Calculation of β in the Fluent User's Guide.

1.5. Swirling and Rotating Flows Many important engineering flows involve swirl or rotation and ANSYS Fluent is well-equipped to model such flows. Swirling flows are common in combustion, with swirl introduced in burners and combustors in order to increase residence time and stabilize the flow pattern. Rotating flows are also encountered in turbomachinery, mixing tanks, and a variety of other applications. When you begin the analysis of a rotating or swirling flow, it is essential that you classify your problem into one of the following five categories of flow: • axisymmetric flows with swirl or rotation • fully three-dimensional swirling or rotating flows • flows requiring a moving reference frame • flows requiring multiple moving reference frames or mixing planes • flows requiring sliding meshes Modeling and solution procedures for the first two categories are presented in this section. The remaining three, which all involve “moving zones”, are discussed in Flows with Moving Reference Frames (p. 17). Information about rotating and swirling flows is provided in the following subsections: 1.5.1. Overview of Swirling and Rotating Flows 1.5.2. Physics of Swirling and Rotating Flows For more information about setting up swirling and rotating flows in ANSYS Fluent, see Swirling and Rotating Flows in the Fluent User's Guide.

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Swirling and Rotating Flows

1.5.1. Overview of Swirling and Rotating Flows 1.5.1.1. Axisymmetric Flows with Swirl or Rotation You can solve a 2D axisymmetric problem that includes the prediction of the circumferential or swirl velocity. The assumption of axisymmetry implies that there are no circumferential gradients in the flow, but that there may be nonzero circumferential velocities. Examples of axisymmetric flows involving swirl or rotation are depicted in Figure 1.3: Rotating Flow in a Cavity (p. 9) and Figure 1.4: Swirling Flow in a Gas Burner (p. 9). Figure 1.3: Rotating Flow in a Cavity

Figure 1.4: Swirling Flow in a Gas Burner

Your problem may be axisymmetric with respect to geometry and flow conditions but still include swirl or rotation. In this case, you can model the flow in 2D (that is, solve the axisymmetric problem) and include the prediction of the circumferential (or swirl) velocity. It is important to note that while the

9

Basic Fluid Flow assumption of axisymmetry implies that there are no circumferential gradients in the flow, there may still be nonzero swirl velocities.

1.5.1.1.1. Momentum Conservation Equation for Swirl Velocity The tangential momentum equation for 2D swirling flows may be written as (1.23)

where is the axial coordinate, and is the swirl velocity.

is the radial coordinate,

is the axial velocity,

is the radial velocity,

1.5.1.2. Three-Dimensional Swirling Flows When there are geometric changes and/or flow gradients in the circumferential direction, your swirling flow prediction requires a three-dimensional model. If you are planning a 3D ANSYS Fluent model that includes swirl or rotation, you should be aware of the setup constraints (Coordinate System Restrictions in the Fluent User's Guide). In addition, you may want to consider simplifications to the problem which might reduce it to an equivalent axisymmetric problem, especially for your initial modeling effort. Because of the complexity of swirling flows, an initial 2D study, in which you can quickly determine the effects of various modeling and design choices, can be very beneficial.

Important For 3D problems involving swirl or rotation, there are no special inputs required during the problem setup and no special solution procedures. Note, however, that you may want to use the cylindrical coordinate system for defining velocity-inlet boundary condition inputs, as described in Defining the Velocity in the User's Guide. Also, you may find the gradual increase of the rotational speed (set as a wall or inlet boundary condition) helpful during the solution process. For more information, see Improving Solution Stability by Gradually Increasing the Rotational or Swirl Speed in the User's Guide.

1.5.1.3. Flows Requiring a Moving Reference Frame If your flow involves a rotating boundary that moves through the fluid (for example, an impeller blade or a grooved or notched surface), you will need to use a moving reference frame to model the problem. Such applications are described in detail in Flow in a Moving Reference Frame (p. 18). If you have more than one rotating boundary (for example, several impellers in a row), you can use multiple reference frames (described in The Multiple Reference Frame Model (p. 22)) or mixing planes (described in The Mixing Plane Model (p. 25)).

1.5.2. Physics of Swirling and Rotating Flows In swirling flows, conservation of angular momentum ( or = constant) tends to create a free vortex flow, in which the circumferential velocity, , increases sharply as the radius, , decreases (with finally decaying to zero near as viscous forces begin to dominate). A tornado is one example of a free vortex. Figure 1.5: Typical Radial Distribution of Circumferential Velocity in a Free Vortex (p. 11) depicts the radial distribution of in a typical free vortex.

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Compressible Flows Figure 1.5: Typical Radial Distribution of Circumferential Velocity in a Free Vortex

It can be shown that for an ideal free vortex flow, the centrifugal forces created by the circumferential motion are in equilibrium with the radial pressure gradient: (1.24) As the distribution of angular momentum in a non-ideal vortex evolves, the form of this radial pressure gradient also changes, driving radial and axial flows in response to the highly non-uniform pressures that result. Thus, as you compute the distribution of swirl in your ANSYS Fluent model, you will also notice changes in the static pressure distribution and corresponding changes in the axial and radial flow velocities. It is this high degree of coupling between the swirl and the pressure field that makes the modeling of swirling flows complex. In flows that are driven by wall rotation, the motion of the wall tends to impart a forced vortex motion to the fluid, wherein or is constant. An important characteristic of such flows is the tendency of fluid with high angular momentum (for example, the flow near the wall) to be flung radially outward (see Figure 1.6: Stream Function Contours for Rotating Flow in a Cavity (p. 11) using the geometry of Figure 1.3: Rotating Flow in a Cavity (p. 9)). This is often referred to as “radial pumping”, since the rotating wall is pumping the fluid radially outward. Figure 1.6: Stream Function Contours for Rotating Flow in a Cavity

1.6. Compressible Flows Compressibility effects are encountered in gas flows at high velocity and/or in which there are large pressure variations. When the flow velocity approaches or exceeds the speed of sound of the gas or when the pressure change in the system ( ) is large, the variation of the gas density with pressure has a significant impact on the flow velocity, pressure, and temperature. Compressible flows create a Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Basic Fluid Flow unique set of flow physics for which you must be aware of the special input requirements and solution techniques described in this section. Figure 1.7: Transonic Flow in a Converging-Diverging Nozzle (p. 12) and Figure 1.8: Mach 0.675 Flow Over a Bump in a 2D Channel (p. 12) show examples of compressible flows computed using ANSYS Fluent. Figure 1.7: Transonic Flow in a Converging-Diverging Nozzle

Figure 1.8: Mach 0.675 Flow Over a Bump in a 2D Channel

For more information about setting up compressible flows in ANSYS Fluent, see Compressible Flows in the User's Guide. Information about compressible flows is provided in the following subsections: 1.6.1. When to Use the Compressible Flow Model 1.6.2. Physics of Compressible Flows 12

Compressible Flows

1.6.1. When to Use the Compressible Flow Model Compressible flows can be characterized by the value of the Mach number: (1.25) Here,

is the speed of sound in the gas: (1.26)

and

is the ratio of specific heats

.

When the Mach number is less than 1.0, the flow is termed subsonic. At Mach numbers much less than 1.0 ( or so), compressibility effects are negligible and the variation of the gas density with pressure can safely be ignored in your flow modeling. As the Mach number approaches 1.0 (which is referred to as the transonic flow regime), compressibility effects become important. When the Mach number exceeds 1.0, the flow is termed supersonic, and may contain shocks and expansion fans that can impact the flow pattern significantly. ANSYS Fluent provides a wide range of compressible flow modeling capabilities for subsonic, transonic, and supersonic flows.

1.6.2. Physics of Compressible Flows Compressible flows are typically characterized by the total pressure and total temperature of the flow. For an ideal gas, these quantities can be related to the static pressure and temperature by the following:

(1.27)

For constant

, Equation 1.27 (p. 13) reduces to (1.28) (1.29)

These relationships describe the variation of the static pressure and temperature in the flow as the velocity (Mach number) changes under isentropic conditions. For example, given a pressure ratio from inlet to exit (total to static), Equation 1.28 (p. 13) can be used to estimate the exit Mach number that would exist in a one-dimensional isentropic flow. For air, Equation 1.28 (p. 13) predicts a choked flow (Mach number of 1.0) at an isentropic pressure ratio, , of 0.5283. This choked flow condition will be established at the point of minimum flow area (for example, in the throat of a nozzle). In the subsequent area expansion the flow may either accelerate to a supersonic flow in which the pressure will continue to drop, or return to subsonic flow conditions, decelerating with a pressure rise. If a supersonic flow is exposed to an imposed pressure increase, a shock will occur, with a sudden pressure rise and deceleration accomplished across the shock.

1.6.2.1. Basic Equations for Compressible Flows Compressible flows are described by the standard continuity and momentum equations solved by ANSYS Fluent, and you do not need to enable any special physical models (other than the compressible

13

Basic Fluid Flow treatment of density as detailed below). The energy equation solved by ANSYS Fluent correctly incorporates the coupling between the flow velocity and the static temperature, and should be enabled whenever you are solving a compressible flow. In addition, if you are using the pressure-based solver, you should enable the viscous dissipation terms in Equation 5.1 (p. 140), which become important in high-Mach-number flows.

1.6.2.2. The Compressible Form of the Gas Law For compressible flows, the ideal gas law is written in the following form: (1.30) where

is the operating pressure defined in the Operating Conditions Dialog Box,

static pressure relative to the operating pressure, is the universal gas constant, and weight. The temperature, , will be computed from the energy equation.

is the local is the molecular

Some compressible flow problems involve fluids that do not behave as ideal gases. For example, flow under very high-pressure conditions cannot typically be modeled accurately using the ideal-gas assumption. Therefore, the real gas model described in Real Gas Models in the User's Guide should be used instead.

1.7. Inviscid Flows Inviscid flow analyses neglect the effect of viscosity on the flow and are appropriate for high-Reynoldsnumber applications where inertial forces tend to dominate viscous forces. One example for which an inviscid flow calculation is appropriate is an aerodynamic analysis of some high-speed projectile. In a case like this, the pressure forces on the body will dominate the viscous forces. Hence, an inviscid analysis will give you a quick estimate of the primary forces acting on the body. After the body shape has been modified to maximize the lift forces and minimize the drag forces, you can perform a viscous analysis to include the effects of the fluid viscosity and turbulent viscosity on the lift and drag forces. Another area where inviscid flow analyses are routinely used is to provide a good initial solution for problems involving complicated flow physics and/or complicated flow geometry. In a case like this, the viscous forces are important, but in the early stages of the calculation the viscous terms in the momentum equations will be ignored. Once the calculation has been started and the residuals are decreasing, you can turn on the viscous terms (by enabling laminar or turbulent flow) and continue the solution to convergence. For some very complicated flows, this may be the only way to get the calculation started. For more information about setting up inviscid flows in ANSYS Fluent, see Inviscid Flows in the User's Guide. Information about inviscid flows is provided in the following section. 1.7.1. Euler Equations

1.7.1. Euler Equations For inviscid flows, ANSYS Fluent solves the Euler equations. The mass conservation equation is the same as for a laminar flow, but the momentum and energy conservation equations are reduced due to the absence of molecular diffusion. In this section, the conservation equations for inviscid flow in an inertial (non-rotating) reference frame are presented. The equations that are applicable to non-inertial reference frames are described in Flows

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Inviscid Flows with Moving Reference Frames (p. 17). The conservation equations relevant for species transport and other models will be discussed in the chapters where those models are described.

1.7.1.1. The Mass Conservation Equation The equation for conservation of mass, or continuity equation, can be written as follows: (1.31) Equation 1.31 (p. 15) is the general form of the mass conservation equation and is valid for incompressible as well as compressible flows. The source is the mass added to the continuous phase from the dispersed second phase (for example, due to vaporization of liquid droplets) and any user-defined sources. For 2D axisymmetric geometries, the continuity equation is given by (1.32) where is the axial coordinate, velocity.

is the radial coordinate,

is the axial velocity, and

is the radial

1.7.1.2. Momentum Conservation Equations Conservation of momentum is described by (1.33) where

is the static pressure and

and

are the gravitational body force and external body forces

(for example, forces that arise from interaction with the dispersed phase), respectively. other model-dependent source terms such as porous-media and user-defined sources.

also contains

For 2D axisymmetric geometries, the axial and radial momentum conservation equations are given by (1.34) and (1.35) where (1.36)

1.7.1.3. Energy Conservation Equation Conservation of energy is described by (1.37)

15

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Chapter 2: Flows with Moving Reference Frames This chapter describes the theoretical background for modeling flows in moving reference frames. Information about using the various models in this chapter can be found in Modeling Flows with Moving Reference Frames in the User's Guide. The information in this chapter is presented in the following sections: 2.1. Introduction 2.2. Flow in a Moving Reference Frame 2.3. Flow in Multiple Reference Frames

2.1. Introduction ANSYS Fluent solves the equations of fluid flow and heat transfer by default in a stationary (or inertial) reference frame. However, there are many problems where it is advantageous to solve the equations in a moving (or non-inertial) reference frame. These problems typically involve moving parts, such as rotating blades, impellers, and moving walls, and it is the flow around the moving parts that is of interest. In most cases, the moving parts render the problem unsteady when viewed from a stationary frame. With a moving reference frame, however, the flow around the moving part can (with certain restrictions) be modeled as a steady-state problem with respect to the moving frame. ANSYS Fluent’s moving reference frame modeling capability allows you to model problems involving moving parts by allowing you to activate moving reference frames in selected cell zones. When a moving reference frame is activated, the equations of motion are modified to incorporate the additional acceleration terms that occur due to the transformation from the stationary to the moving reference frame. For many problems, it may be possible to refer the entire computational domain to a single moving reference frame (see Figure 2.1: Single Component (Blower Wheel Blade Passage) (p. 18)). This is known as the single reference frame (or SRF) approach. The use of the SRF approach is possible; provided the geometry meets certain requirements (as discussed in Flow in a Moving Reference Frame (p. 18)). For more complex geometries, it may not be possible to use a single reference frame (see Figure 2.2: Multiple Component (Blower Wheel and Casing) (p. 18)). In such cases, you must break up the problem into multiple cell zones, with well-defined interfaces between the zones. The manner in which the interfaces are treated leads to two approximate, steady-state modeling methods for this class of problem: the multiple reference frame (or MRF) approach, and the mixing plane approach. These approaches will be discussed in The Multiple Reference Frame Model (p. 22) and The Mixing Plane Model (p. 25). If unsteady interaction between the stationary and moving parts is important, you can employ the sliding mesh approach to capture the transient behavior of the flow. The sliding meshing model will be discussed in Flows Using Sliding and Dynamic Meshes (p. 33).

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Flows with Moving Reference Frames Figure 2.1: Single Component (Blower Wheel Blade Passage)

Figure 2.2: Multiple Component (Blower Wheel and Casing)

2.2. Flow in a Moving Reference Frame The principal reason for employing a moving reference frame is to render a problem that is unsteady in the stationary (inertial) frame steady with respect to the moving frame. For a steadily moving frame 18

Flow in a Moving Reference Frame (for example, the rotational speed is constant), it is possible to transform the equations of fluid motion to the moving frame such that steady-state solutions are possible. It should also be noted that you can run an unsteady simulation in a moving reference frame with constant rotational speed. This would be necessary if you wanted to simulate, for example, vortex shedding from a rotating fan blade. The unsteadiness in this case is due to a natural fluid instability (vortex generation) rather than induced from interaction with a stationary component. It is also possible in ANSYS Fluent to have frame motion with unsteady translational and rotational speeds. Again, the appropriate acceleration terms are added to the equations of fluid motion. Such problems are inherently unsteady with respect to the moving frame due to the unsteady frame motion For more information, see the following section: 2.2.1. Equations for a Moving Reference Frame

2.2.1. Equations for a Moving Reference Frame Consider a coordinate system that is translating with a linear velocity and rotating with angular velocity relative to a stationary (inertial) reference frame, as illustrated in Figure 2.3: Stationary and Moving Reference Frames (p. 19). The origin of the moving system is located by a position vector . Figure 2.3: Stationary and Moving Reference Frames

The axis of rotation is defined by a unit direction vector

such that (2.1)

The computational domain for the CFD problem is defined with respect to the moving frame such that an arbitrary point in the CFD domain is located by a position vector from the origin of the moving frame. The fluid velocities can be transformed from the stationary frame to the moving frame using the following relation: (2.2) where (2.3) Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

19

Flows with Moving Reference Frames In the above equations, is the relative velocity (the velocity viewed from the moving frame), is the absolute velocity (the velocity viewed from the stationary frame), is the velocity of the moving frame relative to the inertial reference frame, is the translational frame velocity, and is the angular velocity. It should be noted that both and can be functions of time. When the equations of motion are solved in the moving reference frame, the acceleration of the fluid is augmented by additional terms that appear in the momentum equations [30] (p. 776). Moreover, the equations can be formulated in two different ways: • Expressing the momentum equations using the relative velocities as dependent variables (known as the relative velocity formulation). • Expressing the momentum equations using the absolute velocities as dependent variables in the momentum equations (known as the absolute velocity formulation). The governing equations for these two formulations will be provided in the sections below. It can be noted here that ANSYS Fluent's pressure-based solvers provide the option to use either of these two formulations, whereas the density-based solvers always use the absolute velocity formulation. For more information about the advantages of each velocity formulation, see Choosing the Relative or Absolute Velocity Formulation in the User's Guide.

2.2.1.1. Relative Velocity Formulation For the relative velocity formulation, the governing equations of fluid flow in a moving reference frame can be written as follows: Conservation of mass: (2.4) Conservation of momentum: (2.5)

where

and

Conservation of energy: (2.6) The momentum equation contains four additional acceleration terms. The first two terms are the Coriolis acceleration ( ) and the centripetal acceleration ( ), respectively. These terms appear for both steadily moving reference frames (that is, and are constant) and accelerating reference frames (that is, and/or are functions of time). The third and fourth terms are due to the unsteady change of the rotational speed and linear velocity, respectively. These terms vanish for constant translation and/or rotational speeds. In addition, the viscous stress ( ) is identical to Equation 1.4 (p. 3) except that relative velocity derivatives are used. The energy equation is written in terms of the relative internal energy ( ) and the relative total enthalpy ( ), also known as the rothalpy. These variables are defined as: (2.7)

20

Flow in a Moving Reference Frame (2.8)

2.2.1.2. Absolute Velocity Formulation For the absolute velocity formulation, the governing equations of fluid flow for a steadily moving frame can be written as follows: Conservation of mass: (2.9) Conservation of momentum: (2.10) Conservation of energy: (2.11) In this formulation, the Coriolis and centripetal accelerations can be simplified into a single term ( ). Notice that the momentum equation for the absolute velocity formulation contains no explicit terms involving or .

2.2.1.3. Relative Specification of the Reference Frame Motion ANSYS Fluent allows you to specify the frame of motion relative to an already moving (rotating and translating) reference frame. In this case, the resulting velocity vector is computed as (2.12) where (2.13) and (2.14) Equation 2.13 (p. 21) is known as the Galilei transformation. The rotation vectors are added together as in Equation 2.14 (p. 21), since the motion of the reference frame can be viewed as a solid body rotation, where the rotation rate is constant for every point on the body. In addition, it allows the formulation of the rotation to be an angular velocity axial (also known as pseudo) vector, describing infinitesimal instantaneous transformations. In this case, both rotation rates obey the commutative law. Note that such an approach is not sufficient when dealing with finite rotations. In this case, the formulation of rotation matrices based on Eulerian angles is necessary [393] (p. 796). To learn how to specify a moving reference frame within another moving reference frame, refer to Setting Up Multiple Reference Frames in the User's Guide.

21

Flows with Moving Reference Frames

2.3. Flow in Multiple Reference Frames Problems that involve multiple moving parts cannot be modeled with the Single Reference Frame approach. For these problems, you must break up the model into multiple fluid/solid cell zones, with interface boundaries separating the zones. Zones that contain the moving components can then be solved using the moving reference frame equations (Equations for a Moving Reference Frame (p. 19)), whereas stationary zones can be solved with the stationary frame equations. The manner in which the equations are treated at the interface lead to two approaches that are supported in ANSYS Fluent: • Multiple Moving Reference Frames – Multiple Reference Frame model (MRF) (see The Multiple Reference Frame Model (p. 22)) – Mixing Plane Model (MPM) (see The Mixing Plane Model (p. 25)) • Sliding Mesh Model (SMM) Both the MRF and mixing plane approaches are steady-state approximations, and differ primarily in the manner in which conditions at the interfaces are treated. These approaches will be discussed in the sections below. The sliding mesh model approach is, on the other hand, inherently unsteady due to the motion of the mesh with time. This approach is discussed in Flows Using Sliding and Dynamic Meshes (p. 33).

2.3.1. The Multiple Reference Frame Model 2.3.1.1. Overview The MRF model [292] (p. 791) is, perhaps, the simplest of the two approaches for multiple zones. It is a steady-state approximation in which individual cell zones can be assigned different rotational and/or translational speeds. The flow in each moving cell zone is solved using the moving reference frame equations. (For details, see Flow in a Moving Reference Frame (p. 18)). If the zone is stationary ( ), the equations reduce to their stationary forms. At the interfaces between cell zones, a local reference frame transformation is performed to enable flow variables in one zone to be used to calculate fluxes at the boundary of the adjacent zone. The MRF interface formulation will be discussed in more detail in The MRF Interface Formulation (p. 24). It should be noted that the MRF approach does not account for the relative motion of a moving zone with respect to adjacent zones (which may be moving or stationary); the mesh remains fixed for the computation. This is analogous to freezing the motion of the moving part in a specific position and observing the instantaneous flow field with the rotor in that position. Hence, the MRF is often referred to as the “frozen rotor approach.” While the MRF approach is clearly an approximation, it can provide a reasonable model of the flow for many applications. For example, the MRF model can be used for turbomachinery applications in which rotor-stator interaction is relatively weak, and the flow is relatively uncomplicated at the interface between the moving and stationary zones. In mixing tanks, since the impeller-baffle interactions are relatively weak, large-scale transient effects are not present and the MRF model can be used. Another potential use of the MRF model is to compute a flow field that can be used as an initial condition for a transient sliding mesh calculation. This eliminates the need for a startup calculation. The multiple reference frame model should not be used, however, if it is necessary to actually simulate the transients that may occur in strong rotor-stator interactions, as the sliding mesh model alone should be used (see Modeling Flows Using Sliding and Dynamic Meshes in the User's Guide).

22

Flow in Multiple Reference Frames

2.3.1.2. Examples For a mixing tank with a single impeller, you can define a moving reference frame that encompasses the impeller and the flow surrounding it, and use a stationary frame for the flow outside the impeller region. An example of this configuration is illustrated in Figure 2.4: Geometry with One Rotating Impeller (p. 23). (The dashes denote the interface between the two reference frames.) Steady-state flow conditions are assumed at the interface between the two reference frames. That is, the velocity at the interface must be the same (in absolute terms) for each reference frame. The mesh does not move. Figure 2.4: Geometry with One Rotating Impeller

You can also model a problem that includes more than one moving reference frame. Figure 2.5: Geometry with Two Rotating Impellers (p. 24) shows a geometry that contains two rotating impellers side by side. This problem would be modeled using three reference frames: the stationary frame outside both impeller regions and two separate moving reference frames for the two impellers. (As noted above, the dashes denote the interfaces between reference frames.)

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Flows with Moving Reference Frames Figure 2.5: Geometry with Two Rotating Impellers

2.3.1.3. The MRF Interface Formulation The MRF formulation that is applied to the interfaces will depend on the velocity formulation being used. The specific approaches will be discussed below for each case. It should be noted that the interface treatment applies to the velocity and velocity gradients, since these vector quantities change with a change in reference frame. Scalar quantities, such as temperature, pressure, density, turbulent kinetic energy, and so on, do not require any special treatment, and therefore are passed locally without any change.

Note The interface formulation used by ANSYS Fluent does not account for different normal (to the interface) cell zone velocities. You should specify the zone motion of both adjacent cell zones in a way that the interface-normal velocity difference is zero.

2.3.1.3.1. Interface Treatment: Relative Velocity Formulation In ANSYS Fluent’s implementation of the MRF model, the calculation domain is divided into subdomains, each of which may be rotating and/or translating with respect to the laboratory (inertial) frame. The governing equations in each subdomain are written with respect to that subdomain’s reference frame. Thus, the flow in stationary and translating subdomains is governed by the equations in Continuity and Momentum Equations (p. 2), while the flow in moving subdomains is governed by the equations presented in Equations for a Moving Reference Frame (p. 19). At the boundary between two subdomains, the diffusion and other terms in the governing equations in one subdomain require values for the velocities in the adjacent subdomain (see Figure 2.6: Interface Treatment for the MRF Model (p. 25)). ANSYS Fluent enforces the continuity of the absolute velocity, , to provide the correct neighbor values of velocity for the subdomain under consideration. (This approach differs from the mixing plane approach described in The Mixing Plane Model (p. 25), where a circumferential averaging technique is used.)

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Flow in Multiple Reference Frames When the relative velocity formulation is used, velocities in each subdomain are computed relative to the motion of the subdomain. Velocities and velocity gradients are converted from a moving reference frame to the absolute inertial frame using Equation 2.15 (p. 25). Figure 2.6: Interface Treatment for the MRF Model

For a translational velocity

, we have (2.15)

From Equation 2.15 (p. 25), the gradient of the absolute velocity vector can be shown to be (2.16) Note that scalar quantities such as density, static pressure, static temperature, species mass fractions, and so on, are simply obtained locally from adjacent cells.

2.3.1.3.2. Interface Treatment: Absolute Velocity Formulation When the absolute velocity formulation is used, the governing equations in each subdomain are written with respect to that subdomain’s reference frame, but the velocities are stored in the absolute frame. Therefore, no special transformation is required at the interface between two subdomains. Again, scalar quantities are determined locally from adjacent cells.

2.3.2. The Mixing Plane Model The mixing plane model in ANSYS Fluent provides an alternative to the multiple reference frame and sliding mesh models for simulating flow through domains with one or more regions in relative motion. This section provides a brief overview of the model and a list of its limitations.

25

Flows with Moving Reference Frames

2.3.2.1. Overview As discussed in The Multiple Reference Frame Model (p. 22), the MRF model is applicable when the flow at the interface between adjacent moving/stationary zones is nearly uniform (“mixed out”). If the flow at this interface is not uniform, the MRF model may not provide a physically meaningful solution. The sliding mesh model (see Modeling Flows Using Sliding and Dynamic Meshes in the User's Guide) may be appropriate for such cases, but in many situations it is not practical to employ a sliding mesh. For example, in a multistage turbomachine, if the number of blades is different for each blade row, a large number of blade passages is required in order to maintain circumferential periodicity. Moreover, sliding mesh calculations are necessarily unsteady, and therefore require significantly more computation to achieve a final, time-periodic solution. For situations where using the sliding mesh model is not feasible, the mixing plane model can be a cost-effective alternative. In the mixing plane approach, each fluid zone is treated as a steady-state problem. Flow-field data from adjacent zones are passed as boundary conditions that are spatially averaged or “mixed” at the mixing plane interface. This mixing removes any unsteadiness that would arise due to circumferential variations in the passage-to-passage flow field (for example, wakes, shock waves, separated flow), therefore yielding a steady-state result. Despite the simplifications inherent in the mixing plane model, the resulting solutions can provide reasonable approximations of the time-averaged flow field.

2.3.2.2. Rotor and Stator Domains Consider the turbomachine stages shown schematically in Figure 2.7: Axial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept) (p. 26) and Figure 2.8: Radial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept) (p. 27), each blade passage contains periodic boundaries. Figure 2.7: Axial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept) (p. 26) shows a constant radial plane within a single stage of an axial machine, while Figure 2.8: Radial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept) (p. 27) shows a constant plane within a mixed-flow device. In each case, the stage consists of two flow domains: the rotor domain, which is rotating at a prescribed angular velocity, followed by the stator domain, which is stationary. The order of the rotor and stator is arbitrary (that is, a situation where the rotor is downstream of the stator is equally valid). Figure 2.7: Axial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept)

26

Flow in Multiple Reference Frames Figure 2.8: Radial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept)

In a numerical simulation, each domain will be represented by a separate mesh. The flow information between these domains will be coupled at the mixing plane interface (as shown in Figure 2.7: Axial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept) (p. 26) and Figure 2.8: Radial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept) (p. 27)) using the mixing plane model. Note that you may couple any number of fluid zones in this manner; for example, four blade passages can be coupled using three mixing planes.

Important Note that the stator and rotor passages are separate cell zones, each with their own inlet and outlet boundaries. You can think of this system as a set of SRF models for each blade passage coupled by boundary conditions supplied by the mixing plane model.

2.3.2.3. The Mixing Plane Concept The essential idea behind the mixing plane concept is that each fluid zone is solved as a steady-state problem. At some prescribed iteration interval, the flow data at the mixing plane interface are averaged in the circumferential direction on both the stator outlet and the rotor inlet boundaries. The ANSYS Fluent implementation gives you the choice of three types of averaging methods: area-weighted averRelease 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

27

Flows with Moving Reference Frames aging, mass averaging, and mixed-out averaging. By performing circumferential averages at specified radial or axial stations, “profiles” of boundary condition flow variables can be defined. These profiles—which will be functions of either the axial or the radial coordinate, depending on the orientation of the mixing plane—are then used to update boundary conditions along the two zones of the mixing plane interface. In the examples shown in Figure 2.7: Axial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept) (p. 26) and Figure 2.8: Radial Rotor-Stator Interaction (Schematic Illustrating the Mixing Plane Concept) (p. 27), profiles of averaged total pressure ( ), direction cosines of the local flow angles in the radial, tangential, and axial directions ( ), total temperature ( ), turbulence kinetic energy ( ), and turbulence dissipation rate ( ) are computed at the rotor exit and used to update boundary conditions at the stator inlet. Likewise, a profile of static pressure ( ), direction cosines of the local flow angles in the radial, tangential, and axial directions ( ), are computed at the stator inlet and used as a boundary condition on the rotor exit. Passing profiles in the manner described above assumes specific boundary condition types have been defined at the mixing plane interface. The coupling of an upstream outlet boundary zone with a downstream inlet boundary zone is called a “mixing plane pair”. In order to create mixing plane pairs in ANSYS Fluent, the boundary zones must be of the following types: Upstream

Downstream

pressure outlet

pressure inlet

pressure outlet

velocity inlet

pressure outlet

mass flow inlet

For specific instructions about setting up mixing planes, see Setting Up the Mixing Plane Model in the User's Guide.

2.3.2.4. Choosing an Averaging Method Three profile averaging methods are available in the mixing plane model: • area averaging • mass averaging • mixed-out averaging

2.3.2.4.1. Area Averaging Area averaging is the default averaging method and is given by (2.17)

Important The pressure and temperature obtained by the area average may not be representative of the momentum and energy of the flow.

2.3.2.4.2. Mass Averaging Mass averaging is given by

28

Flow in Multiple Reference Frames

(2.18) where (2.19) This method provides a better representation of the total quantities than the area-averaging method. Convergence problems could arise if severe reverse flow is present at the mixing plane. Therefore, for solution stability purposes, it is best if you initiate the solution with area averaging, then switch to mass averaging after reverse flow dies out.

Important Mass averaging is not available with multiphase flows.

2.3.2.4.3. Mixed-Out Averaging The mixed-out averaging method is derived from the conservation of mass, momentum and energy:

(2.20)

Because it is based on the principles of conservation, the mixed-out average is considered a better representation of the flow since it reflects losses associated with non-uniformities in the flow profiles. However, like the mass-averaging method, convergence difficulties can arise when severe reverse flow is present across the mixing plane. Therefore, it is best if you initiate the solution with area averaging, then switch to mixed-out averaging after reverse flow dies out. Mixed-out averaging assumes that the fluid is a compressible ideal-gas with constant specific heat,

.

Important Mixed-out averaging is not available with multiphase flows.

2.3.2.5. Mixing Plane Algorithm of ANSYS Fluent ANSYS Fluent’s basic mixing plane algorithm can be described as follows: 1. Update the flow field solutions in the stator and rotor domains.

29

Flows with Moving Reference Frames 2. Average the flow properties at the stator exit and rotor inlet boundaries, obtaining profiles for use in updating boundary conditions. 3. Pass the profiles to the boundary condition inputs required for the stator exit and rotor inlet. 4. Repeat steps 1–3 until convergence.

Important Note that it may be desirable to under-relax the changes in boundary condition values in order to prevent divergence of the solution (especially early in the computation). ANSYS Fluent allows you to control the under-relaxation of the mixing plane variables.

2.3.2.6. Mass Conservation Note that the algorithm described above will not rigorously conserve mass flow across the mixing plane if it is represented by a pressure outlet and pressure inlet mixing plane pair. If you use a pressure outlet and mass flow inlet pair instead, ANSYS Fluent will force mass conservation across the mixing plane. The basic technique consists of computing the mass flow rate across the upstream zone (pressure outlet) and adjusting the mass flux profile applied at the mass flow inlet such that the downstream mass flow matches the upstream mass flow. This adjustment occurs at every iteration, therefore ensuring rigorous conservation of mass flow throughout the course of the calculation.

Important Note that, since mass flow is being fixed in this case, there will be a jump in total pressure across the mixing plane. The magnitude of this jump is usually small compared with total pressure variations elsewhere in the flow field.

2.3.2.7. Swirl Conservation By default, ANSYS Fluent does not conserve swirl across the mixing plane. For applications such as torque converters, where the sum of the torques acting on the components should be zero, enforcing swirl conservation across the mixing plane is essential, and is available in ANSYS Fluent as a modeling option. Ensuring conservation of swirl is important because, otherwise, sources or sinks of tangential momentum will be present at the mixing plane interface. Consider a control volume containing a stationary or moving component (for example, a pump impeller or turbine vane). Using the moment of momentum equation from fluid mechanics, it can be shown that for steady flow, (2.21) where is the torque of the fluid acting on the component, is the radial distance from the axis of rotation, is the absolute tangential velocity, is the total absolute velocity, and is the boundary surface. (The product is referred to as swirl.) For a circumferentially periodic domain, with well-defined inlet and outlet boundaries, Equation 2.21 (p. 30) becomes

30

Flow in Multiple Reference Frames

(2.22) where inlet and outlet denote the inlet and outlet boundary surfaces. Now consider the mixing plane interface to have a finite streamwise thickness. Applying Equation 2.22 (p. 31) to this zone and noting that, in the limit as the thickness shrinks to zero, the torque should vanish, the equation becomes (2.23) where upstream and downstream denote the upstream and downstream sides of the mixing plane interface. Note that Equation 2.23 (p. 31) applies to the full area (360 degrees) at the mixing plane interface. Equation 2.23 (p. 31) provides a rational means of determining the tangential velocity component. That is, ANSYS Fluent computes a profile of tangential velocity and then uniformly adjusts the profile such that the swirl integral is satisfied. Note that interpolating the tangential (and radial) velocity component profiles at the mixing plane does not affect mass conservation because these velocity components are orthogonal to the face-normal velocity used in computing the mass flux.

2.3.2.8. Total Enthalpy Conservation By default, ANSYS Fluent does not conserve total enthalpy across the mixing plane. For some applications, total enthalpy conservation across the mixing plane is very desirable, because global parameters such as efficiency are directly related to the change in total enthalpy across a blade row or stage. This is available in ANSYS Fluent as a modeling option. The procedure for ensuring conservation of total enthalpy simply involves adjusting the downstream total temperature profile such that the integrated total enthalpy matches the upstream integrated total enthalpy. For multiphase flows, conservation of mass, swirl, and enthalpy are calculated for each phase. However, for the Eulerian multiphase model, since mass flow inlets are not permissible, conservation of the above quantities does not occur.

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Chapter 3: Flows Using Sliding and Dynamic Meshes This chapter describes the theoretical background of the sliding and dynamic mesh models in ANSYS Fluent. To learn more about using sliding meshes in ANSYS Fluent, see Using Sliding Meshes in the User’s Guide. For more information about using dynamic meshes in ANSYS Fluent, see Using Dynamic Meshes in the User's Guide. Theoretical information about sliding and dynamic mesh models is presented in the following sections: 3.1. Introduction 3.2. Dynamic Mesh Theory 3.3. Sliding Mesh Theory

3.1. Introduction The dynamic mesh model allows you to move the boundaries of a cell zone relative to other boundaries of the zone, and to adjust the mesh accordingly. The motion of the boundaries can be rigid, such as pistons moving inside an engine cylinder (see Figure 3.1: A Mesh Associated With Moving Pistons (p. 33)) or a flap deflecting on an aircraft wing, or deforming, such as the elastic wall of a balloon during inflation or a flexible artery wall responding to the pressure pulse from the heart. In either case, the nodes that define the cells in the domain must be updated as a function of time, and hence the dynamic mesh solutions are inherently unsteady. The governing equations describing the fluid motion (which are different from those used for moving reference frames, as described in Flows with Moving Reference Frames (p. 17)) are described in Dynamic Mesh Theory (p. 34). Figure 3.1: A Mesh Associated With Moving Pistons

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Flows Using Sliding and Dynamic Meshes An important special case of dynamic mesh motion is called the sliding mesh in which all of the boundaries and the cells of a given mesh zone move together in a rigid-body motion. In this situation, the nodes of the mesh move in space (relative to the fixed, global coordinates), but the cells defined by the nodes do not deform. Furthermore, mesh zones moving adjacent to one another can be linked across one or more non-conformal interfaces. As long as the interfaces stay in contact with one another (that is, “slide” along a common overlap boundary at the interface), the non-conformal interfaces can be dynamically updated as the meshes move, and fluid can pass from one zone to the other. Such a scenario is referred to as the sliding mesh model in ANSYS Fluent. Examples of sliding mesh model usage include modeling rotor-stator interaction between a moving blade and a stationary vane in a compressor or turbine, modeling a blower with rotating blades and a stationary casing (see Figure 3.2: Blower (p. 34)), and modeling a train moving in a tunnel by defining sliding interfaces between the train and the tunnel walls. Figure 3.2: Blower

3.2. Dynamic Mesh Theory The dynamic mesh model in ANSYS Fluent can be used to model flows where the shape of the domain is changing with time due to motion on the domain boundaries. The dynamic mesh model can be applied to single or multiphase flows (and multi-species flows). The generic transport equation (Equation 3.1 (p. 35)) applies to all applicable model equations, such as turbulence, energy, species, phases, and so on. The dynamic mesh model can also be used for steady-state applications, when it is beneficial to move the mesh in the steady-state solver. The motion can be a prescribed motion (for example, you can specify the linear and angular velocities about the center of gravity of a solid body with time) or an unprescribed motion where the subsequent motion is determined based on the solution at the current time (for example, the linear and angular velocities are calculated from the force balance on a solid body, as is done by the six degree of freedom (six DOF) solver; see Six DOF Solver Settings in the User's Guide). The update of the volume mesh is handled automatically by ANSYS Fluent at each time step based on the new positions of the boundaries. To use the dynamic mesh model, you need to provide a starting volume mesh and the description of the motion of any moving zones in the model. ANSYS Fluent allows you to describe the motion using either boundary profiles, user-defined functions (UDFs), or the six degree of freedom solver. ANSYS Fluent expects the description of the motion to be specified on either face or cell zones. If the model contains moving and non-moving regions, you need to identify these regions by grouping them 34

Dynamic Mesh Theory into their respective face or cell zones in the starting volume mesh that you generate. Furthermore, regions that are deforming due to motion on their adjacent regions must also be grouped into separate zones in the starting volume mesh. The boundary between the various regions need not be conformal. You can use the non-conformal or sliding interface capability in ANSYS Fluent to connect the various zones in the final model. Information about dynamic mesh theory is presented in the following sections: 3.2.1. Conservation Equations 3.2.2. Six DOF Solver Theory

3.2.1. Conservation Equations With respect to dynamic meshes, the integral form of the conservation equation for a general scalar, , on an arbitrary control volume, , whose boundary is moving can be written as (3.1) where is the fluid density is the flow velocity vector is the mesh velocity of the moving mesh is the diffusion coefficient is the source term of Here,

is used to represent the boundary of the control volume,

.

By using a first-order backward difference formula, the time derivative term in Equation 3.1 (p. 35) can be written as (3.2) where

and

denote the respective quantity at the current and next time level, respectively. The

th time level volume,

, is computed from (3.3)

where is the volume time derivative of the control volume. In order to satisfy the mesh conservation law, the volume time derivative of the control volume is computed from (3.4)

where

is the number of faces on the control volume and

is the

face area vector. The dot product

on each control volume face is calculated from (3.5) where

is the volume swept out by the control volume face

over the time step

. 35

Flows Using Sliding and Dynamic Meshes By using a second-order backward difference formula, the time derivative in Equation 3.1 (p. 35) can be written as (3.6)

where , , and denote the respective quantities from successive time levels with the current time level.

denoting

In the case of a second-order difference scheme the volume time derivative of the control volume is computed in the same manner as in the first-order scheme as shown in Equation 3.4 (p. 35). For the second-order differencing scheme, the dot product from

on each control volume face is calculated

(3.7)

where and are the volumes swept out by control volume faces at the current and previous time levels over a time step.

3.2.2. Six DOF Solver Theory The six DOF solver in ANSYS Fluent uses the object’s forces and moments in order to compute the translational and angular motion of the center of gravity of an object. The governing equation for the translational motion of the center of gravity is solved for in the inertial coordinate system: (3.8) where

is the translational motion of the center of gravity,

is the mass, and

is the force vector

due to gravity. The angular motion of the object,

, is more easily computed using body coordinates: (3.9)

where is the inertia tensor, velocity vector.

is the moment vector of the body, and

is the rigid body angular

The moments are transformed from inertial to body coordinates using (3.10) where,

represents the following transformation matrix:

where, in generic terms, and resent the following sequence of rotations:

36

. The angles

, , and

are Euler angles that rep-

Sliding Mesh Theory • rotation about the Z axis (for example, yaw for airplanes) • rotation about the Y axis (for example, pitch for airplanes) • rotation about the X axis (for example, roll for airplanes) After the angular and the translational accelerations are computed from Equation 3.8 (p. 36) and Equation 3.9 (p. 36), the rates are derived by numerical integration [450] (p. 799). The angular and translational velocities are used in the dynamic mesh calculations to update the rigid body position.

3.3. Sliding Mesh Theory As mentioned previously, the sliding mesh model is a special case of general dynamic mesh motion wherein the nodes move rigidly in a given dynamic mesh zone. Additionally, multiple cells zones are connected with each other through non-conformal interfaces. As the mesh motion is updated in time, the non-conformal interfaces are likewise updated to reflect the new positions each zone. It is important to note that the mesh motion must be prescribed such that zones linked through non-conformal interfaces remain in contact with each other (that is, “slide” along the interface boundary) if you want fluid to be able to flow from one mesh to the other. Any portion of the interface where there is no contact is treated as a wall, as described in Non-Conformal Meshes in the User's Guide. The general conservation equation formulation for dynamic meshes, as expressed in Equation 3.1 (p. 35), is also used for sliding meshes. Because the mesh motion in the sliding mesh formulation is rigid, all cells retain their original shape and volume. As a result, the time rate of change of the cell volume is zero, and Equation 3.3 (p. 35) simplifies to: (3.11) and Equation 3.2 (p. 35) becomes: (3.12) Additionally, Equation 3.4 (p. 35) simplifies to (3.13)

Equation 3.1 (p. 35), in conjunction with the above simplifications, permits the flow in the moving mesh zones to be updated, provided that an appropriate specification of the rigid mesh motion is defined for each zone (usually this is simple linear or rotation motion, but more complex motions can be used). Note that due to the fact that the mesh is moving, the solutions to Equation 3.1 (p. 35) for sliding mesh applications will be inherently unsteady (as they are for all dynamic meshes).

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Chapter 4: Turbulence This chapter provides theoretical background about the turbulence models available in ANSYS Fluent. Information is presented in the following sections: 4.1. Underlying Principles of Turbulence Modeling 4.2. Spalart-Allmaras Model 4.3. Standard, RNG, and Realizable k-ε Models 4.4. Standard, BSL, and SST k-ω Models 4.5. k-kl-ω Transition Model 4.6.Transition SST Model 4.7. Intermittency Transition Model 4.8.The V2F Model 4.9. Reynolds Stress Model (RSM) 4.10. Scale-Adaptive Simulation (SAS) Model 4.11. Detached Eddy Simulation (DES) 4.12. Shielded Detached Eddy Simulation (SDES) 4.13. Stress-Blended Eddy Simulation (SBES) 4.14. Large Eddy Simulation (LES) Model 4.15. Embedded Large Eddy Simulation (ELES) 4.16. Near-Wall Treatments for Wall-Bounded Turbulent Flows 4.17. Curvature Correction for the Spalart-Allmaras and Two-Equation Models 4.18. Production Limiters for Two-Equation Models 4.19. Definition of Turbulence Scales For more information about using these turbulence models in ANSYS Fluent, see Modeling Turbulence in the User's Guide .

4.1. Underlying Principles of Turbulence Modeling The following sections provide an overview of the underlying principles for turbulence modeling. 4.1.1. Reynolds (Ensemble) Averaging 4.1.2. Filtered Navier-Stokes Equations 4.1.3. Hybrid RANS-LES Formulations 4.1.4. Boussinesq Approach vs. Reynolds Stress Transport Models

4.1.1. Reynolds (Ensemble) Averaging In Reynolds averaging, the solution variables in the instantaneous (exact) Navier-Stokes equations are decomposed into the mean (ensemble-averaged or time-averaged) and fluctuating components. For the velocity components: (4.1) where

and

are the mean and fluctuating velocity components (

).

Likewise, for pressure and other scalar quantities: (4.2) Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

39

Turbulence where

denotes a scalar such as pressure, energy, or species concentration.

Substituting expressions of this form for the flow variables into the instantaneous continuity and momentum equations and taking a time (or ensemble) average (and dropping the overbar on the mean velocity, ) yields the ensemble-averaged momentum equations. They can be written in Cartesian tensor form as: (4.3)

(4.4)

Equation 4.3 (p. 40) and Equation 4.4 (p. 40) are called Reynolds-averaged Navier-Stokes (RANS) equations. They have the same general form as the instantaneous Navier-Stokes equations, with the velocities and other solution variables now representing ensemble-averaged (or time-averaged) values. Additional terms now appear that represent the effects of turbulence. These Reynolds stresses, modeled in order to close Equation 4.4 (p. 40).

, must be

For variable-density flows, Equation 4.3 (p. 40) and Equation 4.4 (p. 40) can be interpreted as Favreaveraged Navier-Stokes equations [183] (p. 785), with the velocities representing mass-averaged values. As such, Equation 4.3 (p. 40) and Equation 4.4 (p. 40) can be applied to variable-density flows.

4.1.2. Filtered Navier-Stokes Equations The governing equations employed for LES are obtained by filtering the time-dependent Navier-Stokes equations in either Fourier (wave-number) space or configuration (physical) space. The filtering process effectively filters out the eddies whose scales are smaller than the filter width or grid spacing used in the computations. The resulting equations therefore govern the dynamics of large eddies. A filtered variable (denoted by an overbar) is defined by Equation 4.5 (p. 40): (4.5) where

is the fluid domain, and

is the filter function that determines the scale of the resolved eddies.

In ANSYS Fluent, the finite-volume discretization itself implicitly provides the filtering operation: (4.6) where

is the volume of a computational cell. The filter function,

, implied here is then (4.7)

The LES capability in ANSYS Fluent is applicable to compressible and incompressible flows. For the sake of concise notation, however, the theory that follows is limited to a discussion of incompressible flows. Filtering the continuity and momentum equations, one obtains (4.8)

40

Underlying Principles of Turbulence Modeling and (4.9) where

is the stress tensor due to molecular viscosity defined by (4.10)

and

is the subgrid-scale stress defined by and

is the subgrid-scale stress defined by (4.11)

Filtering the energy equation, one obtains: (4.12)

where

and

are the sensible enthalpy and thermal conductivity, respectively.

The subgrid enthalpy flux term in the Equation 4.12 (p. 41) is approximated using the gradient hypothesis: (4.13) where

is a subgrid viscosity, and

is a subgrid Prandtl number equal to 0.85.

4.1.3. Hybrid RANS-LES Formulations At first, the concepts of Reynolds Averaging and Spatial Filtering seem incompatible, as they result in different additional terms in the momentum equations (Reynolds Stresses and sub-grid stresses). This would preclude hybrid models like Scale-Adaptive Simulation (SAS), Detached Eddy Simulation (DES), Shielded DES (SDES), or Stress-Blended Eddy Simulation (SBES), which are based on one set of momentum equations throughout the RANS and LES portions of the domain. However, it is important to note that once a turbulence model is introduced into the momentum equations, they no longer carry any information concerning their derivation (averaging). Case in point is that the most popular models, both in RANS and LES, are eddy viscosity models that are used to substitute either the Reynolds- or the subgrid stress tensor. After the introduction of an eddy viscosity (turbulent viscosity), both the RANS and LES momentum equations are formally identical. The difference lies exclusively in the size of the eddyviscosity provided by the underlying turbulence model. This allows the formulation of turbulence models that can switch from RANS to LES mode, by lowering the eddy viscosity in the LES zone appropriately, without any formal change to the momentum equations.

4.1.4. Boussinesq Approach vs. Reynolds Stress Transport Models The Reynolds-averaged approach to turbulence modeling requires that the Reynolds stresses in Equation 4.4 (p. 40) are appropriately modeled. A common method employs the Boussinesq hypothesis [183] (p. 785) to relate the Reynolds stresses to the mean velocity gradients: (4.14)

41

Turbulence The Boussinesq hypothesis is used in the Spalart-Allmaras model, the - models, and the - models. The advantage of this approach is the relatively low computational cost associated with the computation of the turbulent viscosity, . In the case of the Spalart-Allmaras model, only one additional transport equation (representing turbulent viscosity) is solved. In the case of the - and - models, two additional transport equations (for the turbulence kinetic energy, , and either the turbulence dissipation rate, , or the specific dissipation rate, ) are solved, and is computed as a function of and or and . The disadvantage of the Boussinesq hypothesis as presented is that it assumes is an isotropic scalar quantity, which is not strictly true. However the assumption of an isotropic turbulent viscosity typically works well for shear flows dominated by only one of the turbulent shear stresses. This covers many technical flows, such as wall boundary layers, mixing layers, jets, and so on. The alternative approach, embodied in the RSM, is to solve transport equations for each of the terms in the Reynolds stress tensor. An additional scale-determining equation (normally for or ) is also required. This means that five additional transport equations are required in 2D flows and seven additional transport equations must be solved in 3D. In many cases, models based on the Boussinesq hypothesis perform very well, and the additional computational expense of the Reynolds stress model is not justified. However, the RSM is clearly superior in situations where the anisotropy of turbulence has a dominant effect on the mean flow. Such cases include highly swirling flows and stress-driven secondary flows.

4.2. Spalart-Allmaras Model This section describes the theory behind the Spalart-Allmaras model. Information is presented in the following sections: 4.2.1. Overview 4.2.2.Transport Equation for the Spalart-Allmaras Model 4.2.3. Modeling the Turbulent Viscosity 4.2.4. Modeling the Turbulent Production 4.2.5. Modeling the Turbulent Destruction 4.2.6. Model Constants 4.2.7. Wall Boundary Conditions 4.2.8. Convective Heat and Mass Transfer Modeling For details about using the model in ANSYS Fluent, see Modeling Turbulence and Setting Up the SpalartAllmaras Model in the User's Guide .

4.2.1. Overview The Spalart-Allmaras model [456] (p. 800) is a one-equation model that solves a modeled transport equation for the kinematic eddy (turbulent) viscosity. The Spalart-Allmaras model was designed specifically for aerospace applications involving wall-bounded flows and has been shown to give good results for boundary layers subjected to adverse pressure gradients. It is also gaining popularity in turbomachinery applications. In its original form, the Spalart-Allmaras model is effectively a low-Reynolds number model, requiring the viscosity-affected region of the boundary layer to be properly resolved ( meshes). In ANSYS Fluent, the Spalart-Allmaras model has been extended with a -insensitive wall treatment, which allows the application of the model independent of the near-wall resolution. The formulation blends automatically from a viscous sublayer formulation to a logarithmic formulation based on . On intermediate grids, , the formulation maintains its integrity and provides consistent wall shear stress and

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Spalart-Allmaras Model heat transfer coefficients. While the sensitivity is removed, it still should be ensured that the boundary layer is resolved with a minimum resolution of 10-15 cells. The Spalart-Allmaras model was developed for aerodynamic flows. It is not calibrated for general industrial flows, and does produce relatively larger errors for some free shear flows, especially plane and round jet flows. In addition, it cannot be relied on to predict the decay of homogeneous, isotropic turbulence.

4.2.2. Transport Equation for the Spalart-Allmaras Model The transported variable in the Spalart-Allmaras model, , is identical to the turbulent kinematic viscosity except in the near-wall (viscosity-affected) region. The transport equation for the modified turbulent viscosity is (4.15) where is the production of turbulent viscosity, and is the destruction of turbulent viscosity that occurs in the near-wall region due to wall blocking and viscous damping. and are the constants and is the molecular kinematic viscosity. is a user-defined source term. Note that since the turbulence kinetic energy, , is not calculated in the Spalart-Allmaras model, the last term in Equation 4.14 (p. 41) is ignored when estimating the Reynolds stresses.

4.2.3. Modeling the Turbulent Viscosity The turbulent viscosity,

, is computed from (4.16)

where the viscous damping function,

, is given by (4.17)

and (4.18)

4.2.4. Modeling the Turbulent Production The production term,

, is modeled as (4.19)

where (4.20) and (4.21)

43

Turbulence and are constants, is the distance from the wall, and is a scalar measure of the deformation tensor. By default in ANSYS Fluent, as in the original model proposed by Spalart and Allmaras, is based on the magnitude of the vorticity: (4.22) where

is the mean rate-of-rotation tensor and is defined by (4.23)

The justification for the default expression for is that, for shear flows, vorticity and strain rate are identical. Vorticity has the advantage of being zero in inviscid flow regions like stagnation lines, where turbulence production due to strain rate can be unphysical. However, an alternative formulation has been proposed [94] (p. 780) and incorporated into ANSYS Fluent. This modification combines the measures of both vorticity and the strain tensors in the definition of : (4.24) where

with the mean strain rate,

, defined as (4.25)

Including both the rotation and strain tensors reduces the production of eddy viscosity and consequently reduces the eddy viscosity itself in regions where the measure of vorticity exceeds that of strain rate. One such example can be found in vortical flows, that is, flow near the core of a vortex subjected to a pure rotation where turbulence is known to be suppressed. Including both the rotation and strain tensors more accurately accounts for the effects of rotation on turbulence. The default option (including the rotation tensor only) tends to overpredict the production of eddy viscosity and hence over-predicts the eddy viscosity itself inside vortices. You can select the modified form for calculating production in the Viscous Model Dialog Box.

4.2.5. Modeling the Turbulent Destruction The destruction term is modeled as (4.26) where (4.27) (4.28) (4.29)

44

Spalart-Allmaras Model ,

, and

are constants, and

is given by Equation 4.20 (p. 43). Note that the modification

described above to include the effects of mean strain on .

will also affect the value of

used to compute

4.2.6. Model Constants The model constants

, and

have the following default values [456] (p. 800):

4.2.7. Wall Boundary Conditions The Spalart-Allmaras model has been extended within ANSYS Fluent with a -insensitive wall treatment, which automatically blends all solution variables from their viscous sublayer formulation (4.30) to the corresponding logarithmic layer values depending on

. (4.31)

where is the velocity parallel to the wall, is the von Kármán constant (0.4187), and

is the friction velocity, .

The blending is calibrated to also cover intermediate

is the distance from the wall,

values in the buffer layer

.

4.2.7.1. Treatment of the Spalart-Allmaras Model for Icing Simulations On the basis of the standard Spalart-Allmaras model equation, the Boeing Extension [22] (p. 776) has been adopted to account for wall roughness. In this model, the non-zero wall value of the transported variable (directly solved from the S-A equation), , is estimated to mimic roughness effects by replacing the wall condition with: (4.32) where is the wall normal, is the minimum cell-to-face distance between the wall and the first cell near the wall, and is a length introduced to impose an offset, depending on the local roughness height, : (4.33) Then the wall turbulent kinematic viscosity,

, is obtained as follows: (4.34)

where

is the standard model function in the Spalart-Allmaras model. As the roughness effect is

strong, the turbulent viscosity should be large compared to the laminar viscosity at the wall, then

→

1, and therefore: Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

45

Turbulence (4.35) Also in Equation 4.34 (p. 45),

is the Von Karmen constant, and

is the wall friction velocity: (4.36)

Furthermore, to achieve good predictions for smaller roughness, Aupoix and Spalart [22] (p. 776) proposed that the function should be altered by modifying the quantity in the Spalart-Allmaras model equation: (4.37)

4.2.8. Convective Heat and Mass Transfer Modeling In ANSYS Fluent, turbulent heat transport is modeled using the concept of the Reynolds analogy to turbulent momentum transfer. The “modeled” energy equation is as follows: (4.38)

where , in this case, is the thermal conductivity,

is the total energy, and

is the deviatoric

stress tensor, defined as

4.3. Standard, RNG, and Realizable k-ε Models This section describes the theory behind the Standard, RNG, and Realizable - models. Information is presented in the following sections: 4.3.1. Standard k-ε Model 4.3.2. RNG k-ε Model 4.3.3. Realizable k-ε Model 4.3.4. Modeling Turbulent Production in the k-ε Models 4.3.5. Effects of Buoyancy on Turbulence in the k-ε Models 4.3.6. Effects of Compressibility on Turbulence in the k-ε Models 4.3.7. Convective Heat and Mass Transfer Modeling in the k-ε Models For details about using the models in ANSYS Fluent, see Modeling Turbulence and Setting Up the kε Model in the User's Guide . This section presents the standard, RNG, and realizable - models. All three models have similar forms, with transport equations for and . The major differences in the models are as follows: • the method of calculating turbulent viscosity • the turbulent Prandtl numbers governing the turbulent diffusion of • the generation and destruction terms in the

and

equation

The transport equations, the methods of calculating turbulent viscosity, and model constants are presented separately for each model. The features that are essentially common to all models follow,

46

Standard, RNG, and Realizable k-ε Models including turbulent generation due to shear buoyancy, accounting for the effects of compressibility, and modeling heat and mass transfer.

4.3.1. Standard k-ε Model 4.3.1.1. Overview Two-equation turbulence models allow the determination of both, a turbulent length and time scale by solving two separate transport equations. The standard - model in ANSYS Fluent falls within this class of models and has become the workhorse of practical engineering flow calculations in the time since it was proposed by Launder and Spalding [252] (p. 789). Robustness, economy, and reasonable accuracy for a wide range of turbulent flows explain its popularity in industrial flow and heat transfer simulations. It is a semi-empirical model, and the derivation of the model equations relies on phenomenological considerations and empiricism. The standard - model [252] (p. 789) is a model based on model transport equations for the turbulence kinetic energy ( ) and its dissipation rate ( ). The model transport equation for is derived from the exact equation, while the model transport equation for was obtained using physical reasoning and bears little resemblance to its mathematically exact counterpart. In the derivation of the - model, the assumption is that the flow is fully turbulent, and the effects of molecular viscosity are negligible. The standard - model is therefore valid only for fully turbulent flows. As the strengths and weaknesses of the standard - model have become known, modifications have been introduced to improve its performance. Two of these variants are available in ANSYS Fluent: the RNG - model [535] (p. 804) and the realizable - model [430] (p. 798).

4.3.1.2. Transport Equations for the Standard k-ε Model The turbulence kinetic energy, , and its rate of dissipation, , are obtained from the following transport equations: (4.39) and (4.40) In these equations, represents the generation of turbulence kinetic energy due to the mean velocity gradients, calculated as described in Modeling Turbulent Production in the k-ε Models (p. 54). is the generation of turbulence kinetic energy due to buoyancy, calculated as described in Effects of Buoyancy on Turbulence in the k-ε Models (p. 54). represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, calculated as described in Effects of Compressibility on Turbulence in the k-ε Models (p. 55). , , and are constants. and are the turbulent Prandtl numbers for and , respectively. and are user-defined source terms.

4.3.1.3. Modeling the Turbulent Viscosity The turbulent (or eddy) viscosity,

, is computed by combining

and

as follows:

47

Turbulence (4.41) where

is a constant.

4.3.1.4. Model Constants The model constants

and

have the following default values [252] (p. 789):

These default values have been determined from experiments for fundamental turbulent flows including frequently encountered shear flows like boundary layers, mixing layers and jets as well as for decaying isotropic grid turbulence. They have been found to work fairly well for a wide range of wall-bounded and free shear flows. Although the default values of the model constants are the standard ones most widely accepted, you can change them (if needed) in the Viscous Model Dialog Box.

4.3.2. RNG k-ε Model 4.3.2.1. Overview The RNG - model was derived using a statistical technique called renormalization group theory. It is similar in form to the standard - model, but includes the following refinements: • The RNG model has an additional term in its flows.

equation that improves the accuracy for rapidly strained

• The effect of swirl on turbulence is included in the RNG model, enhancing accuracy for swirling flows. • The RNG theory provides an analytical formula for turbulent Prandtl numbers, while the standard - model uses user-specified, constant values. • While the standard - model is a high-Reynolds number model, the RNG theory provides an analyticallyderived differential formula for effective viscosity that accounts for low-Reynolds number effects. Effective use of this feature does, however, depend on an appropriate treatment of the near-wall region. These features make the RNG - model more accurate and reliable for a wider class of flows than the standard - model. The RNG-based - turbulence model is derived from the instantaneous Navier-Stokes equations, using a mathematical technique called “renormalization group” (RNG) methods. The analytical derivation results in a model with constants different from those in the standard - model, and additional terms and functions in the transport equations for and . A more comprehensive description of RNG theory and its application to turbulence can be found in [359] (p. 795).

4.3.2.2. Transport Equations for the RNG k-ε Model The RNG - model has a similar form to the standard - model: (4.42)

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Standard, RNG, and Realizable k-ε Models and (4.43) In these equations, represents the generation of turbulence kinetic energy due to the mean velocity gradients, calculated as described in Modeling Turbulent Production in the k-ε Models (p. 54). is the generation of turbulence kinetic energy due to buoyancy, calculated as described in Effects of Buoyancy on Turbulence in the k-ε Models (p. 54). represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, calculated as described in Effects of Compressibility on Turbulence in the k-ε Models (p. 55). The quantities and are the inverse effective Prandtl numbers for and , respectively. and are user-defined source terms.

4.3.2.3. Modeling the Effective Viscosity The scale elimination procedure in RNG theory results in a differential equation for turbulent viscosity: (4.44)

where

Equation 4.44 (p. 49) is integrated to obtain an accurate description of how the effective turbulent transport varies with the effective Reynolds number (or eddy scale), allowing the model to better handle low-Reynolds number and near-wall flows. In the high-Reynolds number limit, Equation 4.44 (p. 49) gives (4.45) with , derived using RNG theory. It is interesting to note that this value of to the empirically-determined value of 0.09 used in the standard - model.

is very close

In ANSYS Fluent, by default, the effective viscosity is computed using the high-Reynolds number form in Equation 4.45 (p. 49). However, there is an option available that allows you to use the differential relation given in Equation 4.44 (p. 49) when you need to include low-Reynolds number effects.

4.3.2.4. RNG Swirl Modification Turbulence, in general, is affected by rotation or swirl in the mean flow. The RNG model in ANSYS Fluent provides an option to account for the effects of swirl or rotation by modifying the turbulent viscosity appropriately. The modification takes the following functional form: (4.46) where

is the value of turbulent viscosity calculated without the swirl modification using either

Equation 4.44 (p. 49) or Equation 4.45 (p. 49). is a characteristic swirl number evaluated within ANSYS Fluent, and is a swirl constant that assumes different values depending on whether the flow is swirlRelease 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

49

Turbulence dominated or only mildly swirling. This swirl modification always takes effect for axisymmetric, swirling flows and three-dimensional flows when the RNG model is selected. For mildly swirling flows (the default in ANSYS Fluent), is set to 0.07. For strongly swirling flows, however, a higher value of can be used.

4.3.2.5. Calculating the Inverse Effective Prandtl Numbers The inverse effective Prandtl numbers, analytically by the RNG theory:

and

, are computed using the following formula derived (4.47)

where

. In the high-Reynolds number limit (

),

.

4.3.2.6. The R-ε Term in the ε Equation The main difference between the RNG and standard - models lies in the additional term in the equation given by (4.48) where

,

,

.

The effects of this term in the RNG equation can be seen more clearly by rearranging Equation 4.43 (p. 49). Using Equation 4.48 (p. 50), the third and fourth terms on the right-hand side of Equation 4.43 (p. 49) can be merged, and the resulting equation can be rewritten as (4.49) where

is given by (4.50)

In regions where

, the

term makes a positive contribution, and

becomes larger than

. In

the logarithmic layer, for instance, it can be shown that , giving , which is close in magnitude to the value of in the standard - model (1.92). As a result, for weakly to moderately strained flows, the RNG model tends to give results largely comparable to the standard - model. In regions of large strain rate (

), however, the

term makes a negative contribution, making the

value of less than . In comparison with the standard - model, the smaller destruction of augments , reducing and, eventually, the effective viscosity. As a result, in rapidly strained flows, the RNG model yields a lower turbulent viscosity than the standard - model. Thus, the RNG model is more responsive to the effects of rapid strain and streamline curvature than the standard - model, which explains the superior performance of the RNG model for certain classes of flows.

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Standard, RNG, and Realizable k-ε Models

4.3.2.7. Model Constants The model constants and in Equation 4.43 (p. 49) have values derived analytically by the RNG theory. These values, used by default in ANSYS Fluent, are

4.3.3. Realizable k-ε Model 4.3.3.1. Overview The realizable - model [430] (p. 798) differs from the standard - model in two important ways: • The realizable - model contains an alternative formulation for the turbulent viscosity. • A modified transport equation for the dissipation rate, , has been derived from an exact equation for the transport of the mean-square vorticity fluctuation. The term “realizable” means that the model satisfies certain mathematical constraints on the Reynolds stresses, consistent with the physics of turbulent flows. Neither the standard - model nor the RNG - model is realizable. To understand the mathematics behind the realizable - model, consider combining the Boussinesq relationship (Equation 4.14 (p. 41)) and the eddy viscosity definition (Equation 4.41 (p. 48)) to obtain the following expression for the normal Reynolds stress in an incompressible strained mean flow: (4.51) Using Equation 4.41 (p. 48) for , one obtains the result that the normal stress, , which by definition is a positive quantity, becomes negative, that is, “non-realizable”, when the strain is large enough to satisfy (4.52) Similarly, it can also be shown that the Schwarz inequality for shear stresses (

; no summation

over and ) can be violated when the mean strain rate is large. The most straightforward way to ensure the realizability (positivity of normal stresses and Schwarz inequality for shear stresses) is to make variable by sensitizing it to the mean flow (mean deformation) and the turbulence ( , ). The notion of variable is suggested by many modelers including Reynolds [400] (p. 797), and is well substantiated by experimental evidence. For example, is found to be around 0.09 in the logarithmic layer of equilibrium boundary layers, and 0.05 in a strong homogeneous shear flow. Both the realizable and RNG - models have shown substantial improvements over the standard - model where the flow features include strong streamline curvature, vortices, and rotation. Since the model is still relatively new, it is not clear in exactly which instances the realizable - model consistently outperforms the RNG model. However, initial studies have shown that the realizable model provides the best performance of all the - model versions for several validations of separated flows and flows with complex secondary flow features. One of the weaknesses of the standard - model or other traditional - models lies with the modeled equation for the dissipation rate ( ). The well-known round-jet anomaly (named based on the finding Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

51

Turbulence that the spreading rate in planar jets is predicted reasonably well, but prediction of the spreading rate for axisymmetric jets is unexpectedly poor) is considered to be mainly due to the modeled dissipation equation. The realizable - model proposed by Shih et al. [430] (p. 798) was intended to address these deficiencies of traditional - models by adopting the following: • A new eddy-viscosity formula involving a variable

originally proposed by Reynolds [400] (p. 797).

• A new model equation for dissipation ( ) based on the dynamic equation of the mean-square vorticity fluctuation. One limitation of the realizable - model is that it produces non-physical turbulent viscosities in situations when the computational domain contains both rotating and stationary fluid zones (for example, multiple reference frames, rotating sliding meshes). This is due to the fact that the realizable - model includes the effects of mean rotation in the definition of the turbulent viscosity (see Equation 4.55 (p. 53) – Equation 4.57 (p. 53)). This extra rotation effect has been tested on single moving reference frame systems and showed superior behavior over the standard - model. However, due to the nature of this modification, its application to multiple reference frame systems should be taken with some caution. See Modeling the Turbulent Viscosity (p. 53) for information about how to include or exclude this term from the model.

4.3.3.2. Transport Equations for the Realizable k-ε Model The modeled transport equations for

and

in the realizable - model are (4.53)

and (4.54)

where

In these equations, represents the generation of turbulence kinetic energy due to the mean velocity gradients, calculated as described in Modeling Turbulent Production in the k-ε Models (p. 54). is the generation of turbulence kinetic energy due to buoyancy, calculated as described in Effects of Buoyancy on Turbulence in the k-ε Models (p. 54). represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, calculated as described in Effects of Compressibility on Turbulence in the k-ε Models (p. 55). and are constants. and are the turbulent Prandtl numbers for and , respectively. and are user-defined source terms. Note that the equation (Equation 4.53 (p. 52)) is the same as that in the standard - model (Equation 4.39 (p. 47)) and the RNG - model (Equation 4.42 (p. 48)), except for the model constants. However, the form of the equation is quite different from those in the standard and RNG-based - models (Equation 4.40 (p. 47) and Equation 4.43 (p. 49)). One of the noteworthy features is that the production term in the equation (the second term on the right-hand side of Equation 4.54 (p. 52)) does not involve

52

Standard, RNG, and Realizable k-ε Models the production of ; that is, it does not contain the same term as the other - models. It is believed that the present form better represents the spectral energy transfer. Another desirable feature is that the destruction term (the third term on the right-hand side of Equation 4.54 (p. 52)) does not have any singularity; that is, its denominator never vanishes, even if vanishes or becomes smaller than zero. This feature is contrasted with traditional - models, which have a singularity due to in the denominator. This model has been extensively validated for a wide range of flows [230] (p. 787), [430] (p. 798), including rotating homogeneous shear flows, free flows including jets and mixing layers, channel and boundary layer flows, and separated flows. For all these cases, the performance of the model has been found to be substantially better than that of the standard - model. Especially noteworthy is the fact that the realizable - model resolves the round-jet anomaly; that is, it predicts the spreading rate for axisymmetric jets as well as that for planar jets.

4.3.3.3. Modeling the Turbulent Viscosity As in other - models, the eddy viscosity is computed from (4.55) The difference between the realizable - model and the standard and RNG - models is that no longer constant. It is computed from

is (4.56)

where (4.57) and

where velocity

is the mean rate-of-rotation tensor viewed in a moving reference frame with the angular . The model constants and are given by (4.58)

where (4.59)

53

Turbulence It can be seen that is a function of the mean strain and rotation rates, the angular velocity of the system rotation, and the turbulence fields ( and ). in Equation 4.55 (p. 53) can be shown to recover the standard value of 0.09 for an inertial sublayer in an equilibrium boundary layer.

Important In ANSYS Fluent, the term is, by default, not included in the calculation of . This is an extra rotation term that is not compatible with cases involving sliding meshes or multiple reference frames. If you want to include this term in the model, you can enable it by using the define/models/viscous/turbulence-expert/rke-cmu-rotation-term? text command and entering yes at the prompt.

4.3.3.4. Model Constants The model constants , , and have been established to ensure that the model performs well for certain canonical flows. The model constants are

4.3.4. Modeling Turbulent Production in the k-ε Models The term , representing the production of turbulence kinetic energy, is modeled identically for the standard, RNG, and realizable - models. From the exact equation for the transport of , this term may be defined as (4.60) To evaluate

in a manner consistent with the Boussinesq hypothesis, (4.61)

where

is the modulus of the mean rate-of-strain tensor, defined as (4.62)

Important When using the high-Reynolds number - versions,

is used in lieu of

in Equa-

tion 4.61 (p. 54).

4.3.5. Effects of Buoyancy on Turbulence in the k-ε Models When a nonzero gravity field and temperature gradient are present simultaneously, the - models in ANSYS Fluent account for the generation of due to buoyancy ( in Equation 4.39 (p. 47), Equation 4.42 (p. 48), and Equation 4.53 (p. 52)), and the corresponding contribution to the production of in Equation 4.40 (p. 47), Equation 4.43 (p. 49), and Equation 4.54 (p. 52). The generation of turbulence due to buoyancy is given by

54

Standard, RNG, and Realizable k-ε Models

(4.63) where

is the turbulent Prandtl number for energy and

is the component of the gravitational

vector in the th direction. For the standard and realizable - models, the default value of is 0.85. For non-premixed and partially premixed combustion models, is set equal to the PDF Schmidt number to ensure a Lewis number equal to unity. In the case of the RNG - model, = , where is given by Equation 4.47 (p. 50), but with . The coefficient of thermal expansion, , is defined as (4.64) For ideal gases, Equation 4.63 (p. 55) reduces to (4.65) It can be seen from the transport equations for (Equation 4.39 (p. 47), Equation 4.42 (p. 48), and Equation 4.53 (p. 52)) that turbulence kinetic energy tends to be augmented ( ) in unstable stratification. For stable stratification, buoyancy tends to suppress the turbulence ( ). In ANSYS Fluent, the effects of buoyancy on the generation of are always included when you have both a nonzero gravity field and a nonzero temperature (or density) gradient. While the buoyancy effects on the generation of are relatively well understood, the effect on is less clear. In ANSYS Fluent, by default, the buoyancy effects on are neglected simply by setting to zero in the transport equation for (Equation 4.40 (p. 47), Equation 4.43 (p. 49), or Equation 4.54 (p. 52)). However, you can include the buoyancy effects on in the Viscous Model Dialog Box. In this case, the value of given by Equation 4.65 (p. 55) is used in the transport equation for (Equation 4.40 (p. 47), Equation 4.43 (p. 49), or Equation 4.54 (p. 52)). The degree to which is affected by the buoyancy is determined by the constant . In ANSYS Fluent, is not specified, but is instead calculated according to the following relation [177] (p. 784): (4.66) where is the component of the flow velocity parallel to the gravitational vector and is the component of the flow velocity perpendicular to the gravitational vector. In this way, will become 1 for buoyant shear layers for which the main flow direction is aligned with the direction of gravity. For buoyant shear layers that are perpendicular to the gravitational vector, will become zero.

4.3.6. Effects of Compressibility on Turbulence in the k-ε Models For high-Mach-number flows, compressibility affects turbulence through so-called “dilatation dissipation”, which is normally neglected in the modeling of incompressible flows [526] (p. 804). Neglecting the dilatation dissipation fails to predict the observed decrease in spreading rate with increasing Mach number for compressible mixing and other free shear layers. To account for these effects in the - models in ANSYS Fluent, the dilatation dissipation term, , can be included in the equation. This term is modeled according to a proposal by Sarkar [414] (p. 797): (4.67)

55

Turbulence where

is the turbulent Mach number, defined as (4.68)

where

(

) is the speed of sound.

Note The Sarkar model has been tested for a very limited number of free shear test cases, and should be used with caution (and only when truly necessary), as it can negatively affect the wall boundary layer even at transonic and supersonic Mach numbers. It is disabled by default. For details, see Model Enhancements in the Fluent User's Guide.

4.3.7. Convective Heat and Mass Transfer Modeling in the k-ε Models In ANSYS Fluent, turbulent heat transport is modeled using the concept of Reynolds’ analogy to turbulent momentum transfer. The “modeled” energy equation is therefore given by (4.69) where

is the total energy,

is the effective thermal conductivity, and

is the deviatoric stress tensor, defined as

The term involving

represents the viscous heating, and is always computed in the density-based

solvers. It is not computed by default in the pressure-based solver, but it can be enabled in the Viscous Model Dialog Box. Additional terms may appear in the energy equation, depending on the physical models you are using. See Heat Transfer Theory (p. 139) for more details. For the standard and realizable - models, the effective thermal conductivity is given by

where , in this case, is the thermal conductivity. The default value of the turbulent Prandtl number is 0.85. You can change the value of the turbulent Prandtl number in the Viscous Model Dialog Box. For the RNG - model, the effective thermal conductivity is

where

56

is calculated from Equation 4.47 (p. 50), but with

.

Standard, BSL, and SST k-ω Models The fact that

varies with

, as in Equation 4.47 (p. 50), is an advantage of the RNG

- model. It is consistent with experimental evidence indicating that the turbulent Prandtl number varies with the molecular Prandtl number and turbulence [220] (p. 787). Equation 4.47 (p. 50) works well across a very broad range of molecular Prandtl numbers, from liquid metals ( ) to paraffin oils ( ), which allows heat transfer to be calculated in low-Reynolds number regions. Equation 4.47 (p. 50) smoothly predicts the variation of effective Prandtl number from the molecular value ( ) in the viscosity-dominated region to the fully turbulent value ( ) in the fully turbulent regions of the flow. Turbulent mass transfer is treated similarly. For the standard and realizable - models, the default turbulent Schmidt number is 0.7. This default value can be changed in the Viscous Model Dialog Box. For the RNG model, the effective turbulent diffusivity for mass transfer is calculated in a manner that is analogous to the method used for the heat transport. The value of in Equation 4.47 (p. 50) is , where Sc is the molecular Schmidt number.

4.4. Standard, BSL, and SST k-ω Models This section describes the theory behind the Standard, BSL, and SST presented in the following sections: 4.4.1. Standard k-ω Model 4.4.2. Baseline (BSL) k-ω Model 4.4.3. Shear-Stress Transport (SST) k-ω Model 4.4.4.Turbulence Damping 4.4.5. Wall Boundary Conditions

models. Information is

For details about using the models in ANSYS Fluent, see Modeling Turbulence and Setting Up the kω Model in the User's Guide . This section presents the standard [526] (p. 804), baseline (BSL) [311] (p. 792), and shear-stress transport (SST) [311] (p. 792) - models. All three models have similar forms, with transport equations for and . The major ways in which the BSL and SST models [314] (p. 792) differ from the standard model are as follows: • gradual change from the standard - model in the inner region of the boundary layer to a high-Reynolds number version of the - model in the outer part of the boundary layer (BSL, SST) • modified turbulent viscosity formulation to account for the transport effects of the principal turbulent shear stress (SST only) The transport equations, methods of calculating turbulent viscosity, and methods of calculating model constants and other terms are presented separately for each model. Low Reynolds number modifications have been proposed by Wilcox for the k- model and are available in ANSYS Fluent. It is important to note that all k- models can be integrated through the viscous sublayer without these terms. The terms were mainly added to reproduce the peak in the turbulence kinetic energy observed in DNS data very close to the wall. In addition, these terms affect the laminarturbulent transition process. The low-Reynolds number terms can produce a delayed onset of the turbulent wall boundary layer and constitute therefore a very simple model for laminar-turbulent transition. In general, the use of the low-Reynolds number terms in the k- models is not recommended, and it is advised to use the more sophisticated, and more widely calibrated, models for laminar-turbulent transition instead.

57

Turbulence

4.4.1. Standard k-ω Model 4.4.1.1. Overview The standard - model in ANSYS Fluent is based on the Wilcox - model [526] (p. 804), which incorporates modifications for low-Reynolds number effects, compressibility, and shear flow spreading. One of the weak points of the Wilcox model is the sensitivity of the solutions to values for k and outside the shear layer (freestream sensitivity). While the new formulation implemented in ANSYS Fluent has reduced this dependency, it can still have a significant effect on the solution, especially for free shear flows [312] (p. 792). The standard - model is an empirical model based on model transport equations for the turbulence kinetic energy ( ) and the specific dissipation rate ( ), which can also be thought of as the ratio of to [526] (p. 804). As the - model has been modified over the years, production terms have been added to both the and equations, which have improved the accuracy of the model for predicting free shear flows.

4.4.1.2. Transport Equations for the Standard k-ω Model The turbulence kinetic energy, , and the specific dissipation rate, transport equations:

, are obtained from the following (4.70)

and (4.71) In these equations, represents the generation of turbulence kinetic energy due to mean velocity gradients. represents the generation of . and represent the effective diffusivity of and , respectively. and represent the dissipation of and due to turbulence. All of the above terms are calculated as described below. and are user-defined source terms.

4.4.1.3. Modeling the Effective Diffusivity The effective diffusivities for the -

model are given by (4.72)

where and are the turbulent Prandtl numbers for , is computed by combining and as follows:

and

, respectively. The turbulent viscosity,

(4.73)

4.4.1.3.1. Low-Reynolds Number Correction The coefficient by

58

damps the turbulent viscosity causing a low-Reynolds number correction. It is given

Standard, BSL, and SST k-ω Models

(4.74) where (4.75) (4.76) (4.77) (4.78) Note that in the high-Reynolds number form of the -

model,

.

4.4.1.4. Modeling the Turbulence Production 4.4.1.4.1. Production of k The term represents the production of turbulence kinetic energy. From the exact equation for the transport of , this term may be defined as (4.79) To evaluate

in a manner consistent with the Boussinesq hypothesis, (4.80)

where is the modulus of the mean rate-of-strain tensor, defined in the same way as for the model (see Equation 4.62 (p. 54)).

4.4.1.4.2. Production of ω The production of

is given by (4.81)

where

is given by Equation 4.79 (p. 59).

The coefficient

is given by (4.82)

where

= 2.95.

and

are given by Equation 4.74 (p. 59) and Equation 4.75 (p. 59), respectively.

Note that in the high-Reynolds number form of the -

model,

.

4.4.1.5. Modeling the Turbulence Dissipation 4.4.1.5.1. Dissipation of k The dissipation of

is given by (4.83) Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

59

Turbulence where (4.84)

where (4.85) and (4.86) (4.87) (4.88) (4.89) (4.90) where

is given by Equation 4.75 (p. 59).

4.4.1.5.2. Dissipation of ω The dissipation of

is given by (4.91)

where (4.92) (4.93) (4.94) The strain rate tensor,

is defined in Equation 4.25 (p. 44). Also, (4.95)

and

are defined by Equation 4.87 (p. 60) and Equation 4.96 (p. 60), respectively.

4.4.1.5.3. Compressibility Effects The compressibility function,

, is given by (4.96)

where (4.97)

60

Standard, BSL, and SST k-ω Models (4.98) (4.99) Note that, in the high-Reynolds number form of the -

model,

. In the incompressible form,

.

Note The compressibility effects have been calibrated for a very limited number of free shear flow experiments, and it is not recommended for general use. It is disabled by default. For details, see Model Enhancements in the Fluent User's Guide.

4.4.1.6. Model Constants

4.4.2. Baseline (BSL) k-ω Model 4.4.2.1. Overview The main problem with the Wilcox model is its well known strong sensitivity to freestream conditions. The baseline (BSL) - model was developed by Menter [311] (p. 792) to effectively blend the robust and accurate formulation of the - model in the near-wall region with the freestream independence of the - model in the far field. To achieve this, the - model is converted into a - formulation. The BSL - model is similar to the standard - model, but includes the following refinements: • The standard - model and the transformed - model are both multiplied by a blending function and both models are added together. The blending function is designed to be one in the near-wall region, which activates the standard - model, and zero away from the surface, which activates the transformed model. • The BSL model incorporates a damped cross-diffusion derivative term in the

equation.

• The modeling constants are different.

4.4.2.2. Transport Equations for the BSL k-ω Model The BSL -

model has a similar form to the standard -

model: (4.100)

and (4.101)

61

Turbulence In these equations, the term represents the production of turbulence kinetic energy, and is defined in the same manner as in the standard k- model. represents the generation of , calculated as described in a section that follows. and represent the effective diffusivity of and , respectively, which are calculated as described in the section that follows. and represent the dissipation of and due to turbulence, calculated as described in Modeling the Turbulence Dissipation (p. 59). represents the cross-diffusion term, calculated as described in the section that follows. and are user-defined source terms.

4.4.2.3. Modeling the Effective Diffusivity The effective diffusivities for the BSL -

model are given by (4.102) (4.103)

where and are the turbulent Prandtl numbers for , is computed as defined in Equation 4.73 (p. 58), and

and

, respectively. The turbulent viscosity, (4.104) (4.105)

The blending function

is given by (4.106) (4.107) (4.108)

where is the distance to the next surface and (see Equation 4.116 (p. 63)).

is the positive portion of the cross-diffusion term

4.4.2.4. Modeling the Turbulence Production 4.4.2.4.1. Production of k The term represents the production of turbulence kinetic energy, and is defined in the same manner as in the standard - model. See Modeling the Turbulence Production (p. 59) for details.

4.4.2.4.2. Production of ω The term

represents the production of

and is given by (4.109)

Note that this formulation differs from the standard - model (this difference is important for the SST model described in a later section). It also differs from the standard - model in the way the term is evaluated. In the standard - model, is defined as a constant (0.52). For the BSL - model, is given by (4.110)

62

Standard, BSL, and SST k-ω Models where (4.111) (4.112) where

is 0.41.

4.4.2.5. Modeling the Turbulence Dissipation 4.4.2.5.1. Dissipation of k The term represents the dissipation of turbulence kinetic energy, and is defined in a similar manner as in the standard - model (see Modeling the Turbulence Dissipation (p. 59)). The difference is in the way the term is evaluated. In the standard - model, is defined as a piecewise function. For the BSL -

model,

is a constant equal to 1. Thus, (4.113)

4.4.2.5.2. Dissipation of ω The term represents the dissipation of , and is defined in a similar manner as in the standard model (see Modeling the Turbulence Dissipation (p. 59)). The difference is in the way the terms and are evaluated. In the standard -

model,

is defined as a constant (0.072) and

Equation 4.91 (p. 60). For the BSL -

model,

is a constant equal to 1. Thus,

is defined in

(4.114) Instead of having a constant value,

is given by (4.115)

and

is obtained from Equation 4.106 (p. 62).

Note that the constant value of 0.072 is still used for to define in Equation 4.77 (p. 59).

in the low-Reynolds number correction for BSL

4.4.2.6. Cross-Diffusion Modification The BSL - model is based on both the standard - model and the standard - model. To blend these two models together, the standard - model has been transformed into equations based on and , which leads to the introduction of a cross-diffusion term ( in Equation 4.101 (p. 61)). is defined as (4.116)

4.4.2.7. Model Constants

63

Turbulence

All additional model constants ( , , , , , standard - model (see Model Constants (p. 61)).

,

,

, and

) have the same values as for the

4.4.3. Shear-Stress Transport (SST) k-ω Model 4.4.3.1. Overview The SST - model includes all the refinements of the BSL - model, and in addition accounts for the transport of the turbulence shear stress in the definition of the turbulent viscosity. These features make the SST - model (Menter [311] (p. 792)) more accurate and reliable for a wider class of flows (for example, adverse pressure gradient flows, airfoils, transonic shock waves) than the standard and the BSL - models.

4.4.3.2. Modeling the Turbulent Viscosity The BSL model described previously combines the advantages of the Wilcox and the - model, but still fails to properly predict the onset and amount of flow separation from smooth surfaces. The main reason is that both models do not account for the transport of the turbulent shear stress. This results in an overprediction of the eddy-viscosity. The proper transport behavior can be obtained by a limiter to the formulation of the eddy-viscosity: (4.117) where

is the strain rate magnitude and

is defined in Equation 4.74 (p. 59).

is given by (4.118) (4.119)

where

is the distance to the next surface.

4.4.3.3. Model Constants

All additional model constants ( , , , , , standard - model (see Model Constants (p. 61)).

,

,

, and

) have the same values as for the

4.4.3.4. Treatment of the SST Model for Icing Simulations An alternative SST roughness model has been implemented based on the Colebrook correlation by Aupoix [21] (p. 776). As in the Spalart-Allmaras model, the concept of wall turbulent viscosity has been adopted, and it is estimated by modelling the wall values of k and ω. Specifically, Aupoix proposed the following formulations to compute the non-dimensional k and ω on a wall, , [21] (p. 776):

64

Standard, BSL, and SST k-ω Models

(4.120)

(4.121)

where

, a standard constant in the SST k-ω model.

and

are defined as: (4.122) (4.123)

Therefore, all the wall values of k and ω are known: (4.124) (4.125)

4.4.4. Turbulence Damping In free surface flows, a high velocity gradient at the interface between two fluids results in high turbulence generation, in both phases. Hence, turbulence damping is required in the interfacial area to model such flows correctly.

Important Turbulence damping is available only with the The following term is added as a source to the

models.

-equation [119] (p. 781) (4.126)

where = Interfacial area density for phase = Cell height normal to interface = -

model closure coefficient of destruction term, which is equal to 0.075

= Damping factor = Viscosity of phase = Density of phase The interfacial area density for phase is calculated as (4.127) where = Volume fraction of phase

65

Turbulence = Magnitude of gradient of volume fraction The grid size is calculated internally using grid information. You can specify the damping factor in the Viscous Model dialog box. The default value for the damping factor is 10. Turbulence damping is available with the VOF and Mixture models. Note that it is also available with the Eulerian multiphase model when using the immiscible fluid model. To use this option, refer to Turbulence Damping. If the Eulerian multiphase model is enabled, you can specify a turbulence multiphase model. If the Per Phase turbulence model is used, then the source term is added to the -equation of each phase. If the VOF or mixture model is enabled, or the Eulerian multiphase model is used with the Mixture turbulence model, the term is summed over all the phases and added as a source term to the mixture level -equation. For information about turbulence multiphase modeling, see Modeling Turbulence.

4.4.5. Wall Boundary Conditions The wall boundary conditions for the equation in the - models are treated in the same way as the equation is treated when enhanced wall treatments are used with the - models. This means that all boundary conditions for wall-function meshes will correspond to the wall function approach, while for the fine meshes, the appropriate low-Reynolds number boundary conditions will be applied. In ANSYS Fluent the value of

at the wall is specified as (4.128)

Analytical solutions can be given for both the laminar sublayer (4.129) and the logarithmic region: (4.130) Therefore, a wall treatment can be defined for the -equation, which switches automatically from the viscous sublayer formulation to the wall function, depending on the grid. This blending has been optimized using Couette flow in order to achieve a grid independent solution of the skin friction value and wall heat transfer. This improved blending is the default behavior for near-wall treatment.

4.5. k-kl-ω Transition Model This section describes the theory behind the - - Transition model. For details about using the model in ANSYS Fluent, see Modeling Turbulence and Setting Up the Transition k-kl-ω Model in the User's Guide . For more information, see the following sections: 4.5.1. Overview 4.5.2. Transport Equations for the k-kl-ω Model

66

k-kl-ω Transition Model

4.5.1. Overview The - - transition model [509] (p. 803) is used to predict boundary layer development and calculate transition onset. This model can be used to effectively address the transition of the boundary layer from a laminar to a turbulent regime.

4.5.2. Transport Equations for the k-kl-ω Model The - - model is considered to be a three-equation eddy-viscosity type, which includes transport equations for turbulent kinetic energy ( ), laminar kinetic energy ( ), and the inverse turbulent time scale ( ) (4.131) (4.132)

(4.133)

The inclusion of the turbulent and laminar fluctuations on the mean flow and energy equations via the eddy viscosity and total thermal diffusivity is as follows: (4.134) (4.135) The effective length is defined as (4.136) where

is the turbulent length scale and is defined by (4.137)

and the small scale energy is defined by (4.138) (4.139) (4.140) The large scale energy is given by (4.141) Note that the sum of Equation 4.138 (p. 67) and Equation 4.141 (p. 67) yields the turbulent kinetic energy . The turbulence production term generated by turbulent fluctuations is given by Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

67

Turbulence (4.142) where the small-scale turbulent viscosity is (4.143) and (4.144) (4.145) A damping function defining the turbulent production due to intermittency is given by (4.146) (4.147) In Equation 4.132 (p. 67), ations, such that

is the production of laminar kinetic energy by large scale turbulent fluctu(4.148)

The large-scale turbulent viscosity

is modeled as (4.149)

where (4.150) The limit in Equation 4.149 (p. 68) binds the realizability such that it is not violated in the two-dimensional developing boundary layer. The time-scale-based damping function is (4.151) where

from Equation 4.150 (p. 68) is (4.152) (4.153)

Near-wall dissipation is given by (4.154)

68

k-kl-ω Transition Model

(4.155) In Equation 4.131 (p. 67) – Equation 4.133 (p. 67), represents the averaged effect of the breakdown of streamwise fluctuations into turbulence during bypass transition: (4.156) , which is the threshold function controls the bypass transition process: (4.157) (4.158) The breakdown to turbulence due to instabilities is considered to be a natural transition production term, given by (4.159) (4.160)

(4.161) The use of as the scale-determining variable can lead to a reduced intermittency effect in the outer region of a turbulent boundary layer, and consequently an elimination of the wake region in the velocity profile. From Equation 4.133 (p. 67), the following damping is defined as (4.162) The total eddy viscosity and eddy diffusivity included in Equation 4.134 (p. 67) and Equation 4.135 (p. 67) are given by (4.163) (4.164) The turbulent scalar diffusivity in Equation 4.131 (p. 67) – Equation 4.133 (p. 67) is defined as (4.165) (4.166) A compressibility effects option, similar to the one in the - model (Effects of Compressibility on Turbulence in the k-ε Models) is available for the - - model. By default, this compressibility effects option is turned off. For details see, Model Enhancements in the Fluent User's Guide.

4.5.2.1. Model Constants The model constants for the - -

transition model are listed below [509] (p. 803)

69

Turbulence

4.6. Transition SST Model This section describes the theory behind the Transition SST model. Information is presented in the following sections: 4.6.1. Overview 4.6.2.Transport Equations for the Transition SST Model 4.6.3. Mesh Requirements 4.6.4. Specifying Inlet Turbulence Levels For details about using the model in ANSYS Fluent, see Modeling Turbulence and Setting Up the Transition SST Model in the User's Guide .

4.6.1. Overview The Transition SST model (also known as the model) is based on the coupling of the SST transport equations with two other transport equations, one for the intermittency and one for the transition onset criteria, in terms of momentum-thickness Reynolds number. An ANSYS empirical correlation (Langtry and Menter) has been developed to cover standard bypass transition as well as flows in low freestream turbulence environments. In addition, a very powerful option has been included to allow you to enter your own user-defined empirical correlation, which can then be used to control the transition onset momentum thickness Reynolds number equation. Note the following limitations: • The Transition SST model is only applicable to wall-bounded flows. Like all other engineering transition models, the model is not applicable to transition in free shear flows. The model will predict free shear flows as fully turbulent. • The Transition SST model is not Galilean invariant and should therefore not be applied to surfaces that move relative to the coordinate system for which the velocity field is computed; for such cases the Intermittency Transition model should be used instead. • The Transition SST model is designed for flows with a defined nonzero freestream velocity (that is, the classical boundary layer situation). It is not suitable for fully developed pipe / channel flows where no freestream is present. For the same reason, it is also not suitable for wall jet flows. For such scenarios, the Intermittency Transition model should be used instead. Note, however, that it might be necessary to adjust the Intermittency Transition model for such flows by modifying the underlying correlations.

70

Transition SST Model • The Transition SST model has not been calibrated in combination with other physical effects that affect the source terms of the turbulence model, such as: – buoyancy – multiphase turbulence To learn how to set up the Transition SST model, see Setting Up the Transition SST Model (in the User's Guide ).

4.6.2. Transport Equations for the Transition SST Model The transport equation for the intermittency

is defined as: (4.167)

The transition sources are defined as follows: (4.168)

where is the strain rate magnitude, is an empirical correlation that controls the length of the transition region, and and hold the values of 2 and 1, respectively. The destruction/relaminarization sources are defined as follows: (4.169) where

is the vorticity magnitude. The transition onset is controlled by the following functions: (4.170)

(4.171)

(4.172)

where

is the wall distance and

is the critical Reynolds number where the intermittency first starts

to increase in the boundary layer. This occurs upstream of the transition Reynolds number the difference between the two must be obtained from an empirical correlation. Both the correlations are functions of

and and

.

The constants for the intermittency equation are: (4.173) The transport equation for the transition momentum thickness Reynolds number Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

is 71

Turbulence

(4.174) The source term is defined as follows: (4.175)

(4.176)

(4.177)

(4.178)

The model constants for the

equation are: (4.179)

The boundary condition for at a wall is zero flux. The boundary condition for be calculated from the empirical correlation based on the inlet turbulence intensity.

at an inlet should

The model contains three empirical correlations. is the transition onset as observed in experiments. This has been modified from Menter et al. [315] (p. 792) in order to improve the predictions for natural transition. It is used in Equation 4.174 (p. 72). is the length of the transition zone and is substituted in Equation 4.167 (p. 71). is the point where the model is activated in order to match both and , and is used in Equation 4.171 (p. 71). These empirical correlations are provided by Langtry and Menter [246] (p. 788).

(4.180)

The first empirical correlation is a function of the local turbulence intensity,

: (4.181)

where

is the turbulent energy.

The Thwaites’ pressure gradient coefficient

is defined as (4.182)

where

72

is the acceleration in the streamwise direction.

Transition SST Model

4.6.2.1. Separation-Induced Transition Correction The modification for separation-induced transition is:

(4.183)

Here,

is a constant with a value of 2.

The model constants in Equation 4.183 (p. 73) have been adjusted from those of Menter et al. [315] (p. 792) in order to improve the predictions of separated flow transition. The main difference is that the constant that controls the relation between and was changed from 2.193, its value for a Blasius boundary layer, to 3.235, the value at a separation point where the shape factor is 3.5 [315] (p. 792). The boundary condition for at a wall is zero normal flux, while for an inlet, is equal to 1.0.

4.6.2.2. Coupling the Transition Model and SST Transport Equations The transition model interacts with the SST turbulence model by modification of the -equation (Equation 4.100 (p. 61)), as follows: (4.184) (4.185) (4.186) where and are the original production and destruction terms for the SST model. Note that the production term in the -equation is not modified. The rationale behind the above model formulation is given in detail in Menter et al. [315] (p. 792). In order to capture the laminar and transitional boundary layers correctly, the mesh must have a of approximately one. If the is too large (that is, > 5), then the transition onset location moves upstream with increasing . It is recommended that you use the bounded second order upwind based discretization for the mean flow, turbulence and transition equations.

4.6.2.3. Transition SST and Rough Walls When the Transition SST Model is used together with rough walls, the roughness correlation must be enabled in the Viscous Model Dialog Box. This correlation requires the geometric roughness height as an input parameter, since, for the transition process from laminar to turbulent flow, the geometric roughness height is more important than the equivalent sand-grain roughness height . Guidance to determine the appropriate equivalent sand-grain roughness height (based on the geometric roughness height, shape and distribution of the roughness elements) can be obtained, for example, from Schlichting and Gersten [422] (p. 798) and Coleman et al. [86] (p. 779). The roughness correlation is a modification of the built-in correlation for

and is defined as: (4.187)

73

Turbulence The new defined is then used in the correlations for and . represents the transition momentum thickness Reynolds number. The value specified for the geometric roughness will apply to all walls. In case a different value is required for different walls, a user-defined function can be specified. It is important to note that the function for K is used in the volume (not at the wall). The function therefore must cover the region of the boundary layer and beyond where it should be applied. As an example, assume a roughness strip on a flat plate (x-streamwise direction, y wall normal z-spanwise) at the location (m) and a boundary layer thickness in that region of perhaps (spanwise extent (m)). The following pseudo-code will switch between roughness K0 everywhere (could be zero) and K1 at the transition strip. The height in the y-direction does not have to be exactly the boundary layer thickness – it can be much larger – as long as it does not impact other walls in the vicinity. (4.188)

4.6.3. Mesh Requirements The effect of increasing and decreasing for a flat plate test case (T3A) is shown in Figure 4.1: Effect of Increasing y+ for the Flat Plate T3A Test Case (p. 74) and Figure 4.2: Effect of Decreasing y+ for the Flat Plate T3A Test Case (p. 75). For values between 0.001 and 1, there is almost no effect on the solution. Once the maximum increases above 8, the transition onset location begins to move upstream. At a maximum of 25, the boundary layer is almost completely turbulent. For values below 0.001, the transition location appears to move downstream. This is presumably caused by the large surface value of the specific turbulence frequency , which scales with the first mesh node height. For these reasons, very small values (below 0.001) should be avoided. Figure 4.1: Effect of Increasing y+ for the Flat Plate T3A Test Case

74

Transition SST Model Figure 4.2: Effect of Decreasing y+ for the Flat Plate T3A Test Case

The effect of the wall normal expansion ratio from a value of 1 is shown in Figure 4.3: Effect of Wall Normal Expansion Ratio for the Flat Plate T3A Test Case (p. 76). For expansion factors of 1.05 and 1.1, there is no effect on the solution. For larger expansion factors of 1.2 and 1.4, there is a small but noticeable upstream shift in the transition location. Because the sensitivity of the solution to wall-normal mesh resolution can increase for flows with pressure gradients, it is recommended that you apply meshes with and expansion factors smaller than 1.1.

75

Turbulence Figure 4.3: Effect of Wall Normal Expansion Ratio for the Flat Plate T3A Test Case

The effect of streamwise mesh refinement is shown in Figure 4.4: Effect of Streamwise Mesh Density for the Flat Plate T3A Test Case (p. 76). Surprisingly, the model was not very sensitive to the number of streamwise nodes. The solution differed significantly from the mesh-independent one only for the case of 25 streamwise nodes where there was only one cell in the transitional region. Nevertheless, the meshindependent solution appears to occur when there is approximately 75–100 streamwise mesh nodes on the flat plate. Also, separation-induced transition occurs over a very short length; for cases where this is important, a fine mesh is necessary. Figure 4.4: Effect of Streamwise Mesh Density for the Flat Plate T3A Test Case

76

Transition SST Model One point to note is that for sharp leading edges, often transition can occur due to a small leading edge separation bubble. If the mesh is too coarse, the rapid transition caused by the separation bubble is not captured. Based on the mesh sensitivity study, the recommended best practice mesh guidelines are a max of 1, a wall normal expansion ratio that is less than 1.1, and about 75–100 mesh nodes in the streamwise direction. Note that if separation-induced transition is present, additional mesh nodes in the streamwise direction are most likely needed. For a turbine blade, that would translate into 100–150 cells in the streamwise direction on each side of the blade.

4.6.4. Specifying Inlet Turbulence Levels It has been observed that the turbulence intensity specified at an inlet can decay quite rapidly depending on the inlet viscosity ratio ( ) (and hence turbulence eddy frequency). As a result, the local turbulence intensity downstream of the inlet can be much smaller than the inlet value (see Figure 4.5: Exemplary Decay of Turbulence Intensity (Tu) as a Function of Streamwise Distance (x) (p. 78)). Typically, the larger the inlet viscosity ratio, the smaller the turbulent decay rate. However, if too large a viscosity ratio is specified (that is, > 100), the skin friction can deviate significantly from the laminar value. There is experimental evidence that suggests that this effect occurs physically; however, at this point it is not clear how accurately the transition model reproduces this behavior. For this reason, if possible, it is desirable to have a relatively low (that is, 1 – 10) inlet viscosity ratio and to estimate the inlet value of turbulence intensity such that at the leading edge of the blade/airfoil, the turbulence intensity has decayed to the desired value. The decay of turbulent kinetic energy can be calculated with the following analytical solution: (4.189) For the SST turbulence model in the freestream the constants are: (4.190) The time scale can be determined as follows: (4.191) where is the streamwise distance downstream of the inlet and The eddy viscosity is defined as:

is the mean convective velocity. (4.192)

The decay of turbulent kinetic energy equation can be rewritten in terms of inlet turbulence intensity ( ) and inlet eddy viscosity ratio ( ) as follows: (4.193)

77

Turbulence Figure 4.5: Exemplary Decay of Turbulence Intensity (Tu) as a Function of Streamwise Distance (x)

You should ensure that the values around the body of interest roughly satisfy >0.1%. For smaller values of , the reaction of the SST model production terms to the transition onset becomes too slow, and transition can be delayed past the physically correct location.

4.7. Intermittency Transition Model This section describes the theory behind the Intermittency Transition model. Information is presented in the following sections: 4.7.1. Overview 4.7.2.Transport Equations for the Intermittency Transition Model 4.7.3. Coupling with the Other Models 4.7.4. Intermittency Transition Model and Rough Walls For details about using the model in ANSYS Fluent, see Modeling Turbulence and Setting Up the Intermittency Transition Model in the User's Guide .

4.7.1. Overview The (intermittency) transition model is a further development based on the transition model (referred to as the Transition SST model in ANSYS Fluent, as described in Transition SST Model (p. 70)). The transition model solves only one transport equation for the turbulence intermittency , and avoids the need for the second equation of the transition model. The transition model has the following advantages over the transition model: • It reduces the computational effort (by solving one transport equation instead of two). • It avoids the dependency of the equation on the velocity . This makes the transition model Galilean invariant. It can therefore be applied to surfaces that move relative to the coordinate system for which the velocity field is computed. • The model has provisions for crossflow instability that are not available for the model. 78

or the -

transition

Intermittency Transition Model • The model formulation is simple and can be fine-tuned based on a small number of user parameters. Like the transition model, the transition model is based strictly on local variables. The transition model is also only available in combination with the following turbulence models: • BSL -

model

• SST -

model

• Scale-Adaptive Simulation with BSL or SST • Detached Eddy Simulation with BSL or SST • Shielded Detached Eddy Simulation (SDES) with BSL or SST • Stress-Blended Eddy Simulation (SBES) with BSL or SST Note the following limitations: • The transition model is only applicable to wall-bounded flows. Like all other engineering transition models, the model is not applicable to transition in free shear flows. The model will predict free shear flows as fully turbulent. • The transition model has only been calibrated for classical boundary layer flows. Application to other types of wall-bounded flows is possible, but might require a modification of the underlying correlations. • The transition model has not been calibrated in combination with other physical effects that affect the source terms of the turbulence model, such as: – buoyancy – multiphase turbulence

4.7.2. Transport Equations for the Intermittency Transition Model The transport equation for intermittency is the following: (4.194) The formulation of the source terms is similar in nature to the corresponding terms in the transition model, but the precise details are proprietary. As in the model.

transition model, the source terms includes a correlation to trigger the transition (4.195)

In this correlation, is the critical momentum thickness Reynolds number, and and are locally defined variables that approximate the freestream turbulence intensity and the pressure gradient parameter, respectively (see Transport Equations for the Transition SST Model (p. 71)). The function accounts for the influence of the pressure gradient on transition, and is defined as: (4.196)

79

Turbulence The transition model has been calibrated against a wide range of generic as well as turbomachinery and external aeronautical test cases. While you should not have to adjust the model coefficients in most situations, the model provides access to the coefficients so that you can fine-tune the model. This should only be done based on detailed experimental data. You can adjust the following constants in the correlations:

,

,

,

,

,

,

,

,

, and

The

constant defines the minimal value of the critical number, whereas the sum of and defines the maximal value of . The constant controls how fast decreases as the freestream turbulence intensity ( ) increases. The and constants adjust the value of the critical number in areas with favorable and adverse pressure gradients, respectively. The constant becomes active in regions with separation, correcting the value if necessary. Additionally, the size of the separation bubble can be tuned by another constant, , which enters the model through the source terms directly rather than the correlations. It should be noted, however, that the constant has an effect only for the bubbles developing under low conditions ( 100 species) due to the sparse-matrix solution algorithm used by the solver. The source term in the conservation equation for the mean species , Equation 7.1 (p. 193), is modeled as (7.30)

where

is the fine-scale species mass fraction after reacting over the time

.

The EDC model can incorporate detailed chemical mechanisms into turbulent reacting flows. However, typical mechanisms are invariably stiff and their numerical integration is computationally costly. Hence, the model should be used only when the assumption of fast chemistry is invalid, such as modeling lowtemperature or high-pressure combustion, the slow CO burnout in rapidly quenched flames, or the NO conversion in selective non-catalytic reduction (SNCR). For guidelines on obtaining a solution using the EDC model, see Solution of Stiff Chemistry Systems in the User's Guide.

201

Species Transport and Finite-Rate Chemistry

7.1.2.6. The Thickened Flame Model Premixed flames have typical laminar flame thicknesses of the order of a millimeter. Since the laminar flame propagation speed is determined by species diffusion, heat conduction and chemical reaction within the flame, sufficient grid resolution inside the flame is necessary to predict the correct laminar flame velocity. In contrast, the combustor dimensions are usually much larger than the laminar flame thickness and the flame cannot be affordably resolved, even with unstructured and solution-adaptive grids. The premixed laminar flame speed, denoted , is proportional to is a reaction rate. The laminar flame thickness is proportional to

where is a diffusivity and . Hence, the laminar flame can be

artificially thickened, without altering the laminar flame speed, by increasing the diffusivity and decreasing the reaction rate proportionally. The thickened flame can then be feasibly resolved on a coarse mesh while still capturing the correct laminar flame speed. The thickening factor

is calculated in ANSYS Fluent as (7.31)

where

is the grid size,

is the laminar flame thickness, and

is the user-specified number of points

in the flame (default of 8). The grid-size, , is determined as where is the cell volume and is the spatial dimension (2 or 3). The laminar flame thickness, , is a user-input, and may be specified as a constant, a User-Defined Function, or calculated as , where is the thermal diffusivity evaluated as

( is the thermal conductivity,

is the density and

is the specific heat).

All species diffusion coefficients, as well as the thermal conductivity, are multiplied by the thickening factor, , and all reaction rates are divided by . However, away from the flame, these enhanced diffusivities can cause erroneous mixing and heat-transfer, so the flame is dynamically thickened only in a narrow band around the reaction front. This band is calculated by multiplying with, a factor calculated as (7.32)

In Equation 7.32 (p. 202),

is the spatially filtered absolute value of the reaction rate, and

with a default value of 10. The absolute reaction rate is filtered several times, and maximum value of this band.

in the domain.

is a constant is the

ranges from unity in the band around the flame to zero outside

The Thickened Flame Model (TFM) [358] (p. 795) is most often used with a single step chemical mechanism where the global reaction has been tuned to provide the correct laminar flame speed. The TFM can, in principal, be used with multi-step reaction mechanisms, however all composition profiles should be adequately resolved within the flame. The stiff-chemistry solver is recommended for numerical stability. The TFM is available in both the pressure-based as well as density-based solvers. While the TFM can be used to model laminar flames, its most common application is as an LES combustion model for turbulent premixed and partially-premixed flames. The turbulent flame speed of a premixed flame is determined principally by the flame wrinkling, which increases the flame surface area. In a turbulent flow-field, a thick flame wrinkles less than a thin flame, so the diffusivities and reaction rates in the TFM are both multiplied by an efficiency factor, denoted , to increase the flame speed. This ef-

202

Volumetric Reactions ficiency factor is calculated using the turbulent flame speed closure model of Zimont (see Equation 9.9 (p. 265)), which can be written as (7.33) where is the turbulent flame speed, is a constant, and and are turbulent velocity and length scales. Note that is the turbulent flame speed due to the wrinkling effect of turbulence with eddy length scales less than . In a RANS simulation, is the RMS velocity and is the turbulence integral scale, as Equation 9.9. In an LES, and are the sub-grid velocity fluctuation and length scales, as in Equation 9.13 (p. 266) and Equation 9.14 (p. 267). The TFM efficiency factor, , is then calculated as the ratio of the sub-grid turbulent flame speeds at length scales of and : (7.34)

All species reaction rates are multiplied by . The effective species diffusivities (and thermal conductivity) in each cell are dynamically determined as (7.35) where is the molecular (laminar) diffusivity and is the turbulent diffusivity. may be computed with any of the available methods in ANSYS Fluent, including kinetic theory and User Defined Functions (UDFs). Since the kinetic coefficients of 1-step reactions are invariably adjusted to capture the actual laminar flame speed, the same transport properties that are used in this tuning simulation should be used in the TFM simulation. In the narrow band around the flame where is one, the turbulent diffusivities are switched off and the molecular diffusivities are enhanced by a factor . Away from the flame where is zero, the effective diffusivity is the non-thickened value of .

7.1.2.7. The Relaxation to Chemical Equilibrium Model When using the finite-rate (FR) chemistry, or when using eddy-dissipation (ED), finite-rate/eddy-dissipation (FR/ED) or EDC model, as described above, chemical species evolve according to the prescribed kinetic mechanism. In the Relaxation to Chemical Equilibrium model, the species composition is driven to its equilibrium state. The reaction source term in the tion 7.1 (p. 193)), is modeled as

th

mean species conservation equation (Equa(7.36)

where, = the density = the mean mass fraction of species superscript

denotes chemical equilibrium

= a characteristic time-scale Equation 7.36 (p. 203) implies that species react toward their chemical equilibrium state over the characteristic time, , as in the Characteristic Time model [237] (p. 788). Since chemical equilibrium does not depend on reactions or reaction rates, for a given , the reaction source term in Equation 7.36 (p. 203) is independent of the reaction mechanism. Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

203

Species Transport and Finite-Rate Chemistry The Relaxation to Chemical Equilibrium model is an option available for any of the turbulence-chemistry interaction options. When no turbulence-chemistry model is used, the characteristic time is calculated as (7.37) where

is a convection/diffusion time-scale in a cell.

is a cell chemical time-scale modeled as (7.38)

In Equation 7.38 (p. 204),

is a model constant (default of 1), index

denotes the user-specified fuel

species, is the fuel species mass fraction, and is the rate of change of the fuel species mass fraction, while denotes the minimum over all specified fuel species. Hence, represents a chemical ignition time-scale for the fuel species, and is used to prevent a premixed flame from autoigniting, similar to the FR/ED model. Note that a kinetic mechanism is required to evaluate . For the ED model, the characteristic time-scale is evaluated as

. where the turbulent time-

scale is , and the default turbulent rate constant is . For the FR/ED model, the characteristic time-scale is calculated as . Typically, the ED and FR/ED models employ a 1step reaction, or a 2-step reaction where hydrocarbons pyrolize to , which then oxidizes to . The Relaxation to Chemical Equilibrium model can be considered as an extension of the ED type models where species react towards chemical equilibrium instead of complete reaction. The model should provide more accurate predictions of intermediate species such as and radicals required for NOx modeling such as and . Since chemical equilibrium calculations are typically less computationally expensive than detailed chemistry simulations, the Relaxation to Chemical equilibrium can be used to provide a good initial condition for full-kinetic steady-state simulations. The model can also be used in lieu of the Eddy-Dissipation model where the solution tends to chemical equilibrium instead of complete reaction. The assumption of chemical equilibrium can lead to large errors in rich zones for hydrocarbon fuels. ANSYS Fluent offers the option of reducing the reaction rate in Equation 7.36 (p. 203) by increasing the characteristic time-scale in rich zones. With this option, the local equivalence ratio in a cell is calculated as (7.39) where

,

, and

denote the atomic mass fraction of oxygen, hydrogen and carbon. The time-

scale, , is then multiplied by a factor when the local equivalence ratio exceeds the specified rich equivalence ratio . The default rich equivalence ratio is 1, and the default exponential factor, , is 2. The option to enable the slow reaction rate in rich mixtures is available in the text interface. Since chemical equilibrium calculations can consume computational resources, ISAT tabulation is the recommended (and default) solution method. The initial ANSYS Fluent iterations speed up significantly as the table is built up with sufficient entries and the majority of queries to the table are interpolated. Since chemical equilibrium compositions are uniquely determined by the initial temperature, pressure and elemental compositions of the mixture, the ISAT table dimensions are for isobaric systems. In most combustion cases, the number of elements is much smaller than the number of chemical species, and the ISAT table is built up relatively quickly. The Direct Integration option may also be selected, in which case equilibrium calculations will always be performed at all cells.

204

Wall Surface Reactions and Chemical Vapor Deposition

7.2. Wall Surface Reactions and Chemical Vapor Deposition For gas-phase reactions, the reaction rate is defined on a volumetric basis and the net rate of creation and destruction of chemical species becomes a source term in the species conservation equations. For surface reactions, the rate of adsorption and desorption is governed by both chemical kinetics and diffusion to and from the surface. Wall surface reactions therefore create sources and sinks of chemical species in the gas phase, as well as on the reacting surface. Theoretical information about wall surface reactions and chemical vapor deposition is presented in this section. Information can be found in the following sections: 7.2.1. Surface Coverage Reaction Rate Modification 7.2.2. Reaction-Diffusion Balance for Surface Chemistry 7.2.3. Slip Boundary Formulation for Low-Pressure Gas Systems For more information about using wall surface reactions and chemical vapor deposition, see Wall Surface Reactions and Chemical Vapor Deposition in the User's Guide. th

Consider the

wall surface reaction written in general form as follows: (7.40)

where

,

, and

represent the gas phase species, the bulk (or solid) species, and the surface-adsorbed

(or site) species, respectively.

,

, and

are the total numbers of these species.

are the stoichiometric coefficients for each reactant species , and coefficients for each product species . respectively.

and

,

, and

,

, and

are the stoichiometric

are the forward and backward rate of reactions,

The summations in Equation 7.40 (p. 205) are for all chemical species in the system, but only species involved as reactants or products will have nonzero stoichiometric coefficients. Hence, species that are not involved will drop out of the equation. The rate of the

th

reaction is (7.41)

where

represents molar concentrations of surface-adsorbed species on the wall.

are the rate exponents for the The variables

and

and

gaseous species as reactant and product, respectively, in the reaction.

are the rate exponents for the

site species as reactant and product in

the reaction. It is assumed that the reaction rate does not depend on concentrations of the bulk (solid) species. From this, the net molar rate of production or consumption of each species is given by (7.42) (7.43) (7.44)

205

Species Transport and Finite-Rate Chemistry The forward rate constant for reaction

(

) is computed using the Arrhenius expression, (7.45)

where, = pre-exponential factor (consistent units) = temperature exponent (dimensionless) = activation energy for the reaction (J/kmol) = universal gas constant (J/kmol-K) You (or the database) will provide values for

,

,

,

,

,

,

,

, and

.

To include the mass transfer effects and model heat release, refer to Including Mass Transfer To Surfaces in Continuity, Wall Surface Mass Transfer Effects in the Energy Equation, and Modeling the Heat Release Due to Wall Surface Reactions in the User's Guide. If the reaction is reversible, the backward rate constant for reaction , rate constant using the following relation:

is computed from the forward (7.46)

where

is the equilibrium constant for

th

reaction computed from (7.47)

where denotes the atmospheric pressure (101325 Pa). The term within the exponential function represents the change in the Gibbs free energy, and its components are computed per Equation 7.12 (p. 197) and Equation 7.13 (p. 197). is the number of different types of sites, are the stoichiometric coefficients of the

th

is the site density of site type .

site species of type

and

in reaction .

7.2.1. Surface Coverage Reaction Rate Modification ANSYS Fluent has the option to modify the surface reaction rate as a function of species site coverages. In such cases, the forward rate constant for the

th

reaction is evaluated as, (7.48)

In Equation 7.48 (p. 206), the three surface coverage rate modification parameters for species in reaction are , and . These parameters default to zero for reaction species that are not surface rate modifying. The surface (coverage) site fraction, and is defined as,

is the fraction of surface sites covered by species , (7.49)

where

206

is the surface site concentration and

is the surface site density (see Equation 7.54 (p. 207)).

Wall Surface Reactions and Chemical Vapor Deposition

7.2.2. Reaction-Diffusion Balance for Surface Chemistry Reactions at surfaces change gas-phase, surface-adsorbed (site) and bulk (solid) species. On reacting surfaces, the mass flux of each gas species due to diffusion and convection to/from the surface is balanced with its rate of consumption/production on the surface, (7.50) (7.51) The wall mass fraction

is related to concentration by (7.52)

is the net rate of mass deposition or etching as a result of surface reaction; that is, (7.53)

is the site species concentration at the wall, and is defined as (7.54) where

is the site density and

is the site coverage of species .

Equation 7.50 (p. 207) and Equation 7.51 (p. 207) are solved for the dependent variables and using a point-by-point coupled Newton solver. When the Newton solver fails, Equation 7.50 (p. 207) and Equation 7.51 (p. 207) are solved by time marching in an ODE solver until convergence. If the ODE solver fails, reaction-diffusion balance is disabled, is assumed equal to the cell-center value , and only the site coverages are advanced in the ODE solver to convergence. You can manually disable reaction-diffusion balance with the text interface command define/models/species/disablediffusion-reaction-balance. The effective gas-phase reaction source terms are then available for solution of the gas-phase species transport Equation 7.1 (p. 193). The diffusion term in Equation 7.50 (p. 207) is calculated as the difference in the species mass fraction at the cell center and the wall-face center, divided by the normal distance between these center points. ANSYS Fluent can model unresolved surface washcoats, which greatly increase the catalytic surface area, with the Surface Area Washcoat Factor. The surface washcoat increases the area available for surface reaction. ANSYS Fluent can also model surface chemistry on unresolved walls using a porous media model. This model is appropriate for catalytic tube-banks or porous foam matrices where it is not feasible to resolve the individual walls. When surface reaction is enabled in porous cell zones, the Surface-to-Volume Ratio must be specified. The wall normal distance required for the diffusion term in Equation 7.50 (p. 207) is calculated as the inverse of the surface-to-volume ratio.

7.2.3. Slip Boundary Formulation for Low-Pressure Gas Systems Most semiconductor fabrication devices operate far below atmospheric pressure, typically only a few millitorrs. At such low pressures, the fluid flow is in the slip regime and the normally used no-slip boundary conditions for velocity and temperature are no longer valid.

207

Species Transport and Finite-Rate Chemistry The Knudsen number, denoted , and defined as the ratio of mean free path to a characteristic length scale of the system, is used to quantify continuum flow regimes. Since the mean free path increases as the pressure is lowered, the high end of values represents free molecular flow and the low end the continuum regime. The range in between these two extremes is called the slip regime (0.01< 3000), Gnielinski's correlation for turbulent flow is used: (7.100) where

is the friction factor. For smooth tubes of circular cross-section,

is given by (7.101)

The plug flow equations are solved with a stiff ODE solver using time-steps based on the grid size (size of channel element) and the local channel velocity.

7.5.2.2. Surface Reactions in the Reacting Channel When the surface reaction option is enabled, additional source terms due to surface reactions are added to Equation 7.96 (p. 217). Details of the heat and mass sources due to surface reactions are described in Wall Surface Reactions and Chemical Vapor Deposition (p. 205). The reacting surface area of a reacting channel element is calculated as (7.102)

218

Reacting Channel Model where is the volume of the channel element and catalytic surface.

is the area to volume ratio of the

Note In the reacting channel model, the heat source due to surface reaction and the heat and mass source due to mass deposition are always included.

7.5.2.3. Porous Medium Inside Reacting Channel The porous medium option in the reacting channel model uses the superficial velocity porous formulation. ANSYS Fluent calculates the superficial mixture velocities based on the volumetric flow rate in a porous region. When the Porous Medium option is enabled, the heat transfer coefficient calculated from Equation 7.98 (p. 218) is modified to account for the solid material of the porous medium in the following manner. (7.103) where is the porosity and are modified by multiplying the solid.

is the conductivity of the solid material. Also, the reaction rates by medium porosity to account for the displacement of the gas by

The Porous Medium option provides the capability to model pressure drop inside the reacting channel. The pressure drop in the channel is calculated only in the axial direction and has contributions from viscous and inertial resistances: (7.104) The pressure drop due to viscous resistance is calculated using Darcy's law (7.105) where

is the viscous resistance along axial direction and

is the length of the channel element.

The pressure drop due to inertial losses is modeled as (7.106) where

is the inertial resistance along axial direction and

is the length of the channel element.

7.5.2.4. Outer Flow in the Shell The outer shell can be any arbitrarily shaped 3D geometry, but must resolve the channel walls. Note that while this outer shell is meshed in ANSYS Fluent with finite-volume cells, the inside of the channels should not be meshed. This outer flow can be a reacting or non-reacting mixture, which is typically a different mixture material than the complex mechanism material used for the reacting channel plug flow. The ANSYS Fluent energy solution of the outer shell flow uses a prescribed heat flux boundary condition at the channel walls from the solution of the reacting channel. The value of this heat flux is calculated as an under-relaxation of the heat gained from the channel: Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

219

Species Transport and Finite-Rate Chemistry (7.107) where is the channel heat gain/loss, is a user-specified under-relaxation parameter, and is the heat flux from the previous iteration. At convergence, the heat loss/gain from the channel is equal to the heat gain/loss in the shell outer flow.

7.6. Reactor Network Model The reactor network model is used to simulate species and temperature fields in a combustor using a detailed chemical kinetic mechanism. The reactor network is constructed from a converged ANSYS Fluent simulation modeled with a fast chemistry combustion model, such as the Non-Premixed, PartiallyPremixed, or Eddy-Dissipation model. A full chemical mechanism in CHEMKIN format can be imported into Fluent and solved on the reactor network. The combustor volume is automatically subdivided into a small number of connected perfectly stirred reactors. The mass fluxes through the network are determined from the CFD solution, and the species and temperatures in the reactor network are solved together. Hence, the reactor network model is used to simulate finite-rate chemistry effects with detailed kinetic mechanisms, in particular pollutant emissions such as NOx, CO, and unburnt hydro-carbons. Since the specified number of stirred reactors is much smaller than the number of CFD cells, the reactor network model allows much faster simulations of the species and temperature fields than solving for detailed chemistry in every cell, as in the Laminar, EDC, and PDF Transport models. Typically, the reactor network model is executed on a converged steady-state RANS solution or a timeaveraged unsteady solution. The model can also be run on an unsteady flow representing a “snap-shot” in time. Since there is no backward coupling of the reactor-network solution to the flow, the model is useful for predictions where the detailed chemistry has little impact on the flow, such as pollutant formation. The model is inappropriate for highly unsteady flows such as flame ignition or global extinction and also for flows that are strongly influenced by the chemistry, such as soot with significant soot-radiation interaction. For more information about using reactor network, see Reactor Network Model in the Fluent User's Guide.

7.6.1. Reactor Network Model Theory The first step in a reactor network simulation involves agglomerating the CFD cells into the specified number of reactors, . Since each reactor is a perfectly mixed representation of a region of the combustor, ideally cells that are closest in composition space should be grouped together. For optimal performance, the CFD cells grouped together in each reactor should have temperatures and species mass fractions that are as similar as possible. By default, for Non-Premixed and Partially-Premixed cases, Fluent groups cells that have similar temperatures and mixture fractions, and for Species Transport cases, Fluent groups cells that have similar temperatures and mass fractions of and . These defaults should provide good cell clustering in most cases. However, Fluent allows user-controlled clustering through custom-field functions when these defaults are not sufficient. After similar cells have been clustered, Fluent splits non-contiguous groups, then agglomerates clusters with the smallest number of cells to their closest neighbors until the specified number of reactors, , is obtained.

220

Reactor Network Model The second step in a reactor network simulation involves the solution of the reactor network, which proceeds as follows. The mass flux matrix,

, is calculated from the cell fluxes in the CFD solution, where each matrix com-

ponent is the mass flux from reactor to reactor . The is governed by the algebraic equation:

th

species mass fraction in reactor ,

,

(7.108) where is the volume of reactor , is the th species reaction rate in the mass source term. is the net mass flux into reactor and is calculated as:

th

reactor, and

is a

(7.109)

The mass source term, , accounts for contributions from the CFD inlet species mass fluxes through the CFD boundaries and from volume sources, such as the Discrete Phase Model (DPM). The system of equations for , Equation 7.108 (p. 221), has the dimension of , where is the user-specified number of reactors, and is the number of species in the chemical mechanism. ANSYS Fluent solves this system with a segregated algorithm by default, although the option to use a fully coupled algorithm is available.

7.6.1.1. Reactor network temperature solution An energy equation is not solved in the reactor network. Instead, by default, the temperature in each reactor is calculated from the equation of state. The reactor pressure is fixed and determined as the mass-averaged pressure of the CFD cells in the reactor. Note that the density of each reactor is fixed since both the reactor volume and the reactor mass are constant. This approach ensures that heat loss (or gain) in the CFD simulation is appropriately accounted for in the reactor network. Fluent also provides an option to not solve for temperature, in which case the temperature is fixed as the mass-average temperature of the CFD cells in the reactor.

221

222

Chapter 8: Non-Premixed Combustion In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. This is in contrast to premixed systems, in which reactants are mixed at the molecular level before burning. Examples of non-premixed combustion include pulverized coal furnaces, diesel internal-combustion engines and pool fires. Under certain assumptions, the thermochemistry can be reduced to a single parameter: the mixture fraction. The mixture fraction, denoted by , is the mass fraction that originated from the fuel stream. In other words, it is the local mass fraction of burnt and unburnt fuel stream elements ( , , and so on) in all the species ( , , , and so on). The approach is elegant because atomic elements are conserved in chemical reactions. In turn, the mixture fraction is a conserved scalar quantity, and therefore its governing transport equation does not have a source term. Combustion is simplified to a mixing problem, and the difficulties associated with closing nonlinear mean reaction rates are avoided. Once mixed, the chemistry can be modeled as being in chemical equilibrium with the Equilibrium model, being near chemical equilibrium with the Steady Diffusion Flamelet model, or significantly departing from chemical equilibrium with the Unsteady Diffusion Flamelet model. For more information about using the non-premixed combustion model, see Modeling Non-Premixed Combustion in the User's Guide. Theoretical information about the non-premixed combustion model is presented in the following sections: 8.1. Introduction 8.2. Non-Premixed Combustion and Mixture Fraction Theory 8.3. Restrictions and Special Cases for Using the Non-Premixed Model 8.4.The Diffusion Flamelet Models Theory 8.5.The Steady Diffusion Flamelet Model Theory 8.6.The Unsteady Diffusion Flamelet Model Theory

8.1. Introduction Non-premixed modeling involves the solution of transport equations for one or two conserved scalars (the mixture fractions). Equations for individual species are not solved. Instead, species concentrations are derived from the predicted mixture fraction fields. The thermochemistry calculations are preprocessed and then tabulated for look-up in ANSYS Fluent. Interaction of turbulence and chemistry is accounted for with an assumed-shape Probability Density Function (PDF).

8.2. Non-Premixed Combustion and Mixture Fraction Theory Information about non-premixed combustion and mixture fraction theory are presented in the following sections: 8.2.1. Mixture Fraction Theory 8.2.2. Modeling of Turbulence-Chemistry Interaction 8.2.3. Non-Adiabatic Extensions of the Non-Premixed Model 8.2.4. Chemistry Tabulation

223

Non-Premixed Combustion

8.2.1. Mixture Fraction Theory 8.2.1.1. Definition of the Mixture Fraction The basis of the non-premixed modeling approach is that under a certain set of simplifying assumptions, the instantaneous thermochemical state of the fluid is related to a conserved scalar quantity known as the mixture fraction, . The mixture fraction can be written in terms of the atomic mass fraction as [439] (p. 799): (8.1) where is the elemental mass fraction for element, . The subscript ox denotes the value at the oxidizer stream inlet and the subscript fuel denotes the value at the fuel stream inlet. If the diffusion coefficients for all species are equal, then Equation 8.1 (p. 224) is identical for all elements, and the mixture fraction definition is unique. The mixture fraction is therefore the elemental mass fraction that originated from the fuel stream. If a secondary stream (another fuel or oxidant, or a non-reacting stream) is included, the fuel and secondary mixture fractions are simply the elemental mass fractions of the fuel and secondary streams, respectively. The sum of all three mixture fractions in the system (fuel, secondary stream, and oxidizer) is always equal to 1: (8.2) This indicates that only points on the plane ABC (shown in Figure 8.1: Relationship of Mixture Fractions (Fuel, Secondary Stream, and Oxidizer) (p. 225)) in the mixture fraction space are valid. Consequently, the two mixture fractions, and , cannot vary independently; their values are valid only if they are both within the triangle OBC shown in Figure 8.1: Relationship of Mixture Fractions (Fuel, Secondary Stream, and Oxidizer) (p. 225).

224

Non-Premixed Combustion and Mixture Fraction Theory Figure 8.1: Relationship of Mixture Fractions (Fuel, Secondary Stream, and Oxidizer)

Figure 8.2: Relationship of Mixture Fractions (Fuel, Secondary Stream, and Normalized Secondary Mixture Fraction)

ANSYS Fluent discretizes the triangle OBC as shown in Figure 8.2: Relationship of Mixture Fractions (Fuel, Secondary Stream, and Normalized Secondary Mixture Fraction) (p. 225). Essentially, the primary mixture

225

Non-Premixed Combustion fraction,

, is allowed to vary between zero and one, as for the single mixture fraction case, while

the secondary mixture fraction lies on lines with the following equation: (8.3) where

is the normalized secondary mixture fraction and is the value at the intersection of a line

with the secondary mixture fraction axis. Note that unlike regardless of the

,

is bounded between zero and one,

value.

An important characteristic of the normalized secondary mixture fraction,

, is its assumed statistical

independence from the fuel mixture fraction,

is not a conserved scalar.

. Note that unlike

,

This normalized mixture fraction definition, , is used everywhere in ANSYS Fluent when prompted for Secondary Mixture Fraction except when defining the rich limit for a secondary fuel stream, which is defined in terms of .

8.2.1.2. Transport Equations for the Mixture Fraction Under the assumption of equal diffusivities, the species equations can be reduced to a single equation for the mixture fraction, . The reaction source terms in the species equations cancel (since elements are conserved in chemical reactions), and therefore is a conserved quantity. While the assumption of equal diffusivities is problematic for laminar flows, it is generally acceptable for turbulent flows where turbulent convection overwhelms molecular diffusion. The Favre mean (density-averaged) mixture fraction equation is (8.4) where is laminar thermal conductivity of the mixture, number, and is the turbulent viscosity.

is the mixture specific heat,

is the Prandtl

The source term is due solely to transfer of mass into the gas phase from liquid fuel droplets or reacting particles (for example, coal). is any user-defined source term. In addition to solving for the Favre mean mixture fraction, ANSYS Fluent solves a conservation equation for the mixture fraction variance,

[210] (p. 786): (8.5)

where

is laminar thermal conductivity of the mixture,

number, and ively, and

is the mixture specific heat,

. The default values for the constants is any user-defined source term.

,

, and

is the Prandtl

are 0.85, 2.86, and 2.0, respect-

The mixture fraction variance is used in the closure model describing turbulence-chemistry interactions (see Modeling of Turbulence-Chemistry Interaction (p. 229)). For a two-mixture-fraction problem, tion 8.5 (p. 226) by substituting by substituting 226

for .

and for

and

are obtained from Equation 8.4 (p. 226) and Equafor

.

is obtained from Equation 8.4 (p. 226)

is then calculated using Equation 8.3 (p. 226), and

is obtained by

Non-Premixed Combustion and Mixture Fraction Theory solving Equation 8.5 (p. 226) with and as

substituted for . To a first-order approximation, the variances in

are relatively insensitive to

, and therefore the equation for

is essentially the same

.

Important The equation for instead of is valid when the mass flow rate of the secondary stream is relatively small compared with the total mass flow rate.

8.2.1.3. The Non-Premixed Model for LES For Large Eddy Simulations, the transport equation is not solved for the mixture fraction variance. Instead, it is modeled as (8.6) where = constant = subgrid length scale (see Equation 4.285 (p. 105)) The constant is computed dynamically when the Dynamic Stress option is enabled in the Viscous dialog box, else a constant value (with a default of 0.5) is used. If the Dynamic Scalar Flux option is enabled, the turbulent Sc ( dynamically.

in Equation 8.4 (p. 226)) is computed

8.2.1.4. Mixture Fraction vs. Equivalence Ratio The mixture fraction definition can be understood in relation to common measures of reacting systems. Consider a simple combustion system involving a fuel stream (F), an oxidant stream (O), and a product stream (P) symbolically represented at stoichiometric conditions as (8.7) where

is the air-to-fuel ratio on a mass basis. Denoting the equivalence ratio as

, where (8.8)

the reaction in Equation 8.7 (p. 227), under more general mixture conditions, can then be written as (8.9) Looking at the left side of this equation, the mixture fraction for the system as a whole can then be deduced to be (8.10) Equation 8.10 (p. 227) allows the computation of the mixture fraction at stoichiometric conditions ( 1) or at fuel-rich conditions (for example, > 1), or fuel-lean conditions (for example, < 1). Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

=

227

Non-Premixed Combustion

8.2.1.5. Relationship of Mixture Fraction to Species Mass Fraction, Density, and Temperature The power of the mixture fraction modeling approach is that the chemistry is reduced to one or two conserved mixture fractions. Under the assumption of chemical equilibrium, all thermochemical scalars (species fractions, density, and temperature) are uniquely related to the mixture fraction(s). For a single mixture fraction in an adiabatic system, the instantaneous values of mass fractions, density, and temperature depend solely on the instantaneous mixture fraction, : (8.11) If a secondary stream is included, the instantaneous values will depend on the instantaneous fuel mixture fraction, , and the secondary partial fraction, : (8.12) In Equation 8.11 (p. 228) and Equation 8.12 (p. 228), represents the instantaneous species mass fraction, density, or temperature. In the case of non-adiabatic systems, the effect of heat loss/gain is parameterized as (8.13) for a single mixture fraction system, where

is the instantaneous enthalpy (see Equation 5.7 (p. 141)).

If a secondary stream is included, (8.14) Examples of non-adiabatic flows include systems with radiation, heat transfer through walls, heat transfer to/from discrete phase particles or droplets, and multiple inlets at different temperatures. Additional detail about the mixture fraction approach in such non-adiabatic systems is provided in NonAdiabatic Extensions of the Non-Premixed Model (p. 232). In many reacting systems, the combustion is not in chemical equilibrium. ANSYS Fluent offers several approaches to model chemical non-equilibrium, including the finite-rate (see The Generalized FiniteRate Formulation for Reaction Modeling (p. 195)), EDC (see The Eddy-Dissipation-Concept (EDC) Model (p. 200)), and PDF transport (see Composition PDF Transport (p. 289)) models, where detailed kinetic mechanisms can be incorporated. There are three approaches in the non-premixed combustion model to simulate chemical non-equilibrium. The first is to use the Rich Flammability Limit (RFL) option in the Equilibrium model, where rich regions are modeled as a mixed-but-unburnt mixture of pure fuel and a leaner equilibrium burnt mixture (see Enabling the Rich Flammability Limit (RFL) Option in the User’s Guide). The second approach is the Steady Diffusion Flamelet model, where chemical non-equilibrium due to diffusion flame stretching by turbulence can be modeled. The third approach is the Unsteady Diffusion Flamelet model where slowforming product species that are far from chemical equilibrium can be modeled. See The Diffusion Flamelet Models Theory (p. 246) and The Unsteady Diffusion Flamelet Model Theory (p. 254) for details about the Steady and Unsteady Diffusion Flamelet models in ANSYS Fluent.

228

Non-Premixed Combustion and Mixture Fraction Theory

8.2.2. Modeling of Turbulence-Chemistry Interaction Equation 8.11 (p. 228) through Equation 8.14 (p. 228) describe the instantaneous relationships between mixture fraction and species fractions, density, and temperature under the assumption of chemical equilibrium. The ANSYS Fluent prediction of the turbulent reacting flow, however, is concerned with prediction of the averaged values of these fluctuating scalars. How these averaged values are related to the instantaneous values depends on the turbulence-chemistry interaction model. ANSYS Fluent applies the assumed-shape probability density function (PDF) approach as its closure model when the non-premixed model is used. The assumed shape PDF closure model is described in this section.

8.2.2.1. Description of the Probability Density Function The Probability Density Function, written as , can be thought of as the fraction of time that the fluid spends in the vicinity of the state . Figure 8.3: Graphical Description of the Probability Density Function (p. 229) plots the time trace of mixture fraction at a point in the flow (right-hand side) and the probability density function of (left-hand side). The fluctuating value of , plotted on the right side of the figure, spends some fraction of time in the range denoted as . , plotted on the left side of the figure, takes on values such that the area under its curve in the band denoted, , is equal to the fraction of time that spends in this range. Written mathematically, (8.15) where

is the time scale and

is the amount of time that

spends in the

band. The shape of

the function depends on the nature of the turbulent fluctuations in . In practice, is unknown and is modeled as a mathematical function that approximates the actual PDF shapes that have been observed experimentally. Figure 8.3: Graphical Description of the Probability Density Function

8.2.2.2. Derivation of Mean Scalar Values from the Instantaneous Mixture Fraction The probability density function , describing the temporal fluctuations of in the turbulent flow, can be used to compute averaged values of variables that depend on . Density-weighted mean species mass fractions and temperature can be computed (in adiabatic systems) as (8.16)

229

Non-Premixed Combustion for a single-mixture-fraction system. When a secondary stream exists, mean values are calculated as (8.17)

where

is the PDF of

and

is the PDF of

is assumed, so that

. Here, statistical independence of

and

.

Similarly, the mean time-averaged fluid density, , can be computed as (8.18)

for a single-mixture-fraction system, and (8.19)

when a secondary stream exists.

or

is the instantaneous density obtained using the

instantaneous species mass fractions and temperature in the ideal gas law equation. Using Equation 8.16 (p. 229) and Equation 8.18 (p. 230) (or Equation 8.17 (p. 230) and Equation 8.19 (p. 230)), it remains only to specify the shape of the function

(or

and

) in order to determine

the local mean fluid state at all points in the flow field.

8.2.2.3. The Assumed-Shape PDF The shape of the assumed PDF,

, is described in ANSYS Fluent by one of two mathematical functions:

• the double-delta function (two-mixture-fraction cases only) • the -function (single- and two-mixture-fraction cases) The double-delta function is the most easily computed, while the -function most closely represents experimentally observed PDFs. The shape produced by this function depends solely on the mean mixture fraction, , and its variance,

. A detailed description of each function follows.

8.2.2.3.1. The Double Delta Function PDF The double delta function is given by

(8.20)

with suitable bounding near = 1 and = 0. One example of the double delta function is illustrated in Figure 8.4: Example of the Double Delta Function PDF Shape (p. 231). As noted above, the double

230

Non-Premixed Combustion and Mixture Fraction Theory delta function PDF is very easy to compute but is invariably less accurate than the alternate -function PDF because it assumes that only two states occur in the turbulent flow. For this reason, it is available only for two-mixture-fraction simulations where the savings in computational cost is significant. Figure 8.4: Example of the Double Delta Function PDF Shape

8.2.2.3.2. The β-Function PDF The -function PDF shape is given by the following function of

and

: (8.21)

where (8.22) and (8.23)

Importantly, the PDF shape

is a function of only its first two moments, namely the mean mixture

fraction, , and the mixture fraction variance, . Thus, given ANSYS Fluent’s prediction of and at each point in the flow field (Equation 8.4 (p. 226) and Equation 8.5 (p. 226)), the assumed PDF shape can be computed and used as the weighting function to determine the mean values of species mass fractions, density, and temperature using, Equation 8.16 (p. 229) and Equation 8.18 (p. 230) (or, for a system with a secondary stream, Equation 8.17 (p. 230) and Equation 8.19 (p. 230)). This logical dependence is depicted visually in Figure 8.5: Logical Dependence of Averaged Scalars on Mean Mixture Fraction, the Mixture Fraction Variance, and the Chemistry Model (Adiabatic, Single-MixtureFraction Systems) (p. 232) for a single mixture fraction.

231

Non-Premixed Combustion Figure 8.5: Logical Dependence of Averaged Scalars on Mean Mixture Fraction, the Mixture Fraction Variance, and the Chemistry Model (Adiabatic, Single-Mixture-Fraction Systems)

8.2.3. Non-Adiabatic Extensions of the Non-Premixed Model Many reacting systems involve heat transfer through wall boundaries, droplets, and/or particles. In such flows the local thermochemical state is no longer related only to , but also to the enthalpy, . The system enthalpy impacts the chemical equilibrium calculation and the temperature and species of the reacting flow. Consequently, changes in enthalpy due to heat loss must be considered when computing scalars from the mixture fraction, as in Equation 8.13 (p. 228). In such non-adiabatic systems, turbulent fluctuations should be accounted for by means of a joint PDF, . The computation of , however, is not practical for most engineering applications. The problem can be simplified significantly by assuming that the enthalpy fluctuations are independent of the enthalpy level (that is, heat losses do not significantly impact the turbulent enthalpy fluctuations). With this assumption,

and mean scalars are calculated as (8.24)

Determination of in the non-adiabatic system therefore requires solution of the modeled transport equation for mean enthalpy: (8.25) where accounts for source terms due to radiation, heat transfer to wall boundaries, and heat exchange with the dispersed phase. Figure 8.6: Logical Dependence of Averaged Scalars on Mean Mixture Fraction, the Mixture Fraction Variance, Mean Enthalpy, and the Chemistry Model (Non-Adiabatic, Single-Mixture-Fraction Systems) (p. 233) depicts the logical dependence of mean scalar values (species mass fraction, density, and

232

Non-Premixed Combustion and Mixture Fraction Theory temperature) on ANSYS Fluent’s prediction of , systems.

, and

in non-adiabatic single-mixture-fraction

Figure 8.6: Logical Dependence of Averaged Scalars on Mean Mixture Fraction, the Mixture Fraction Variance, Mean Enthalpy, and the Chemistry Model (Non-Adiabatic, Single-Mixture-Fraction Systems)

When a secondary stream is included, the mean values are calculated from (8.26)

As noted above, the non-adiabatic extensions to the PDF model are required in systems involving heat transfer to walls and in systems with radiation included. In addition, the non-adiabatic model is required in systems that include multiple fuel or oxidizer inlets with different inlet temperatures. Finally, the nonadiabatic model is required in particle-laden flows (for example, liquid fuel systems or coal combustion systems) when such flows include heat transfer to the dispersed phase. Figure 8.7: Reacting Systems Requiring Non-Adiabatic Non-Premixed Model Approach (p. 234) illustrates several systems that must include the non-adiabatic form of the PDF model.

233

Non-Premixed Combustion Figure 8.7: Reacting Systems Requiring Non-Adiabatic Non-Premixed Model Approach

8.2.4. Chemistry Tabulation 8.2.4.1. Look-Up Tables for Adiabatic Systems For an equilibrium, adiabatic, single-mixture-fraction case, the mean temperature, density, and species fraction are functions of the and only (see Equation 8.16 (p. 229) and Equation 8.21 (p. 231)). Significant computational time can be saved by computing these integrals once, storing them in a look-up table, and retrieving them during the ANSYS Fluent simulation. Figure 8.8: Visual Representation of a Look-Up Table for the Scalar (Mean Value of Mass Fractions, Density, or Temperature) as a Function of Mean Mixture Fraction and Mixture Fraction Variance in Adiabatic Single-Mixture-Fraction Systems (p. 235) illustrates the concept of the look-up tables generated 234

Non-Premixed Combustion and Mixture Fraction Theory for a single-mixture-fraction system. Given ANSYS Fluent’s predicted value for and at a point in the flow domain, the mean value of mass fractions, density, or temperature ( ) at that point can be obtained by table interpolation. The table, Figure 8.8: Visual Representation of a Look-Up Table for the Scalar (Mean Value of Mass Fractions, Density, or Temperature) as a Function of Mean Mixture Fraction and Mixture Fraction Variance in Adiabatic Single-Mixture-Fraction Systems (p. 235), is the mathematical result of the integration of Equation 8.16 (p. 229). There is one look-up table of this type for each scalar of interest (species mass fractions, density, and temperature). In adiabatic systems, where the instantaneous enthalpy is a function of only the instantaneous mixture fraction, a two-dimensional look-up table, like that in Figure 8.8: Visual Representation of a Look-Up Table for the Scalar (Mean Value of Mass Fractions, Density, or Temperature) as a Function of Mean Mixture Fraction and Mixture Fraction Variance in Adiabatic Single-MixtureFraction Systems (p. 235), is all that is required. Figure 8.8: Visual Representation of a Look-Up Table for the Scalar (Mean Value of Mass Fractions, Density, or Temperature) as a Function of Mean Mixture Fraction and Mixture Fraction Variance in Adiabatic Single-Mixture-Fraction Systems

For systems with two mixture fractions, the creation and interpolation costs of four-dimensional lookup tables are computationally expensive. By default, the instantaneous properties are tabulated as a function of the fuel mixture fraction

and the secondary partial fraction

(see Equa-

tion 8.12 (p. 228)), and the PDF integrations (see Equation 8.14 (p. 228)) are performed at run time. This two-dimensional table is illustrated in Figure 8.9: Visual Representation of a Look-Up Table for the Scalar φ_I as a Function of Fuel Mixture Fraction and Secondary Partial Fraction in Adiabatic Two-MixtureFraction Systems (p. 236). Alternatively, 4D look-up tables can be created before the simulation and interpolated at run time (see Full Tabulation of the Two-Mixture-Fraction Model in the User's Guide).

235

Non-Premixed Combustion Figure 8.9: Visual Representation of a Look-Up Table for the Scalar φ_I as a Function of Fuel Mixture Fraction and Secondary Partial Fraction in Adiabatic Two-Mixture-Fraction Systems

8.2.4.2. 3D Look-Up Tables for Non-Adiabatic Systems In non-adiabatic systems, where the enthalpy is not linearly related to the mixture fraction, but depends also on wall heat transfer and/or radiation, a look-up table is required for each possible enthalpy value in the system. The result, for single mixture fraction systems, is a three-dimensional look-up table, as illustrated in Figure 8.10: Visual Representation of a Look-Up Table for the Scalar as a Function of Mean Mixture Fraction and Mixture Fraction Variance and Normalized Heat Loss/Gain in Non-Adiabatic SingleMixture-Fraction Systems (p. 237), which consists of layers of two-dimensional tables, each one corresponding to a normalized heat loss or gain. The first slice corresponds to the maximum heat loss from the system, the last slice corresponds to the maximum heat gain to the system, and the zero heat loss/gain slice corresponds to the adiabatic table. Slices interpolated between the adiabatic and maximum slices correspond to heat gain, and those interpolated between the adiabatic and minimum slices correspond to heat loss. The three-dimensional look-up table allows ANSYS Fluent to determine the value of each mass fraction, density, and temperature from calculated values of , , and . This three-dimensional table in Figure 8.10: Visual Representation of a Look-Up Table for the Scalar as a Function of Mean Mixture Fraction and Mixture Fraction Variance and Normalized Heat Loss/Gain in Non-Adiabatic Single-Mixture-Fraction Systems (p. 237) is the visual representation of the integral in Equation 8.24 (p. 232).

236

Non-Premixed Combustion and Mixture Fraction Theory Figure 8.10: Visual Representation of a Look-Up Table for the Scalar as a Function of Mean Mixture Fraction and Mixture Fraction Variance and Normalized Heat Loss/Gain in Non-Adiabatic Single-Mixture-Fraction Systems

For non-adiabatic, two-mixture-fraction problems, it is very expensive to tabulate and retrieve Equation 8.26 (p. 233) since five-dimensional tables are required. By default, 3D lookup tables of the instantaneous state relationship given by Equation 8.14 (p. 228) are created. The 3D table in Figure 8.11: Visual Representation of a Look-Up Table for the Scalar φ_I as a Function of Fuel Mixture Fraction and Secondary Partial Fraction, and Normalized Heat Loss/Gain in Non-Adiabatic Two-Mixture-Fraction Systems (p. 238) is the visual representation of Equation 8.14 (p. 228). The mean density during the ANSYS Fluent solution is calculated by integrating the instantaneous density over the fuel and secondary mixture fraction space (see Equation 8.26 (p. 233)). Alternatively, 5D look-up tables can be created before the simulation and interpolated at run time (see Full Tabulation of the Two-Mixture-Fraction Model in the User's Guide). The one-time pre-generation of the 5D look-up table is very expensive, but, once built, interpolating the table during an ANSYS Fluent solution is usually significantly faster than performing the integrations at run time. This is especially true for cases with a large number of cells that require many iterations or time steps to converge.

Important Note that the computation time in ANSYS Fluent for a two-mixture-fraction case will be much greater than for a single-mixture-fraction problem. This expense should be carefully considered Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

237

Non-Premixed Combustion before choosing the two-mixture-fraction model. Also, it is usually expedient to start a twomixture-fraction simulation from a converged single-mixture-fraction solution. Figure 8.11: Visual Representation of a Look-Up Table for the Scalar φ_I as a Function of Fuel Mixture Fraction and Secondary Partial Fraction, and Normalized Heat Loss/Gain in Non-Adiabatic Two-Mixture-Fraction Systems

8.2.4.3. Generating Lookup Tables Through Automated Grid Refinement ANSYS Fluent has the capability of generating lookup tables using Automated Grid Refinement. This is where an adaptive algorithm inserts grid points in all table dimensions so that changes in the values of tabulated variables (such as mean temperature, density and species mass fractions) between successive grid points, as well as changes in their slopes, are less than a user specified tolerance. The advantage of automated grid refinement is that tabulated quantities are resolved only in regions of rapid change, therefore, more accurate and/or smaller PDF tables are generated, compared to the alternative of an a priori fixed grid.

238

Non-Premixed Combustion and Mixture Fraction Theory When using automated grid refinement, table points are calculated on a coarse grid with a user specified Initial Number of Grid Points (default of ). A new grid point is inserted midway between a point and its neighbor if, (8.27) where is the value of a table variable at grid point , is a user specified Maximum Change in Value Ratio (default of ), and ( ) are the maximum (minimum) table values over all grid points. In addition to ensuring gradual change in value between successive table points, a grid point is added if (8.28) where the slope at any point

is defined as, (8.29)

In Equation 8.28 (p. 239) and Equation 8.29 (p. 239), is a user specified Maximum Change in Slope Ratio (default of ), ( ) are the maximum (minimum) slopes over all grid points, and is the value of the independent grid variable being refined (that is, mean mixture fraction, mixture fraction variance or mean enthalpy). The automated grid refinement algorithm is summarized as follows: an initial grid in the mean mixture fraction dimension is created with the specified Initial Number of Grid Points. Grid points are then inserted when the change in table mean temperature, and mean H2, CO and OH mole fractions, between two grid points, or the change in their slope between three grid points, is more than what you specified in Maximum Change in Value/Slope Ratio. This process is repeated until all points satisfy the specified value and slope change requirements, or the Maximum Number of Grid Points is exceeded. This procedure is then repeated for the mixture fraction variance dimension, which is calculated at the mean stoichiometric mixture fraction. Finally, if the non-adiabatic option is enabled, the procedure is repeated for the mean enthalpy grid, evaluated at the mean stoichiometric mixture fraction and zero mixture fraction variance. When steady diffusion flamelets are used, the Automated Grid Refinement can generate different mean mixture fraction grid points than specified in the diffusion flamelets. For such cases, a 4th order polynomial interpolation is used to obtain the flamelet solution at the inserted points.

Note The second order interpolation option is faster than fourth order interpolation, especially for tables with high dimensions, such as non-adiabatic flamelets. However, under-resolved tables that are interpolated with second order can lead to convergence difficulties. Therefore, the Automated Grid Refinement (AGR) option places grid points optimally when generating PDF tables. Hence, AGR should be used in general, and especially for second order interpolation.

Note Automated grid refinement is not available with two mixture fractions.

239

Non-Premixed Combustion For more information about the input parameters used to generate lookup tables, refer to Calculating the Look-Up Tables in the User's Guide.

8.3. Restrictions and Special Cases for Using the Non-Premixed Model For information about restrictions and special cases for using the non-premixed model, see the following sections: 8.3.1. Restrictions on the Mixture Fraction Approach 8.3.2. Using the Non-Premixed Model for Liquid Fuel or Coal Combustion 8.3.3. Using the Non-Premixed Model with Flue Gas Recycle 8.3.4. Using the Non-Premixed Model with the Inert Model

8.3.1. Restrictions on the Mixture Fraction Approach The unique dependence of (species mass fractions, density, or temperature) on (Equation 8.11 (p. 228) or Equation 8.13 (p. 228)) requires that the reacting system meet the following conditions: • The chemical system must be of the diffusion type with discrete fuel and oxidizer inlets (spray combustion and pulverized fuel flames may also fall into this category). • The Lewis number must be unity. (This implies that the diffusion coefficients for all species and enthalpy are equal, a good approximation in turbulent flow). • When a single mixture fraction is used, the following conditions must be met: – Only one type of fuel is involved. The fuel may be made up of a burnt mixture of reacting species (for example, 90% and 10% ) and you may include multiple fuel inlets. The multiple fuel inlets must have the same composition; two or more fuel inlets with different fuel composition are not allowed (for example, one inlet of and one inlet of ). Similarly, in spray combustion systems or in systems involving reacting particles, only one off-gas is permitted. – Only one type of oxidizer is involved. The oxidizer may consist of a mixture of species (for example, 21% and 79% ) and you may have multiple oxidizer inlets. The multiple oxidizer inlets must, however, have the same composition. Two or more oxidizer inlets with different compositions are not allowed (for example, one inlet of air and a second inlet of pure oxygen). • When two mixture fractions are used, three streams can be involved in the system. Valid systems are as follows: – Two fuel streams with different compositions and one oxidizer stream. Each fuel stream may be made up of a mixture of reacting species (for example, 90% and 10% ). You may include multiple inlets of each fuel stream, but each fuel inlet must have one of the two defined compositions (for example, one inlet of and one inlet of ). – Mixed fuel systems including gas-liquid, gas-coal, or liquid-coal fuel mixtures with a single oxidizer. In systems with a gas-coal or liquid-coal fuel mixture, the coal volatiles and char can be treated as a single composite fuel stream and the secondary stream can represent another fuel. Alternatively, for coal combustion, the volatile and char off-gases are tracked separately as distinct fuel streams. – Two oxidizer streams with different compositions and one fuel stream. Each oxidizer stream may consist of a mixture of species (for example 21% and 79% ). You may have multiple inlets of each oxidizer stream, but each oxidizer inlet must have one of the two defined compositions (for example, one inlet of air and a second inlet of pure oxygen).

240

Restrictions and Special Cases for Using the Non-Premixed Model – A fuel stream, an oxidizer stream, and a non-reacting secondary stream. • The flow must be turbulent. It is important to emphasize that these restrictions eliminate the use of the non-premixed approach for directly modeling premixed combustion. This is because the unburned premixed stream is far from chemical equilibrium. Note, however, that an extended mixture fraction formulation, the partially premixed model (see Partially Premixed Combustion (p. 279)), can be applied to non-premixed (with mixed-butunburnt regions), as well as partially premixed flames. Figure 8.12: Chemical Systems That Can Be Modeled Using a Single Mixture Fraction (p. 242) and Figure 8.13: Chemical System Configurations That Can Be Modeled Using Two Mixture Fractions (p. 243) illustrate typical reacting system configurations that can be handled by the non-premixed model in ANSYS Fluent. Figure 8.14: Premixed Systems That Cannot Be Modeled Using the Non-Premixed Model (p. 243) shows a premixed configuration that cannot be modeled using the non-premixed model.

241

Non-Premixed Combustion Figure 8.12: Chemical Systems That Can Be Modeled Using a Single Mixture Fraction

242

Restrictions and Special Cases for Using the Non-Premixed Model Figure 8.13: Chemical System Configurations That Can Be Modeled Using Two Mixture Fractions

Figure 8.14: Premixed Systems That Cannot Be Modeled Using the Non-Premixed Model

8.3.2. Using the Non-Premixed Model for Liquid Fuel or Coal Combustion You can use the non-premixed model if your ANSYS Fluent simulation includes liquid droplets and/or coal particles. In this case, fuel enters the gas phase within the computational domain at a rate determined by the evaporation, devolatilization, and char combustion laws governing the dispersed phase. In the case of coal, the volatiles and the products of char can be defined as two different types of fuel

243

Non-Premixed Combustion (using two mixture fractions) or as a single composite off-gas (using one mixture fraction), as described in Modeling Coal Combustion Using the Non-Premixed Model in the User’s Guide.

8.3.3. Using the Non-Premixed Model with Flue Gas Recycle While most problems you solve using the non-premixed model will involve inlets that contain either pure oxidant or pure fuel ( or 1), you can include an inlet that has an intermediate value of mixture fraction ( ) provided that this inlet represents a completely reacted mixture. Such cases arise when there is flue gas recirculation, as depicted schematically in Figure 8.15: Using the Non-Premixed Model with Flue Gas Recycle (p. 244). Since is a conserved quantity, the mixture fraction at the flue gas recycle inlet can be computed as (8.30) or (8.31) where

is the exit mixture fraction (and the mixture fraction at the flue gas recycle inlet),

mass flow rate of the oxidizer inlet, rate of the recycle inlet.

is the mass flow rate of the fuel inlet,

is the

is the mass flow

If a secondary stream is included, (8.32) and (8.33) Figure 8.15: Using the Non-Premixed Model with Flue Gas Recycle

8.3.4. Using the Non-Premixed Model with the Inert Model To model the effect of dilution on combustion without the expense of using two mixture fractions, ANSYS Fluent allows the introduction of an inert stream into the domain. Unlike a secondary mixture fraction, the inert does not chemically equilibrate with the primary fuel and oxidizer - instead, its com-

244

Restrictions and Special Cases for Using the Non-Premixed Model position remains constant after mixing. However the inert stream does affect the solution due to its influence on enthalpy, specific heat, and density of the mixture. The equation for conservation of inert is written as: (8.34) where = inert stream tracer = turbulent Schmidt number = turbulent viscosity = density Equation 8.34 (p. 245) has no sources or sinks, because the problem is reduced to tracking a conserved scalar when it is assumed that the inert components have the same turbulent diffusivities.

8.3.4.1. Mixture Composition The mixture properties are computed from the mean ( ) and variance ( ) of the mixture fraction in the cell, the reaction progress variable ( , when the partially premixed model is enabled), the cell enthalpy ( , for non-adiabatic flows), and the inert tracer ( ). The mixture is modeled as a blend of inert and active species, but the PDF tables need to be accessed with conditioned variables. Conditioning is necessary to take into account the volume taken up by the inert fraction, yet still be able to use previously built tables by straightforward lookup. The mean mixture fraction and mixture fraction variance used to access the PDF table is given by: (8.35)

(8.36) The reaction progress variable is not conditioned, however the cell enthalpy must be conditioned to account for the inert enthalpy. The inert enthalpy and active enthalpy are obtained from the following relationships: (8.37) where is the enthalpy of the cell at temperature , is the enthalpy of the active mixture-fraction stream and is the enthalpy of the inert stream. Here it is assumed that the inert and the active streams have the same temperature, but different enthalpies. To calculate the temperature in the cell, Equation 8.37 (p. 245) is solved for the temperature and for energy between the inert and active streams.

, which gives the partitioning of the

The inert enthalpy is defined as (8.38)

where ature,

refers to the mass fraction of species defined in the inert stream, the specific heat of species , and is the number of inert species.

is the reference temper-

245

Non-Premixed Combustion The inert and PDF enthalpies are defined further in Equation 33.12 in the User's Guide.

8.3.4.1.1. Property Evaluation The specific heat of the mixture is evaluated by mixing the inert and active streams in the following way: (8.39) The density of the mixture is calculated by using a harmonic average of the densities of the active and inert streams, weighted by the inert tracer: (8.40)

Here, the inert density ( ) is calculated from the ideal gas law. For information on how to set up the inert model, see Setting Up the Inert Model in the User's Guide.

8.4. The Diffusion Flamelet Models Theory Information about the flamelet models are presented in the following sections: 8.4.1. Restrictions and Assumptions 8.4.2.The Flamelet Concept 8.4.3. Flamelet Generation 8.4.4. Flamelet Import

8.4.1. Restrictions and Assumptions The following restrictions apply to all diffusion flamelet models in ANSYS Fluent: • Only a single mixture fraction can be modeled; two-mixture-fraction flamelet models are not allowed. • The mixture fraction is assumed to follow the -function PDF, and scalar dissipation fluctuations are ignored. • Empirically-based streams cannot be used with the flamelet model.

8.4.2. The Flamelet Concept 8.4.2.1. Overview The flamelet concept views the turbulent flame as an ensemble of thin, laminar, locally one-dimensional flamelet structures embedded within the turbulent flow field [55] (p. 778), [370] (p. 795), [371] (p. 795) (see Figure 8.16: Laminar Opposed-Flow Diffusion Flamelet (p. 247)).

246

The Diffusion Flamelet Models Theory Figure 8.16: Laminar Opposed-Flow Diffusion Flamelet

A common laminar flame type used to represent a flamelet in a turbulent flow is the counterflow diffusion flame. This geometry consists of opposed, axisymmetric fuel and oxidizer jets. As the distance between the jets is decreased and/or the velocity of the jets increased, the flame is strained and increasingly departs from chemical equilibrium until it is eventually extinguished. The species mass fraction and temperature fields can be measured in laminar counterflow diffusion flame experiments, or, most commonly, calculated. For the latter, a self-similar solution exists, and the governing equations can be simplified to one dimension along the axis of the fuel and oxidizer jets, where complex chemistry calculations can be affordably performed. In the laminar counterflow flame, the mixture fraction, , (see Definition of the Mixture Fraction (p. 224)) decreases monotonically from unity at the fuel jet to zero at the oxidizer jet. If the species mass fraction and temperature along the axis are mapped from physical space to mixture fraction space, they can be uniquely described by two parameters: the mixture fraction and the strain rate (or, equivalently, the Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

247

Non-Premixed Combustion scalar dissipation, , defined in Equation 8.42 (p. 248)). Hence, the chemistry is reduced and completely described by the two quantities, and . This reduction of the complex chemistry to two variables allows the flamelet calculations to be preprocessed, and stored in look-up tables. By preprocessing the chemistry, computational costs are reduced considerably. The balance equations, solution methods, and sample calculations of the counterflow laminar diffusion flame can be found in several references. Comprehensive reviews and analyses are presented in the works of Bray and Peters, and Dixon-Lewis [55] (p. 778), [104] (p. 780).

8.4.2.2. Strain Rate and Scalar Dissipation A characteristic strain rate for a counterflow diffusion flamelet can be defined as , where the relative speed of the fuel and oxidizer jets, and is the distance between the jet nozzles.

is

Instead of using the strain rate to quantify the departure from equilibrium, it is expedient to use the scalar dissipation, denoted by . The scalar dissipation is defined as (8.41) where

is a representative diffusion coefficient.

Note that the scalar dissipation, , varies along the axis of the flamelet. For the counterflow geometry, the flamelet strain rate can be related to the scalar dissipation at the position where is stoichiometric by [370] (p. 795): (8.42)

where = scalar dissipation at = characteristic strain rate = stoichiometric mixture fraction = inverse complementary error function Physically, as the flame is strained, the width of the reaction zone diminishes, and the gradient of at the stoichiometric position increases. The instantaneous stoichiometric scalar dissipation, , is used as the essential non-equilibrium parameter. It has the dimensions and may be interpreted as the inverse of a characteristic diffusion time. In the limit the chemistry tends to equilibrium, and as increases due to aerodynamic straining, the non-equilibrium increases. Local quenching of the flamelet occurs when exceeds a critical value.

8.4.2.3. Embedding Diffusion Flamelets in Turbulent Flames A turbulent flame brush is modeled as an ensemble of discrete diffusion flamelets. Since, for adiabatic systems, the species mass fraction and temperature in the diffusion flamelets are completely parameterized by and , density-weighted mean species mass fractions and temperature in the turbulent flame can be determined from the PDF of

248

and

as

The Diffusion Flamelet Models Theory (8.43) where

represents species mass fractions and temperature.

In ANSYS Fluent, be simplified as

and

are assumed to be statistically independent, so the joint PDF .A

PDF shape is assumed for

are solved in ANSYS Fluent to specify function: Fluent as

. Fluctuations in

can

, and transport equations for

are ignored so that the PDF of

. The first moment, namely the mean scalar dissipation,

and is a delta

, is modeled in ANSYS

(8.44) where

is a constant with a default value of 2.

For LES, the mean scalar dissipation is modeled as (8.45) To avoid the PDF convolutions at ANSYS Fluent run time, the integrations in Equation 8.43 (p. 249) are preprocessed and stored in look-up tables. For adiabatic flows, flamelet tables have three dimensions: ,

and

.

For non-adiabatic steady diffusion flamelets, the additional parameter of enthalpy is required. However, the computational cost of modeling steady diffusion flamelets over a range of enthalpies is prohibitive, so some approximations are made. Heat gain/loss to the system is assumed to have a negligible effect on the species mass fractions, and adiabatic mass fractions are used [41] (p. 777), [334] (p. 793). The temperature is then calculated from Equation 5.7 (p. 141) for a range of mean enthalpy gain/loss, . Accordingly, mean temperature and density PDF tables have an extra dimension of mean enthalpy. The approximation of constant adiabatic species mass fractions is, however, not applied for the case corresponding to a scalar dissipation of zero. Such a case is represented by the non-adiabatic equilibrium solution. For

, the species mass fractions are computed as functions of ,

, and

.

In ANSYS Fluent, you can either generate your own diffusion flamelets, or import them as flamelet files calculated with other stand-alone packages. Such stand-alone codes include OPPDIF [293] (p. 791), CFXRIF [28] (p. 776), [29] (p. 776), [378] (p. 796) and RUN-1DL [374] (p. 795). ANSYS Fluent can import flamelet files in standard flamelet file format. Instructions for generating and importing diffusion flamelets are provided in Flamelet Generation (p. 249) and Flamelet Import (p. 250).

8.4.3. Flamelet Generation The laminar counterflow diffusion flame equations can be transformed from physical space (with as the independent variable) to mixture fraction space (with as the independent variable) [379] (p. 796). In ANSYS Fluent, a simplified set of the mixture fraction space equations are solved [378] (p. 796). Here, equations are solved for the species mass fractions, ,

249

Non-Premixed Combustion

(8.46) and one equation for temperature: (8.47)

The notation in Equation 8.46 (p. 250) and Equation 8.47 (p. 250) is as follows:

, , , and

species mass fraction, temperature, density, and mixture fraction, respectively. species specific heat and mixture-averaged specific heat, respectively. and

is the specific enthalpy of the

th

is the

and th

are the

are the

th

th

species reaction rate,

species.

The scalar dissipation, , must be modeled across the flamelet. An extension of Equation 8.42 (p. 248) to variable density is used [225] (p. 787): (8.48)

where

is the density of the oxidizer stream.

Using the definition of the strain rate given by Equation 8.48 (p. 250), the scalar dissipation in the mixture fraction space can be expressed as: (8.49)

where

is the stoichiometric mixture fraction,

is the mixture density, and

at . is the user input for flamelet generation. Equation 8.48 (p. 250).

and

is the scalar dissipation are evaluated using

8.4.4. Flamelet Import ANSYS Fluent can import one or more flamelet files, convolute these diffusion flamelets with the assumedshape PDFs (see Equation 8.43 (p. 249)), and construct look-up tables. The flamelet files can be generated in ANSYS Fluent, or with separate stand-alone computer codes. The following types of flamelet files can be imported into ANSYS Fluent: • ASCII files generated by the CFX-RIF code [28] (p. 776), [29] (p. 776), [378] (p. 796) • Standard format files described in Standard Files for Diffusion Flamelet Modeling in the User's Guide and in Peters and Rogg [374] (p. 795) When diffusion flamelets are generated in physical space, the species and temperature vary in one spatial dimension. The species and temperature must then be mapped from physical space to mixture fraction space. If the diffusion coefficients of all species are equal, a unique definition of the mixture fraction exists. However, with differential diffusion, the mixture fraction can be defined in a number of ways.

250

The Steady Diffusion Flamelet Model Theory ANSYS Fluent computes the mixture fraction profile along the diffusion flamelet in one of the following ways: • Read from a file (standard format files only) This option is for diffusion flamelets solved in mixture fraction space. If you choose this method, ANSYS Fluent will search for the mixture fraction keyword Z, as specified in Peter and Roggs’s work [374] (p. 795), and retrieve the data. If ANSYS Fluent does not find mixture fraction data in the flamelet file, it will instead use the hydrocarbon formula method described below. • Hydrocarbon formula Following the work of Bilger et al. [40] (p. 777), the mixture fraction is calculated as (8.50) where (8.51) ,

, and are the mass fractions of carbon, hydrogen, and oxygen atoms, and , , and are the molecular weights. and are the values of at the oxidizer and fuel inlets.

The flamelet profiles in the multiple-flamelet data set should vary only in the strain rate imposed; the species and the boundary conditions should be the same. In addition, it is recommended that an extinguished flamelet is excluded from the multiple-flamelet data set. The formats for multiple diffusion flamelets are as follows: • Standard format: If you have a set of standard format flamelet files, you can import them all at the same time, and ANSYS Fluent will merge them internally into a multiple-flamelet file. When you import the set of flamelet files, ANSYS Fluent will search for and count the occurrences of the HEADER keyword to determine the number of diffusion flamelets in the file. • CFX-RIF format: A CFX-RIF flamelet file contains multiple diffusion flamelets at various strains and the file should not be modified manually. Only one CFX-RIF flamelet file should be imported. For either type of file, ANSYS Fluent will determine the number of flamelet profiles and sort them in ascending strain-rate order. For diffusion flamelets generated in physical space, you can select one of the four methods available for the calculation of mixture fraction. The scalar dissipation will be calculated from the strain rate using Equation 8.42 (p. 248).

8.5. The Steady Diffusion Flamelet Model Theory The steady flamelet approach models a turbulent flame brush as an ensemble of discrete, steady laminar flames, called diffusion flamelets. The individual diffusion flamelets are assumed to have the same structure as laminar flames in simple configurations, and are obtained by experiments or calculations. Using detailed chemical mechanisms, ANSYS Fluent can calculate laminar opposed-flow diffusion flamelets for non-premixed combustion. The diffusion flamelets are then embedded in a turbulent flame using statistical PDF methods. The advantage of the diffusion flamelet approach is that realistic chemical kinetic effects can be incorporated into turbulent flames. The chemistry can then be preprocessed and tabulated, offering tremendous computational savings. However, the steady diffusion flamelet model is limited to modeling comRelease 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

251

Non-Premixed Combustion bustion with relatively fast chemistry. The flame is assumed to respond instantaneously to the aerodynamic strain, and therefore the model cannot capture deep non-equilibrium effects such as ignition, extinction, and slow chemistry (like NOx). Information pertaining strictly to the steady diffusion flamelet model is presented in the following sections: 8.5.1. Overview 8.5.2. Multiple Steady Flamelet Libraries 8.5.3. Steady Diffusion Flamelet Automated Grid Refinement 8.5.4. Non-Adiabatic Steady Diffusion Flamelets For general information about the mixture fraction model, see Introduction (p. 223).

8.5.1. Overview In a diffusion flame, at the molecular level, fuel and oxidizer diffuse into the reaction zone. Here, they encounter high temperatures and radical species and ignite. More heat and radicals are generated in the reaction zone and some diffuse out. In near-equilibrium flames, the reaction rate is much faster than the diffusion rate. However, as the flame is stretched and strained by the turbulence, species and temperature gradients increase, and radicals and heat diffuse more quickly out of the flame. The species have less time to reach chemical equilibrium, and the degree of local non-equilibrium increases. The steady diffusion flamelet model is suited to predict chemical non-equilibrium due to aerodynamic straining of the flame by the turbulence. The chemistry, however, is assumed to respond rapidly to this strain, so as the strain relaxes to zero, the chemistry tends to equilibrium. When the chemical time-scale is comparable to the fluid mixing time-scale, the species can be considered to be in global chemical non-equilibrium. Such cases include NOx formation and low-temperature CO oxidation. The steady diffusion flamelet model is not suitable for such slow-chemistry flames. Instead, you can model slow chemistry using one of the following: • the Unsteady Diffusion Flamelet model (see The Unsteady Diffusion Flamelet Model Theory (p. 254)) • the trace species assumption in the NOx model (see Pollutant Formation (p. 319)) • the Laminar Finite-Rate model (see The Generalized Finite-Rate Formulation for Reaction Modeling (p. 195)), where the turbulence-chemistry interaction is ignored. • the EDC model (see The Eddy-Dissipation-Concept (EDC) Model (p. 200)) • the PDF transport model (see Composition PDF Transport (p. 289)).

8.5.2. Multiple Steady Flamelet Libraries ANSYS Fluent can generate multiple steady diffusion flamelets over a range of strain rates to account for the varying strain field in your multi-dimensional simulation. If you specify the number of diffusion flamelets to be greater than one, diffusion flamelets are generated at scalar dissipation values as determined by Equation 8.52 (p. 252). (8.52)

where ranges from 1 up to the specified maximum number of diffusion flamelets, is the initial scalar dissipation at the stoichiometric mixture fraction from Equation 8.49 (p. 250), and is the 252

The Steady Diffusion Flamelet Model Theory scalar dissipation step. Diffusion flamelets are generated until either the maximum number of flamelets is reached, or the flamelet extinguishes. Extinguished flamelets are excluded from the flamelet library.

8.5.3. Steady Diffusion Flamelet Automated Grid Refinement By default, 1D flamelet grids are discretized by clustering a fixed number of points about the stoichiometric mixture fraction, which is approximated as the location of peak temperature. ANSYS Fluent also has the option for Automated Grid Refinement of steady diffusion flamelets, where an adaptive algorithm inserts grid points so that the change of values, as well as the change of slopes, between successive grid points is less than user specified tolerances. When using automated grid refinement, a steady solution is calculated on a coarse grid with a user specified Initial Number of Grid Points in Flamelet (default of ). At convergence, a new grid point is inserted midway between a point and its neighbor if, (8.53) where is the value for each temperature and species mass fraction at grid point , is a user specified Maximum Change in Value Ratio (default of ), and ( ) are the maximum (minimum) values over all grid points. In addition a grid point is added if, (8.54) where the slope

is defined as, (8.55)

In Equation 8.54 (p. 253) and Equation 8.55 (p. 253), is a user specified Maximum Change in Slope Ratio (default of ), ( ) are the maximum (minimum) slopes over all grid points, and is the mixture fraction value of grid point . The refined flamelet is reconverged, and the refinement process is repeated until no further grid points are added by Equation 8.53 (p. 253) and Equation 8.54 (p. 253), or the user-specified Maximum Number of Grid Points in Flamelet (default of ) is exceeded.

8.5.4. Non-Adiabatic Steady Diffusion Flamelets For non-adiabatic steady diffusion flamelets, ANSYS Fluent follows the approach of [41] (p. 777), [334] (p. 793) and assumes that flamelet species profiles are unaffected by heat loss/gain from the flamelet. No special non-adiabatic flamelet profiles need to be generated, avoiding a very cumbersome preprocessing step. In addition, the compatibility of ANSYS Fluent with external steady diffusion flamelet generation packages (for example, OPPDIF, CFX-RIF, RUN-1DL) is retained. The disadvantage to this model is that the effect of the heat losses on the species mass fractions is not taken into account. Also, the effect of the heat loss on the extinction limits is not taken into account. After diffusion flamelet generation, the flamelet profiles are convoluted with the assumed-shape PDFs as in Equation 8.43 (p. 249), and then tabulated for look-up in ANSYS Fluent. The non-adiabatic PDF tables have the following dimensions:

253

Non-Premixed Combustion for

= 0 (that is, equilibrium

solution) for

During the ANSYS Fluent solution, the equations for the mean mixture fraction, mixture fraction variance, and mean enthalpy are solved. The scalar dissipation field is calculated from the turbulence field and the mixture fraction variance (Equation 8.44 (p. 249)). The mean values of cell temperature, density, and species mass fraction are obtained from the PDF look-up table.

8.6. The Unsteady Diffusion Flamelet Model Theory The steady diffusion flamelet model, described in The Diffusion Flamelet Models Theory (p. 246) and The Steady Diffusion Flamelet Model Theory (p. 251), models local chemical non-equilibrium due to the straining effect of turbulence. In many combustors the strain is small at the outlet and the steady diffusion flamelet model predicts all species, including slow-forming species like NOx, to be near equilibrium, which is often inaccurate. The cause of this inaccuracy is the disparity between the flamelet time-scale, which is the inverse of the scalar dissipation, and the slow-forming species time-scale, which is the residence time since the species started accumulating after mixing in the combustor. The unsteady diffusion flamelet model in ANSYS Fluent can predict slow-forming species, such as gaseous pollutants or product yields in liquid reactors, more accurately than the steady diffusion flamelet model. Computationally expensive chemical kinetics are reduced to one dimension and the model is significantly faster than the laminar-finite-rate, EDC or PDF Transport models where kinetics are calculated in two or three dimensions. There are two variants of the unsteady flamelet model, namely an Eulerian unsteady flamelet model (described in The Eulerian Unsteady Laminar Flamelet Model (p. 254)) and a diesel unsteady flamelet model for predicting combustion in compression-ignition engines (described in The Diesel Unsteady Laminar Flamelet Model (p. 257)). Information pertaining strictly to the unsteady flamelet model is presented in the following sections: 8.6.1.The Eulerian Unsteady Laminar Flamelet Model 8.6.2.The Diesel Unsteady Laminar Flamelet Model 8.6.3. Multiple Diesel Unsteady Flamelets 8.6.4. Multiple Diesel Unsteady Flamelets with Flamelet Reset

8.6.1. The Eulerian Unsteady Laminar Flamelet Model The Eulerian unsteady laminar flamelet model can be used to predict slow-forming intermediate and product species that are not in chemical equilibrium. Typical examples of slow-forming species are gasphase pollutants like NOx, and product compounds in liquid reactors. By reducing the chemistry computations to one dimension, detailed kinetics with multiple species and stiff reactions can be economically simulated in complex 3D geometries. The model, following the work of Barths et al. [29] (p. 776) and Coelho and Peters [81] (p. 779), postprocesses unsteady marker probability equations on a steady-state converged flow field. The marker field represents the probability of finding a flamelet at any point in time and space. A probability marker transport equation is solved for each flamelet. In ANSYS Fluent, the steady flow solution must be computed with the steady diffusion flamelet model (see The Steady Diffusion Flamelet Model Theory (p. 251)) before starting the unsteady flamelets simula-

254

The Unsteady Diffusion Flamelet Model Theory tion. Since the unsteady flamelet equations are postprocessed on a steady-state flamelet solution, the effect of the unsteady flamelet species on the flow-field is neglected. When multiple flamelets are enabled, ANSYS Fluent solves Eulerian transport equations representing the probability of fuel from

th

flamelet,

, as follows (8.56)

where is laminar thermal conductivity of the mixture, Prandtl number.

is the mixture specific heat, and

is the

Each marker probability is initialized as, (8.57) where

is the scalar dissipation,

values for

th

flamelet marker probability,

are the minimum and maximum scalar dissipation is the mean mixture fraction, and

is a user-specified

constant that should be set greater than the stoichiometric mixture fraction. Hence the marker probabilities are initialized to unity in regions of the domain where the mean mixture fraction is greater than a user specified value (typically greater than stoichiometric), and multiple flamelets sub-divide this initial region by the scalar dissipation. As previously mentioned, the Eulerian Unsteady Laminar Flamelet Model is only available for steadystate simulations. However, marker probability transport equations (Equation 8.56 (p. 255)) are always solved time-accurately as the initial marker probabilities convect and diffuse through the steady flow field. At the inlet boundaries, is set to zero, and hence the field decreases to zero with time as it is convected and diffused out of the domain (for cases with outlet boundaries). The unsteady flamelet species Equation 8.46 (p. 250)) is integrated simultaneously with the marker probability Equation 8.56 (p. 255) for each marker probability . For liquid-phase chemistry, the initial flamelet field is the mixed-but-unburnt flamelet, as liquid reactions are assumed to proceed immediately upon mixing. Gas-phase chemistry involves ignition, so the initial flamelet field is calculated from a steady diffusion flamelet solution. All of the slow-forming species, such as NOx, must be identified before solving the unsteady flamelet equations. The mass fractions of all slow-forming species are set to zero in this initial flamelet profile, since, at ignition, little residence time has elapsed for any significant formation. The scalar dissipation at stoichiometric mixture fraction ( ) is required by each flamelet species equation. This is calculated from the steady-state ANSYS Fluent field at each time step as a probabilityweighted volume integral:

(8.58)

where is defined in Equation 8.44 (p. 249), and denotes the fluid volume. ANSYS Fluent provides the option of limiting to a user-specified maximum value, which should be approximately equal to the flamelet extinction scalar dissipation (the steady diffusion flamelet solver can be used to calculate this extinction scalar dissipation in a separate simulation).

255

Non-Premixed Combustion The unsteady flamelet energy equation is not solved in order to avoid flamelet extinction for high scalar dissipation, and to account for non-adiabatic heat loss or gain. For adiabatic cases, the flamelet temperature, , is calculated at each time step from the steady diffusion flamelet library at the probability-weighted scalar dissipation from Equation 8.58 (p. 255). For non-adiabatic cases, the flamelet temperature at time is calculated from (8.59) where

(8.60)

Here, the subscript referring to th flamelet has been omitted for simplicity, and represents the ANSYS Fluent steady-state mean cell temperature conditioned on the local cell mixture fraction. Unsteady flamelet mean species mass fractions in each cell are accumulated over time and can be expressed as:

(8.61)

where

denotes the

th

flamelet mass fraction of

species unsteady flamelet mass fraction, and th

is the

th

unsteady

flamelet and is calculated using a Beta pdf as, (8.62)

The probability marker equation (Equation 8.56 (p. 255)) and the flamelet species equation (Equation 8.46 (p. 250)) are advanced together in time until the probability marker has substantially convected and diffused out of the domain. The unsteady flamelet mean species, calculated from Equation 8.60 (p. 256), reach steady-state as the probability marker vanishes.

8.6.1.1. Liquid Reactions Liquid reactors are typically characterized by: • Near constant density and temperature. • Relatively slow reactions and species far from chemical equilibrium. • High Schmidt number (

) and hence reduced molecular diffusion.

The Eulerian unsteady laminar flamelet model can be used to model liquid reactions. When the Liquid Micro-Mixing model is enabled, ANSYS Fluent uses the volume-weighted-mixing-law formula to calculate the density. The effect of high is to decrease mixing at the smallest (micro) scales and increase the mixture fraction variance, which is modeled with the Turbulent Mixer Model [24] (p. 776). Three transport

256

The Unsteady Diffusion Flamelet Model Theory equations are solved for the inertial-convective ( (

), viscous-convective (

), and viscous-diffusive

) subranges of the turbulent scalar spectrum, (8.63) (8.64) (8.65)

where is laminar thermal conductivity of the mixture, is the mixture specific heat, and is the Prandtl number. The constants through have values of 2, 1.86, 0.058, 0.303, and 17050, respectively. The total mixture fraction variance is the sum of

,

and

.

In Equation 8.65 (p. 257), the cell Schmidt number, , is calculated as where the density, and the mass diffusivity as defined for the pdf-mixture material.

is the viscosity,

8.6.2. The Diesel Unsteady Laminar Flamelet Model In diesel engines, fuel sprayed into the cylinder evaporates, mixes with the surrounding gases, and then auto-ignites as compression raises the temperature and pressure. The diesel unsteady laminar flamelet model, based on the work of Pitsch et al. and Barths et al. [378] (p. 796), [28] (p. 776), models the chemistry by a finite number of one-dimensional laminar flamelets. By reducing the costly chemical kinetic calculation to 1D, substantial savings in run time can be achieved over the laminar-finite-rate, EDC or PDF Transport models. The flamelet species and energy equations (Equation 8.46 (p. 250) and Equation 8.47 (p. 250)) are solved simultaneously with the flow. The flamelet equations are advanced for a fractional step using properties from the flow, and then the flow is advanced for the same fractional time step using properties from the flamelet. The initial condition of each flamelet at the time of its introduction into computational domain is a mixed-but-unburnt distribution. For the flamelet fractional time step, the volume-averaged scalar dissipation and pressure, as well as the fuel and oxidizer temperatures, are passed from the flow solver to the flamelet solver. To account for temperature rise during compression, the flamelet energy equation (Equation 8.47 (p. 250)) has an additional term on the right-hand side as (8.66) where is the specific heat and is the volume-averaged pressure in the cylinder. This rise in flamelet temperature due to compression eventually leads to ignition of the flamelet. After the flamelet equations have been advanced for the fractional time step, the PDF Table is created as a Non-Adiabatic Steady Flamelet table (see Non-Adiabatic Steady Diffusion Flamelets (p. 253)). Using the properties from this table, the CFD flow field is then advanced for the same fractional time step.

8.6.3. Multiple Diesel Unsteady Flamelets In certain applications where the ignition in different regions of the combustion domain occurs at different times, the chemistry cannot be accurately represented by a single flamelet. Among the examples

257

Non-Premixed Combustion are split-injections and sprays with high residence times. In these cases, evaporated spray injected at early stage ignites before spray injected at later stage due to the longer residence time. A single flamelet cannot model the local ignition delay for the late spray as the single flamelet represents a burnt state. This deficiency is overcome with the use of multiple flamelets, which are generated in the reacting domain at user-specified times during the simulation. The new flamelet inherits the preceding flamelet boundary temperature calculated at the time of the new flamelet introduction, and the flamelet species field is initialized as mixed-but-unburnt. The marker probability equations (Equation 8.56 (p. 255)) are solved for flamelets, where is the total number of unsteady flamelets. The marker probability of the last flamelet is obtained as follows: (8.67)

where

is the mean mixture fraction, and

The scalar dissipation of the

th

is the marker probability of the

th

unsteady flamelet.

flamelet is calculated using Equation 8.58 (p. 255).

The properties from the PDF tables (such as mass fraction, specific heat, and so on) are calculated using the weighted contribution from each flamelet:

(8.68)

where

is the property for

th

flamelet.

The diesel unsteady flamelet approach can model ignition as well as formation of product, intermediate, and pollutant species. The setting of the Diesel Unsteady Flamelet model is described in Using the Diesel Unsteady Laminar Flamelet Model in the Fluent User's Guide.

8.6.4. Multiple Diesel Unsteady Flamelets with Flamelet Reset As the multiple diesel unsteady flamelets ignite, their species and temperature fields tend toward the same chemical equilibrium state as the scalar dissipation (mixing) decreases. To model multiple engine cycles, ANSYS Fluent allows flamelet reset events, which resets multiple flamelets to a single flamelet. Additionally, at the end of a cycle, some burnt gases typically remain trapped and mix later with the fresh charge introduced in the next cycle. The presence of the exhaust gas in the engine chamber can be modeled using either a Diesel Unsteady Flamelet Reset or the inert model (via Inert EGR Reset). For information on how to use and set up the Diesel Unsteady Flamelet Reset and Inert EGR Reset options, see Resetting Diesel Unsteady Flamelets in the Fluent User's Guide.

8.6.4.1. Resetting the Flamelets At the end of combustion stroke and just before the inlet valves open, all existing flamelets, except the first one, are deleted, and the probability of the first flamelet is set as:

258

The Unsteady Diffusion Flamelet Model Theory where, is the probability of the th flamelet, and is the total number of flamelets. The mixture composition (and similarly the temperature) of the reset flamelet is set as:

where

is the species mass fraction of the

th

species from the

th

flamelet.

After resetting the flamelets, the computations are performed with this single, typically burnt, flamelet. The flamelet start time of subsequent flamelets should be set to just before the fuel injection. Hence, the injected fuel is modeled in the second and higher flamelets. The process is controlled through dynamic mesh events described in Diesel Unsteady Flamelet Reset in the Fluent User's Guide.

259

260

Chapter 9: Premixed Combustion ANSYS Fluent has several models to simulate premixed turbulent combustion. For more information about using the premixed turbulent combustion models, see Modeling Premixed Combustion in the User's Guide. Theoretical information about these models is provided in the following sections: 9.1. Overview and Limitations 9.2. C-Equation Model Theory 9.3. G-Equation Model Theory 9.4.Turbulent Flame Speed Models 9.5. Extended Coherent Flamelet Model Theory 9.6. Calculation of Properties

9.1. Overview and Limitations For more information, see the following sections: 9.1.1. Overview 9.1.2. Limitations

9.1.1. Overview In premixed combustion, fuel and oxidizer are mixed at the molecular level prior to ignition. Combustion occurs as a flame front propagating into the unburnt reactants. Examples of premixed combustion include aspirated internal combustion engines, lean-premixed gas turbine combustors, and gas-leak explosions. Premixed combustion is much more difficult to model than non-premixed combustion. The reason for this is that premixed combustion usually occurs as a thin, propagating flame that is stretched and contorted by turbulence. For subsonic flows, the overall rate of propagation of the flame is determined by both the laminar flame speed and the turbulent eddies. The laminar flame speed is determined by the rate that species and heat diffuse upstream into the reactants and burn. To capture the laminar flame speed, the internal flame structure would need to be resolved, as well as the detailed chemical kinetics and molecular diffusion processes. Since practical laminar flame thicknesses are of the order of millimeters or smaller, resolution requirements are usually unaffordable. The effect of turbulence is to wrinkle and stretch the propagating laminar flame sheet, increasing the sheet area and, in turn, the effective flame speed. The large turbulent eddies tend to wrinkle and corrugate the flame sheet, while the small turbulent eddies, if they are smaller than the laminar flame thickness, may penetrate the flame sheet and modify the laminar flame structure. Non-premixed combustion, in comparison, can be greatly simplified to a mixing problem (see the mixture fraction approach in Introduction (p. 223)). The essence of premixed combustion modeling lies in capturing the turbulent flame speed, which is influenced by both the laminar flame speed and the turbulence. In premixed flames, the fuel and oxidizer are intimately mixed before they enter the combustion device. Reaction then takes place in a combustion zone that separates unburnt reactants and burnt combustion products. Partially premixed flames exhibit the properties of both premixed and diffusion flames. They occur when an additional oxidizer or fuel stream enters a premixed system, or when a diffusion flame becomes lifted off the burner so that some premixing takes place prior to combustion. Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

261

Premixed Combustion Premixed and partially premixed flames can be modeled using ANSYS Fluent’s finite-rate/eddy-dissipation formulation (see Species Transport and Finite-Rate Chemistry (p. 193)). If finite-rate chemical kinetic effects are important, the Laminar Finite-Rate model (see Direct Use of Finite-Rate Kinetics (no TCI) (p. 195)), the EDC model (see The Eddy-Dissipation-Concept (EDC) Model (p. 200)) or the composition PDF transport model (see Composition PDF Transport (p. 289)) can be used. For information about ANSYS Fluent’s partially premixed combustion model, see Partially Premixed Combustion (p. 279). If the flame is perfectly premixed (all streams entering the combustor have the same equivalence ratio), it is possible to use the premixed combustion model, as described in this chapter.

9.1.2. Limitations The following limitations apply to the premixed combustion model: • You must use the pressure-based solver. The premixed combustion model is not available with either of the density-based solvers. • The premixed combustion model is valid only for turbulent, subsonic flows. These types of flames are called deflagrations. Explosions, also called detonations, where the combustible mixture is ignited by the heat behind a shock wave, can be modeled with the finite-rate model using the density-based solver. See Species Transport and Finite-Rate Chemistry (p. 193) for information about the finite-rate model. • The premixed combustion model cannot be used in conjunction with the pollutant (that is, soot and NOx) models. However, a perfectly premixed system can be modeled with the partially premixed model (see Partially Premixed Combustion (p. 279)), which can be used with the pollutant models. • You cannot use the premixed combustion model to simulate reacting discrete-phase particles, since these would result in a partially premixed system. Only inert particles can be used with the premixed combustion model.

9.2. C-Equation Model Theory A scalar variable representing the progress of reaction from unburnt to burnt is denoted by c. The transport equation for c describes the spatial and temporal evolution of the reaction progress in a turbulent flow field. Ahead of the flame, c is defined as zero in the unburnt reactants, and behind the flame c is unity in the burnt products. Within the flame brush c varies between zero and one. The flame brush propagates upstream at a modeled turbulent flame speed. Fluent offers two models for the turbulent flame speed, namely the Zimont model [543] (p. 805), [544] (p. 805), [546] (p. 805) and the Peters model [372] (p. 795), as detailed in Peters Flame Speed Model (p. 268). For more information, see the following section: 9.2.1. Propagation of the Flame Front

9.2.1. Propagation of the Flame Front In many industrial premixed systems, combustion takes place in a thin flame sheet. As the flame front moves, combustion of unburnt reactants occurs, converting unburnt premixed reactants to burnt products. The premixed combustion model therefore considers the reacting flow field to be divided into regions of burnt and unburnt species, separated by the flame sheet. Note that the C-equation model assumes that the laminar flame is thin in comparison to the turbulent flame brush, so a value of reaction progress between 0 and 1 implies that the fluctuating flame spends some time at the unburnt state and the remainder at the burnt state; it does not represent an intermediate reaction state between unburnt and burnt.

262

C-Equation Model Theory The flame front propagation is modeled by solving a transport equation for the density-weighted mean reaction progress variable, denoted by : (9.1) where = mean reaction progress variable = turbulent Schmidt number = 0.7 = reaction progress source term (

)

= laminar thermal conductivity of the mixture = mixture specific heat The progress variable is defined as a normalized sum of the product species mass fractions,

(9.2)

where superscript

denotes the unburnt reactant

denotes the superscript

species mass fraction

denotes chemical equilibrium

are constants that are typically zero for reactants and unity for a few product species Based on this definition, •

: unburnt mixture

•

: burnt mixture

where the mixture is unburnt and

where the mixture is burnt:

The value of is defined as a boundary condition at all flow inlets. It is usually specified as either 0 (unburnt) or 1 (burnt). The mean reaction rate in Equation 9.1 (p. 263) is modeled as [544] (p. 805): (9.3) where = density of unburnt mixture = turbulent flame speed ANSYS Fluent provides two models for the turbulent flame speed. Many other models for exist [55] (p. 778) and can be specified using user-defined functions. More information about user-defined functions can be found in the Fluent Customization Manual.

263

Premixed Combustion

9.3. G-Equation Model Theory The G-equation is a premixed flame-front tracking model. The transport equation governing the unsteady evolution of a propagating flame interface is (derivation can be found in [372] (p. 795)), (9.4) where is the fluid density, is the fluid velocity vector, is the laminar flame speed (interface normal propagation), is the diffusivity and is the flame curvature. For turbulent flames, Equation 9.4 (p. 264) can be Favre Reynolds-averaged or spatially-filtered to provide transport equations for the flame mean position, (9.5) and the variance of the flame position, (9.6) where is the turbulent flame speed is a diffusivity term defined as is the turbulent velocity scale is a modeling constant taken from the

turbulence model (default=0.09)

is a modeling constant = 2.0 is the turbulent Schmidt number is the turbulent length scale is the flame curvature, defined as

, where

In Equation 9.6 (p. 264), indicates that the diffusion term is only applied parallel to the flame front [372] (p. 795), the normal component being accounted for in the turbulent burning velocity. In practice this makes only a minor difference to results and can lead to convergence problems, and is therefore disabled by default. ANSYS Fluent also offers the option of using an algebraic expression to calculate the flame position variance,

, instead of Equation 9.6 (p. 264): (9.7)

where

is the effective turbulent viscosity.

For more information, see the following section: 9.3.1. Numerical Solution of the G-equation

264

Turbulent Flame Speed Models

9.3.1. Numerical Solution of the G-equation Equation 9.4 (p. 264) and Equation 9.5 (p. 264) do not contain a diffusion term and sharp interfaces remain sharp at all times. Special numerical techniques such as the Volume-of-Fluid (VOF) and Marker-and-Cell (MAC) have been devised to solve such equations. In ANSYS Fluent, the mean G-equation Equation 9.5 (p. 264) is solved using a Level-set method. Here, represents the signed mean distance to the flame front, and hence the flame front is the isosurface. Since is the distance to the flame front, is constrained to be unity everywhere in the flow field. The standard ANSYS Fluent transport equation machinery is used to solve for (Equation 9.5 (p. 264)) over a time step. However, at the end of the time step, the field is typically is not exactly equal to the mean flame distance (and is not identically equal to 1), and this condition is enforced by a process called re-initialization. In ANSYS Fluent this is done by constructing a faceted representation of the flame front from the field. Then, in every cell, is set to the geometric distance to the nearest flame front facet. is positive in the burnt region downstream of the flame front and negative in the unburnt region upstream of the flame front. Given the mean flame front position, the mean progress variable is calculated according to a Gaussian distribution depending on proximity to the flame front and the G-equation variance: (9.8)

Mean properties, such as the mean density, temperature and species mass fractions, are calculated from the mean reaction progress variable, as in the C-equation model.

9.4. Turbulent Flame Speed Models The key to the premixed combustion model is the prediction of , the turbulent flame speed normal to the mean surface of the flame. The turbulent flame speed is influenced by the following: • laminar flame speed, which is, in turn, determined by the fuel concentration, temperature, and molecular diffusion properties, as well as the detailed chemical kinetics • flame front wrinkling and stretching by large eddies, and flame thickening by small eddies ANSYS Fluent has two turbulent flame speed models, namely the Zimont turbulent flame speed closure model and the Peters flame speed model. For more information, see the following sections: 9.4.1. Zimont Turbulent Flame Speed Closure Model 9.4.2. Peters Flame Speed Model

9.4.1. Zimont Turbulent Flame Speed Closure Model In ANSYS Fluent, the Zimont turbulent flame speed closure is computed using a model for wrinkled and thickened flame fronts [544] (p. 805): (9.9) (9.10)

265

Premixed Combustion where = model constant = RMS (root-mean-square) velocity (m/s) = laminar flame speed (m/s) = unburnt thermal diffusivity (

)

= turbulence length scale (m) = turbulence time scale (s) = chemical time scale (s) The turbulence length scale, , is computed from (9.11) where

is the turbulence dissipation rate.

The model is based on the assumption of equilibrium small-scale turbulence inside the laminar flame, resulting in a turbulent flame speed expression that is purely in terms of the large-scale turbulent parameters. The default value of 0.52 for is recommended [544] (p. 805), and is suitable for most premixed flames. The default value of 0.37 for should also be suitable for most premixed flames. The model is strictly applicable when the smallest turbulent eddies in the flow (the Kolmogorov scales) are smaller than the flame thickness, and penetrate into the flame zone. This is called the thin reaction zone combustion region, and can be quantified by Karlovitz numbers, , greater than unity. is defined as (9.12) where = characteristic flame time scale = smallest (Kolmogorov) turbulence time scale = Kolmogorov velocity = kinematic viscosity Lastly, the model is valid for premixed systems where the flame brush width increases in time, as occurs in most industrial combustors. Flames that propagate for a long period of time equilibrate to a constant flame width, which cannot be captured in this model.

9.4.1.1. Zimont Turbulent Flame Speed Closure for LES For simulations that use the LES turbulence model, the Reynolds-averaged quantities in the turbulent flame speed expression (Equation 9.9 (p. 265)) are replaced by their equivalent subgrid quantities. In particular, the large eddy length scale is modeled as (9.13) where

266

is the Smagorinsky constant and

is the cell characteristic length.

Turbulent Flame Speed Models The RMS velocity in Equation 9.9 (p. 265) is replaced by the subgrid velocity fluctuation, calculated as (9.14) where is the subgrid scale mixing rate (inverse of the subgrid scale time scale), given in Equation 7.27 (p. 200).

9.4.1.2. Flame Stretch Effect Since industrial low-emission combustors often operate near lean blow-off, flame stretching will have a significant effect on the mean turbulent heat release intensity. To take this flame stretching into account, the source term for the progress variable ( in Equation 9.1 (p. 263)) is multiplied by a stretch factor, [546] (p. 805). This stretch factor represents the probability that the stretching will not quench the flame; if there is no stretching ( ), the probability that the flame will be unquenched is 100%. The stretch factor, , is obtained by integrating the log-normal distribution of the turbulence dissipation rate, : (9.15) where erfc is the complementary error function, and

and

are defined below.

is the standard deviation of the distribution of : (9.16) where is the stretch factor coefficient for dissipation pulsation, is the turbulent integral length scale, and is the Kolmogorov micro-scale. The default value of 0.26 for (measured in turbulent non-reacting flows) is recommended by [544] (p. 805), and is suitable for most premixed flames. is the turbulence dissipation rate at the critical rate of strain [544] (p. 805): (9.17) By default, is set to a very high value ( ) so no flame stretching occurs. To include flame stretching effects, the critical rate of strain should be adjusted based on experimental data for the burner. Numerical models can suggest a range of physically plausible values [544] (p. 805), or an appropriate value can be determined from experimental data. A reasonable model for the critical rate of strain is (9.18) where is a constant (typically 0.5) and is the unburnt thermal diffusivity. Equation 9.18 (p. 267) can be implemented in ANSYS Fluent using a property user-defined function. More information about userdefined functions can be found in the Fluent Customization Manual.

9.4.1.3. Gradient Diffusion Volume expansion at the flame front can cause counter-gradient diffusion. This effect becomes more pronounced when the ratio of the reactant density to the product density is large, and the turbulence intensity is small. It can be quantified by the ratio

, where

,

,

, and

are the

unburnt and burnt densities, laminar flame speed, and turbulence intensity, respectively. Values of this Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

267

Premixed Combustion ratio greater than one indicate a tendency for counter-gradient diffusion, and the premixed combustion model may be inappropriate. Recent arguments for the validity of the turbulent-flame-speed model in such regimes can be found in Zimont et al. [545] (p. 805).

9.4.1.4. Wall Damping High turbulent kinetic energy levels at the walls in some problems can cause an unphysical acceleration of the flame along the wall. In reality, radical quenching close to walls decreases reaction rates and therefore the flame speed, but is not included in the model. To approximate this effect, ANSYS Fluent includes a constant multiplier for the turbulent flame speed, ,which modifies the flame speed in the vicinity of wall boundaries: (9.19) The default for this constant is 1 which does not change the flame speed. Values of larger than 1 increase the flame speed, while values less than 1 decrease the flame speed in the cells next to the wall boundary. ANSYS Fluent will solve the transport equation for the reaction progress variable computing the source term, , based on the theory outlined above:

(Equation 9.1 (p. 263)),

(9.20)

9.4.2. Peters Flame Speed Model The Peters model [372] (p. 795) for turbulent flame speed is used in the form proposed by Ewald [123] (p. 781): (9.21) where (9.22)

The term is Ewald's corrector and may be disabled by you, in which case reduces to that of Peters flame speed model.

and the formulation

In Equation 9.22 (p. 268): is the laminar flame speed

is the laminar flame thickness is the turbulent velocity scale is the flame brush thickness, given below

268

Turbulent Flame Speed Models

is an algebraic flame brush thickness

is a constant with a default value of 1.0 is a constant with a default value of 2.0 is a constant with a default value of 0.66 is a modeling constant taken from the

turbulence model with a default value of 0.09

is a constant with a default value of 2.0 is the turbulent Schmidt number with a default value of 0.7 If the Blint modifier is being used

where = 2.0 = 0.7 is the unburned density is the burned density The flame brush thickness is calculated differently for the C-equation and G-equation models. For the G equation model [372] (p. 795): (9.23)

For the C-equation model an algebraic form of this is used giving: (9.24) Note that for the C-equation model the Ewald corrector has no affect. To reduce the flame speed along walls, ANSYS Fluent includes a constant multiplier for the turbulent flame speed, , which modifies the flame speed in the vicinity of wall boundaries by multiplying the expression in Equation 9.21 (p. 268) by a constant between 0 and 1.

9.4.2.1. Peters Flame Speed Model for LES For LES the Peters model [372] (p. 795) for turbulent flame speed must be modified to use the subgrid quantities. Here we use the form derived by Pitsch [377] (p. 796). (9.25)

269

Premixed Combustion

(9.26)

where

is the thermal diffusivity.

The turbulent velocity scale

is the subgrid quantity as given in Equation 9.14 (p. 267).

9.5. Extended Coherent Flamelet Model Theory The Extended Coherent Flamelet Model (ECFM) [380] (p. 796) is a more refined premixed combustion model than the C-equation or G-equation models. It has theoretically greater accuracy, but is less robust and requires greater computational effort to converge. Information in this section is provided in the following sections: 9.5.1. Closure for ECFM Source Terms 9.5.2.Turbulent Flame Speed in ECFM 9.5.3. LES and ECFM The ECFM model solves an additional equation for the flame area density, denoted , which is ultimately used to model the mean reaction rate in Equation 9.1 (p. 263). The model assumes that the smallest turbulence length scales (Kolmogorov eddies) are larger than the laminar flame thickness, so the effect of turbulence is to wrinkle the laminar flame sheet, however the internal laminar flame profile is not distorted. The increased surface area of the flame results in increased net fuel consumption and an increased flame speed. The range of applicability of the ECFM model is illustrated on the Borghi diagram in Figure 9.1: Borghi Diagram for Turbulent Combustion (p. 271), where the wrinkled flamelets regime is indicated below the line. Typical Internal Combustion (IC) engines typically operate in this wrinkled flamelet range.

270

Extended Coherent Flamelet Model Theory Figure 9.1: Borghi Diagram for Turbulent Combustion

An expression for the transport of the net flame area per unit volume, or flame area density, , can be derived based on these assumptions [65] (p. 778): (9.27) where = mean flame area density = turbulent Schmidt number = laminar thermal conductivity of the mixture = mixture specific heat = turbulent viscosity = density = Source due to turbulence interaction = Source due to dilatation in the flame [500] (p. 802) = Source due to expansion of burned gas [500] (p. 802) = Source due to normal propagation [500] (p. 802) = Dissipation of flame area

271

Premixed Combustion Equation 9.27 (p. 271) requires closure terms for the production and destruction terms for flame area density. Several families of closure terms have been put forth in the literature [380] (p. 796). ANSYS Fluent uses the closure described in the following section.

9.5.1. Closure for ECFM Source Terms There are four ECFM model variants available. The default Veynante scheme is the recommended scheme, because it provides the best accuracy in most situations. You can access the other models by using the define/models/species/ecfm-controls text command, and revise the model constants as necessary. The source terms for each model are shown in Table 9.1: Source Terms for ECFM Models (p. 272): Table 9.1: Source Terms for ECFM Models Model Name Veynante [500] (p. 802) (default) Meneveau [87] (p. 779)

N/A

Poinsot [87] (p. 779)

N/A

Teraji [481] (p. 801)

N/A

N/A

The values of the constants shown in Table 9.1: Source Terms for ECFM Models (p. 272) are detailed in Table 9.2: Values of Constants for ECFM Model Source Terms (p. 272): Table 9.2: Values of Constants for ECFM Model Source Terms Model Name Veynante (default)

1.6

1.0 0.5 1.0 0.4 N/A

Meneveau

1.6

1.0 1.0 N/A 1.0 N/A

Poinsot

1.6

1.0 1.0 N/A 1.0 N/A

Teraji

0.012 8.5 1.4 N/A 1.0 0.1

In Table 9.1: Source Terms for ECFM Models (p. 272), in the following manner:

is the Karlovitz number, and

is calculated from

(9.28)

where

is the Favre-averaged progress variable, which is provided by ANSYS Fluent.

represents the flame area density that results from turbulent flame stretching, which is calculated using the turbulent time scale : (9.29)

272

Extended Coherent Flamelet Model Theory where is the turbulent dissipation rate and is the turbulent kinetic energy. The constant is a user-defined coefficient that determines the weight of the intermediate turbulent net flame stretch (ITNFS) term, , relative to a straightforward turbulent time scale. The value of is 1 for low turbulence levels (the default), and 0 for high turbulence levels. The ITNFS term, and

, where

, can be specified either as a constant or calculated as a function of the two parameters is the turbulent velocity fluctuation,

turbulent length scale, and The expression for

is the laminar flame speed,

is the integral

is the laminar flame thickness.

is given by: (9.30)

where

is defined as (9.31)

and

is (9.32)

ANSYS Fluent allows you to set the ITNFS term (

) directly, in a manner that is referred to as the constant

ITNFS treatment. Otherwise, you can influence the following:

by defining the laminar flame thickness

as one of

• a constant, user-defined value This is referred to as the constant delta ITNFS treatment. • the Meneveau flame thickness [310] (p. 792) In this case, the laminar flame thickness is calculated as: (9.33) where

is the local unburned thermal diffusivity.

• the Poinsot flame thickness [380] (p. 796) In this case, the flame thickness is evaluated as in Equation 9.33 (p. 273), but is replaced by . is calculated as:

in Equation 9.29 (p. 272)

(9.34) where (9.35)

273

Premixed Combustion

(9.36) (9.37) and

is defined in Equation 9.31 (p. 273).

• the Blint Correction flame thickness [47] (p. 777) In this case, a correction is added in order to account for the rapid expansion of the gases: (9.38) where

is the unburned temperature and

denotes the burned temperature.

As formulated, a singularity can result when , , , or , and so ANSYS Fluent limits and in order to avoid such values. Furthermore, the production terms and can be nonzero values in regions where the mixture is outside of the flammability limits, which is not physically possible. Accordingly, ANSYS Fluent sets the production terms to zero when the laminar flame speed is less than a very small value. The stability of the solution is enhanced by ensuring that the laminar flame speed in the destruction term is always greater than a small finite value. Inspection of the function for shows that a singularity exists in Equation 9.30 (p. 273) when , which can occur when the turbulent length scale is small compared to the laminar flame thickness. To prevent such a singularity, the quantity ( ) is limited to a small positive number. As a result, the net turbulent flame stretch is small in laminar zones. These numerical limiting constants can be adjusted via the text user interface.

9.5.2. Turbulent Flame Speed in ECFM The mean reaction rate term in the reaction progress variable Equation 9.1 (p. 263) is closed as (9.39) which is the product of the unburnt density, .

, laminar flame speed,

, and flame surface area density,

9.5.3. LES and ECFM When LES turbulence modeling is used in conjunction with the ECFM model, it is necessary to recast Equation 9.27 (p. 271) using terms that are resolved by the mesh and those that are unresolved (that is, subgrid terms):

(9.40)

The resolved terms remain the same as given previously in Table 9.1: Source Terms for ECFM Models (p. 272). The subgrid terms are derived from those defined by Veynante et al. [404] (p. 797) [499] (p. 802). The term takes the same form as the term given in Table 9.1: Source Terms for ECFM Models (p. 272), but is now defined as:

274

Extended Coherent Flamelet Model Theory

(9.41)

where is the combustion filter size and is the turbulent velocity fluctuation at scale . The ECFM equation is filtered at a larger scale than the momentum equations, because it has been shown that eddies smaller than this size do not wrinkle the flame front. Consequently,

is defined as: (9.42)

where is a user-defined coefficient with a default value of 5, and defined as:

is the grid size.

is then (9.43)

where

is the usual LES subgrid scale turbulent velocity fluctuation.

With LES turbulence modeling, the definition of in Equation 9.41 (p. 275) also changes to become a sub-filter scale function, as given by Meneveau et al. [72] (p. 779): (9.44)

The denominator on the right side of Equation 9.44 (p. 275) is defined by the following: (9.45)

(9.46)

(9.47)

(9.48) (9.49) where: (9.50) (9.51) The coefficient in Equation 9.40 (p. 274) and Equation 9.41 (p. 275) is designed to ensure that a correct flame brush thickness is maintained. This coefficient is applied not only to the diffusion term of the ECFM equation, but also to the diffusion terms of the progress variable and mixture fraction equations: (9.52) where

is the flame brush thickness of a 1D steady flame:

275

Premixed Combustion

(9.53) In the previous equation,

represents the unburnt density,

represents the burnt density, and

is

calculated as follows: (9.54) is the equilibrium wrinkling factor given by the following: (9.55)

where (9.56) (9.57) represents the turbulent viscosity. Similarly, in Equation 9.40 (p. 274): (9.58) The remaining subgrid terms are: (9.59) (9.60) where (9.61) The defaults for the user-defined constants used in the subgrid terms are: (9.62) (9.63) (9.64)

9.6. Calculation of Properties The C-equation, G-equation and ECFM turbulent premixed combustion models require properties for temperature, density, unburnt density, unburnt thermal diffusivity and laminar flame speed, which are modeled as described in the sections that follow. For more information, see the following sections: 9.6.1. Calculation of Temperature 9.6.2. Calculation of Density

276

Calculation of Properties 9.6.3. Laminar Flame Speed 9.6.4. Unburnt Density and Thermal Diffusivity

9.6.1. Calculation of Temperature The calculation method for temperature will depend on whether the model is adiabatic or non-adiabatic.

9.6.1.1. Adiabatic Temperature Calculation For the adiabatic premixed combustion model, the temperature is assumed to be a linear function of reaction progress between the lowest temperature of the unburnt mixture, , and the highest adiabatic burnt temperature : (9.65)

9.6.1.2. Non-Adiabatic Temperature Calculation For the non-adiabatic premixed combustion model, ANSYS Fluent solves an energy transport equation in order to account for any heat losses or gains within the system. The energy equation in terms of sensible enthalpy, , for the fully premixed fuel (see Equation 5.3 (p. 140)) is as follows: (9.66) represents the heat losses due to radiation and reaction:

represents the heat gains due to chemical (9.67)

where = normalized average rate of product formation (

)

= heat of combustion for burning 1 kg of fuel (J/kg) = fuel mass fraction of unburnt mixture

9.6.2. Calculation of Density ANSYS Fluent calculates the premixed density using the ideal gas law. For the adiabatic model, pressure variations are neglected and the mean molecular weight is assumed to be constant. The burnt gas density is then calculated from the following relation: (9.68) where the subscript refers to the unburnt cold mixture, and the subscript refers to the burnt hot mixture. The required inputs are the unburnt density ( ), the unburnt temperature ( ), and the burnt adiabatic flame temperature ( ). For the non-adiabatic model, you can choose to either include or exclude pressure variations in the ideal gas equation of state. If you choose to ignore pressure fluctuations, ANSYS Fluent calculates the density from (9.69)

277

Premixed Combustion where is computed from the energy transport equation, Equation 9.66 (p. 277). The required inputs are the unburnt density ( ) and the unburnt temperature ( ). Note that, from the incompressible ideal gas equation, the expression may be considered to be the effective molecular weight of the gas, where

is the gas constant and

is the operating pressure.

If you want to include pressure fluctuations for a compressible gas, you will need to specify the effective molecular weight of the gas, and the density will be calculated from the ideal gas equation of state.

9.6.3. Laminar Flame Speed The laminar flame speed ( in Equation 9.3 (p. 263)) can be specified as constant, or as a user-defined function. A third option appears for non-adiabatic premixed and partially-premixed flames and is based on the correlation proposed by Metghalchi and Keck [317] (p. 792), (9.70) In Equation 9.70 (p. 278), flame, and

and

The reference laminar flame speed,

are the unburnt reactant temperature and pressure ahead of the . , is calculated from (9.71)

where is the equivalence ratio ahead of the flame front, and The exponents and are calculated from,

,

and

are fuel-specific constants.

(9.72) The Metghalchi-Keck laminar flame speeds are available for fuel-air mixtures of methane, methanol, propane, iso-octane and indolene fuels.

9.6.4. Unburnt Density and Thermal Diffusivity The unburnt density ( in Equation 9.3 (p. 263)) and unburnt thermal diffusivity ( in Equation 9.9 (p. 265) and Equation 9.10 (p. 265)) are specified constants that are set in the Materials dialog box. However, for compressible cases, such as in-cylinder combustion, these can change significantly in time and/or space. When the ideal gas model is selected for density, the unburnt density and thermal diffusivity are calculated by evaluating the local cell at the unburnt state c=0.

278

Chapter 10: Partially Premixed Combustion ANSYS Fluent provides a partially premixed combustion model that is based on the non-premixed combustion model described in Non-Premixed Combustion (p. 223) and the premixed combustion model described in Premixed Combustion (p. 261). For information about using the partially premixed combustion model, see Modeling Partially Premixed Combustion in the User's Guide. Information about the theory behind the partially premixed combustion model is presented in the following sections: 10.1. Overview 10.2. Limitations 10.3. Partially Premixed Combustion Theory

10.1. Overview Partially premixed combustion systems are premixed flames with non-uniform fuel-oxidizer mixtures (equivalence ratios). Such flames include premixed jets discharging into a quiescent atmosphere, lean premixed combustors with diffusion pilot flames and/or cooling air jets, and imperfectly premixed inlets. ANSYS Fluent has three types of partially premixed models, namely Chemical Equilibrium, Steady Diffusion Flamelet, and Flamelet Generated Manifold. The Chemical Equilibrium and Steady Diffusion Flamelet partially-premixed models assume that the premixed flame front is infinitely thin, with unburnt reactants ahead and burnt products behind the flame front. The composition of the burnt products can be modeled assuming chemical equilibrium or with steady laminar diffusion flamelets. The flame-brush is indicated by a value of mean reaction progress between 0 and 1. Note that at a point within the turbulent premixed flame brush, , the fluctuating thin flame spends some time at the unburnt state ( ) and the remaining time at the burnt state ( ), with a mean reaction progress between zero and one. It should not be interpreted that the instantaneous premixed flame reaction progress is intermediate between the unburnt and burnt state. The Flamelet Generated Manifold (FGM) model [494] (p. 802) assumes that the thermochemical states in a turbulent flame are similar to the states in a laminar flame, and parameterize these by mixture fraction and reaction progress. Within the laminar flame, reaction progress increases from in the unburnt reactants to in the burnt products, over a nonzero flame thickness. A point within the turbulent flame brush with has contributions from both fluctuating flame fronts, as well as intermediate reaction progress. ANSYS Fluent has the option to model the FGM with either premixed or diffusion laminar flames.

10.2. Limitations The underlying theory, assumptions, and limitations of the non-premixed and premixed models apply directly to the partially premixed model. In particular, the single-mixture-fraction approach is limited to two inlet streams, which may be pure fuel, pure oxidizer, or a mixture of fuel and oxidizer. The twomixture-fraction model extends the number of inlet streams to three, but incurs a major computational overhead. See Limitations (p. 262) for additional limitations.

279

Partially Premixed Combustion

10.3. Partially Premixed Combustion Theory The partially premixed model solves a transport equation for the mean reaction progress variable, , or the mean flame position, (to determine the position of the flame front), as well as the mean mixture fraction,

and the mixture fraction variance,

. The Flamelet Generated Manifold model has an option

to solve a transport equation for the reaction progress variable variance, , or to use an algebraic expression. Ahead of the flame ( ), the fuel and oxidizer are mixed but unburnt and behind the flame ( ), the mixture is burnt. For more information, see the following sections: 10.3.1. Chemical Equilibrium and Steady Diffusion Flamelet Models 10.3.2. Flamelet Generated Manifold (FGM) model 10.3.3. FGM Turbulent Closure 10.3.4. Calculation of Mixture Properties 10.3.5. Calculation of Unburnt Properties 10.3.6. Laminar Flame Speed

10.3.1. Chemical Equilibrium and Steady Diffusion Flamelet Models Density weighted mean scalars (such as species fractions and temperature), denoted by from the probability density function (PDF) of and as

, are calculated

(10.1)

Under the assumption of thin flames, so that only unburnt reactants and burnt products exist, the mean scalars are determined from (10.2)

where the subscripts The burnt scalars,

and

denote burnt and unburnt, respectively.

, are functions of the mixture fraction and are calculated by mixing a mass

of

fuel with a mass of oxidizer and allowing the mixture to equilibrate. When non-adiabatic mixtures and/or diffusion laminar flamelets are considered, is also a function of enthalpy and/or strain, but this does not alter the basic formulation. The unburnt scalars, mass

of fuel with a mass

, are calculated similarly by mixing a

of oxidizer, but the mixture is not reacted.

Just as in the non-premixed model, the chemistry calculations and PDF integrations for the burnt mixture are performed in ANSYS Fluent, and look-up tables are constructed. It is important to understand that in the limit of perfectly premixed combustion, the equivalence ratio and hence mixture fraction is constant. Hence, the mixture fraction variance and its scalar dissipation are zero. If you are using laminar diffusion flamelets, the flamelet at the lowest strain will always be interpolated, and if you have Include Equilibrium Flamelet enabled, the ANSYS Fluent solution will be identical to a calculation with a chemical equilibrium PDF table.

280

Partially Premixed Combustion Theory

10.3.2. Flamelet Generated Manifold (FGM) model The Laminar Flamelet model (see The Diffusion Flamelet Models Theory (p. 246)) postulates that a turbulent flame is an ensemble of laminar flames that have an internal structure not significantly altered by the turbulence. These laminar flamelets are embedded in the turbulent flame brush using statistical averaging. The Flamelet Generated Manifold (FGM) [494] (p. 802) model assumes that the scalar evolution (that is the realized trajectories on the thermochemical manifold) in a turbulent flame can be approximated by the scalar evolution in a laminar flame. Both Laminar Flamelet and FGM parameterize all species and temperature by a few variables, such as mixture-fraction, scalar-dissipation and/or reaction-progress, and solve transport equations for these parameters in a 3D CFD simulation. Note that the FGM model is fundamentally different from the Laminar Flamelet model. For instance, since Laminar Flamelets are parameterized by strain, the thermochemistry always tends to chemical equilibrium as the strain rate decays towards the outlet of the combustor. In contrast, the FGM model is parameterized by reaction progress and the flame can be fully quenched, for example, by adding dilution air. No assumption of thin and intact flamelets is made by FGM, and the model can theoretically be applied to the stirred reactor limit, as well as to ignition and extinction modeling. Any type of laminar flame can be used to parameterize an FGM. ANSYS Fluent can either import an FGM calculated in a third-party flamelet code and written in Standard file format (see Standard Files for Flamelet Generated Manifold Modeling in the User's Guide), or calculate an FGM from 1D steady premixed flamelets or 1D diffusion flamelets. In general, premixed FGMs should be used for turbulent partially-premixed flames that are predominantly premixed. Similarly, diffusion FGMs should be used for turbulent partially-premixed flames that are predominantly non-premixed.

10.3.2.1. Premixed FGMs While the only possible configuration for a diffusion flamelet in 1D is opposed flow, 1D steady premixed flamelets can have several configurations. These include unstrained adiabatic freely-propagating, unstrained non-adiabatic burner-stabilized, as well as a strained opposed flow premixed flames. 1D premixed flamelets can be solved in physical space (for example, [395] (p. 796), [70] (p. 778)), then transformed to reaction-progress space in the ANSYS Fluent premixed flamelet file format, and imported into ANSYS Fluent. Alternatively 1D premixed flamelets can be generated in ANSYS Fluent, which solves the flamelets in reaction-progress space. The reaction progress variable is defined by Equation 9.2 (p. 263) where the sum is over all species in the chemical mechanism, denotes the species mass fraction, superscript denotes the unburnt reactant at the flame inlet, and superscript denotes chemical equilibrium at the flame outlet. The coefficients should be prescribed so that the reaction progress, , increases monotonically through the flame. By default, for all species other than , which are selected for hydrocarbon combustion. In the case where the element is missing from the chemical mechanism, such as combustion, ANSYS Fluent uses except , by default. The coefficients can be set in the text user interface when generating FGM using the text user interface. The 1D adiabatic premixed flame equations can be transformed from physical-space to reaction-progress space [342] (p. 794), [285] (p. 790). Neglecting differential-diffusion, these equations are (10.3) (10.4)

281

Partially Premixed Combustion where

is the

species mass fraction,

the species mass reaction rate, constant pressure. The scalar-dissipation rate,

is the temperature,

is the total enthalpy, and

is the fluid density, is the

is time,

is

species specific heat at

in Equation 10.3 (p. 281) and Equation 10.4 (p. 281) is defined as (10.5)

where

is the thermal conductivity. Note that

varies with

and is an input to the equation set. If

is taken from a 1D physical-space, adiabatic, equi-diffusivity flamelet calculation, either freelypropagating (unstrained) or opposed-flow (strained), the -space species and temperature distributions would be identical to the physical-space solution. However, is generally not known and is modeled in ANSYS Fluent as (10.6) where is a user-specified maximum scalar dissipation within the premixed flamelet, and the inverse complementary error function.

is

A 1D premixed flamelet is calculated at a single equivalence ratio, which can be directly related to a corresponding mixture fraction. For partially-premixed combustion, premixed laminar flamelets must be generated over a range of mixture fractions. Premixed flamelets at different mixture fractions have different maximum scalar dissipations, . In ANSYS Fluent, the scalar dissipation at any mixture fraction

is modeled as (10.7)

where

indicates stoichiometric mixture fraction.

Hence, the only model input to the premixed flamelet generator in ANSYS Fluent is the scalar dissipation at stoichiometric mixture fraction, . The default value is , which reasonably matches solutions of unstrained (freely propagating) physical-space flamelets for rich, lean, and stoichiometric hydrocarbon and hydrogen flames at standard temperature and pressure. The following are important points to consider about premixed flamelet generation in ANSYS Fluent in general and when specifying in particular: • It is common in the FGM approach to use unstrained, freely-propagating premixed flamelets. However, for highly turbulent flames where the instantaneous premixed flame front is stretched and distorted by the turbulence, a strained premixed flamelet may be a better representation of the manifold. The ANSYS Fluent FGM model allows a premixed flamelet generated manifold at a single, representative strain rate, . • Calculating flamelets in physical-space can be compute-intensive and difficult to converge over the entire mixture fraction range, especially with large kinetic mechanisms at the flammability limits. The ANSYS Fluent solution in reaction-progress space is substantially faster and more robust than a corresponding physical-space solution. However, if physical-space solutions (for example, from [395] (p. 796), [70] (p. 778)) are preferred, they can be generated with an external code, transformed to reaction-progress space, and imported in ANSYS Fluent's Standard Flamelet file format. • A more appropriate value of than the default (1000/s) for the specific fuel and operating conditions of your combustor can be determined from a physical-space premixed flamelet solution at stoichiometric

282

Partially Premixed Combustion Theory mixture fraction. This premixed flamelet solution can be unstrained or strained. flamelet is then calculated from Equation 10.5 (p. 282) and

through the

is the maximum value of

.

• As is increased, the solution of temperature and species fractions as a function of tends to the thin-flamelet chemical equilibrium solution, which is linear between unburnt at and chemical equilibrium at . The minimum specified should be that in a freely-propagating flame. For smaller values of , the ANSYS Fluent premixed flamelet generator will have difficulties converging. When ANSYS Fluent fails to converge steady premixed flamelets at rich mixture fractions, equilibrium thin-flamelet solutions are used. • The ANSYS Fluent partially-premixed model can be used to model partially-premixed flames ranging from the non-premixed to the perfectly-premixed limit. In the non-premixed limit, reaction progress is unity everywhere in the domain and all premixed streams are fully burnt. In the premixed limit, the mixture fraction is constant in the domain. When creating a partially-premixed PDF table, the interface requires specification of the fuel and oxidizer compositions and temperatures. For partially-premixed flames, the fuel composition can be modeled as pure fuel, in which case any premixed inlet would be set to the corresponding mixture fraction, which is less than one. Alternatively, the fuel composition can be specified in the interface as a premixed inlet composition, containing both fuel and oxidizer components. In this latter case, the mixture fraction at premixed inlets would be set to one.

10.3.2.2. Diffusion FGMs For turbulent partially-premixed flames that are predominantly non-premixed, diffusion FGMs are a better representation of the thermochemistry than premixed FGMs. An example of this is modeling emissions from a gas-turbine combustor where the primary combustion zone is quenched by rapid mixing with dilution air. If the outlet equivalence ratio is less than the flammability limit of a corresponding premixed flamelet, the premixed FGM will predict sub-equilibrium , even if the combustor is quenched ( ). A diffusion FGM, however, will better predict super-equilibrium for . Diffusion FGMs are calculated in ANSYS Fluent using the diffusion laminar flamelet generator as detailed in Flamelet Generation (p. 249). Steady diffusion flamelets are generated over a range of scalar dissipation rates by starting from a very small strain (0.01/s by default) and increasing this in increments (5/s by default) until the flamelet extinguishes. The diffusion FGM is calculated from the steady diffusion laminar flamelets by converting the flamelet species fields to reaction progress, (see Equation 10.2 (p. 280)). As the strain rate increases, the flamelet chemistry departs further from chemical equilibrium and decreases from unity towards the extinction reaction, . The FGM between , and the unburnt state of the final, extinguishing diffusion flamelet.

, is determined from the thermochemical states

10.3.3. FGM Turbulent Closure The reaction progress variable is defined as a normalized fraction of product species (see Equation 9.2 (p. 263)), namely

. Note that the denominator,

, is only a function of the local mixture

fraction. When the Flamelet Generated Manifold model is enabled, ANSYS Fluent solves a transport equation for the un-normalized progress variable, , and not the normalized progress variable, . This has two advantages. Firstly, since there are usually no products in the oxidizer stream, is zero and is undefined here (in other words, burnt air is the same as unburnt air). This can lead to difficulties in specifying oxidizer boundary conditions where oxidizer is mixed into unburnt reactants before the flame, as well as into burnt products behind the flame. Solving for avoids these issues and the Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

283

Partially Premixed Combustion solution is independent of the specified boundary value of for pure oxidizer inlets. The second advantage of solving for is that flame quenching can be modeled naturally. Consider a burnt stream in chemical equilibrium ( ) that is rapidly quenched with an air jet. Since the equation in ANSYS Fluent does not have a source term dependent on changes in mixture fraction, remains at unity and hence the diluted mixture remains at chemical equilibrium. Solving for can capture quenching since changes with mixture fraction, and the normalized reaction progress is correctly less than unity after mixing. To model these two effects with the normalized equation, additional terms involving derivatives and cross-derivatives of mixture fraction are required [54] (p. 778). These terms do not appear in the transport equation for : (10.8) where

is laminar thermal conductivity of the mixture,

Typically, in the FGM model, the mean source term

is the mixture specific heat.

is modeled as: (10.9)

where is the Finite-Rate flamelet source term from the flamelet library, and reaction-progress ( ) and mixture fraction ( ).

is the joint PDF of

The source term determines the turbulent flame position. Errors in both the approximation of the variance, as well as the assumed shape Beta PDF, can cause inaccurate flame positions. ANSYS Fluent has the option of two other turbulence-chemistry interaction models for the source term . One option is to use a turbulent flame speed, (10.10) which is essentially the same source term when using the chemical equilibrium and diffusion flamelet partially-premixed models. An advantage of using a turbulent flame speed is that model constants can be calibrated to predict the correct flame position. In contrast, there are no direct parameters to control the FGM finite-rate source (Equation 10.9 (p. 284)), and hence the flame position. The third option is to use the minimum of the Finite-Rate and Turbulent-Flame-Speed expressions: (10.11) The idea behind Equation 10.11 (p. 284) is to use the turbulent flame speed model to predict the flame location and the finite-rate model to predict post-flame quenching, for example, by dilution with cold air. The joint PDF, in Equation 10.9 (p. 284), is specified as the product of two beta PDFs. The beta PDFs require second moments (that is, variances). The variance of the un-normalized reaction progress variable is modeled either with a transport equation (10.12) where

,

or with an algebraic expression (10.13) where

284

is the turbulence length scale and

is a constant with a default value of 0.1.

Partially Premixed Combustion Theory

10.3.4. Calculation of Mixture Properties The flamelets generated using the premixed or diffusion FGM model store instantaneous species mass fractions and temperature as a function of the local mixture fraction and the progress variable . The mean thermochemical properties of the mixture are determined by averaging the instantaneous thermochemical property values using the modeled PDF

as: (10.14)

where denotes the species mass fraction or temperature from the flamelet files. In addition to temperature and species mass fraction, other mixture properties, such as specific heat, density, and molecular weight, can also be evaluated using Equation 10.14 (p. 285). The average mixture properties for an adiabatic system are represented as a function of mean mixture fraction , mean progress variable , and their variances and and are stored inside a four-dimensional PDF table: (10.15)

Non-adiabatic Extension for Average Mixture Properties Calculation Similar to Non-Adiabatic Extensions of the Non-Premixed Model (p. 232), the fluctuations of enthalpy are ignored with the non-adiabatic extension to the PDF model. The average mixture properties for non-adiabatic partially premixed combustion using FGM can then be calculated as: (10.16) where

is the mean enthalpy.

For non-adiabatic systems, each of the average mixture properties is a function of five independent variables: (10.17) Calculation of average mixture properties using Equation 10.16 (p. 285) and Equation 10.17 (p. 285) requires a five-dimensional PDF table, which places an enormous demand on memory and could be computationally expensive. In order to optimize the run-time memory requirements and computational cost, the non-adiabatic PDF tables are generated with the following assumptions: • Species mass fractions are not sensitive to the enthalpy change • The mixture-averaged properties other than species mass fractions are computed using average progress variable With these assumptions, the average species mass fraction and, therefore, molecular weight can be calculated using Equation 10.14 (p. 285), ignoring the enthalpy level. The other mixture properties, such as temperature, specific heat, and density, account for the enthalpy changes and are estimated as: (10.18) (10.19)

285

Partially Premixed Combustion

10.3.5. Calculation of Unburnt Properties Turbulent fluctuations are neglected for the unburnt mixture, so the mean unburnt scalars, functions of

, are

only. The unburnt density, temperature, specific heat, and thermal diffusivity are fitted

in ANSYS Fluent to third-order polynomials of

using linear least squares: (10.20)

Since the unburnt scalars are smooth and slowly-varying functions of , these polynomial fits are generally accurate. Access to polynomials is provided in case you want to modify them. When the secondary mixture fraction model is enabled, the unburnt density, temperature, specific heat, thermal diffusivity, and laminar flame speed are calculated as follows: polynomial functions are calculated for a mixture of pure primary fuel and oxidizer, as described above, and are a function of the mean primary mixture fraction, . Similar polynomial functions are calculated for a mixture of pure secondary fuel and oxidizer, and are a function of the normalized secondary mixture fraction, . The unburnt properties in a cell are then calculated as a weighted function of the mean primary mixture fraction and mean secondary normalized mixture fraction as, (10.21)

10.3.6. Laminar Flame Speed The premixed models require the laminar flame speed (see Equation 9.9 (p. 265)), which depends strongly on the composition, temperature, and pressure of the unburnt mixture. For adiabatic perfectly premixed systems as in Premixed Combustion (p. 261), the reactant stream has one composition, and the laminar flame speed is constant throughout the domain. However, in partially premixed systems, the laminar flame speed will change as the reactant composition (equivalence ratio) changes, and this must be taken into account. Accurate laminar flame speeds are difficult to determine analytically, and are usually measured from experiments or computed from 1D simulations. For the partially-premixed model, in addition to the laminar flame speed model options described in Laminar Flame Speed (p. 278), namely constant, userdefined function, and Metghalchi-Keck, ANSYS Fluent has fitted curves obtained from numerical simulations of the laminar flame speed [160] (p. 783). These curves were determined for hydrogen ( ), methane ( ), acetylene ( ), ethylene ( ), ethane ( ), and propane ( ) fuels. They are valid for inlet compositions ranging from the lean limit through unity equivalence ratio (stoichiometric), for unburnt temperatures from 298 K to 800 K, and for pressures from 1 bar to 40 bars. ANSYS Fluent fits these curves to a piecewise-linear polynomial. Mixtures leaner than the lean limit or richer than the rich limit will not burn, and have zero flame speed. The required inputs are values for the laminar flame speed at ten mixture fraction ( ) points. The first (minimum) and last (maximum) inputs are the flammability limits of the mixture and the laminar flame speed is zero outside these values. For non-adiabatic simulations, such as heat transfer at walls or compressive heating, the unburnt mixture temperature may deviate from its adiabatic value. The piecewise-linear function of mixture fraction is unable to account for this effect on the laminar flame speed. You can include non-adiabatic effects on the laminar flame speed by enabling Non-Adiabatic Laminar Flame Speed, which tabulates the laminar speeds in the PDF table by evaluating the curve fits from [160] (p. 783) at the enthalpy levels in the 286

Partially Premixed Combustion Theory PDF table. Note that the tabulated mean laminar flame speed accounts for fluctuations in the mixture fraction.

Important These flame speed fits are accurate for air mixtures with pure fuels of , , , , , and . If an oxidizer other than air or a different fuel is used, or if the unburnt temperature or pressure is outside the range of validity, then the curve fits will be incorrect. Although ANSYS Fluent defaults to a methane-air mixture, the laminar flame speed polynomial and the rich and lean limits are most likely incorrect for your specified fuel/oxidizer and unburnt temperature/pressure conditions. The laminar flame speed polynomial should be determined from other sources, such as measurements from the relevant literature or detailed 1D simulations, and then input into ANSYS Fluent. Alternatively, you can use a user-defined function for the laminar flame speed.

287

288

Chapter 11: Composition PDF Transport ANSYS Fluent provides a composition PDF transport model for modeling finite-rate chemistry effects in turbulent flames. For information about using the composition PDF transport model, see Modeling a Composition PDF Transport Problem in the User's Guide. Information about the theory behind this model is presented in the following sections: 11.1. Overview and Limitations 11.2. Composition PDF Transport Theory 11.3.The Lagrangian Solution Method 11.4.The Eulerian Solution Method

11.1. Overview and Limitations The composition PDF transport model, like the Laminar Finite-Rate (see Direct Use of Finite-Rate Kinetics (no TCI) (p. 195)) and EDC model (see The Eddy-Dissipation-Concept (EDC) Model (p. 200)), should be used when you are interested in simulating finite-rate chemical kinetic effects in turbulent reacting flows. With an appropriate chemical mechanism, kinetically-controlled species such as CO and NOx, as well as flame extinction and ignition, can be predicted. PDF transport simulations are computationally expensive, and it is recommended that you start your modeling with small meshes, and preferably in 2D. A limitation that applies to the composition PDF transport model is that you must use the pressurebased solver as the model is not available with the density-based solver. ANSYS Fluent has two different discretizations of the composition PDF transport equation, namely Lagrangian and Eulerian. The Lagrangian method is strictly more accurate than the Eulerian method, but requires significantly longer run time to converge.

11.2. Composition PDF Transport Theory Turbulent combustion is governed by the reacting Navier-Stokes equations. While this equation set is accurate, its direct solution (where all turbulent scales are resolved) is far too expensive for practical turbulent flows. In Species Transport and Finite-Rate Chemistry (p. 193), the species equations are Reynolds-averaged, which leads to unknown terms for the turbulent scalar flux and the mean reaction rate. The turbulent scalar flux is modeled in ANSYS Fluent by gradient diffusion, treating turbulent convection as enhanced diffusion. The mean reaction rate can be modeled with the Laminar, EddyDissipation or EDC Finite-Rate chemistry models. Since the reaction rate is invariably highly nonlinear, modeling the mean reaction rate in a turbulent flow is difficult and prone to error. An alternative to Reynolds-averaging the species and energy equations is to derive a transport equation for their single-point, joint probability density function (PDF). This PDF, denoted by , can be considered to represent the fraction of the time that the fluid spends at each species, temperature and pressure state. has dimensions for the species, temperature and pressure spaces. From the PDF, any single-point thermo-chemical moment (for example, mean or RMS temperature, mean reaction rate) can be calculated. The composition PDF transport equation is derived from the Navier-Stokes equations as [382] (p. 796):

289

Composition PDF Transport

(11.1)

where = Favre joint PDF of composition = mean fluid density = Favre mean fluid velocity vector = reaction rate for species = composition space vector = fluid velocity fluctuation vector = molecular diffusion flux vector The notation of denotes expectations, and that event occurs.

is the conditional probability of event , given

In Equation 11.1 (p. 290), the terms on the left-hand side are closed, while those on the right-hand side are not and require modeling. The first term on the left-hand side is the unsteady rate of change of the PDF, the second term is the change of the PDF due to convection by the mean velocity field, and the third term is the change due to chemical reactions. The principal strength of the PDF transport approach is that the highly-nonlinear reaction term is completely closed and requires no modeling. The two terms on the right-hand side represent the PDF change due to scalar convection by turbulence (turbulent scalar flux), and molecular mixing/diffusion, respectively. The turbulent scalar flux term is unclosed, and is modeled in ANSYS Fluent by the gradient-diffusion assumption (11.2) where is the turbulent viscosity and is the turbulent Schmidt number. A turbulence model, as described in Turbulence (p. 39), is required for composition PDF transport simulations, and this determines . Since single-point PDFs are described, information about neighboring points is missing and all gradient terms, such as molecular mixing, are unclosed and must be modeled. The mixing model is critical because combustion occurs at the smallest molecular scales when reactants and heat diffuse together. Modeling mixing in PDF methods is not straightforward, and is the weakest link in the PDF transport approach. See Particle Mixing (p. 292) for a description of the mixing models.

11.3. The Lagrangian Solution Method A Lagrangian Monte Carlo method is used to solve for the dimensional PDF Transport equation. Monte Carlo methods are appropriate for high-dimensional equations since the computational cost increases linearly with the number of dimensions. The disadvantage is that statistical errors are introduced, and these must be carefully controlled.

290

The Lagrangian Solution Method To solve the modeled PDF transport equation, an analogy is made with a stochastic differential equation (SDE) that has identical solutions. The Monte Carlo algorithm involves notional particles that move randomly through physical space due to particle convection, and also through composition space due to molecular mixing and reaction. The particles have mass and, on average, the sum of the particle masses in a cell equals the cell mass (cell density times cell volume). Since practical meshes have large changes in cell volumes, the particle masses are adjusted so that the number of particles in a cell is controlled to be approximately constant and uniform. The processes of convection, diffusion, and reaction are treated in fractional steps as described in the sections that follow. For information on the fractional step method, refer to [66] (p. 778). Information about this method is described in the following sections: 11.3.1. Particle Convection 11.3.2. Particle Mixing 11.3.3. Particle Reaction

11.3.1. Particle Convection A spatially second-order-accurate Lagrangian method is used in ANSYS Fluent, consisting of two steps. At the first convection step, particles are advanced to a new position (11.3) where = particle position vector = Favre mean fluid-velocity vector at the particle position = particle time step For unsteady flows, the particle time step is the physical time step. For steady-state flows, local time steps are calculated for each cell as (11.4) where = convection number = diffusion number = mixing number

/ (cell fluid velocity) / (cell turbulent diffusivity)

turbulent time scale

= characteristic cell length =

where

is the problem dimension

After the first convection step, all other sub-processes, including diffusion and reaction are treated. Finally, the second convection step is calculated as (11.5)

where = mean cell fluid density = mean fluid-velocity vector at the particle position

291

Composition PDF Transport = effective viscosity = turbulent Schmidt number = standardized normal random vector

11.3.2. Particle Mixing Molecular mixing of species and heat must be modeled and is usually the source of the largest modeling error in the PDF transport approach. ANSYS Fluent provides three models for molecular diffusion: the Modified Curl model [205] (p. 786), [345] (p. 794), the IEM model (which is sometimes called the LSME model) [107] (p. 780) and the EMST model [468] (p. 800).

11.3.2.1. The Modified Curl Model For the Modified Curl model, a few particle pairs are selected at random from all the particles in a cell, and their individual compositions are moved toward their mean composition. For the special case of equal particle mass, the number of particle pairs selected is calculated as (11.6) where = total number of particles in the cell = mixing constant (default = 2) = turbulent time scale (for the -

model this is

)

The algorithm in [345] (p. 794) is used for the general case of variable particle mass. For each particle pair, a uniform random number is selected and each particle’s composition moved toward the pair’s mean composition by a factor proportional to :

is

(11.7)

where

and

are the composition vectors of particles and , and

and

are the masses of

particles and .

11.3.2.2. The IEM Model For the Interaction by Exchange with the Mean (IEM) model, the composition of all particles in a cell are moved a small distance toward the mean composition: (11.8) where is the composition before mixing, composition vector at the particle’s location.

292

is the composition after mixing, and

is the Favre mean-

The Lagrangian Solution Method

11.3.2.3. The EMST Model Physically, mixing occurs between fluid particles that are adjacent to each other. The Modified Curl and IEM mixing models take no account of this localness, which can be a source of error. The Euclidean Minimum Spanning Tree (EMST) model mixes particle pairs that are close to each other in composition space. Since scalar fields are locally smooth, particles that are close in composition space are likely to be close in physical space. The particle pairing is determined by a Euclidean Minimum Spanning Tree, which is the minimum length of the set of edges connecting one particle to at least one other particle. The EMST mixing model is more accurate than the Modified Curl and IEM mixing models, but incurs a slightly greater computational expense. Details on the EMST model can be found in reference [468] (p. 800).

11.3.2.4. Liquid Reactions Reactions in liquids often occur at low turbulence levels (small Re), among reactants with low diffusivities (large Sc). For such flows, the mixing constant default of overestimates the mixing rate. The Liquid Micro-Mixing option interpolates from model turbulence [384] (p. 796) and scalar [139] (p. 782) spectra.

11.3.3. Particle Reaction The particle composition vector is represented as (11.9) where

is the

th

species mass fraction,

is the temperature and

the pressure.

For the reaction fractional step, the reaction source term is integrated as (11.10)

where is the chemical source term. Most realistic chemical mechanisms consist of tens of species and hundreds of reactions. Typically, a reaction does not occur until an ignition temperature is reached, but then proceeds very quickly until reactants are consumed. Hence, some reactions have very fast time scales, in the order of s, while others have much slower time scales, on the order of 1 s. This timescale disparity results in numerical stiffness, which means that extensive computational work is required to integrate the chemical source term in Equation 11.10 (p. 293). In ANSYS Fluent, the reaction step (that is, the calculation of ) can be performed either by Direct Integration or by In-Situ Adaptive Tabulation (ISAT), as described in the following paragraphs. A typical steady-state PDF transport simulation in ANSYS Fluent may have 50000 cells, with 20 particles per cell, and requires 1000 iterations to converge. Hence, at least stiff ODE integrations are required. Since each integration typically takes tens or hundreds of milliseconds, on average, the direct integration of the chemistry is extremely CPU-demanding. For a given reaction mechanism, Equation 11.10 (p. 293) may be considered as a mapping. With an initial composition vector , the final state depends only on and the mapping time . In theory, if a table could be built before the simulation, covering all realizable states and time steps, the integrations could be avoided by table look-ups. In practice, this a priori tabulation is not feasible since a full table in dimensions ( species, temperature, pressure and time step) is required. To illustrate this, consider

293

Composition PDF Transport a structured table with vative estimate of

points in each dimension. The required table size is discretization points and

, and for a conser-

species, the table would contain

entries.

On closer examination, the full storage of the entire realizable space is very wasteful because most regions are never accessed. For example, it would be unrealistic to find a composition of and in a real combustor. In fact, for steady-state, 3D laminar simulations, the chemistry can be parameterized by the spatial position vector. Thus, mappings must lie on a three dimensional manifold within the dimensional composition space. It is, hence, sufficient to tabulate only this accessed region of the composition space. The accessed region, however, depends on the particular chemical mechanism, molecular transport properties, flow geometry, and boundary conditions. For this reason, the accessed region is not known before the simulation and the table cannot be preprocessed. Instead, the table must be built during the simulation, and this is referred to as in-situ tabulation. ANSYS Fluent employs ISAT [383] (p. 796) to dynamically tabulate the chemistry mappings and accelerate the time to solution. ISAT is a method to tabulate the accessed composition space region “on the fly” (in-situ) with error control (adaptive tabulation). When ISAT is used correctly, accelerations of two to three orders of magnitude are typical. However, it is important to understand how ISAT works in order to use it optimally.

11.4. The Eulerian Solution Method The Lagrangian solution method solves the composition PDF transport equation by stochastically tracking Lagrangian particles through the domain. It is computationally expensive since a large number of particles are required to represent the PDF, and a large number of iterations are necessary to reduce statistical errors and explicitly convect the particles through the domain. The Eulerian PDF transport model overcomes these limitations by assuming a shape for the PDF, which allows Eulerian transport equations to be derived. Stochastic errors are eliminated and the transport equations are solved implicitly, which is computationally economical. The multi-dimensional PDF shape is assumed as a product of delta functions. As with the Lagrangian PDF model, the highly nonlinear chemical source term is closed. However, the turbulent scalar flux and molecular mixing terms must be modeled, and are closed with the gradient diffusion and the IEM models, respectively. The composition PDF of dimension ( species and enthalpy) is represented as a collection of delta functions (or modes). This presumed PDF has the following form: (11.11)

where the

th

is the probability in each mode, mode,

is the conditional mean composition of species

is the composition space variable of species , and

in

is the delta function.

The Eulerian PDF transport equations are derived by substituting Equation 11.11 (p. 294) into the closed composition PDF transport equation (Equation 11.1 (p. 290) with Equation 11.2 (p. 290) and Equation 11.8 (p. 292)). The unknown terms, and , are determined by forcing lower moments of this transported PDF to match the RANS lower moment transport equations, using the Direct Quadrature Method of Moments (DQMOM) approach [139] (p. 782), [302] (p. 791). The resulting transport equations are: • Probability (magnitude of the

th

delta function): (11.12)

294

The Eulerian Solution Method • Probability weighted conditional mean of composition : (11.13) where

is the probability of the

th

conditional mean composition of the terms the

,

and

mode, and th

is the

mode.

th

species probability weighted

is the effective turbulent diffusivity. The

represent mixing, reaction and correction terms respectively. Note that only

probability equations are solved and the solved probabilities.

th

probability is calculated as one minus the sum of

For more information, see the following sections: 11.4.1. Reaction 11.4.2. Mixing 11.4.3. Correction 11.4.4. Calculation of Composition Mean and Variance

11.4.1. Reaction The reaction source term calculated as,

in Equation 11.13 (p. 295) for the

th

composition and the

th

mode is (11.14)

where

is the net reaction rate for the

th

component.

11.4.2. Mixing The micro-mixing term

is modeled with the IEM mixing model: (11.15)

where

is the turbulence time-scale and

is the mixing constant.

Hence, for the two-mode DQMOM-IEM model, the mixing terms for component

are, (11.16)

The default value of is 2, which is appropriate for gas-phase combustion. For reactions in liquids, where the diffusivities are much smaller than gases, the Liquid Micro-Mixing option interpolates from model turbulence [384] (p. 796) and scalar [139] (p. 782) spectra.

11.4.3. Correction Using assumptions to ensure realizability and boundedness, the correction terms tion 11.13 (p. 295) for the

th

in Equa-

composition are determined from the linear system, (11.17)

295

Composition PDF Transport where are the non-negative integer lower moments (1– ) for each component . Note that the condition of the matrix decreases with increasing , which reduces the stability of higher mode simulations. The dissipation term

in Equation 11.17 (p. 295) is calculated as, (11.18)

For the two-mode DQMOM-IEM model, the correction terms for the

th

component are, (11.19)

11.4.4. Calculation of Composition Mean and Variance The mean composition (species

or energy) is calculated as, (11.20)

and its variance is calculated as (11.21)

296

Chapter 12: Chemistry Acceleration ANSYS Fluent provides several methods to reduce the computational expense of solving detailed chemistry in multi-dimensional CFD. For information about using these chemistry acceleration methods, see Using Chemistry Acceleration in the User's Guide. Information about the theory behind these approaches is presented in the following sections: 12.1. Overview and Limitations 12.2. In-Situ Adaptive Tabulation (ISAT) 12.3. Dynamic Mechanism Reduction 12.4. Chemistry Agglomeration 12.5. Chemical Mechanism Dimension Reduction 12.6. Dynamic Cell Clustering with ANSYS CHEMKIN-CFD Solver

12.1. Overview and Limitations ANSYS Fluent has the following turbulence-chemistry interaction options for simulating combustors with detailed chemical kinetics: • Finite-Rate chemistry with no turbulence-chemistry interaction model (for laminar flows or for turbulent flows where turbulence-chemistry interaction may be neglected) • Eddy-Dissipation Concept (for turbulent flames) • Lagrangian Composition PDF Transport (for turbulent flames) • Eulerian Composition PDF Transport (for turbulent flames) Using detailed chemistry is appropriate when modeling kinetically controlled phenomena, such as slowly forming product and pollutant species, as well as flame ignition and extinction. Comprehensive chemical mechanisms contain a multitude of intermediate species in addition to the major fuel, oxidizer, and product species. These intermediate species evolve at widely different reaction rates resulting in disparate species formation and destruction time-scales. The numerical time integration to accurately calculate the species evolution requires very small time substeps, called stiffness, and causes large computational run times. The chemistry acceleration tools in ANSYS Fluent can mitigate this cost, but with some loss of accuracy. Care must be taken to set controlling parameters so that these inaccuracies are within acceptable limits.

12.2. In-Situ Adaptive Tabulation (ISAT) ISAT is ANSYS Fluent’s most powerful tool to accelerate detailed stiff chemistry. A speedup of two to three orders of magnitude is common, which is significant in that a simulation that would take months without ISAT can be run in days instead. For kinetic mechanisms that are deterministic, the final reacted state is a unique function of the initial unreacted state and the time step. This reaction mapping can, in theory, be performed once and tabulated. The table can then be interpolated with run-time speedup as long as interpolation is more efficient than chemistry integration. In practice, pre-tabulation is prohibitive since the table dimensions are too

297

Chemistry Acceleration large: the number of species (N), plus the temperature, pressure, and time step. However, in a reacting flow, only a very small sub-space of the full composition space is accessed. For example, it is unlikely that a state consisting of pure radical OH (mass fraction of one) at a temperature of 300 K will exist, and it is unnecessary to pre-tabulate this state. The chemistry is said to lie on a low-dimensional manifold. In fact, for steady-state, 3D, laminar simulations, the chemistry can be parameterized by the spatial position vector. Thus, mappings must lie on a three dimensional manifold within the N+3 dimensional composition space. It is, hence, sufficient to tabulate only this accessed region of the composition space. The accessed region, however, depends on the particular chemical mechanism, the thermodynamic and transport properties, the flow geometry, and the boundary conditions. For this reason, the accessed region is not known before the simulation and the table cannot be preprocessed. Instead, the table must be built during the simulation, and this is referred to as in-situ tabulation. ANSYS Fluent employs ISAT [383] (p. 796) to dynamically tabulate the chemistry mappings and accelerate the time to solution. ISAT is a method to tabulate the accessed composition space region “on the fly” (in-situ) with error control (adaptive tabulation). It is important to understand how ISAT works in order to use it optimally. Reaction over a time step

from an initial composition

to a final composition

is calculated as: (12.1)

where

is the chemical source term.

At the start of an ANSYS Fluent simulation using ISAT, the ISAT table is empty. For the first reaction step, Equation 12.1 (p. 298) is integrated with a stiff ODE solver. This is called Direct Integration (DI). The first table entry is created and consists of: • the initial composition • the mapping

(where the superscript 0 denotes the composition vector before the reaction)

(where the superscript 1 denotes the composition vector after the reaction)

• the mapping gradient matrix • a hyper-ellipsoid of accuracy The next reaction mapping is calculated as follows: The initial composition vector for this particle is denoted by , where the subscript denotes a query. The existing table (consisting of one entry at this stage) is queried by interpolating the new mapping as (12.2) The mapping gradient is hence used to linearly interpolate the table when queried. The ellipsoid of accuracy (EOA) is the elliptical space around the table point where the linear approximation to the mapping is accurate to the specified tolerance, . If the query point

is within the EOA, then the linear interpolation by Equation 12.2 (p. 298) is sufficiently

accurate, and the mapping is retrieved. Otherwise, a direct integration (DI) is performed and the mapping error error tolerance (

is calculated (here,

is a scaling matrix). If this error is smaller than the specified

), then the original interpolation

is accurate and the EOA is grown so as to include

. If not, and if the table size is smaller than the user-specified maximum RAM (Max. Storage), a new table entry is added.

298

Dynamic Mechanism Reduction Table entries are stored as leaves in a binary tree. When a new table entry is added, the original leaf becomes a node with two leaves—the original leaf and the new entry. A cutting hyper-plane is created at the new node, so that the two leaves are on either side of this cutting plane. A composition vector will hence lie on either side of this hyper-plane. The ISAT algorithm is summarized as follows: 1. The ISAT table is queried for every composition vector during the reaction step. 2. For each query

the table is traversed to identify a leaf whose composition

3. If the query composition

lies within the EOA of the leaf, then the mapping

is close to

.

is retrieved using interpol-

ation by Equation 12.2 (p. 298). Otherwise, Direct Integration (DI) is performed and the error DI and the linear interpolation is measured.

between the

4. If the error is less than the tolerance, then the ellipsoid of accuracy is grown and the DI result is returned. Otherwise, a new table entry is added. At the start of the simulation, most operations are adds and grows. Later, as more of the composition space is tabulated, retrieves become frequent. Since adds and grows are very slow whereas retrieves are relatively quick, initial ANSYS Fluent iterations are slow but accelerate as the table is built. There are two inputs to ISAT, namely the ISAT error tolerance ( ) and the maximum ISAT table size (in Mbytes). Large values of provide faster run times, but larger error. For steady-state simulations, it is advised to start the simulation with a large and reduce as the solution stabilizes and nears convergence. For this reason, the default ISAT error tolerance of 0.001 is relatively large and should be decreased for unsteady simulations or when a steady-state simulation converges. It is recommended that you monitor the species of interest and re-converge with reduced until these species change in acceptably small increments. The maximum ISAT table size should be set to just below the available RAM memory on the computer.

12.3. Dynamic Mechanism Reduction The solution time for a chemically reacting flow problem increases with the size of the reaction mechanism used. Typically, this relationship is

where

,

,

, and

are constants. However,

if finite-difference Jacobian is applied.

Dynamic Mechanism Reduction can accelerate the simulation by decreasing the number of species ( ) and number of reactions ( ) in the chemical mechanism. In general, more reduction results in faster, but less accurate simulations. Mechanism reduction seeks to decrease the mechanism size while limiting accuracy loss to some predefined tolerances. Unlike skeletal reduction where mechanism reduction is completed at the pre-solution stage to create a single reduced mechanism that is used throughout the simulation, Dynamic Mechanism Reduction is performed "on the fly" in every cell (or particle), at every flow iteration (for steady simulations) or time step (for transient simulations). Since the mechanism is only required to be accurate at the local cell conditions, Dynamic Mechanism Reduction can be used with a higher level of reduction yet less accuracy loss than skeletal mechanism [[290] (p. 791), [353] (p. 794)]. Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

299

Chemistry Acceleration Mechanism reduction in ANSYS Fluent is performed using the Directed Relation Graph (DRG) approach [[288] (p. 791), [289] (p. 791)], described next.

12.3.1. Directed Relation Graph (DRG) Method for Mechanism Reduction Given a list of species that need to be modeled accurately (called “targets”), DRG eliminates all species and reactions in the mechanism that do not contribute significantly (directly or indirectly) to predicting the evolution of the targets. To generate the reduced mechanism, DRG implements the following steps. 1.

DRG considers normalized contribution of each non-target species species within a single cell:

to overall production of each target

(12.3)

where

is a chemical rate of elementary reaction at a local cell condition defined by the temperature , the pressure , and the species mass fraction is a stoichiometric coefficient of species

in reaction

Species is retained in the mechanism if and only if the largest reaction rate involving both species, and , is larger than a fraction of the largest reaction rate involving species within the cell: (12.4) where

is a specified error tolerance.

Normalized contribution value is computed for every non-target species in the mechanism, to identify all non-target species that directly contribute significantly to modeling target species . These species constitute the dependent set of . This process is repeated for all target species, and a comprehensive dependent set is created as the union of individual dependent sets for all targets. 2.

In the next step, DRG algorithm, in a similar manner, identifies indirect contributors, which are species that directly impact the dependent set, rather than the targets. In other words, if species is included in the comprehensive dependent set created in step 1, then all the remaining species with must also be included. The procedure outlined in step 1 is applied to each species in the dependent set to generate the list of species that contribute to its production or consumption. This process is continued for each species added to the mechanism, until no new species qualifies to be added to the comprehensive dependent set. The resulting set of species (including the targets) constitutes the species retained in the mechanism. All the other species, that is all the species for which for all species included in the resulting set of species, are considered unimportant and are eliminated from the mechanism.

300

Chemistry Agglomeration 3.

Finally, all reactions that do not involve any of the retained species are also eliminated from the mechanism. The resulting mechanism is the final reduced mechanism. A lower-dimensional ODE system is solved, involving only the retained species and reactions. The eliminated species mass fractions are stored for computing mixture quantities such as density and heat capacity.

It is important to note that computational expense of the DRG method has been shown to be linear in the number of reactions in the full detailed mechanism. This small additional overhead is typically significantly outweighed by the acceleration due to Dynamic Mechanism Reduction using DRG. Mechanism reduction is controlled by the following two parameters in ANSYS Fluent: • Error Tolerance The default value for error tolerance

is 0.01.

• Target Species List (adjustable only in expert mode, accessible in the TUI) The default list of target species consists of 3 constituents. Hydrogen radical is explicitly defined as the first target species. Hydrogen radical is used as a default target species because it is strongly linked to heat release in combustion; accurate prediction of this species should ensure accurate prediction of heat release. Two other species with the largest mass fractions are added to the target species list by DRG algorithm at each time step or flow iteration. There is also an option to remove a species from the target list whenever its mass fraction is below a given threshold. This option is disabled by default in ANSYS Fluent (that is, the default value for minimum mass fraction is 0). In general, a smaller value of error tolerance and a larger value of number of targets yield larger, more accurate mechanisms, but slower simulations. The default settings should balance accuracy and efficiency well for most simulations. However, these parameters may require more careful selection for some problems, such as auto-ignition of highly complex fuels. For further instructions on how to use Dynamic Mechanism Reduction, see Using Dynamic Mechanism Reduction in the Fluent User's Guide.

12.4. Chemistry Agglomeration Reacting flow computations with detailed mechanisms can be computationally demanding, even with the speedup provided by ISAT. Chemistry agglomeration (CA) provides additional run-time improvement, with a corresponding additional decrease in accuracy. The idea behind chemistry agglomeration is to collect cells (or particles for the Lagrangian PDF Transport model) that are close in composition space, average these to a single composition, call the reaction step integrator, then map this reaction step back to the cells. In summary: 1. Just before the reaction step, bin CFD cells that are close in composition space. 2. Average their compositions. 3. Call ISAT to perform the chemistry integration with the single, averaged composition. 4. Map this reaction step back to the cells in the group. The number of calls to the relatively costly chemistry integration routine (ISAT) is less than the number of cells in the domain. Chemistry agglomeration is hence similar to Reactor Network, or Multi-Zone Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

301

Chemistry Acceleration models, which calculate reaction on a smaller number of zones than the number of cells in the CFD simulation. A reaction mapping is represented as, (12.5) where and are the thermo-chemical compositions (temperature, pressure, and species fractions) before and after reaction, respectively. By default, ANSYS Fluent loops over all cells (or particles for the Lagrangian PDF Transport model) and calculates the reaction mapping. Chemistry agglomeration loops through all CFD cells before reaction and bins those that are ‘close’ in composition space to within a specified tolerance. The composition in each bin is averaged as, (12.6)

where and

represents the agglomerated composition, subscript is an index over the cells in the bin, and denote the density and volume of the

th

cell, respectively.

After agglomeration, the ISAT reaction mapping routine is called, (12.7) Finally, the reacted cluster composition is mapped back to all the CFD cells in the corresponding bin, (12.8) for every cell within the bin. To ensure an iso-enthalpic reaction, the enthalpy of each cell after reaction,

, is set according to: (12.9)

where

denotes the enthalpy of cell before the reaction step,

, is the mass fraction after reaction for the cell cluster, and action.

is the formation enthalpy of species is the cell mass fraction before re-

To learn how to enable Chemistry Agglomeration, see Using Chemistry Agglomeration in the User's Guide. For more information, see the following section: 12.4.1. Binning Algorithm

12.4.1. Binning Algorithm A uniform Cartesian mesh is used to bin cells at every reaction step. The composition space has dimensions, where are the number of species and the additional two dimensions are temperature and pressure. To reduce the cost of tabulating in -dimensions, a subset of representative composition space coordinates is selected.

302

Chemistry Agglomeration For tabulation purposes, each coordinate ized as,

of the

-dimensional reduced composition space is normal-

(12.10)

where

and

are the minimum and maximum of composition space variable

in the CFD

computational domain. Hence, the reduced composition space binning mesh extends from zero to one in each of the coordinates. The hyper-cube is discretized into uniform intervals along each of the species or pressure coordinates and uniform temperature intervals. Here, (dimensionless) and (units of Kelvin) are user specified CA tolerances that indicate maximum bin sizes for species and temperature dimensions, respectively. All CFD cell compositions that fall within a reduced composition space bin are agglomerated. Since most of the bins in the -dimensional hyper-cube are likely to be empty, a dynamic hash table [20] (p. 776) is employed to efficiently store the bins. The hash table maps a unique bin index in the -dimensional hyper-cube to a 1D line. Since the reduced space hyper-cube is discretized into equispaced bins, a unique index for every bin can be defined as, (12.11)

where

is the bin index in the

The hash table maps

th

dimension.

to a 1D table of size

reduced space hyper-cube,

, which is much smaller than the number of entries in the

. A simple hash mapping function is the modulo function: (12.12)

Different -space indices may have the same hash mapping index , which results in collisions. In the dynamic hash table, the number of collisions, as well as the number of empty hash table entries are monitored, and the hash table size, , is adjusted at each iteration to set these to be within acceptable limits. The compositions representing the reduced space coordinates are selected from the dimensional composition space as follows. By default, temperature is included and pressure is excluded. The current algorithm orders the species with largest total mass in the computational domain. For example, is typically first in this list for combustors using air as the oxidizer. The first species of this list are selected as the reduced composition space variables. By selecting species with the largest mass, minor species are invariably neglected. Since and are of interest in many practical applications, and are inserted at the front of the list if they exist in the computational domain. It was found that inclusion of these species in the table coordinates consistently provided increased accuracy for all thermo-chemical variables. In summary, the default coordinate algorithm selects as a reduced space coordinate, then (if is present in the CFD domain), then (if is present in the CFD domain), followed by the species with largest mass in the domain until variables are reached (with =4 as default). The default reduced table size ( ) and species list should be suitable for most reacting flow applications, but can be changed. Contact your technical support engineer to learn how to make the changes.

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Chemistry Acceleration

12.5. Chemical Mechanism Dimension Reduction Dimension Reduction is a chemistry acceleration method in addition to ISAT storage-retrieval, Dynamic Mechanism Reduction, and Cell Agglomeration, providing faster chemistry calculations with a corresponding loss of accuracy. To learn how to use chemical mechanism Dimension Reduction, see Dimension Reduction in the User's Guide. Dimension Reduction solves a smaller number ( ) of species transport equations (see Equation 7.1 (p. 193)) than the number of species in the full chemical mechanism ( ). These are termed the represented species. The remaining species are termed the unrepresented species. In ANSYS Fluent, the unrepresented species are reconstructed assuming that they are in chemical equilibrium [385] (p. 796). The Dimension Reduction algorithm is implemented as follows: • ANSYS Fluent solves transport equations for the mixture enthalpy and the represented species mass fractions. Transport equations are also solved for the unrepresented element mass fractions, which are the cumulative mass fraction of each atomic element in all the unrepresented species. • For the reaction step, the initial unrepresented species in a cell are reconstructed by assuming that they are in chemical equilibrium at the cell pressure and enthalpy, subject to the constraints of the represented species mass fractions and the unrepresented element mass fractions. That is, the unrepresented species are determined as those that give maximum mixture entropy with the represented species held fixed, while also satisfying the unrepresented element mass fractions and mixture enthalpy. At the end of this reconstruction step, all species mass fractions of the full mechanism are available in the cell. • Next, the full detailed reaction mechanism with all species is integrated for the reaction time step. Reaction mappings for the represented species are available after the reaction step, and the mappings for the unrepresented species are discarded. As Dimension Reduction integrates the full chemical mechanism, which is computationally expensive, its advantage lies in coupling with ISAT. An ISAT table with dimensions will cover more of the reaction manifold than ISAT tabulation of the full dimensions in the same time. Consequently, a simulation with Dimension Reduction is initially not much faster than a simulation with the full mechanism, but iterations can be substantially faster at later times when the ISAT table is retrieved. Additionally, the retrieval time with Dimension Reduction enabled (proportional to ), is much smaller than that with the Full Mechanism (proportional to ). For this reason, Dimension Reduction is only available with ISAT. Note that Dimension Reduction allows CFD simulations with full chemical mechanisms consisting of more than the ANSYS Fluent transported species limit of 700, as long as the number of represented species is less than 700. For more information, see the following section: 12.5.1. Selecting the Represented Species

12.5.1. Selecting the Represented Species Judicious selection of the represented species is important for simulation accuracy. Boundary and initial species must be included in the represented species list. Species of interest, especially species that are far from chemical equilibrium, such as pollutants, should also be included. Intermediate species that occur in large mass fractions relative to the fuel and oxidizer species should be included, as well as

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Dynamic Cell Clustering with ANSYS CHEMKIN-CFD Solver species important in the chemical pathway. For example, for methane combustion in air, be included as a represented species since pyrolizes to first.

should

In ANSYS Fluent, you will specify the number of represented species, as well as selected represented species, such as the boundary Fuel and Oxidizer species. When the selected represented species are less than the input number of represented species, the remaining represented species are taken from the full mechanism in the order they appear in the Mixture Species list. After you obtain a preliminary solution with Dimension Reduction, it is a good idea to check the magnitude of all unrepresented species, which are available in the Full Mechanism Species... option in the Contours dialog box. If the mass fraction of an unrepresented species is larger than other represented species, you should repeat the simulation with this species included in the represented species list. In turn, the mass fraction of unrepresented elements should decrease.

12.6. Dynamic Cell Clustering with ANSYS CHEMKIN-CFD Solver For transient simulations that involve direct use of finite-rate chemistry, the chemistry-solution portion of the species equations is solved on a cell-by-cell basis. As a result, the equations solved are independent of the mass and volume of a specific cell. In this way, cells that have the same temperature, pressure, and initial species mass fractions will yield the same result. To take advantage of this fact, when you select the ANSYS CHEMKIN-CFD Solver, ANSYS Fluent uses a Dynamic Cell Clustering (DCC) method to group computational cells of high similarity into clusters using an efficient data-clustering method. That requires solution of the kinetic equations only once for each cluster. The optimal number of clusters is dynamically determined for each CFD time step. The clustering algorithm is solely based on the cell thermochemical states and is independent of their locations in the CFD mesh. In addition, the clustering algorithm is highly automated and requires minimal inputs. The algorithm uses cell temperature and equivalence ratio as the clustering indices. The only user-specified control parameters are the maximum dispersions of temperature and equivalence ratio in each cluster. Smaller values will result in a larger number of clusters, but generally the default settings have been found to provide accurate predictions for a wide range of combustion conditions. The DCC method includes three major steps: 1. Grouping cells into clusters using an evolutionary data-clustering algorithm. 2. Solving chemical kinetic equations based on cluster averaged state variables. 3. Mapping the cluster averaged solution back to the individual cells while preserving the initial temperature and species stratification.

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Chapter 13: Engine Ignition This chapter discusses the theory behind the engine ignition models available in ANSYS Fluent. Information can be found in the following sections. 13.1. Spark Model 13.2. Autoignition Models 13.3. Crevice Model For information about using these ignition models, see Modeling Engine Ignition in the User's Guide.

13.1. Spark Model The spark model in ANSYS Fluent will be described in the context of the premixed turbulent combustion model. For information about using this model, see Spark Model in the User's Guide. Information regarding the theory behind this model is detailed in the following sections: 13.1.1. Overview and Limitations 13.1.2. Spark Model Theory 13.1.3. ECFM Spark Model Variants

13.1.1. Overview and Limitations Initiation of combustion at a desired time and location in a combustion chamber can be accomplished by sending a high voltage across two narrowly separated wires, creating a spark. The spark event in typical engines happens very quickly relative to the main combustion in the engine. The physical description of this simple event is very involved and complex, making it difficult to accurately model the spark in the context of a multidimensional engine simulation. Additionally, the energy from the spark event is several orders of magnitude less than the chemical energy release from the fuel. Despite the amount of research devoted to spark ignition physics and ignition devices, the ignition of a mixture at a point in the domain is more dependent on the local composition than on the spark energy (see Heywood [180] (p. 785)). Thus, for situations in which ANSYS Fluent is utilized for combustion engine modeling, including internal combustion engines, the spark event does not need to be modeled in great detail, but simply as the initiation of combustion over a duration, which you will set. Since spark ignition is inherently transient, the spark model is only available in the transient solver. Additionally, the spark model requires chemical reactions to be solved. The spark model is available for all of the combustion models.

13.1.2. Spark Model Theory Typically, the initial spark size is small relative to the cell size, and the spark is under-resolved on the CFD mesh. Modeling the spark by burning a few cells around the spark location shows strong sensitivity to the grid and time-step size, and flame speed and flame brush diffusion can be erroneous due to the insufficient space and time resolution. In addition, for cases where the initial spark is smaller than the cell size, ignition proceeds too quickly.

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Engine Ignition To mitigate this sensitivity, ANSYS Fluent solves a sub-grid equation for the spark evolution. The spark shape is assumed to be perfectly spherical, with an infinitely thin flame front. The spark radius, , grows in time, , according to the ODE, (13.1) where

is the density of the unburned fluid ahead of the flame front,

fluid behind of the flame, and

is the density of the burnt

is the turbulent flame speed.

The sub-grid spark model is transferred to the CFD grid through a representative volume of CFD cells. This volume is spherical and has a fixed diameter computed as the local turbulent length scale at the spark location. However, in order to ensure that this representative volume is not too large (relative to the size of the combustor) or too small (relative to the cell size), the radius of the representative sphere, , is calculated as, (13.2) where scale.

is the user-specified initial spark radius,

Alternatively

is the cell length scale, and

is the turbulent length

may be specified as a fixed value via the spark-model text interface.

is taken to define the representative volume because, once the spark diameter reaches this size, the flame speed is affected by all the turbulent scales present. The spark radius will continue to increase until the length scale is reached, even if the simulation time exceeds the user-specified duration. Note, that the specified duration is only used to calculate the rate of spark energy input and the time when this input ceases. At this condition ANSYS Fluent automatically switches the spark flame speed model off, so that the flame speed is modeled using the flame speed model that you have selected in the Species dialog box, throughout the domain. For the Extended Coherent Flamelet model (ECFM) and C-Equation model, ANSYS Fluent fixes the reaction progress in the representative volume, , as, (13.3) For the G-Equation model, reaction progress in the spark is calculated in the usual way for this model as given by Equation 9.8 (p. 265). The temperature and species composition (denoted by at every time-step are calculated as,

) within the representative spherical volume (13.4)

where denotes the equilibrium burnt composition, and compositions are fixed in time and uniform in space.

is the unburned composition. These

Since the thermo-chemical state behind the spark flame front is instantaneously equilibrated as the spark propagates, spark energy is not required to ignite the mixture. By default, the spark energy is set to zero for all combustion models, and the burnt temperature is the equilibrium temperature. However, spark energy can be set to a positive value in the user interface, in which case the temperature behind the spark will be higher than the equilibrium temperature. ANSYS Fluent offers the following models for the turbulent flame speed,

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in Equation 13.1 (p. 308):

Spark Model

Turbulent Curvature In the Turbulent Curvature model, the turbulent flame speed is calculated as, (13.5) where is the laminar flame speed, is the current spark radius, is the laminar diffusivity, and is the turbulent diffusivity. and are evaluated at the spark location, and is the turbulent flame speed evaluated at the turbulent length scale of the spark radius. Since turbulent scales larger than the spark radius convect the spark but do not increase its area and flame speed, only turbulent length scales up to the spark radius can affect the turbulent flame speed of the spark. For the premixed and partiallypremixed models, the spark flame speed model is the same flame speed model as selected for the main combustion. For Species Transport cases, by default the Zimont flame speed model is used. and are added as additional material inputs in the interface for the species transport models. Note that the effect of the flame curvature is to decrease both the laminar and turbulent flames speeds. Since the initial spark radius is a user-input, decreasing slows the spark propagation and increases the burning time.

Turbulent Length In the Turbulent Length model, the turbulent flame speed is calculated as, (13.6) That is, the Turbulent Length model ignores the effects of flame curvature on the flame speed.

Herweg-Maly The turbulent flame speed is calculated using the model proposed by Herweg and Maly [181] (p. 785): (13.7)

where

is a function for effect of strain on the laminar burning velocity calculated as,

and = current time = start time = turbulent velocity scale

= laminar flame thickness = If the Blint modifier is being used

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Engine Ignition

where = 2.0 = 0.7 is the unburned density is the burned density

Laminar In the Laminar model, the turbulent flame speed in Equation 13.1 (p. 308) is modeled as the laminar flame speed, . Since the Laminar flame speed can be modeled with a user-defined function (UDF), the Laminar option can be used to define your own function for the turbulent flame speed.

13.1.3. ECFM Spark Model Variants When the ECFM combustion model is used the value of the flame surface density must be set within the spark region. The following ECFM spark model variants provide different methods of doing this:

Turbulent Model The flame surface density is calculated as:

with

being calculated using the spark flame speed model that you choose. In this equation

resents a flame wrinkling factor and

rep-

is the ratio of the spherical flame area to its volume.

Zimont Model The flame surface density is calculated as:

Constant Value Model In this case you supply the value of flame surface density.

User-Defined Sigma Source In this instance you supply a source for the flame surface density via a UDF. This is detailed in the .

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Autoignition Models

13.2. Autoignition Models Autoignition phenomena in engines are due to the effects of chemical kinetics of the reacting flow inside the cylinder. There are two types of autoignition models considered in ANSYS Fluent: • knock model in spark-ignited (SI) engines • ignition delay model in diesel engines For information regarding using autoignition models, see Autoignition Models in the User's Guide. The theory behind the autoignition models is described in the following sections: 13.2.1. Model Overview 13.2.2. Model Limitations 13.2.3. Ignition Model Theory

13.2.1. Model Overview The concept of knock has been studied extensively in the context of premixed engines, as it defines a limit in terms of efficiency and power production of that type of engine. As the compression ratio increases, the efficiency of the engine as a function of the work extracted from the fuel increases. However, as the compression ratio increases, the temperature and pressure of the air/fuel mixture in the cylinder also increase during the cycle compressions. The temperature and pressure increase can be large enough for the mixture to spontaneously ignite and release its heat before the spark plug fires. The premature release of all of the energy in the air/fuel charge is almost never desirable, as this results in the spark event no longer controlling the combustion. As a result of the premature release of the energy, catastrophic damage to the engine components can occur. The sudden, sharp rise in pressure inside the engine can be heard clearly through the engine block as a knocking sound, hence the term “knock”. For commonly available gasoline pumps, knock usually limits the highest practical compression ratio to less than 11:1 for premium fuels and around 9:1 for less expensive fuels. By comparison, ignition delay in diesel engines has not been as extensively studied as SI engines, mainly because it does not have such a sharply defining impact on engine efficiency. Ignition delay in diesel engines refers to the time between when the fuel is injected into the combustion chamber and when the pressure starts to increase as the fuel releases its energy. The fuel is injected into a gas that is usually air, however, it can have a considerable amount of exhaust gas mixed in (or EGR) to reduce nitrogen oxide emissions (NOx). Ignition delay depends on the composition of the gas in the cylinder, the temperature of the gas, the turbulence level, and other factors. Since ignition delay changes the combustion phasing, which in turn impacts efficiency and emissions, it is important to account for it in a diesel engine simulation.

13.2.2. Model Limitations The main difference between the knock model and the ignition delay model is the manner in which the model is coupled with the chemistry. The knock model always releases energy from the fuel while the ignition delay model prevents energy from being released prematurely. The knock model in ANSYS Fluent is compatible with the premixed and partially premixed combustion models. The autoignition model is compatible with any volumetric combustion model, with the exception of the purely premixed models. The autoignition models are inherently transient and so are not available with steady simulations.

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Engine Ignition The autoignition models in general require adjustment of parameters to reproduce engine data and are likely to require tuning to improve accuracy. Once the model is calibrated to a particular engine configuration, then different engine speeds and loads can be reasonably well represented. Detailed chemical kinetics may be more applicable over a wider range of conditions, though are more expensive to solve. The single equation autoignition models are appropriate for the situation where geometric fidelity or resolution of particular flow details is more important than chemical effects on the simulation.

13.2.3. Ignition Model Theory Both the knock and the ignition delay models are treated similarly in ANSYS Fluent, in that they share the same infrastructure. These models belong to the family of single equation autoignition models and use correlations to account for complex chemical kinetics. They differ from the eight step reaction models, such as Halstead’s “Shell” model [171] (p. 784), in that only a single transport equation is solved. The source term in the transport equation is typically not stiff, therefore making the equation relatively inexpensive to solve. This approach is appropriate for large simulations where geometric accuracy is more important than fully resolved chemical kinetics. The model can be used on less resolved meshes to explore a range of designs quickly, and to obtain trends before utilizing more expensive and presumably more accurate chemical mechanisms in multidimensional simulations.

13.2.3.1. Transport of Ignition Species Autoignition is modeled using the transport equation for an Ignition Species,

, which is given by (13.8)

where is a “mass fraction” of a passive species representing radicals that form when the fuel in the domain breaks down. is the turbulent Schmidt number. The term is the source term for the ignition species, which has a form,

where corresponds to the time at which fuel is introduced into the domain. The term is a correlation of ignition delay with the units of time. Ignition has occurred when the ignition species reaches a value of 1 in the domain. It is assumed that all the radical species represented by diffuse at the same rate as the mean flow. Note that the source term for these radical species is treated differently for knock and ignition delay. Furthermore, the form of the correlation of ignition delay differs between the two models. Details of how the source term is treated are covered in the following sections.

13.2.3.2. Knock Modeling When modeling knock or ignition delay, chemical energy in the fuel is released when the ignition species reaches a value of 1 in the domain. For the knock model, two correlations are built into ANSYS Fluent. One is given by Douaud [108] (p. 780), while the other is a generalized model that reproduces several correlations, given by Heywood [180] (p. 785).

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Autoignition Models

13.2.3.2.1. Modeling of the Source Term In order to model knock in a physically realistic manner, the source term is accumulated under appropriate conditions in a cell. Consider the one dimensional flame in Figure 13.1: Flame Front Showing Accumulation of Source Terms for the Knock Model (p. 313). Here, the flame is propagating from left to right, and the temperature is relatively low in front of the flame and high behind the flame. In this figure, and represent the temperatures at the burned and unburned states, respectively. The ignition species accumulates only when there is fuel. In the premixed model, the fuel is defined as , where is the progress variable. If the progress variable has a value of zero, the mixture is considered unburned. If the progress variable is 1, then the mixture is considered burned. Figure 13.1: Flame Front Showing Accumulation of Source Terms for the Knock Model

When the ignition species reaches a value of 1 in the domain, knock has occurred at that point. The value of the ignition species can exceed unity. In fact, values well above that can be obtained in a short time. The ignition species will continue to accumulate until there is no more fuel present.

13.2.3.2.2. Correlations An extensively tested correlation for knock in SI engines is given by Douaud and Eyzat [108] (p. 780): (13.9) where is the octane number of the fuel, temperature in Kelvin.

is the absolute pressure in atmospheres and

is the

A generalized expression for is also available that can reproduce many existing Arrhenius correlations. The form of the correlation is (13.10) where is the pre-exponential (with units in seconds), RPM is the engine speed in cycles per minute and is the fuel/air equivalence ratio.

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Engine Ignition

13.2.3.2.3. Energy Release Once ignition has occurred in the domain, the knock event is modeled by releasing the remaining fuel energy with a single-step Arrhenius reaction. An additional source term, which burns the remaining fuel in that cell, is added to the rate term in the premixed model. The reaction rate is given by (13.11) where = 8.6×1011, and = 15078 These values are chosen to reflect single-step reaction rates appropriate for propane as described in Amsden [7] (p. 775). The rate at which the fuel is consumed is limited such that a completely unburned cell will burn during three of the current time steps. Limiting the reaction rate is done purely for numerical stability.

13.2.3.3. Ignition Delay Modeling When modeling ignition delay in diesel engines, chemical reactions are allowed to occur when the ignition species reaches a value of 1 in the domain. For the ignition delay model, two correlations are built into ANSYS Fluent, one given by Hardenburg and Hase [175] (p. 784) and the other, a generalized model that reproduces several Arrhenius correlations from the literature. If the ignition species is less than 1 when using the ignition delay model, the chemical source term is suppressed by not activating the combustion model at that particular time step; therefore, the energy release is delayed. This approach is reasonable if you have a good high-temperature chemical model, but do not want to solve for typically expensive low temperature chemistry.

13.2.3.3.1. Modeling of the Source Term In order to model ignition in a physically realistic manner, the source term is accumulated under appropriate conditions in a cell. Consider the one dimensional spray in Figure 13.2: Propagating Fuel Cloud Showing Accumulation of Source Terms for the Ignition Delay Model (p. 314). Figure 13.2: Propagating Fuel Cloud Showing Accumulation of Source Terms for the Ignition Delay Model

314

Crevice Model Here, the spray is propagating from left to right and the fuel mass fraction is relatively low in front of the spray and high behind the spray. If there is no fuel in the cell, the model will set the local source term to zero, nevertheless, the value of can be nonzero due to convection and diffusion.

13.2.3.3.2. Correlations If fuel is present in the cell, there are two built-in options in ANSYS Fluent to calculate the local source term. The first correlation was done by Hardenburg and Hase and was developed at Daimler Chrysler for heavy duty diesel engines. The correlation works over a reasonably wide range of conditions and is given by (13.12) where is in seconds, is 0.36, is engine speed in revolutions per minute, is the effective activation energy and is the pressure exponent. The expression for the effective activation energy is given by (13.13) Table 13.1: Default Values of the Variables in the Hardenburg Correlation Variable Default

618,840 25

0.36 0.63

The second correlation, which is the generalized correlation, is given by Equation 13.10 (p. 313) and is available for ignition delay calculations.

13.2.3.3.3. Energy Release If the ignition species is greater than or equal to 1 anywhere in the domain, ignition has occurred and combustion is no longer delayed. The ignition species acts as a switch to turn on the volumetric reactions in the domain. Note that the ignition species “mass fraction” can exceed 1 in the domain, therefore, it is not truly a mass fraction, but rather a passive scalar that represents the integrated correlation as a function of time.

13.3. Crevice Model This section describes the theory behind the crevice model. Information can be found in the following sections: 13.3.1. Overview 13.3.2. Limitations 13.3.3. Crevice Model Theory For information regarding using the crevice model, see Crevice Model in the User's Guide.

13.3.1. Overview The crevice model implemented in ANSYS Fluent is a zero-dimensional ring-flow model based on the model outlined in Namazian and Heywood [339] (p. 793) and Roberts and Matthews [406] (p. 797). The model is geared toward in-cylinder specific flows, and more specifically, direct-injection (DI) diesel engines, and therefore is available only for time-dependent simulations.

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Engine Ignition The model takes mass, momentum, and energy from cells adjoining two boundaries and accounts for the storage of mass in the volumes of the crevices in the piston. Detailed geometric information regarding the ring and piston—typically a ring pack around the bore of an engine—is necessary to use the crevice model. An example representation is shown in Figure 13.3: Crevice Model Geometry (Piston) (p. 316) — Figure 13.5: Crevice Model “Network” Representation (p. 316). Figure 13.3: Crevice Model Geometry (Piston)

Figure 13.4: Crevice Model Geometry (Ring)

Figure 13.5: Crevice Model “Network” Representation

13.3.1.1. Model Parameters • The piston to bore clearance is the distance between the piston and the bore. Typical values are 2 to 5 mil (80 to 120 μm) in a spark engine (SI) and 4 to 7 mil (100 to 240 μm) in some diesel engines (DI). • The ring thickness is the variable in Figure 13.4: Crevice Model Geometry (Ring) (p. 316). Typical values range from 1 to 3 mm for SI engines and 2 to 4 mm for DI engines.

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Crevice Model • The ring width is the variable in Figure 13.4: Crevice Model Geometry (Ring) (p. 316). Typical values range from 3 to 3.5 mm for SI engines and 4 to 6 mm for DI diesel engines. • The ring spacing is the distance between the bottom of one ring land and the top of the next ring land. Typical values of the ring spacing are 3 to 5 mm for SI engines and 4 to 8 mm for DI diesel engines. • The land length is the depth of the ring land (that is, the cutout into the piston); always deeper than the width of the ring by about 1 mm. Typical values are 4 to 4.5 mm for SI engines and 5 to 7 mm for DI diesel engines. • The top gap is the clearance between the ring land and the top of the ring (40 to 80 μm). • The middle gap is the distance between the ring and the bore (10 to 40 μm). • The bottom gap is the clearance between the ring land and the bottom of the ring (40 to 80 μm). • The shared boundary and leaking wall is the piston (for example, wall-8) and the cylinder wall (for example, wall.1) in most in-cylinder simulations. Cells that share a boundary with the top of the piston and the cylinder wall are defined as the crevice cells. The ring pack is the set of rings that seal the piston in the cylinder bore. As the piston moves upward in the cylinder when the valves are closed (for example, during the compression stroke in a four-stroke cycle engine), the pressure in the cylinder rises and flow begins to move past the rings. The pressure distribution in the ring pack is modeled by assuming either fully-developed compressible flow through the spaces between the rings and the piston, or choked compressible flow between the rings and the cylinder wall. Since the temperature in the ring pack is fixed and the geometry is known, once a pressure distribution is calculated, the mass in each volume can be found using the ideal gas equation of state. The overall mass flow out of the ring pack (that is, the flow past the last ring specified) is also calculated at each discrete step in the ANSYS Fluent solution.

13.3.2. Limitations The limitations of the crevice model are that it is zero dimensional, transient, and currently limited to two threads that share a boundary. A zero-dimensional approach is used because it is difficult to accurately predict lateral diffusion of species in the crevice. If the lateral diffusion of species is important in the simulation, as in when a spray plume in a DI engine is in close proximity to the boundary and the net mass flow is into the crevice, it is recommended that the full multidimensional crevice geometry be simulated in ANSYS Fluent using a nonconformal mesh. Additionally, this approach does not specifically track individual species, as any individual species would be instantly distributed over the entire ring pack. The mass flux into the domain from the crevice is assumed to have the same composition as the cell into which mass is flowing. The formulation of the crevice flow equations is inherently transient and is solved using ANSYS Fluent’s stiff-equation solver. A steady problem with leakage flow can be solved by running the transient problem to steady state. Additional limitations of the crevice model in its current form are that only a single crevice is allowed and only one thread can have leakage. Ring dynamics are not explicitly accounted for, although ring positions can be set during the simulation. In this context, the crevice model solution is a stiff initial boundary-value problem. The stiffness increases as the pressure difference between the ring crevices increases and also as the overall pressure difference across the ring pack increases. Thus, if the initial conditions are very far from the solution during a time Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Engine Ignition step, the ODE solver may not be able to integrate the equations successfully. One solution to this problem is to decrease the flow time step for several iterations. Another solution is to start with initial conditions that are closer to the solution at the end of the time step.

13.3.3. Crevice Model Theory ANSYS Fluent solves the equations for mass conservation in the crevice geometry by assuming laminar compressible flow in the region between the piston and the top and bottom faces of the ring, and by assuming an orifice flow between the ring and the cylinder wall. The equation for the mass flow through the ring end gaps is of the form (13.14) where is the discharge coefficient, is the gap area, sound, and is a compressibility factor given by

is the gas density,

is the local speed of

(13.15)

where

is the ratio of specific heats,

the upstream pressure and

the downstream pressure. The

equation for the mass flow through the top and bottom faces of the ring (that is, into and out of the volume behind the piston ring) is given by (13.16)

where is the cross-sectional area of the gap, is the width of the ring along which the gas is flowing, is the local gas viscosity, is the temperature of the gas and is the universal gas constant. The system of equations for a set of three rings is of the following form: (13.17) (13.18) (13.19) (13.20) (13.21) where is the average pressure in the crevice cells and is the crankcase pressure input from the text interface. The expressions for the mass flows for numerically adjacent zones (for example, 0-1, 1-2, 2-3, and so on) are given by Equation 13.16 (p. 318) and expressions for the mass flows for zones separated by two integers (for example, 0-2, 2-4, 4-6) are given by Equation 13.14 (p. 318) and Equation 13.15 (p. 318). Thus, there are equations needed for the solution to the ring-pack equations, where is the number of rings in the simulation.

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Chapter 14: Pollutant Formation This chapter discusses the theory behind the models available in ANSYS Fluent for modeling pollutant formation. Information is presented in the following sections: 14.1. NOx Formation 14.2. SOx Formation 14.3. Soot Formation 14.4. Decoupled Detailed Chemistry Model For information about using the models in ANSYS Fluent, see Modeling Pollutant Formation in the User's Guide.

14.1. NOx Formation The following sections present the theoretical background of prediction. For information about using the models in ANSYS Fluent, see Using the NOx Model in the User's Guide. 14.1.1. Overview 14.1.2. Governing Equations for NOx Transport 14.1.3.Thermal NOx Formation 14.1.4. Prompt NOx Formation 14.1.5. Fuel NOx Formation 14.1.6. NOx Formation from Intermediate N2O 14.1.7. NOx Reduction by Reburning 14.1.8. NOx Reduction by SNCR 14.1.9. NOx Formation in Turbulent Flows

14.1.1. Overview emission consists of mostly nitric oxide ( ), and to a lesser degree nitrogen dioxide ( ) and nitrous oxide ( ). is a precursor for photochemical smog, contributes to acid rain, and causes ozone depletion. Thus, is a pollutant. The ANSYS Fluent model provides a tool to understand the sources of production and to aid in the design of control measures.

14.1.1.1. NOx Modeling in ANSYS Fluent The ANSYS Fluent model provides the capability to model thermal, prompt, and fuel formation, as well as consumption due to reburning in combustion systems. It uses rate models developed at the Department of Fuel and Energy at The University of Leeds in England, as well as from the open literature. reduction using reagent injection, such as selective non-catalytic reduction (SNCR), can be modeled in ANSYS Fluent, along with an intermediate model that has also been incorporated. To predict When fuel species (

emissions, ANSYS Fluent solves a transport equation for nitric oxide ( ) concentration. sources are present, ANSYS Fluent solves additional transport equations for intermediate and/or ). When the intermediate model is activated, an additional transport Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Pollutant Formation equation for will be solved. The transport equations are solved based on a given flow field and combustion solution. In other words, is postprocessed from a combustion simulation. It is therefore evident that an accurate combustion solution becomes a prerequisite of prediction. For example, thermal production doubles for every 90 K temperature increase when the flame temperature is about 2200 K. Great care must be exercised to provide accurate thermophysical data and boundary condition inputs for the combustion model. Appropriate turbulence, chemistry, radiation, and other submodels must be employed. To be realistic, you can only expect results to be as accurate as the input data and the selected physical models. Under most circumstances, variation trends can be accurately predicted, but the quantity itself cannot be pinpointed. Accurate prediction of parametric trends can cut down on the number of laboratory tests, allow more design variations to be studied, shorten the design cycle, and reduce product development cost. That is truly the power of the ANSYS Fluent model and, in fact, the power of CFD in general.

14.1.1.2. NOx Formation and Reduction in Flames In laminar flames and at the molecular level within turbulent flames, the formation of can be attributed to four distinct chemical kinetic processes: thermal formation, prompt formation, fuel formation, and intermediate . Thermal is formed by the oxidation of atmospheric nitrogen present in the combustion air. Prompt is produced by high-speed reactions at the flame front. Fuel is produced by oxidation of nitrogen contained in the fuel. At elevated pressures and oxygen-rich conditions, may also be formed from molecular nitrogen ( ) via . The reburning and SNCR mechanisms reduce the total formation by accounting for the reaction of with hydrocarbons and ammonia, respectively.

Important The

models cannot be used in conjunction with the premixed combustion model.

14.1.2. Governing Equations for NOx Transport ANSYS Fluent solves the mass transport equation for the species, taking into account convection, diffusion, production, and consumption of and related species. This approach is completely general, being derived from the fundamental principle of mass conservation. The effect of residence time in mechanisms (a Lagrangian reference frame concept) is included through the convection terms in the governing equations written in the Eulerian reference frame. For thermal and prompt mechanisms, only the species transport equation is needed: (14.1) As discussed in Fuel NOx Formation (p. 327), the fuel mechanisms are more involved. The tracking of nitrogen-containing intermediate species is important. ANSYS Fluent solves a transport equation for the , , or species, in addition to the species: (14.2) (14.3) (14.4)

320

NOx Formation where , , , and are mass fractions of is the effective diffusion coefficient. The source terms next for different mechanisms.

,

, ,

,

, and , and

in the gas phase, and are to be determined

14.1.3. Thermal NOx Formation The formation of thermal is determined by a set of highly temperature-dependent chemical reactions known as the extended Zeldovich mechanism. The principal reactions governing the formation of thermal from molecular nitrogen are as follows: (14.5) (14.6) A third reaction has been shown to contribute to the formation of thermal stoichiometric conditions and in fuel-rich mixtures:

, particularly at near(14.7)

14.1.3.1. Thermal NOx Reaction Rates The rate constants for these reactions have been measured in numerous experimental studies [46] (p. 777), [138] (p. 782), [329] (p. 793), and the data obtained from these studies have been critically evaluated by Baulch et al. [31] (p. 776) and Hanson and Salimian [174] (p. 784). The expressions for the rate coefficients for Equation 14.5 (p. 321) – Equation 14.7 (p. 321) used in the model are given below. These were selected based on the evaluation of Hanson and Salimian [174] (p. 784). =

=

=

=

=

=

In the above expressions, , , and are the rate constants for the forward reactions Equation 14.5 (p. 321) – Equation 14.7 (p. 321), respectively, and , , and are the corresponding reverse rate constants. All of these rate constants have units of m3/mol-s. The net rate of formation of given by

via the reactions in Equation 14.5 (p. 321) – Equation 14.7 (p. 321) is

(14.8)

where all concentrations have units of mol/m3. To calculate the formation rates of

and

, the concentrations of , , and

are required.

14.1.3.2. The Quasi-Steady Assumption for [N] The rate of formation of is significant only at high temperatures (greater than 1800 K) because fixation of nitrogen requires the breaking of the strong triple bond (dissociation energy of 941 kJ/mol). This effect is represented by the high activation energy of reaction Equation 14.5 (p. 321), which makes it the rate-limiting step of the extended Zeldovich mechanism. However, the activation energy for oxidation of atoms is small. When there is sufficient oxygen, as in a fuel-lean flame, the rate of Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

321

Pollutant Formation consumption of free nitrogen atoms becomes equal to the rate of its formation, and therefore a quasisteady state can be established. This assumption is valid for most combustion cases, except in extremely fuel-rich combustion conditions. Hence the formation rate becomes (14.9)

14.1.3.3. Thermal NOx Temperature Sensitivity From Equation 14.9 (p. 322) it is clear that the rate of formation of will increase with increasing oxygen concentration. It also appears that thermal formation should be highly dependent on temperature but independent of fuel type. In fact, based on the limiting rate described by , the thermal production rate doubles for every 90 K temperature increase beyond 2200 K.

14.1.3.4. Decoupled Thermal NOx Calculations To solve Equation 14.9 (p. 322), the concentration of atoms and the free radical will be required, in addition to the concentration of stable species (that is, , ). Following the suggestion by Zeldovich, the thermal formation mechanism can be decoupled from the main combustion process by assuming equilibrium values of temperature, stable species, atoms, and radicals. However, radical concentrations ( atoms in particular) are observed to be more abundant than their equilibrium levels. The effect of partial equilibrium atoms on formation rate has been investigated [323] (p. 793) during laminar methane-air combustion. The results of these investigations indicate that the level of emission can be under-predicted by as much as 28% in the flame zone, when assuming equilibrium -atom concentrations.

14.1.3.5. Approaches for Determining O Radical Concentration There has been little detailed study of radical concentration in industrial turbulent flames, but work [111] (p. 781) has demonstrated the existence of this phenomenon in turbulent diffusion flames. Presently, there is no definitive conclusion as to the effect of partial equilibrium on formation rates in turbulent flames. Peters and Donnerhack [373] (p. 795) suggest that partial equilibrium radicals can account for no more than a 25% increase in thermal and that fluid dynamics has the dominant effect on the formation rate. Bilger and Beck [39] (p. 777) suggest that in turbulent diffusion flames, the effect of atom overshoot on the formation rate is very important. To overcome this possible inaccuracy, one approach would be to couple the extended Zeldovich mechanism with a detailed hydrocarbon combustion mechanism involving many reactions, species, and steps. This approach has been used previously for research purposes [319] (p. 792). However, long computer processing time has made the method economically unattractive and its extension to turbulent flows difficult. To determine the radical concentration, ANSYS Fluent uses one of three approaches—the equilibrium approach, the partial equilibrium approach, and the predicted concentration approach—in recognition of the ongoing controversy discussed above.

14.1.3.5.1. Method 1: Equilibrium Approach The kinetics of the thermal formation rate is much slower than the main hydrocarbon oxidation rate, and so most of the thermal is formed after completion of combustion. Therefore, the thermal

322

NOx Formation formation process can often be decoupled from the main combustion reaction mechanism and the formation rate can be calculated by assuming equilibration of the combustion reactions. Using this approach, the calculation of the thermal formation rate is considerably simplified. The assumption of equilibrium can be justified by a reduction in the importance of radical overshoots at higher flame temperature [110] (p. 781). According to Westenberg [523] (p. 804), the equilibrium -atom concentration can be obtained from the expression (14.10) With

included, this expression becomes (14.11)

where

is in Kelvin.

14.1.3.5.2. Method 2: Partial Equilibrium Approach An improvement to method 1 can be made by accounting for third-body reactions in the recombination process:

dissociation(14.12)

Equation 14.11 (p. 323) is then replaced by the following expression [513] (p. 803): (14.13) which generally leads to a higher partial -atom concentration.

14.1.3.5.3. Method 3: Predicted O Approach When the -atom concentration is well predicted using an advanced chemistry model (such as the flamelet submodel of the non-premixed model), [ ] can be taken simply from the local -species mass fraction.

14.1.3.6. Approaches for Determining OH Radical Concentration ANSYS Fluent uses one of three approaches to determine the radical concentration: the exclusion of from the thermal calculation approach, the partial equilibrium approach, and the use of the predicted concentration approach.

14.1.3.6.1. Method 1: Exclusion of OH Approach In this approach, the third reaction in the extended Zeldovich mechanism (Equation 14.7 (p. 321)) is assumed to be negligible through the following observation:

This assumption is justified for lean fuel conditions and is a reasonable assumption for most cases.

14.1.3.6.2. Method 2: Partial Equilibrium Approach In this approach, the concentration of in the third reaction in the extended Zeldovich mechanism (Equation 14.7 (p. 321)) is given by [32] (p. 776), [522] (p. 803):

323

Pollutant Formation (14.14)

14.1.3.6.3. Method 3: Predicted OH Approach As in the predicted approach, when the radical concentration is well predicted using an advanced chemistry model such as the flamelet model, [ ] can be taken directly from the local species mass fraction.

14.1.3.7. Summary To summarize, the thermal formation rate is predicted by Equation 14.9 (p. 322). The -atom concentration needed in Equation 14.9 (p. 322) is computed using Equation 14.11 (p. 323) for the equilibrium assumption, using Equation 14.13 (p. 323) for a partial equilibrium assumption, or using the local -species mass fraction. You will make the choice during problem setup. In terms of the transport equation for (Equation 14.1 (p. 320)), the source term due to thermal mechanisms is (14.15) where is the molecular weight of tion 14.9 (p. 322).

(kg/mol), and

is computed from Equa-

14.1.4. Prompt NOx Formation It is known that during combustion of hydrocarbon fuels, the formation rate can exceed that produced from direct oxidation of nitrogen molecules (that is, thermal ).

14.1.4.1. Prompt NOx Combustion Environments The presence of a second mechanism leading to formation was first identified by Fenimore [125] (p. 781) and was termed “prompt ”. There is good evidence that prompt can be formed in a significant quantity in some combustion environments, such as in low-temperature, fuel-rich conditions and where residence times are short. Surface burners, staged combustion systems, and gas turbines can create such conditions [26] (p. 776). At present, the prompt contribution to total from stationary combustors is small. However, as emissions are reduced to very low levels by employing new strategies (burner design or furnace geometry modification), the relative importance of the prompt can be expected to increase.

14.1.4.2. Prompt NOx Mechanism Prompt is most prevalent in rich flames. The actual formation involves a complex series of reactions and many possible intermediate species. The route now accepted is as follows: (14.16) (14.17) (14.18) (14.19)

324

NOx Formation A number of species resulting from fuel fragmentation have been suggested as the source of prompt in hydrocarbon flames (for example, , , , ), but the major contribution is from (Equation 14.16 (p. 324)) and , via (14.20) The products of these reactions could lead to formation of amines and cyano compounds that subsequently react to form by reactions similar to those occurring in oxidation of fuel nitrogen, for example: (14.21)

14.1.4.3. Prompt NOx Formation Factors Prompt formation is proportional to the number of carbon atoms present per unit volume and is independent of the parent hydrocarbon identity. The quantity of formed increases with the concentration of hydrocarbon radicals, which in turn increases with equivalence ratio. As the equivalence ratio increases, prompt production increases at first, then passes a peak, and finally decreases due to a deficiency in oxygen.

14.1.4.4. Primary Reaction The reaction described by Equation 14.16 (p. 324) is of primary importance. In recent studies [420] (p. 798), comparison of probability density distributions for the location of the peak with those obtained for the peak have shown close correspondence, indicating that the majority of the at the flame base is prompt formed by the reaction. Assuming that the reaction described by Equation 14.16 (p. 324) controls the prompt formation rate, (14.22)

14.1.4.5. Modeling Strategy There are, however, uncertainties about the rate data for the above reaction. From the reactions described by Equation 14.16 (p. 324) – Equation 14.20 (p. 325), it can be concluded that the prediction of prompt formation within the flame requires coupling of the kinetics to an actual hydrocarbon combustion mechanism. Hydrocarbon combustion mechanisms involve many steps and, as mentioned previously, are extremely complex and costly to compute. In the present model, a global kinetic parameter derived by De Soete [102] (p. 780) is used. De Soete compared the experimental values of total formation rate with the rate of formation calculated by numerical integration of the empirical overall reaction rates of and formation. He showed that overall prompt formation rate can be predicted from the expression (14.23) In the early stages of the flame, where prompt is formed under fuel-rich conditions, the concentration is high and the radical almost exclusively forms rather than nitrogen. Therefore, the prompt formation rate will be approximately equal to the overall prompt formation rate: (14.24) For

(ethylene)-air flames, (14.25) Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

325

Pollutant Formation is 251151 , is the oxygen reaction order, is the universal gas constant, and is pressure (all in SI units). The rate of prompt formation is found to be of the first order with respect to nitrogen and fuel concentration, but the oxygen reaction order, , depends on experimental conditions.

14.1.4.6. Rate for Most Hydrocarbon Fuels Equation 14.24 (p. 325) was tested against the experimental data obtained by Backmier et al. [19] (p. 776) for different mixture strengths and fuel types. The predicted results indicated that the model performance declined significantly under fuel-rich conditions and for higher hydrocarbon fuels. To reduce this error and predict the prompt adequately in all conditions, the De Soete model was modified using the available experimental data. A correction factor, , was developed, which incorporates the effect of fuel type, that is, number of carbon atoms, and the air-to-fuel ratio for gaseous aliphatic hydrocarbons. Equation 14.24 (p. 325) now becomes (14.26) so that the source term due to prompt

mechanism is (14.27)

In the above equations, (14.28) (14.29) is 303474.125 , is the number of carbon atoms per molecule for the hydrocarbon fuel, and is the equivalence ratio. The correction factor is a curve fit for experimental data, valid for aliphatic alkane hydrocarbon fuels ( ) and for equivalence ratios between 0.6 and 1.6. For values outside the range, the appropriate limit should be used. Values of of Fuel and Energy at The University of Leeds in England.

and

were developed at the Department

Here, the concept of equivalence ratio refers to an overall equivalence ratio for the flame, rather than any spatially varying quantity in the flow domain. In complex geometries with multiple burners this may lead to some uncertainty in the specification of . However, since the contribution of prompt to the total emission is often very small, results are not likely to be significantly biased.

14.1.4.7. Oxygen Reaction Order Oxygen reaction order depends on flame conditions. According to De Soete [102] (p. 780), oxygen reaction order is uniquely related to oxygen mole fraction in the flame:

(14.30)

326

NOx Formation

14.1.5. Fuel NOx Formation 14.1.5.1. Fuel-Bound Nitrogen It is well known that nitrogen-containing organic compounds present in liquid or solid fossil fuel can contribute to the total formed during the combustion process. This fuel nitrogen is a particularly important source of nitrogen oxide emissions for residual fuel oil and coal, which typically contain 0.3–2% nitrogen by weight. Studies have shown that most of the nitrogen in heavy fuel oils is in the form of heterocycles, and it is thought that the nitrogen components of coal are similar [212] (p. 786). It is believed that pyridine, quinoline, and amine type heterocyclic ring structures are of importance.

14.1.5.2. Reaction Pathways The extent of the conversion of fuel nitrogen to is dependent on the local combustion characteristics and the initial concentration of nitrogen-bound compounds. Fuel-bound compounds that contain nitrogen are released into the gas phase when the fuel droplets or particles are heated during the devolatilization stage. From the thermal decomposition of these compounds (aniline, pyridine, pyrroles, and so on) in the reaction zone, radicals such as , , , , and can be formed and converted to . The above free radicals (that is, secondary intermediate nitrogen compounds) are subject to a double competitive reaction path. This chemical mechanism has been subject to several detailed investigations [320] (p. 792). Although the route leading to fuel formation and destruction is still not completely understood, different investigators seem to agree on a simplified model:

Recent investigations [186] (p. 785) have shown that hydrogen cyanide appears to be the principal product if fuel nitrogen is present in aromatic or cyclic form. However, when fuel nitrogen is present in the form of aliphatic amines, ammonia becomes the principal product of fuel nitrogen conversion. In the ANSYS Fluent model, sources of emission for gaseous, liquid, and coal fuels are considered separately. The nitrogen-containing intermediates are grouped as , , or a combination of both. Transport equations (Equation 14.1 (p. 320) and Equation 14.2 (p. 320) or Equation 14.3 (p. 320)) are solved, after which the source terms , , and are determined for different fuel types. Discussions to follow refer to fuel sources for and intermediate , and sources for and . Contributions from thermal and prompt mechanisms have been discussed in previous sections.

14.1.5.3. Fuel NOx from Gaseous and Liquid Fuels The fuel mechanisms for gaseous and liquid fuels are based on different physics, but the same chemical reaction pathways.

327

Pollutant Formation

14.1.5.3.1. Fuel NOx from Intermediate Hydrogen Cyanide (HCN) When

is used as the intermediate species:

The source terms in the transport equations can be written as follows: (14.31) (14.32)

14.1.5.3.1.1. HCN Production in a Gaseous Fuel The rate of

production is equivalent to the rate of combustion of the fuel: (14.33)

where = source of

(kg/m3–s)

= mean limiting reaction rate of fuel (kg/m3–s) = mass fraction of nitrogen in the fuel The mean limiting reaction rate of fuel, , is calculated from the Magnussen combustion model, so the gaseous fuel option is available only when the generalized finite-rate model is used.

14.1.5.3.1.2. HCN Production in a Liquid Fuel The rate of evaporation:

production is equivalent to the rate of fuel release into the gas phase through droplet (14.34)

where = source of

(kg/m3–s)

= rate of fuel release from the liquid droplets to the gas (kg/s) = mass fraction of nitrogen in the fuel = cell volume (m3)

328

NOx Formation

14.1.5.3.1.3. HCN Consumption The depletion rates from reactions (1) and (2) in the above mechanism are the same for both gaseous and liquid fuels, and are given by De Soete [102] (p. 780) as (14.35) (14.36) where ,

= conversion rates of

(s–1)

= instantaneous temperature (K) = mole fractions = 1.0

s–1

= 3.0

s–1

= 280451.95 J/mol = 251151 J/mol The oxygen reaction order, , is calculated from Equation 14.30 (p. 326). Since mole fraction is related to mass fraction through molecular weights of the species ( mixture ( ),

) and the

(14.37)

14.1.5.3.1.4. HCN Sources in the Transport Equation The mass consumption rates of

that appear in Equation 14.31 (p. 328) are calculated as (14.38) (14.39)

where ,

= consumption rates of

in reactions 1 and 2, respectively (kg/m3–s)

= pressure (Pa) = mean temperature (K) = universal gas constant

14.1.5.3.1.5. NOx Sources in the Transport Equation is produced in reaction 1, but destroyed in reaction 2. The sources for Equation 14.32 (p. 328) are the same for a gaseous as for a liquid fuel, and are evaluated as follows: (14.40) (14.41)

329

Pollutant Formation

14.1.5.3.2. Fuel NOx from Intermediate Ammonia (NH3) When

is used as the intermediate species:

The source terms in the transport equations can be written as follows: (14.42) (14.43)

14.1.5.3.2.1. NH3 Production in a Gaseous Fuel The rate of

production is equivalent to the rate of combustion of the fuel: (14.44)

where = source of

(

)

= mean limiting reaction rate of fuel (kg/m3–s) = mass fraction of nitrogen in the fuel The mean limiting reaction rate of fuel, , is calculated from the Magnussen combustion model, so the gaseous fuel option is available only when the generalized finite-rate model is used.

14.1.5.3.2.2. NH3 Production in a Liquid Fuel The rate of evaporation:

production is equivalent to the rate of fuel release into the gas phase through droplet (14.45)

where = source of

(kg/m3–s)

= rate of fuel release from the liquid droplets to the gas (kg/s) = mass fraction of nitrogen in the fuel = cell volume (m3)

330

NOx Formation

14.1.5.3.2.3. NH3 Consumption The depletion rates from reactions (1) and (2) in the above mechanism are the same for both gaseous and liquid fuels, and are given by De Soete [102] (p. 780) as (14.46)

where ,

= conversion rates of

(s–1)

= instantaneous temperature (K) = mole fractions = 4.0

s–1

= 1.8

s–1

= 133947.2 J/mol = 113017.95 J/mol The oxygen reaction order, , is calculated from Equation 14.30 (p. 326). Since mole fraction is related to mass fraction through molecular weights of the species ( mixture ( ), can be calculated using Equation 14.37 (p. 329).

) and the

14.1.5.3.2.4. NH3 Sources in the Transport Equation The mass consumption rates of

that appear in Equation 14.42 (p. 330) are calculated as (14.47) (14.48)

where ,

= consumption rates of

in reactions 1 and 2, respectively (kg/m3–s)

= pressure (Pa) = mean temperature (K) = universal gas constant

14.1.5.3.2.5. NOx Sources in the Transport Equation is produced in reaction 1, but destroyed in reaction 2. The sources for Equation 14.43 (p. 330) are the same for a gaseous as for a liquid fuel, and are evaluated as follows: (14.49) (14.50)

331

Pollutant Formation

14.1.5.3.3. Fuel NOx from Coal 14.1.5.3.3.1. Nitrogen in Char and in Volatiles For the coal it is assumed that fuel nitrogen is distributed between the volatiles and the char. Since there is no reason to assume that is equally distributed between the volatiles and the char, the fraction of in the volatiles and the char should be specified separately. When is used as the intermediate species, two variations of fuel mechanisms for coal are included. When is used as the intermediate species, two variations of fuel mechanisms for coal are included, much like in the calculation of production from the coal via . It is assumed that fuel nitrogen is distributed between the volatiles and the char.

14.1.5.3.3.2. Coal Fuel NOx Scheme A The first mechanism assumes that all char converts to , which is then converted partially to [449] (p. 799). The reaction pathway is described as follows:

With the first scheme, all char-bound nitrogen converts to

. Thus, (14.51) (14.52)

where = char burnout rate (kg/s) = mass fraction of nitrogen in char = cell volume (m3)

14.1.5.3.3.3. Coal Fuel NOx Scheme B The second mechanism assumes that all char pathway is described as follows:

332

converts to

directly [284] (p. 790). The reaction

NOx Formation

According to Lockwood [284] (p. 790), the char nitrogen is released to the gas phase as directly, mainly as a desorption product from oxidized char nitrogen atoms. If this approach is followed, then (14.53) (14.54)

14.1.5.3.3.4. HCN Scheme Selection The second mechanism tends to produce more it is difficult to say which one outperforms the other.

emission than the first. In general, however,

The source terms for the transport equations are (14.55) (14.56) The source contributions , heterogeneous reaction source, , need to be considered.

, , and are described previously. Therefore, only the , the char source, , and the production source,

14.1.5.3.3.5. NOx Reduction on Char Surface The heterogeneous reaction of following [266] (p. 789):

reduction on the char surface has been modeled according to the (14.57)

where = rate of

reduction (

= mean

partial pressure (

) )

= 142737.485 = 230 = mean temperature ( ) The partial pressure

is calculated using Dalton’s law: Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

333

Pollutant Formation

The rate of

consumption due to reaction 3 will then be

where = BET surface area (

)

= concentration of particles ( =

consumption (

) )

14.1.5.3.3.5.1. BET Surface Area The heterogeneous reaction involving char is mainly an adsorption process, whose rate is directly proportional to the pore surface area. The pore surface area is also known as the BET surface area, due to the researchers who pioneered the adsorption theory (Brunauer, Emmett, and Teller [62] (p. 778)). For commercial adsorbents, the pore (BET) surface areas range from 100,000 to 2 million square meters per kilogram, depending on the microscopic structure. For coal, the BET area is typically 25,000 , which is used as the default in ANSYS Fluent. The overall source of ( ) is a combination of volatile contribution ( ) and char contribution ( ):

14.1.5.3.3.5.2. HCN from Volatiles The source of

from the volatiles is related to the rate of volatile release:

where = source of volatiles originating from the coal particles into the gas phase (kg/s) = mass fraction of nitrogen in the volatiles = cell volume (

)

Calculation of sources related to char-bound nitrogen depends on the fuel

scheme selection.

14.1.5.3.3.6. Coal Fuel NOx Scheme C The first mechanism assumes that all char converts to [449] (p. 799). The reaction pathway is described as follows:

334

which is then converted partially to

NOx Formation

In this scheme, all char-bound nitrogen converts to

. Thus, (14.58) (14.59)

where = char burnout rate (kg/s) = mass fraction of nitrogen in char = cell volume (

)

14.1.5.3.3.7. Coal Fuel NOx Scheme D The second mechanism assumes that all char pathway is described as follows:

converts to

directly [284] (p. 790). The reaction

According to Lockwood [284] (p. 790), the char nitrogen is released to the gas phase as directly, mainly as a desorption product from oxidized char nitrogen atoms. If this approach is followed, then (14.60) (14.61)

335

Pollutant Formation

14.1.5.3.3.8. NH3 Scheme Selection The second mechanism tends to produce more it is difficult to say which one outperforms the other.

emission than the first. In general, however,

The source terms for the transport equations are (14.62) (14.63) The source contributions , , , Therefore, only the production source, The overall production source of tribution ( ):

, , and are described previously. , must be considered.

is a combination of volatile contribution (

), and char con(14.64)

14.1.5.3.3.8.1. NH3 from Volatiles The source of

from the volatiles is related to the rate of volatile release:

where = source of volatiles originating from the coal particles into the gas phase (kg/s) = mass fraction of nitrogen in the volatiles = cell volume (m3) Calculation of sources related to char-bound nitrogen depends on the fuel

scheme selection.

14.1.5.3.4. Fuel Nitrogen Partitioning for HCN and NH3 Intermediates In certain cases, especially when the fuel is a solid, both and can be generated as intermediates at high enough temperatures [341] (p. 794). In particular, low-ranking (lignite) coal has been shown to produce 10 times more compared to the level of , whereas higher-ranking (bituminous) coal has been shown to produce only [340] (p. 793). Studies by Winter et al. [532] (p. 804) have shown that for bituminous coal, using an / partition ratio of 9:1 gave better predictions when compared to measurements than specifying only a single intermediate species. Liu and Gibbs [283] (p. 790) work with woody-biomass (pine wood chips), on the other hand, has suggested an / ratio of 1:9 due to the younger age of the fuel. In total, the above work suggests the importance of being able to specify that portions of the fuel nitrogen will be converted to both and intermediates at the same time. In ANSYS Fluent, fuel nitrogen partitioning can be used whenever or are intermediates for production, though it is mainly applicable to solid fuels such as coal and biomass. The reaction pathways and source terms for and are described in previous sections.

336

NOx Formation

14.1.6. NOx Formation from Intermediate N2O Melte and Pratt [309] (p. 792) proposed the first intermediate mechanism for formation from molecular nitrogen ( ) via nitrous oxide ( ). Nitrogen enters combustion systems mainly as a component of the combustion and dilution air. Under favorable conditions, which are elevated pressures and oxygenrich conditions, this intermediate mechanism can contribute as much as 90% of the formed during combustion. This makes it particularly important in equipment such as gas turbines and compressionignition engines. Because these devices are operated at increasingly low temperatures to prevent formation via the thermal mechanism, the relative importance of the -intermediate mechanism is increasing. It has been observed that about 30% of the formed in these systems can be attributed to the -intermediate mechanism. The -intermediate mechanism may also be of importance in systems operated in flameless mode (for example, diluted combustion, flameless combustion, flameless oxidation, and FLOX systems). In a flameless mode, fuel and oxygen are highly diluted in inert gases so that the combustion reactions and resulting heat release are carried out in the diffuse zone. As a consequence, elevated peaks of temperature are avoided, which prevents thermal . Research suggests that the -intermediate mechanism may contribute about 90% of the formed in flameless mode, and that the remainder can be attributed to the prompt mechanism. The relevance of formation from has been observed indirectly and theoretically speculated for a number of combustion systems, by a number of researchers [25] (p. 776), [89] (p. 779), [156] (p. 783), [466] (p. 800), [473] (p. 801).

14.1.6.1. N2O - Intermediate NOx Mechanism The simplest form of the mechanism [309] (p. 792) takes into account two reversible elementary reactions: (14.65) (14.66) Here, is a general third body. Because the first reaction involves third bodies, the mechanism is favored at elevated pressures. Both reactions involve the oxygen radical , which makes the mechanism favored for oxygen-rich conditions. While not always justified, it is often assumed that the radical atoms originate solely from the dissociation of molecular oxygen, (14.67) According to the kinetic rate laws, the rate of

formation via the

-intermediate mechanism is (14.68)

To solve Equation 14.68 (p. 337), you will need to have first calculated [ ] and [ It is often assumed that

is at quasi-steady-state (that is,

].

), which implies (14.69)

The system of Equation 14.68 (p. 337) – Equation 14.69 (p. 337) can be solved for the rate of formation when the concentration of , , and , the kinetic rate constants for Equation 14.65 (p. 337) and Equation 14.66 (p. 337), and the equilibrium constant of Equation 14.67 (p. 337) are known. The appearance

337

Pollutant Formation of in Equation 14.66 (p. 337) entails that coupling of the mechanism (and other mechanisms). =

=

=

=

In the above expressions, Equation 14.66 (p. 337), and , and

mechanism with the thermal

are

and and , while

are the forward rate constants of Equation 14.65 (p. 337) and are the corresponding reverse rate constants. The units for has units of

,

.

14.1.7. NOx Reduction by Reburning The design of complex combustion systems for utility boilers, based on air- and fuel-staging technologies, involves many parameters and their mutual interdependence. These parameters include the local stoichiometry, temperature and chemical concentration field, residence time distribution, velocity field, and mixing pattern. A successful application of the in-furnace reduction techniques requires control of these parameters in an optimum manner, so as to avoid impairing the boiler performance. In the mid 1990s, global models describing the kinetics of destruction in the reburn zone of a staged combustion system became available. Two of these models are described below.

14.1.7.1. Instantaneous Approach The instantaneous reburning mechanism is a pathway whereby is subsequently reduced. In general:

reacts with hydrocarbons and (14.70)

Three reburn reactions are modeled by ANSYS Fluent for

: (14.71) (14.72) (14.73)

Important If the temperature is outside of this range,

reburn will not be computed.

The rate constants for these reactions are taken from Bowman [50] (p. 777) and have units of

The

:

depletion rate due to reburn is expressed as (14.74)

and the source term for the reburning mechanism in the

338

transport equation can be calculated as

NOx Formation

(14.75)

Important To calculate the depletion rate due to reburning, ANSYS Fluent will obtain the concentrations of , , and from the species mass fraction results of the combustion calculation. When you use this method, you must be sure to include the species , , and in your problem definition.

14.1.7.2. Partial Equilibrium Approach The partial equilibrium approach is based on the model proposed by Kandamby et al. [214] (p. 786), [11] (p. 775). The model adds a reduction path to De Soete’s global model [102] (p. 780) that describes the formation/destruction mechanism in a pulverized coal flame. The additional reduction path accounts for the destruction in the fuel-rich reburn zone by radicals (see Figure 14.1: De Soete’s Global NOx Mechanism with Additional Reduction Path (p. 339)). Figure 14.1: De Soete’s Global NOx Mechanism with Additional Reduction Path

This model can be used in conjunction with the eddy-dissipation combustion model and does not require the specification of radical concentrations, because they are computed based on the -radical partial equilibrium. The reburn fuel itself can be an equivalent of , , , or . How this equivalent fuel is determined is open for debate, and an approximate guide would be to consider the ratio of the fuel itself. A multiplicative constant of has been developed for the partial equilibrium of radicals to reduce the rates of and in the reburn model. This value was obtained by researchers who developed the model, by way of predicting values for a number of test cases for which experimental data exists.

14.1.7.2.1. NOx Reduction Mechanism In the fuel-rich reburn zone, the oxidation is suppressed and the amount of formed in the primary combustion zone is decreased by the reduction reaction from to . However, the concentration may also decrease due to reactions with radicals, which are available in significant amounts in the reburn zone. The following are considered to be the most important reactions of reduction by radicals: (14.76) Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

339

Pollutant Formation (14.77) (14.78) These reactions may be globally described by the addition of pathways (4) and (5) in Figure 14.1: De Soete’s Global NOx Mechanism with Additional Reduction Path (p. 339), leading respectively to the formation of and of minor intermediate nitrogen radicals. Assuming that methane is the reburning gas, the global reduction rates are then expressed as (14.79) (14.80) where

Therefore, the additional source terms of the are given by

and

transport equations due to reburn reactions (14.81) (14.82)

Certain assumptions are required to evaluate the rate constants , , and and the factors and . For hydrocarbon diffusion flames, the following reaction set can be reasonably considered to be in partial equilibrium: (14.83) (14.84) (14.85) (14.86) Thus, the rate constants may be computed as

where , , and are the rate constants for Equation 14.76 (p. 339) – Equation 14.78 (p. 340). The forward and reverse rate constants for Equation 14.83 (p. 340) – Equation 14.86 (p. 340) are – and – , respectively. In addition, it is assumed that , because the -radical concentration in the postflame region of a hydrocarbon diffusion flame has been observed to be of the same order as [ ]. Finally, the -radical concentration is estimated by considering the reaction (14.87) to be partially equilibrated, leading to the relationship

340

NOx Formation Values for the rate constants form (

,

, and

for different equivalent fuel types are given in Arrhenius

) in Table 14.1: Rate Constants for Different Reburn Fuels (p. 341) [264] (p. 789). All rate

constants have units of

, and all values of

have units of

.

Table 14.1: Rate Constants for Different Reburn Fuels Equivalent Fuel Type

0.0

-1.54 27977

-3.33 15090

-2.64 77077

-1.54 27977

-3.33 15090

-2.64 77077

-1.54 27977

-3.33 15090

-2.64 77077

0.0

-3.33 15090

-2.64 77077

0.0

For Equation 14.87 (p. 340),

14.1.8. NOx Reduction by SNCR The selective noncatalytic reduction of (SNCR), first described by Lyon [295] (p. 791), is a method to reduce the emission of from combustion by injecting a selective reductant such as ammonia (

) or urea (

) into the furnace, where it can react with

in the flue gas to form

.

However, the reductant can be oxidized as well to form . The selectivity for the reductive reactions decreases with increasing temperature [318] (p. 792), while the rate of the initiation reaction simultaneously increases. This limits the SNCR process to a narrow temperature interval, or window, where the lower temperature limit for the interval is determined by the residence time.

14.1.8.1. Ammonia Injection Several investigators have modeled the process using a large number of elementary reactions. A simple empirical model has been proposed by Fenimore [126] (p. 781), which is based on experimental measurements. However, the model was found to be unsuitable for practical applications. Ostberg and DamJohansen [361] (p. 795) proposed a two-step scheme describing the SNCR process as shown in Figure 14.2: Simplified Reaction Mechanism for the SNCR Process (p. 341), which is a single initiation step followed by two parallel reaction pathways: one leading to reduction, and the other to formation. Figure 14.2: Simplified Reaction Mechanism for the SNCR Process

341

Pollutant Formation (14.88) (14.89) The reaction orders of and at 4% volume and the empirical rate constants and for Equation 14.88 (p. 342) Equation 14.89 (p. 342), respectively, have been estimated from work done by Brouwer et al. [59] (p. 778). The reaction order of was found to be 1 for Equation 14.88 (p. 342) and the order of was found to be 1 for both reactions. As such, the following reaction rates for and , at 4% volume , were proposed: (14.90) (14.91) have units of m3/mol-s, and are defined as

The rate constants

and

where

J/mol and

J/mol.

This model has been shown to give reasonable predictions of the SNCR process in pulverized coal and fluidized bed combustion applications. The model also captures the influence of the most significant parameters for SNCR, which are the temperature of the flue gas at the injection position, the residence time in the relevant temperature interval, the to molar ratio, and the effect of combustible additives. This model overestimates the reduction for temperatures above the optimum temperature by an amount similar to that of the detailed kinetic model of Miller and Bowman [318] (p. 792).

Important The SNCR process naturally occurs when is present in the flame as a fuel intermediate. For this reason, even if the SNCR model is not activated and there is no reagent injection, the natural SNCR process may still occur in the flame. The temperature range or “window” at which SNCR may occur is 1073 K < T < 1373 K. If you want to model your case without using the natural SNCR process, contact your support engineer for information on how to deactivate it.

14.1.8.2. Urea Injection Urea as a reagent for the SNCR process is similar to that of injecting ammonia, and has been used in power station combustors to reduce emissions successfully. However, both reagents, ammonia and urea, have major limitations as a reducing agent. The narrow temperature “window” of effectiveness and mixing limitations are difficult factors to handle in a large combustor. The use of urea instead of ammonia as the reducing agent is attractive because of the ease of storage and handling of the reagent. The SNCR process using urea is a combination of Thermal DeNOx (SNCR with ammonia) and RAPRENOx (SNCR using cyanuric acid that, under heating, sublimes and decomposes into isocyanic acid), because urea most probably decomposes into ammonia and isocyanic acid [318] (p. 792). One problem of SNCR processes using urea is that slow decay of , as well as the reaction channels leading to and , can significantly increase the emission of pollutants other than . Urea seems to involve a significant emission of carbon-containing pollutants, such as and .

342

NOx Formation Also, some experimental observations [408] (p. 797) show that SNCR using urea is effective in a narrow temperature window that is shifted toward higher temperatures, when compared to Thermal DeNOx processes at the same value of the ratio of nitrogen in the reducing agent and the in the feed, , where is defined as the ratio of nitrogen in the reducing agent and in the feed. The effect of increasing the value is to increase the efficiency of abatement, while the effect of increasing concentration depends on the temperature considered. The model described here is proposed by Brouwer et al. [59] (p. 778) and is a seven-step reduced kinetic mechanism. Brouwer et al. [59] (p. 778) assumes that the breakdown of urea is instantaneous and 1 mole of urea is assumed to produce 1.1 moles of and 0.9 moles of . The work of Rota et al. [408] (p. 797) proposed a finite rate two-step mechanism for the breakdown of urea into ammonia and . The seven-step reduced mechanism is given in Table 14.2: Seven-Step Reduced Mechanism for SNCR with Urea (p. 343), and the two-step urea breakdown mechanism is given in Table 14.3: Two-Step Urea Breakdown Process (p. 343). Table 14.2: Seven-Step Reduced Mechanism for SNCR with Urea Reaction

A

b

E

4.24E+02 5.30 349937.06 3.500E-01 7.65 524487.005 2.400E+08 0.85 284637.8 1.000E+07 0.00 -1632.4815 1.000E+07 0.00

2.000E+06 0.00 41858.5 6.900E+17 -2.5 271075.646 Table 14.3: Two-Step Urea Breakdown Process Reaction

A

b E

1.27E+04 0 65048.109 6.13E+04 0 87819.133 where SI units (m, mol, sec, J) are used in Table 14.2: Seven-Step Reduced Mechanism for SNCR with Urea (p. 343) and Table 14.3: Two-Step Urea Breakdown Process (p. 343).

14.1.8.3. Transport Equations for Urea, HNCO, and NCO When the SNCR model with urea injection is employed in addition to the usual transport equations, ANSYS Fluent solves the following three additional mass transport equations for the urea, , and species. (14.92) (14.93) (14.94) Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

343

Pollutant Formation where

,

and

source terms

,

are mass fractions of urea, , and

, and

in the gas phase. The

are determined according to the rate equations given in

Table 14.2: Seven-Step Reduced Mechanism for SNCR with Urea (p. 343) and Table 14.3: Two-Step Urea Breakdown Process (p. 343) and the additional source terms due to reagent injection. These additional source terms are determined next. The source terms in the transport equations can be written as follows: (14.95) (14.96) (14.97) Apart from the source terms for the above three species, additional source terms for , , and are also determined as follows, which should be added to the previously calculated sources due to fuel : (14.98) (14.99) (14.100) The source terms for species are determined from the rate equations given in Table 14.2: Seven-Step Reduced Mechanism for SNCR with Urea (p. 343) and Table 14.3: Two-Step Urea Breakdown Process (p. 343).

14.1.8.4. Urea Production due to Reagent Injection The rate of urea production is equivalent to the rate of reagent release into the gas phase through droplet evaporation: (14.101) where

is the rate of reagent release from the liquid droplets to the gas phase (kg/s) and

is

3

the cell volume (m ).

14.1.8.5. NH3 Production due to Reagent Injection If the urea decomposition model is set to the user-specified option, then the rate of production is proportional to the rate of reagent release into the gas phase through droplet evaporation: (14.102) where is the rate of reagent release from the liquid droplets to the gas phase (kg/s), is the mole fraction of in the / mixture created from urea decomposition, and

is the

3

cell volume (m ).

14.1.8.6. HNCO Production due to Reagent Injection If the urea decomposition model is set to the user-specified option, then the rate of production is proportional to the rate of reagent release into the gas phase through droplet evaporation: (14.103)

344

NOx Formation where , the injection source term, is the rate of reagent release from the liquid droplets to the gas phase (kg/s), is the mole fraction of in the / mixture created from urea decomposition, and

is the cell volume (m3).

Important The mole conversion fractions (MCF) for species and are determined through the user species values such that if one mole of urea decomposes into 1.1 moles of and 0.9 moles of , then = 0.55 and = 0.45. When the user-specified option is used for urea decomposition, then . However, the default option for urea decomposition is through rate limiting reactions given in Table 14.3: Two-Step Urea Breakdown Process (p. 343), and the source terms are calculated accordingly. In this case, both values of and are zero.

14.1.9. NOx Formation in Turbulent Flows The kinetic mechanisms of formation and destruction described in the preceding sections have all been obtained from laboratory experiments using either a laminar premixed flame or shock-tube studies where molecular diffusion conditions are well defined. In any practical combustion system, however, the flow is highly turbulent. The turbulent mixing process results in temporal fluctuations in temperature and species concentration that will influence the characteristics of the flame. The relationships among formation rate, temperature, and species concentration are highly nonlinear. Hence, if time-averaged composition and temperature are employed in any model to predict the mean formation rate, significant errors will result. Temperature and composition fluctuations must be taken into account by considering the probability density functions that describe the time variation.

14.1.9.1. The Turbulence-Chemistry Interaction Model In turbulent combustion calculations, ANSYS Fluent solves the density-weighted time-averaged NavierStokes equations for temperature, velocity, and species concentrations or mean mixture fraction and variance. To calculate concentration, a time-averaged formation rate must be computed at each point in the domain using the averaged flow-field information. Methods of modeling the mean turbulent reaction rate can be based on either moment methods [531] (p. 804) or probability density function (PDF) techniques [206] (p. 786). ANSYS Fluent uses the PDF approach.

Important The PDF method described here applies to the transport equations only. The preceding combustion simulation can use either the generalized finite-rate chemistry model by Magnussen and Hjertager, the non-premixed or partially premixed combustion model. For details on these models, refer to Species Transport and Finite-Rate Chemistry (p. 193), Non-Premixed Combustion (p. 223), and Partially Premixed Combustion (p. 279).

345

Pollutant Formation

14.1.9.2. The PDF Approach The PDF method has proven very useful in the theoretical description of turbulent flow [207] (p. 786). In the ANSYS Fluent model, a single- or joint-variable PDF in terms of a normalized temperature, species mass fraction, or the combination of both is used to predict the emission. If the non-premixed or partially premixed combustion model is used to model combustion, then a one- or two-variable PDF in terms of mixture fraction(s) is also available. The mean values of the independent variables needed for the PDF construction are obtained from the solution of the transport equations.

14.1.9.3. The General Expression for the Mean Reaction Rate The mean turbulent reaction rate can be described in terms of the instantaneous rate or joint PDF of various variables. In general,

and a single (14.104)

where ,... are temperature and/or the various species concentrations present. density function (PDF).

is the probability

14.1.9.4. The Mean Reaction Rate Used in ANSYS Fluent The PDF is used for weighting against the instantaneous rates of production of (for example, Equation 14.15 (p. 324)) and subsequent integration over suitable ranges to obtain the mean turbulent reaction rate. Hence we have (14.105) or, for two variables (14.106) where

is the mean turbulent rate of production of

,

production, is the instantaneous density, and and and, if relevant, . The same treatment applies for the or

is the instantaneous molar rate of are the PDFs of the variables source terms.

Equation 14.105 (p. 346) or Equation 14.106 (p. 346) must be integrated at every node and at every iteration. For a PDF in terms of temperature, the limits of integration are determined from the minimum and maximum values of temperature in the combustion solution (note that you have several options for how the maximum temperature is calculated, as described in Setting Turbulence Parameters in the User's Guide). For a PDF in terms of mixture fraction, the limits of the integrations in Equation 14.105 (p. 346) or Equation 14.106 (p. 346) are determined from the values stored in the look-up tables.

14.1.9.5. Statistical Independence In the case of the two-variable PDF, it is further assumed that the variables independent, so that

and

are statistically

can be expressed as (14.107)

346

NOx Formation

14.1.9.6. The Beta PDF Option ANSYS Fluent can assume to be a two-moment beta function that is appropriate for combustion calculations [173] (p. 784), [322] (p. 792). The equation for the beta function is (14.108)

where is the Gamma function, and , and its variance, :

and

depend on the mean value of the quantity in question, (14.109) (14.110)

The beta function requires that the independent variable assumes values between 0 and 1. Thus, field variables such as temperature must be normalized. See Setting Turbulence Parameters in the User's Guide for information on using the beta PDF when using single-mixture fraction models and two-mixture fraction models.

14.1.9.7. The Gaussian PDF Option ANSYS Fluent can also assume

to exhibit a clipped Gaussian form with delta functions at the tails.

The cumulative density function for a Gaussian PDF ( function as follows:

) may be expressed in terms of the error (14.111)

where values of function (

is the error function, is the quantity in question, and and are the mean and variance , respectively. The error function may be expressed in terms of the incomplete gamma ): (14.112)

14.1.9.8. The Calculation Method for the Variance The variance, , can be computed by solving the following transport equation during the combustion calculation or pollutant postprocessing stage: (14.113) where the constants

,

, and

take the values 0.85, 2.86, and 2.0, respectively.

Note that the previous equation may only be solved for temperature. This solution may be computationally intensive, and therefore may not always be applicable for a postprocessing treatment of prediction. When this is the case or when solving for species, the calculation of is instead based on an approximate form of the variance transport equation (also referred to as the algebraic form). The approximate form assumes equal production and dissipation of variance, and is as follows: Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

347

Pollutant Formation

(14.114) The term in the brackets is the dissipation rate of the independent variable. For a PDF in terms of mixture fraction, the mixture fraction variance has already been solved as part of the basic combustion calculation, so no additional calculation for is required.

14.2. SOx Formation The following sections include information on the theory used in the model. For information about using the models in ANSYS Fluent, see Using the SOx Model in the User's Guide. 14.2.1. Overview 14.2.2. Governing Equations for SOx Transport 14.2.3. Reaction Mechanisms for Sulfur Oxidation 14.2.4. SO2 and H2S Production in a Gaseous Fuel 14.2.5. SO2 and H2S Production in a Liquid Fuel 14.2.6. SO2 and H2S Production from Coal 14.2.7. SOx Formation in Turbulent Flows

14.2.1. Overview Sulfur exists in coal as organic sulfur, pyretic, and sulfates [1] (p. 775), and exists in liquid fuels mostly in organic form [327] (p. 793), with mass fractions ranging from 0.5% to 3%. All emissions are produced because of the oxidation of fuel-bound sulfur. During the combustion process, fuel sulfur is oxidized to and . A portion of the gaseous will condense on the particles, attaching an amount of water and therefore forming sulfuric acid, or may react further to form sulfates. While emissions are the main cause of acid rain, also contributes to particulate emissions, and is responsible for corrosion of combustion equipment. Furthermore, there is a growing interest in the interaction of sulfur species with the nitrogen oxide chemistry [327] (p. 793), as levels are affected by the presence of sulfur species. The evidence to date indicates that thermal levels (Thermal NOx Formation (p. 321)) are reduced in the presence of . However, the effect of sulfur compounds on the fuel formation is yet to be clarified. Sulfur emissions are regulated from stationary sources and from automotive fuels. Sulfur pollutants can be captured during the combustion process or with after treatment methods, such as wet or dry scrubbing. Coal fired boilers are by far the biggest single emissions source, accounting for over 50% of total emissions [78] (p. 779). For higher sulfur concentrations in the fuel, the concentration field should be resolved together with the main combustion calculation using any of the ANSYS Fluent reaction models. For cases where the sulfur fraction in the fuel is low, the postprocessing option can be used, which solves transport equations for , , , , and .

14.2.1.1. The Formation of SOx The

model incorporates the following stages:

1. Sulfur release from the fuel For liquid fuels, one can conveniently assume that sulfur is released as S [327] (p. 793). However, the process is more complicated in the case of coal; here, some of the sulfur is decomposed into 348

SOx Formation the gas phase during devolatilization as , , , and , while part of the sulfur is retained in the char to be oxidized at a later stage. The percentage of sulfur retained in char is rank dependent [1] (p. 775). 2. Sulfur reaction in the gas phase In oxygen rich flames, the predominant sulfur species are , , and . At lower oxygen concentrations, , , and are also present in significant proportions, while becomes negligible [327] (p. 793). In PCGC-3, as well as in the works of Norman et al. [346] (p. 794), the gas phase sulfur species are assumed to be in equilibrium. 3. Sulfur retention in sorbents Sulfur pollutants can be absorbed by sorbent particles, injected either in situ, or in the post-flame region. For low sulfur fuels, we can assume that sulfur is mainly released as . The rate of release can be determined similarly to that of fuel-bound . For the char it can be assumed that is produced directly at the same rate as that of char burnout. Transport equations for , , , , and species are incorporated and an appropriate reaction set has been developed, as described in the ensuing sections.

14.2.2. Governing Equations for SOx Transport ANSYS Fluent solves the mass transport equations for the species, taking into account convection, diffusion, production, and consumption of and related species. This approach is completely general, being derived from the fundamental principle of mass conservation. The effect of residence time in mechanisms, a Lagrangian reference frame concept, is included through the convection terms in the governing equations written in the Eulerian reference frame. If all fuel sulfur is assumed to convert directly to and the other product and intermediate species are assumed negligible, then only the species transport equation is needed: (14.115) As described in Reaction Mechanisms for Sulfur Oxidation (p. 350), formation mechanisms involve multiple reactions among multiple species, and tracking sulfur-containing intermediate species is important. ANSYS Fluent solves transport equations for the , , , and species in addition to the species: (14.116) (14.117) (14.118) (14.119) where , , , , and are mass fractions of , , , , and in the gas phase. The source terms , , , , and are to be determined depending on the form of fuel sulfur release ( and/or ) and inclusion of , , and in the mechanism.

349

Pollutant Formation

14.2.3. Reaction Mechanisms for Sulfur Oxidation A detailed reaction mechanism for sulfur oxidation has been proposed by Kramlich [239] (p. 788). The mechanism consists of 20 reversible reactions and includes 12 species ( , , , , , , , , , , , and ). The mechanism has been reduced to 8 steps and 10 species (with and removed), and validated in Perfectly Stirred Reactor (PSR) and Plug Flow Reactor (PFR) simulations. Table 14.4: Eight-Step Reduced Mechanism (p. 350) (for a rate constant ) lists the reduced mechanism with the modified rate constants. For reduction calculations, and concentrations have been calculated through partial equilibrium assumptions based on and concentrations, respectively. was used as the dilutant. Since each reaction of the eight-step reduced mechanism is reversible, for each adjacent pair of reactions given in Table 14.4: Eight-Step Reduced Mechanism (p. 350), the second reaction is in fact the reverse reaction of the first. The reduced mechanism given in Table 14.4: Eight-Step Reduced Mechanism (p. 350) closely follows the concentration levels, but slightly over-predicts the concentrations at temperatures below 1500 K. Above 1500 K, both mechanisms are in close agreement for and concentration predictions. However, and are not well correlated by the reduced mechanism when compared against the predictions using the original detailed mechanism. A major concern in these mechanisms is the presence of radicals and the method by which to calculate its concentration in the flow field. At present, the concentration of radicals is assumed to be proportional to the radical concentration, which can be evaluated from one of the existing methods in ANSYS Fluent; viz. Partial Equilibrium (Method 2: Partial Equilibrium Approach (p. 323)) or Equilibrium (Method 1: Equilibrium Approach (p. 322)). You are then given the option of varying the proportionality constant. Although this assumption is open to debate, the lack of simple relation to calculate the radical concentration in a flame has prompted the present choice. The present implementation allows you to either include or remove from the calculations. Also, depending on the form of fuel sulfur release (for example, or ) the species may or may not be present for the calculation. You are also given the extended option of partitioning the intermediate fuel sulfur species to and . However, there is no literature to guide you on how to select a correct partition fraction. Table 14.4: Eight-Step Reduced Mechanism Reaction

350

A

b

E

1.819702E+07

0.0E+00

7.484300E+03

9.375623E+06

0.0E+00

6.253660E+04

1.380385E+02

0.0E+00

3.742150E+03

3.104557E+07

0.0E+00

1.218543E+05

1.621810E+08

0.0E+00

2.565926E+03

7.691299E+09

0.0E+00

1.187023E+05

3.548135E+08

0.0E+00

2.687316E+03

2.985385E+09

0.0E+00

1.694600E+05

4.365162E+03

0.0E+00

1.380493E+04

SOx Formation Reaction

A is in

A

b

E

9.885528E+08

0.0E+00

6.035996E+04

4.466832E+05

0.0E+00

2.703222E+04

1.663412E+06

0.0E+00

7.613643E+04

1.096478E+03

0.0E+00

0.000000E+00

8.669613E+14

0.0E+00

3.819463E+05

8.709647E+09 k

-1.8E+00

0.000000E+00

1.905464E+14

0.0E+00

5.207365E+05

, E is J/mol (assumed 1 cal = 4.18585 J), A units for the thirteenth reaction is , and A units for the fifteenth reaction is

.

In addition, the following two reactions were included in ANSYS Fluent to complete the with the rate constants taken from Hunter’s work [191] (p. 785).

mechanism, (14.120)

= argon, nitrogen, oxygen =

x

exp(+4185.85/RT)

where R = 8.313 J/mol-K =

x

exp(-346123.75/RT)

. (14.121)

=

x

exp(-39765.575/RT)

The reverse rate of Equation 14.121 (p. 351) was determined through the equilibrium constant for that equation.

14.2.4. SO2 and H2S Production in a Gaseous Fuel The rate of

or

production is equivalent to the rate of combustion of the fuel: (14.122)

where = source of (

), where

or

= mean limiting reaction rate of the fuel (kg/m3–s) = mass fraction of sulfur in the fuel The mean limiting reaction rate of the fuel, , is calculated from the Magnussen combustion model, so the gaseous fuel option for formation is available only when the generalized finite-rate model is used. Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

351

Pollutant Formation

14.2.5. SO2 and H2S Production in a Liquid Fuel The rate of or droplet evaporation:

production is equivalent to the rate of fuel release into the gas phase through (14.123)

where = source of (kg/m3–s), where

or

= rate of fuel release from the liquid droplets to the gas (kg/s) = mass fraction of sulfur in the fuel = cell volume (m3)

14.2.6. SO2 and H2S Production from Coal For coal, it is assumed that sulfur is distributed between the volatiles and the char. Since there is no reason to assume that is equally distributed between the volatiles and the char, the fraction of in the volatiles and the char should be specified separately.

14.2.6.1. SO2 and H2S from Char The source of

and

from the char is related to the rate of char combustion: (14.124)

where = char burnout rate (kg/s) = source of (kg/m3–s) in char, where

or

= mass fraction of sulfur in char = cell volume (m3)

14.2.6.2. SO2 and H2S from Volatiles The source of

and

from the volatiles is related to the rate of volatile release: (14.125)

where = source of volatiles originating from the coal particles into the gas phase (kg/s), where or = mass fraction of sulfur in the volatiles = cell volume (m3)

352

Soot Formation

14.2.7. SOx Formation in Turbulent Flows The kinetic mechanisms of formation and destruction are obtained from laboratory experiments in a similar fashion to the model. In any practical combustion system, however, the flow is highly turbulent. The turbulent mixing process results in temporal fluctuations in temperature and species concentration that will influence the characteristics of the flame. The relationships among formation rate, temperature, and species concentration are highly nonlinear. Hence, if time-averaged composition and temperature are employed in any model to predict the mean formation rate, significant errors will result. Temperature and composition fluctuations must be taken into account by considering the probability density functions that describe the time variation.

14.2.7.1. The Turbulence-Chemistry Interaction Model In turbulent combustion calculations, ANSYS Fluent solves the density-weighted time-averaged NavierStokes equations for temperature, velocity, and species concentrations or mean mixture fraction and variance. To calculate concentration, a time-averaged formation rate must be computed at each point in the domain using the averaged flow-field information.

14.2.7.2. The PDF Approach The PDF method has proven very useful in the theoretical description of turbulent flow [207] (p. 786). In the ANSYS Fluent model, a single- or joint-variable PDF in terms of a normalized temperature, species mass fraction, or the combination of both is used to predict the emission. If the non-premixed combustion model is used to model combustion, then a one- or two-variable PDF in terms of mixture fraction(s) is also available. The mean values of the independent variables needed for the PDF construction are obtained from the solution of the transport equations.

14.2.7.3. The Mean Reaction Rate The mean turbulent reaction rate described in The General Expression for the Mean Reaction Rate (p. 346) for the model also applies to the model. The PDF is used for weighting against the instantaneous rates of production of and subsequent integration over suitable ranges to obtain the mean turbulent reaction rate, as described in Equation 14.105 (p. 346) and Equation 14.106 (p. 346) for .

14.2.7.4. The PDF Options As is the case with the model, can be calculated as either a two-moment beta function or as a clipped Gaussian function, as appropriate for combustion calculations [173] (p. 784), [322] (p. 792). Equation 14.108 (p. 347) – Equation 14.112 (p. 347) apply to the model as well, with the variance computed by solving a transport equation during the combustion calculation stage, using Equation 14.113 (p. 347) or Equation 14.114 (p. 348).

14.3. Soot Formation Information about the theory behind soot formation is presented in the following sections. For information about using soot formation models in ANSYS Fluent, see Using the Soot Models in the User's Guide. 14.3.1. Overview and Limitations 14.3.2. Soot Model Theory

353

Pollutant Formation

14.3.1. Overview and Limitations ANSYS Fluent provides five models for the prediction of soot formation in combustion systems. In addition, the predicted soot concentration can be coupled with radiation absorption when you use the P1, discrete ordinates, or discrete transfer radiation model with a variable absorption coefficient.

14.3.1.1. Predicting Soot Formation ANSYS Fluent predicts soot concentrations in a combustion system using one of the following available models: • the one-step Khan and Greeves model [224] (p. 787), in which ANSYS Fluent predicts the rate of soot formation based on a simple empirical rate • the two-step Tesner model [300] (p. 791), [482] (p. 801), in which ANSYS Fluent predicts the formation of nuclei particles, with soot formation on the nuclei • the Moss-Brookes model [58] (p. 778), in which ANSYS Fluent predicts soot formation for methane flames (and higher hydrocarbon species, if appropriately modified) by solving transport equations for normalized radical nuclei concentration and the soot mass fraction • the Moss-Brookes-Hall model [170] (p. 784), which is an extension of the Moss-Brookes model and is applicable for higher hydrocarbon fuels (for example, kerosene) • the Method of Moments model [144] (p. 782), in which ANSYS Fluent predicts soot formation based on the soot particle population balance methodology. The Khan and Greeves model is the default model used by ANSYS Fluent when you include soot formation. In the Khan and Greeves model and the Tesner model, combustion of the soot (and particle nuclei) is assumed to be governed by the Magnussen combustion rate [300] (p. 791). Note that this limits the use of these soot formation models to turbulent flows. Both models are empirically-based, approximate models of the soot formation process in combustion systems. The detailed chemistry and physics of soot formation are quite complex and are only approximated in these models. You should view the results of the Khan and Greeves model and the Tesner model as qualitative indicators of your system performance, unless you can undertake experimental validation of the results. The Moss-Brookes model has less empiricism and should theoretically provide superior accuracy than the Khan and Greeves and Tesner models. The Hall extension provides further options for modeling higher hydrocarbon fuels. Note that the Moss-Brookes-Hall model is only available when the required species are present in the gas phase species list. The Method of Moments considers a soot size distribution where the diameters of the soot particles are dynamically evolving. This approach uses fewer empirical constants for modeling of various soot formation sub-processes, such as nucleation, coagulation, and the surface growth kernels.

14.3.1.2. Restrictions on Soot Modeling The following restrictions apply to soot formation models: • You must use the pressure-based solver. The soot models are not available with either of the density-based solvers.

354

Soot Formation • The Khan and Greeves model and the Tesner model can model soot formation only for turbulent flows (whereas the Moss-Brookes model, the Moss-Brookes-Hall model, and the Method of Moments model can be used with both laminar and turbulent flows). • The soot model cannot be used in conjunction with the premixed combustion model.

14.3.2. Soot Model Theory 14.3.2.1. The One-Step Soot Formation Model In the one-step Khan and Greeves model [224] (p. 787), ANSYS Fluent solves a single transport equation for the soot mass fraction: (14.126) where = soot mass fraction = turbulent Prandtl number for soot transport = net rate of soot generation (kg/m3–s) , the net rate of soot generation, is the balance of soot formation, :

, and soot combustion, (14.127)

The rate of soot formation is given by a simple empirical rate expression: (14.128) where = soot formation constant (kg/N-m-s) = fuel partial pressure (Pa) = equivalence ratio = equivalence ratio exponent = activation temperature (K) The rate of soot combustion is the minimum of two rate expressions [300] (p. 791): (14.129) The two rates are computed as (14.130) and (14.131) where = constant in the Magnussen model Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

355

Pollutant Formation ,

= mass fractions of oxidizer and fuel ,

= mass stoichiometries for soot and fuel combustion

The default constants for the one-step model are valid for a wide range of hydrocarbon fuels.

14.3.2.2. The Two-Step Soot Formation Model The two-step Tesner model [482] (p. 801) predicts the generation of radical nuclei and then computes the formation of soot on these nuclei. ANSYS Fluent therefore solves transport equations for two scalar quantities: the soot mass fraction (Equation 14.126 (p. 355)) and the normalized radical nuclei concentration: (14.132) where = normalized radical nuclei concentration (particles

/kg)

= turbulent Prandtl number for nuclei transport = normalized net rate of nuclei generation (particles

/m3–s)

In these transport equations, the rates of nuclei and soot generation are the net rates, involving a balance between formation and combustion.

14.3.2.2.1. Soot Generation Rate The two-step model computes the net rate of soot generation, model, as a balance of soot formation and soot combustion:

, in the same way as the one-step (14.133)

In the two-step model, however, the rate of soot formation, of radical nuclei, :

, depends on the concentration (14.134)

where = mean mass of soot particle (kg/particle) = concentration of soot particles (particles/m3) = radical nuclei concentration =

(particles/m3)

= empirical constant (s-1) = empirical constant (m3/particle-s) The rate of soot combustion, , is computed in the same way as for the one-step model, using Equation 14.129 (p. 355) – Equation 14.131 (p. 355). The default constants for the two-step model are applicable for the combustion of acetylene ( ). According to Ahmad et al. [4] (p. 775), these values should be modified for other fuels, as the sooting characteristics of acetylene are known to be different from those of saturated hydrocarbon fuels.

356

Soot Formation

14.3.2.2.2. Nuclei Generation Rate The net rate of nuclei generation in the two-step model is given by the balance of the nuclei formation rate and the nuclei combustion rate: (14.135) where /m3-s)

= rate of nuclei formation (particles

/m3-s)

= rate of nuclei combustion (particles The rate of nuclei formation, described by

, depends on a spontaneous formation and branching process, (14.136) (14.137)

where = normalized nuclei concentration (

)

= = pre-exponential rate constant (particles/kg-s) = fuel concentration (kg/m3) = linear branching

termination coefficient (s-1)

= linear termination on soot particles (m3/particle-s) Note that the branching term in Equation 14.136 (p. 357), rate,

, is greater than the limiting formation rate (

, is included only when the kinetic 3

particles/m -s, by default).

The rate of nuclei combustion is assumed to be proportional to the rate of soot combustion: (14.138) where the soot combustion rate,

, is given by Equation 14.129 (p. 355).

14.3.2.3. The Moss-Brookes Model The Moss-Brookes model solves transport equations for normalized radical nuclei concentration and soot mass fraction : (14.139) (14.140) where = soot mass fraction

357

Pollutant Formation = soot mass concentration (kg/m3) = normalized radical nuclei concentration (particles

/kg) =

= soot particle number density (particles/m3) =

particles

The instantaneous production rate of soot particles, subject to nucleation from the gas phase and coagulation in the free molecular regime, is given by (14.141)

where , and are model constants. Here, (= 6.022045 x 1026kmol–1) is the Avogadro number and is the mole fraction of soot precursor (for methane, the precursor is assumed to be acetylene, whereas for kerosene it is a combination of acetylene and benzene). The mass density of soot, , is assumed to be 1800 kg/m3 and is the mean diameter of a soot particle. The nucleation rate for soot particles is taken to be proportional to the local acetylene concentration for methane. The activation temperature for the nucleation reaction is that proposed by Lindstedt [276] (p. 790). The source term for soot mass concentration is modeled by the expression

(14.142)

where , , , , and are additional model constants. The constant (= 144 kg/kmol) is the mass of an incipient soot particle, here taken to consist of 12 carbon atoms. Even though the model is not found to be sensitive to this assumption, a nonzero initial mass is needed to begin the process of surface growth. Here, is the mole fraction of the participating surface growth species. For paraffinic fuels, soot particles have been found to grow primarily by the addition of gaseous species at their surfaces, particularly acetylene that has been found in abundance in the sooting regions of laminar methane diffusion flames. The model assumes that the hydroxyl radical is the dominant oxidizing agent in methane/air diffusion flames and that the surface-specific oxidation rate of soot by the radical may be formulated according to the model proposed by Fenimore and Jones [127] (p. 781). Assuming a collision efficiency ( ) of 0.04, the oxidation rate may be written as Equation 14.142 (p. 358). The process of determination of the exponents , , and are explained in detail by Brookes and Moss [58] (p. 778). The constants and are determined through numerical modeling of a laminar flame for which experimental data exists. The set of constants proposed by Brookes and Moss for methane flames are given below:

358

Soot Formation = 54 s–1 (model constant for soot inception rate) = 21000 K (activation temperature of soot inception) = 1.0 (model constant for coagulation rate) = 11700

. (surface growth rate scaling factor)

= 12100 K (activation temperature of surface growth rate) = 105.8125

(oxidation model constant)

= 0.04 (collisional efficiency parameter) = 0.015 (oxidation rate scaling parameter) Note that the implementation of the Moss-Brookes model in ANSYS Fluent uses the values listed above, except for , which is set to unity by default. The closure for the mean soot source terms in the above equations was also described in detail by Brookes and Moss [58] (p. 778). The uncorrelated closure is the preferred option for a tractable solution of the above transport equations. Moss et al. [333] (p. 793) have shown the above model applied to kerosene flames by modifying only the soot precursor species (in the original model the precursor was acetylene, whereas for kerosene flames the precursor was assumed to be a combination of both acetylene and benzene) and by setting the value of oxidation scaling parameter to unity. A good comparison against the experimental measurements for the lower pressure (7 bar) conditions was observed. The predictions of soot formation within methane flames have shown the Brooks and Moss [58] (p. 778) model to be superior compared with the standard Tesner et al. [482] (p. 801) formulation.

14.3.2.3.1. The Moss-Brookes-Hall Model Since the Moss-Brookes model was mainly developed and validated for methane flames, a further extension for higher hydrocarbon fuels called the Moss-Brookes-Hall model is available in ANSYS Fluent. Here, the extended version is a model reported by Wen et al. [520] (p. 803), based on model extensions proposed by Hall et al. [170] (p. 784) and an oxidation model proposed by Lee et al. [257] (p. 789). The work of Hall [170] (p. 784) is based on a soot inception rate due to two-ringed and three-ringed aromatics, as opposed to the Moss-Brookes assumption of a soot inception due to acetylene or benzene (for higher hydrocarbons). Hall et al. [170] (p. 784) proposed a soot inception rate based on the formation rates of two-ringed and three-ringed aromatics ( and ), from acetylene ( ), benzene ( ), and the phenyl radical ( ) based on the following mechanisms: (14.143) Based on their laminar methane flame data, the inception rate of soot particles was given to be eight times the formation rate of species and , as shown by (14.144)

359

Pollutant Formation where = 127 x s–1, = 178 x s–1, = 4378 K, and = 6390 K as determined by Hall et al. [170] (p. 784). In their model, the mass of an incipient soot particle was assumed to be 1200 kg/kmol (corresponding to 100 carbon atoms, as opposed to 12 carbon atoms used by Brookes and Moss [58] (p. 778)). The mass density of soot was assumed to be 2000 kg/m3, which is also slightly different from the value used by Brookes and Moss [58] (p. 778). Both the coagulation term and the surface growth term were formulated similar to those used by Brookes and Moss [58] (p. 778), with a slight modification to the constant so that the value is 9000.6 (based on the model developed by Lindstedt [277] (p. 790)). For the soot oxidation term, oxidation due to (based on measurements and a model based on Lee et al. [257] (p. 789)) was added, in addition to the soot oxidation due to the hydroxyl radical. By assuming that the kinetics of surface reactions are the limiting mechanism and that the particles are small enough to neglect the diffusion effect on the soot oxidation, they derived the specific rate of soot oxidation by molecular oxygen. Therefore, the full soot oxidation term, including that due to hydroxyl radical, is of the form (14.145)

Here, the collision efficiency is assumed to be 0.13 (compared to the value of 0.04 used by Brookes and Moss) and the oxidation rate scaling parameter is assumed to be unity. The model constants used are as follows: = 105.81

(same as that used by Brookes and Moss)

=8903.51 =19778 K

14.3.2.3.2. Soot Formation in Turbulent Flows The kinetic mechanisms of soot formation and destruction for the Moss-Brookes model and the Hall extension are obtained from laboratory experiments, in a similar fashion to the model. In any practical combustion system, however, the flow is highly turbulent. The turbulent mixing process results in temporal fluctuations in temperature and species concentration that will influence the characteristics of the flame. The relationships among soot formation rate, temperature, and species concentration are highly nonlinear. Hence, if time-averaged composition and temperature are employed in any model to predict the mean soot formation rate, significant errors will result. Temperature and composition fluctuations must be taken into account by considering the probability density functions that describe the time variation.

14.3.2.3.2.1. The Turbulence-Chemistry Interaction Model In turbulent combustion calculations, ANSYS Fluent solves the density-weighted time-averaged NavierStokes equations for temperature, velocity, and species concentrations or mean mixture fraction and variance. To calculate soot concentration for the Moss-Brookes model and the Hall extension, a timeaveraged soot formation rate must be computed at each point in the domain using the averaged flowfield information.

360

Soot Formation

14.3.2.3.2.2. The PDF Approach The PDF method has proven very useful in the theoretical description of turbulent flow [207] (p. 786). In the ANSYS Fluent Moss-Brookes model and the Hall extension, a single- or joint-variable PDF in terms of a normalized temperature, species mass fraction, or the combination of both is used to predict the soot formation. If the non-premixed combustion model is used to model combustion, then a one- or two-variable PDF in terms of mixture fraction(s) is also available. The mean values of the independent variables needed for the PDF construction are obtained from the solution of the transport equations.

14.3.2.3.2.3. The Mean Reaction Rate The mean turbulent reaction rate described in The General Expression for the Mean Reaction Rate (p. 346) for the model also applies to the Moss-Brookes model and the Hall extension. The PDF is used for weighting against the instantaneous rates of production of soot and subsequent integration over suitable ranges to obtain the mean turbulent reaction rate as described in Equation 14.105 (p. 346) and Equation 14.106 (p. 346) for .

14.3.2.3.2.4. The PDF Options As is the case with the model, can be calculated as either a two-moment beta function or as a clipped Gaussian function, as appropriate for combustion calculations [173] (p. 784), [322] (p. 792). Equation 14.108 (p. 347) – Equation 14.112 (p. 347) apply to the Moss-Brookes model and Hall extension as well, with the variance computed by solving a transport equation during the combustion calculation stage, using Equation 14.113 (p. 347) or Equation 14.114 (p. 348).

14.3.2.3.3. The Effect of Soot on the Radiation Absorption Coefficient A description of the modeling of soot-radiation interaction is provided in The Effect of Soot on the Absorption Coefficient (p. 179).

14.3.2.4. The Method of Moments Model To accurately predict soot formation, the detailed kinetic modeling of soot formation and resolution of the particle-size distribution are required. The method of moments, where only few moments are solved, is a computationally efficient approach for modeling soot formation.

14.3.2.4.1. Soot Particle Population Balance During soot formation, the soot particles are present in a wide range of diameters. The particle population balance method uses the particle-size distribution (PSD) for modeling of the soot particle surface area involved in the calculations of important sub steps of soot formation process like soot particles surface growth and oxidation. The various stages of soot formation have an impact on the soot PSD. The evolution of the soot particles’ population can be represented by the population balance equations, also known as the Smoluchowski master equations: (14.146)

361

Pollutant Formation where th

is the particle number density function that denotes the number of the soot particles of the

size class per unit volume,

is the collision coefficient between particles of size classes and .

For the first particle size class (the first equation), the right-hand side contains only one negative source term. This term reflects the reduction in the first size class population due to coagulation between the particles of the current size class and any other particles resulting in formation of particles of higher size classes. For all other higher particle size classes, the positive source term appears in the right-hand side of the second equation indicating the growth of the particle number density function as a result of said coagulation between particles of lower size classes. The principal difficulty in obtaining a solution for Equation 14.146 (p. 361) involves solving an infinite number of particle size classes. The two approaches commonly used for these problems are the sectional method and the method of moments. The sectional method is based on discretization of the entire range of particle size classes into a predefined finite number of intervals. Considering that the particle mass and population will be spread over all intervals and will be also affected by other processes (such as, surface reactions and oxidation), a sufficiently large number of sections (or finite intervals) is required for obtaining an accurate solution. For these reasons, applying the sectional method to Equation 14.146 (p. 361) is computationally demanding. The efficient alternative to the sectional method is the method of moments. This method considers moments of the soot PSD functions described as follows. For the particle of the size class , its mass

can be expressed as:

where is the mass of a monomer, that is, the smallest mass that can be added or removed from the soot particles. is the mass of a single carbon atom. The concentration moment of the particle number density function is written as: (14.147) For =0 and =1, the first and second moments are:

The soot volume fraction

.

Multiplying Equation 14.146 (p. 361) by the soot particle mass tion 14.147 (p. 362), we obtain:

and using the expression of Equa-

(14.148)

362

Soot Formation where

is the right-hand side of Equation 14.146 (p. 361), which is a coagulation term.

In soot formation, in addition to the coagulation process, the nucleation and the surface reactions are the two other important contributing factors. In the presence of nucleation and surface reactions, Equation 14.148 (p. 362) is written as (14.149) where is the nucleation source term, and surface growth and oxidation.

is the source term due to surface reactions including

14.3.2.4.2. Moment Transport Equations Equation 14.149 (p. 363) describes the change in the moments due to various sub-processes at a single point. In a flow system, the moments will also be affected by convection and diffusion. For flow systems, the transport equation for the moments of soot concentration can be written as: (14.150) where = -th moment of soot size distribution = effective diffusion coefficient = turbulent Prandtl number for moment transport equation = source term in the moment transport computed using Equation 14.149 (p. 363) The source term for computation of

is described in the sections that follow.

14.3.2.4.3. Nucleation In ANSYS Fluent, the nucleation process is modeled as coagulation between two soot precursor species. The soot precursor is a user-defined gas phase species. Typically, the soot precursors are poly-cyclic aromatic hydrocarbons (PAH), and the soot nuclei formation is modeled as coagulation of two PAH molecules. The mean diameter of the PAH is calculated from the number of carbon atoms in it and the soot density. The concentration of the PAH is obtained from the gas phase mechanism. In many practical cases, where the chemical mechanism used in simulations is small and does not include PAH species, the smaller species, such as C2H2, which is a building block of PAH molecules, can be used as a precursor species. The nucleation source term for the first moment is calculated as: (14.151) where

is the molar concentration of the precursor species, and

is the constant calculated by: (14.152)

where = 2.2 = van der Waals enhancement factor = Boltzmann constant Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

363

Pollutant Formation = diameter of precursor species = number of carbon atoms in a precursor molecule = mass of a carbon atom (12 amu) = Avogadro number The nucleation rates calculated based on the kinetic theory using Equation 14.151 (p. 363) are generally very large. To properly scale them, Equation 14.151 (p. 363) is adjusted using a sticking coefficient : (14.153) The value of the sticking coefficient varies with the size of the precursor species. Table 14.5: Sticking Coefficient for Different PAH Species (p. 364) lists the sticking coefficient values for different precursors as proposed by Blanquart and Pitsch [43] (p. 777). Table 14.5: Sticking Coefficient for Different PAH Species Species Name

Formula

naphthalene

C10H8

128

0.0010

acenaphthylene

C12H8

152

0.0030

biphenyl

C12H10

154

0.0085

phenathrene

C14H10

178

0.0150

acephenanthrylene

C16H10

202

0.0250

pyrene

C16H10

202

0.0250

fluoranthene

C16H10

202

0.0250

cyclo[cd]pyrene

C18H10

226

0.0390

Other suggestions found in the literature propose using the sticking coefficient that is proportional to the fourth power of the precursor size: (14.154) where is the constant that can be either calculated using curve fitting from Table 14.5: Sticking Coefficient for Different PAH Species (p. 364) or obtained from experimental data. Equation 14.154 (p. 364) could be used to approximate the sticking coefficient values for precursors that are not listed in Table 14.5: Sticking Coefficient for Different PAH Species (p. 364). Alternatively, the nucleation can also be specified as an irreversible kinetic reaction between two precursors: (14.155) (14.156) where

and

are the power exponent and stoichiometric coefficients of the precursor species.

Equation 14.156 (p. 364) is analogues to Equation 14.151 (p. 363) having the same PAH species and with the following values for the constants:

364

Soot Formation

The nucleation source terms for higher moments are calculated from the lower moment source terms using the following expression: (14.157)

14.3.2.4.4. Coagulation Once formed, the soot particles collide with each other affecting the size distribution of the soot particles population. The process of coagulation assumes that the resulting particle remains a sphere with an increased diameter. The coagulation process changes the number density but not the total mass of the particle. The source terms in the moment transport equation due to coagulation are calculated as: (14.158) The coagulation term for the second size class the total mass of soot formed.

is equal to 0 because the coagulation does not change

The coagulation terms for higher size classes are calculated by: (14.159)

where

is the collision efficiency, which is dependent on the coagulation regimes.

The coagulation process can take place in different regimes: • continuum • free molecular • intermediate of the two (or transition) The regimes of coagulation depends on the Knudsen number (14.160) where the gas mixture mean free path, and soot particles are calculated as follows:

is the soot particle diameter. The mean free path of

(14.161) where is the Boltzmann constant, calculated by

is the pressure, and

is the diameter of the gas molecules

(14.162)

365

Pollutant Formation where

is the molecular weight of the gas.

The mean diameter of the soot particles is calculated using the definition of the soot moments in Equation 14.147 (p. 362) in the following manner:

(14.163)

where

is the diameter of smallest particle, which is a single carbon atom.

For different regimes, different treatments are applied when solving Equation 14.159 (p. 365) as further described.

Continuum Coagulation (

< 0.1)

The collision coefficient in the continuum region is expressed as: (14.164)

where is the Cunningham slip correction factor equal to 1+1.257 Kn, factor calculated as:

where

is the continuum collision

is the molecular viscosity of the mixture.

Substituting the collision coefficient (defined in Equation 14.164 (p. 366)) into Equation 14.159 (p. 365) gives the source terms due to coagulation described below. For the first moment, the moment source term due to continuum coagulation is: (14.165) where

.

By introducing the concept of reduced moment as:

, Equation 14.165 (p. 366) can be rewritten (14.166)

Similarly, for = 2, 3 …. the moment source terms due to continuum coagulation are expressed as: (14.167)

The coagulation terms in Equation 14.166 (p. 366) and Equation 14.167 (p. 366) consist of the fractional order moments as well as the negative order moments that need to be calculated for closure. Here, the

366

Soot Formation process of interpolative closure proposed by Frenklach ([144] (p. 782)) has been used. In the interpolative closure, the positive fractional order moments are obtained using a polynomial interpolation. Since the numerical values of the subsequent moments differ by an order of magnitude, the logarithmic interpolation is used to minimize interpolation errors. The fractional moments are obtained using the following expression: (14.168) where,

is the Lagrange interpolating polynomial.

In order to calculate the negative order fractional moments, an extrapolation from integer order moments is used (14.169) where is the Lagrange extrapolation. It has been observed that a quadratic interpolation of Equation 14.168 (p. 367) and a linear extrapolation of equation Equation 14.169 (p. 367) work reasonably well.

Coagulation in the Free Molecular Regime (

> 10)

In the free molecular regime, the collision coefficient is calculated as: (14.170) where

is the coefficient for the free molecular collision efficiency calculation in the following form: (14.171)

Using Equation 14.170 (p. 367) for the collision coefficient, Equation 14.166 (p. 366) and Equation 14.167 (p. 366) give the moment source terms due to coagulation in the free molecular regime. Note that the collision coefficient written in the form of Equation 14.170 (p. 367) is non-additive. Therefore, Equation 14.166 (p. 366) and Equation 14.167 (p. 366) are now expressed as:

(14.172)

where

is the grid function defined as: (14.173)

Since the evaluation of this expression is not straightforward for =1/2, the grid functions are evaluated for integer values of . Then the grid functions for the fractional values are obtained using Lagrangian interpolation. For example, for cases where three moments are solved, the following grid functions need to be calculated:

367

Pollutant Formation • For =0: • For =1:

,

• For =2:

,

,

,

These functions are obtained by first calculating

,

, and

and then applying Lagrangian inter-

polation:

Note that the grid function is symmetrical, that is

Coagulation in Transition Regime (0.1

0.

= 0, Equation 14.186 (p. 371) ) is only

The first particle moment gives the average number of the primary particles (Equation 14.185 (p. 371)) from which the particle mass can be calculated. = 0 because the particles total mass does not change due to coaR16gulation process. For higher moments, in equation Equation 14.186 (p. 371) needs to be computed. Using Equation 14.159 (p. 365) along with the definition of particle moments in Equation 14.182 (p. 370), can be written as (14.187)

where is the aggregate collision coefficient. Its value is dependent on the collision regime, which is calculated using the Knudsen number (Equation 14.160 (p. 365)).

Aggregate Coagulation in Continuum Regime (

< 0.1)

The aggregate collision coefficient in the continuum regime is expressed as:

371

Pollutant Formation

(14.188) where and are the collision diameters of the aggregates of different size classes and , respectively. The rest of notation is the same as in equation Equation 14.164 (p. 366). For coalescent collision, the collision diameter and the particle diameter are assumed to be the same. However, due to aggregation, the collision diameter can differ significantly from the particle diameter depending on the fractal structure of the aggregate. An aggregate comprises a number of primary particles, which are assumed to be spherical and of equal size. The collision diameter of the aggregate is calculated from the following relation: (14.189) where is the diameter of the primary particle, and is the fractal dimension that describes the fractal structure of the aggregate. A value of between 1.7 and 2.0 is found to be reasonably good and suggested in the literature for aggregate coagulation. In the pure coalescent coagulation, = 3. The mass of the aggregate can be specified in the following manner: (14.190) From Equation 14.189 (p. 372) and Equation 14.190 (p. 372), the aggregate collision diameter can be written as: (14.191) Substituting the value of the collision diameter from Equation 14.191 (p. 372) into Equation 14.188 (p. 372) and replacing the value of the collision coefficient in Equation 14.187 (p. 371), we obtain the aggregate coagulation source term for the

th

moment:

(14.192)

where, (14.193) Equation 14.192 (p. 372) involves binary moments of the soot aggregate size and the number of primary particles. The calculations for binary moments are not trivial and require a multi-dimensional PDF of the particle size distribution. Kazakov and Frenklach [222] (p. 787) suggested approximating the binary moment using two one-dimensional moments as follows: (14.194) Thus, Equation 14.192 (p. 372) can be written as:

(14.195)

372

Soot Formation Equation 14.195 (p. 372) contains the fractional and intermediate primary particle moments, which are calculated using the same interpolation method employed to aggregate moments (see Equation 14.168 (p. 367) and Equation 14.169 (p. 367)).

Aggregate Coagulation in Free Molecular Regime (

> 10)

The collision coefficient in the free molecular regime can be written as: (14.196) where and are the mass and collision diameter of the aggregate, respectively. The collision diameter is calculated from Equation 14.191 (p. 372). Similar to the coalescent coagulation in the free molecular regime calculations, difficulties arise when calculating the source due to the coagulation of the aggregates (Equation 14.187 (p. 371)) because of the non-additive collision coefficient term from Equation 14.196 (p. 373). In a manner similar to Equation 14.172 (p. 367), can be expressed as: (14.197) where is the aggregate grid function. Equating the right-hand sides of Equation 14.187 (p. 371) and Equation 14.197 (p. 373) and substituting the collision coefficient from equation Equation 14.196 (p. 373), the aggregate grid function can be written as: (14.198)

The collision diameter in Equation 14.198 (p. 373) is given by equation Equation 14.191 (p. 372). Therefore, Equation 14.198 (p. 373) can be rewritten as (14.199)

Using Equation 14.147 (p. 362), Equation 14.182 (p. 370), and Equation 14.194 (p. 372), the aggregate grid function can be expressed as follows: (14.200)

Aggregate Coagulation in Transition Regime (0.1

. Note that the value for for a given phase can be found from the Phases dialog box in the user interface. User-Defined You can specify your own limiting function by creating a User-Defined Function (UDF) using the DEFINE_EXCHANGE_PROPERTY macro (DEFINE_EXCHANGE_PROPERTY). Note that

must

have a value between 0 and 1, and should vary continuously to ensure numerical stability and physical solutions.

17.5.10. Virtual Mass Force For multiphase flows, you can optionally include the “virtual mass effect” that occurs when a secondary phase accelerates relative to the primary phase . The inertia of the primary-phase mass encountered by the accelerating particles (or droplets or bubbles) exerts a “virtual mass force” on the particles [112] (p. 781). The virtual mass force is defined as: (17.288)

where is the virtual mass coefficient which typically has a value of 0.5. The term phase material time derivative of the form

denotes the (17.289)

The virtual mass force phases (

will be added to the right-hand side of the momentum equation for both

).

The virtual mass effect is significant when the secondary phase density is much smaller than the primary phase density (for example, for a transient bubble column). By default, the virtual mass is applied as an explicit source term. You can also choose to use an implicit approach which is recommended in steady-state coupled simulations where the explicit approach may fail to converge. Refer to Including the Virtual Mass Force for details on how to include virtual mass in your simulation.

17.5.11. Solids Pressure For granular flows in the compressible regime (that is, where the solids volume fraction is less than its maximum allowed value), a solids pressure is calculated independently and used for the pressure gradient term, , in the granular-phase momentum equation. Because a Maxwellian velocity distribution is used for the particles, a granular temperature is introduced into the model, and appears in the expression for the solids pressure and viscosities. The solids pressure is composed of a kinetic term and a second term due to particle collisions: (17.290) where

is the coefficient of restitution for particle collisions,

is the radial distribution function,

and is the granular temperature. ANSYS Fluent uses a default value of 0.9 for , but the value can be adjusted to suit the particle type. The granular temperature is proportional to the kinetic energy 578

Eulerian Model Theory of the fluctuating particle motion, and will be described later in this section. The function

(described

below in more detail) is a distribution function that governs the transition from the “compressible” condition with , where the spacing between the solid particles can continue to decrease, to the “incompressible” condition with , where no further decrease in the spacing can occur. A value of 0.63 is the default for , but you can modify it during the problem setup. Other formulations that are also available in ANSYS Fluent are [472] (p. 801) (17.291) and [296] (p. 791) (17.292) where

is defined in [296] (p. 791).

When more than one solids phase are calculated, the above expression does not take into account the effect of other phases. A derivation of the expressions from the Boltzman equations for a granular mixture are beyond the scope of this manual, however there is a need to provide a better formulation so that some properties may feel the presence of other phases. A known problem is that N solid phases with identical properties should be consistent when the same phases are described by a single solids phase. Equations derived empirically may not satisfy this property and need to be changed accordingly without deviating significantly from the original form. From [153] (p. 783), a general solids pressure formulation in the presence of other phases could be of the form (17.293)

where is the average diameter, , are the number of particles, and are the masses of the particles in phases and , and is a function of the masses of the particles and their granular temperatures. For now, we have to simplify this expression so that it depends only on the granular temperature of phase (17.294) Since all models need to be cast in the general form, it follows that (17.295)

where

is the collisional part of the pressure between phases

and . In Equation 17.294 (p. 579),

. The above expression reverts to the one solids phase expression when property of feeling the presence of other phases.

and

but also has the

579

Multiphase Flows

17.5.11.1. Radial Distribution Function The radial distribution function, , is a correction factor that modifies the probability of collisions between grains when the solid granular phase becomes dense. This function may also be interpreted as the nondimensional distance between spheres: (17.296) where is the distance between grains. From Equation 17.296 (p. 580) it can be observed that for a dilute solid phase , and therefore . In the limit when the solid phase compacts, and . The radial distribution function is closely connected to the factor of Chapman and Cowling’s [71] (p. 778) theory of nonuniform gases. is equal to 1 for a rare gas, and increases and tends to infinity when the molecules are so close together that motion is not possible. In the literature there is no unique formulation for the radial distribution function. ANSYS Fluent has a number of options: • For one solids phase, use [350] (p. 794): (17.297) This is an empirical function and does not extend easily to phases. For two identical phases with the property that , the above function is not consistent for the calculation of the partial pressures and , . In order to correct this problem, ANSYS Fluent uses the following consistent formulation: (17.298)

where is the packing limit, phases, and

is the total number of solid phases,

is the diameter of the

(17.299) and

are solid phases only.

• The following expression is also available [193] (p. 785): (17.300) • Also available [296] (p. 791), slightly modified for

solids phases, is the following: (17.301)

• The following equation [472] (p. 801) is available: (17.302)

580

Eulerian Model Theory When the number of solid phases is greater than 1, Equation 17.298 (p. 580), Equation 17.300 (p. 580) and Equation 17.301 (p. 580) are extended to (17.303) It is interesting to note that Equation 17.300 (p. 580) and Equation 17.301 (p. 580) compare well with [6] (p. 775) experimental data, while Equation 17.302 (p. 580) reverts to the [67] (p. 778) derivation.

17.5.12. Maximum Packing Limit in Binary Mixtures The packing limit is not a fixed quantity and may change according to the number of particles present within a given volume and the diameter of the particles. Small particles accumulate in between larger particles increasing the packing limit. For a binary mixture ANSYS Fluent uses the correlations proposed by [124] (p. 781). For a binary mixture with diameters

>

, the mixture composition is defined as

where (17.304) as this is a condition for application of the maximum packing limit for binary mixtures. The maximum packing limit for the mixture is the minimum of the two expressions (Equation 17.305 (p. 581) and Equation 17.306 (p. 581)): (17.305) and (17.306) The packing limit is used for the calculation of the radial distribution function.

17.5.13. Solids Shear Stresses The solids stress tensor contains shear and bulk viscosities arising from particle momentum exchange due to translation and collision. A frictional component of viscosity can also be included to account for the viscous-plastic transition that occurs when particles of a solid phase reach the maximum solid volume fraction. The collisional and kinetic parts, and the optional frictional part, are added to give the solids shear viscosity: (17.307)

17.5.13.1. Collisional Viscosity The collisional part of the shear viscosity is modeled as [154] (p. 783), [472] (p. 801) (17.308)

581

Multiphase Flows

17.5.13.2. Kinetic Viscosity ANSYS Fluent provides two expressions for the kinetic part. The default expression is from Syamlal et al. [472] (p. 801): (17.309) The following optional expression from Gidaspow et al. [154] (p. 783) is also available: (17.310)

17.5.13.3. Bulk Viscosity The solids bulk viscosity accounts for the resistance of the granular particles to compression and expansion. It has the following form from Lun et al. [291] (p. 791): (17.311) Note that the bulk viscosity is set to a constant value of zero, by default. It is also possible to select the Lun et al. expression or use a user-defined function.

17.5.13.4. Frictional Viscosity In dense flow at low shear, where the secondary volume fraction for a solid phase nears the packing limit, the generation of stress is mainly due to friction between particles. The solids shear viscosity computed by ANSYS Fluent does not, by default, account for the friction between particles. If the frictional viscosity is included in the calculation, ANSYS Fluent uses Schaeffer’s [419] (p. 798) expression: (17.312) where is the solids pressure, is the angle of internal friction, and is the second invariant of the deviatoric stress tensor. It is also possible to specify a constant or user-defined frictional viscosity. In granular flows with high solids volume fraction, instantaneous collisions are less important. The application of kinetic theory to granular flows is no longer relevant since particles are in contact and the resulting frictional stresses need to be taken into account. ANSYS Fluent extends the formulation of the frictional viscosity and employs other models, as well as providing new hooks for UDFs. See the Fluent Customization Manual for details. The frictional stresses are usually written in Newtonian form: (17.313) The frictional stress is added to the stress predicted by the kinetic theory when the solids volume fraction exceeds a critical value. This value is normally set to 0.5 when the flow is three-dimensional and the maximum packing limit is about 0.63. Then (17.314)

582

Eulerian Model Theory (17.315) The derivation of the frictional pressure is mainly semi-empirical, while the frictional viscosity can be derived from the first principles. The application of the modified Coulomb law leads to an expression of the form (17.316) Where

is the angle of internal friction and

is the second invariant of the deviatoric stress tensor.

Two additional models are available in ANSYS Fluent: the Johnson and Jackson [209] (p. 786) model for frictional pressure and Syamlal et al [472] (p. 801). The Johnson and Jackson [209] (p. 786) model for frictional pressure is defined as (17.317)

With coefficient = 0.05, = 2 and = 5 [348] (p. 794). The critical value for the solids volume fraction is 0.5. The coefficient was modified to make it a function of the volume fraction: (17.318) The frictional viscosity for this model is of the form (17.319) The second model that is employed is Syamlal et al. [472] (p. 801). Comparing the two models results in the frictional normal stress differing by orders of magnitude. The radial distribution function is an important parameter in the description of the solids pressure resulting from granular kinetic theory. If we use the models of Lun et al. [291] (p. 791) or Gidaspow [153] (p. 783) the radial function tends to infinity as the volume fraction tends to the packing limit. It would then be possible to use this pressure directly in the calculation of the frictional viscosity, as it has the desired effect. This approach is also available in ANSYS Fluent by default.

Important The introduction of the frictional viscosity helps in the description of frictional flows, however a complete description would require the introduction of more physics to capture the elastic regime with the calculation of the yield stress and the use of the flow-rule. These effects can be added by you via UDFs to model static regime. Small time steps are required to get good convergence behavior.

17.5.14. Granular Temperature The granular temperature for the solids phase is proportional to the kinetic energy of the particles’ random motion. The formal expression is: (17.320)

583

Multiphase Flows In Equation 17.320 (p. 583) represents the component of the fluctuating solids velocity in the Cartesian coordinate system. This is defined as an ensemble average of the particles’ random velocity within a finite volume and time period. The averaging basis is particle number per unit volume following [71] (p. 778) and [103] (p. 780). The transport equation derived from kinetic theory takes the form [103] (p. 780): (17.321) where = the generation of energy by the solid stress tensor = the diffusion of energy (

is the diffusion coefficient)

= the collisional dissipation of energy = the energy exchange between the

fluid or solid phase and the

solid

phase Equation 17.321 (p. 584) contains the term describing the diffusive flux of granular energy. When the default Syamlal et al. model [472] (p. 801) is used, the diffusion coefficient for granular energy, is given by (17.322)

where

ANSYS Fluent uses the following expression if the optional model of Gidaspow et al. [154] (p. 783) is enabled: (17.323)

The collisional dissipation of energy,

, represents the rate of energy dissipation within the

solids

phase due to collisions between particles. This term is represented by the expression derived by Lun et al. [291] (p. 791) (17.324)

The transfer of the kinetic energy of random fluctuations in particle velocity from the to the

fluid or solid phase is represented by

solids phase

[154] (p. 783): (17.325)

ANSYS Fluent allows you to solve for the granular temperature with the following options: • algebraic formulation (the default)

584

Eulerian Model Theory It is obtained by neglecting convection and diffusion in the transport equation, Equation 17.321 (p. 584) [472] (p. 801). • partial differential equation This is given by Equation 17.321 (p. 584) and it is allowed to choose different options for it properties. • dpm-averaged granular temperature An alternative formulation available only with the Dense Discrete Phase Model (DDPM). • constant granular temperature This is useful in very dense situations where the random fluctuations are small. • UDF for granular temperature For a granular phase , we may write the shear force at the wall in the following form: (17.326) Here is the particle slip velocity parallel to the wall, is the specularity coefficient between the particle and the wall, is the volume fraction for the particles at maximum packing, and is the radial distribution function that is model dependent. The general boundary condition for granular temperature at the wall takes the form: [209] (p. 786) (17.327)

17.5.15. Description of Heat Transfer The internal energy balance for phase defined by

is written in terms of the phase enthalpy, Equation 17.171 (p. 557), (17.328)

where is the specific heat at constant pressure of phase . The thermal boundary conditions used with multiphase flows are the same as those for a single-phase flow. See Cell Zone and Boundary Conditions in the User's Guide for details.

17.5.15.1. The Heat Exchange Coefficient The volumetric rate of energy transfer between phases, ature difference and the interfacial area,

, is assumed to be a function of the temper-

: (17.329)

where is the volumetric heat transfer coefficient between the The heat transfer coefficient is related to the phase Nusselt number,

phase and the , by

phase.

(17.330) Here

is the thermal conductivity of the

phase,

is the bubble diameter.

585

Multiphase Flows When using the Eulerian multiphase, ANSYS Fluent provides several methods for determining the volumetric heat transfer coefficient, , detailed in the following sections. 17.5.15.1.1. Constant 17.5.15.1.2. Nusselt Number 17.5.15.1.3. Ranz-Marshall Model 17.5.15.1.4.Tomiyama Model 17.5.15.1.5. Hughmark Model 17.5.15.1.6. Gunn Model 17.5.15.1.7.Two-Resistance Model 17.5.15.1.8. Fixed To Saturation Temperature 17.5.15.1.9. User Defined

17.5.15.1.1. Constant You can specify a constant value for the volumetric heat transfer coefficient, , by selecting the constant-htc method for heat transfer coefficient. If the Two-Resistance model is selected, the constant can be specified for each phase.

17.5.15.1.2. Nusselt Number You can specify the Nusselt number, , used in Equation 17.330 (p. 585) by selecting the nusseltnumber method for heat transfer coefficient. For the Two-Resistance model, the Nusselt number can be specified for each phase. In such cases, the Nusselt number is always defined relative to the physical properties to which it pertains.

17.5.15.1.3. Ranz-Marshall Model The correlation of Ranz and Marshall [391] (p. 796), [392] (p. 796) computes the Nusselt number as follows: (17.331) where velocity

is the relative Reynolds number based on the diameter of the , and Pr is the Prandtl number of the

phase and the relative

phase: (17.332)

17.5.15.1.4. Tomiyama Model Tomiyama [485] (p. 801) proposed a slightly different correlation for the interfacial heat transfer, applicable to turbulent bubbly flows with relatively low Reynolds number. For the Tomiyama model, the Nusselt number, , is expressed: (17.333)

17.5.15.1.5. Hughmark Model In order to extend the Ranz-Marshall model to a wider range of Reynolds numbers, Hughmark [189] (p. 785) proposed the following correction: (17.334) The Reynolds number crossover point is chosen to achieve continuity. The Hughmark model should not be used outside the recommended Prandtl number range

586

Eulerian Model Theory

17.5.15.1.6. Gunn Model In the case of granular flows (where ), you can also choose a Nusselt number correlation by Gunn [165] (p. 784), applicable to a porosity range of 0.35–1.0 and a Reynolds number of up to : (17.335) The Prandtl number is defined as above with

.

17.5.15.1.7. Two-Resistance Model In some special situations, the use of an overall volumetric heat transfer coefficient is not sufficient to model the interphase heat transfer process accurately. A more general approach is to consider separate heat transfer processes with different heat transfer coefficients on either side of the phase interface. This generalization is referred to in Fluent as the two-resistance model. At the interface between the th phase and the th phase, the temperature is assumed to be the same on both sides of the interface and is represented by . Then the volumetric rates of phase heat exchange can be expressed as follows: From the interface to the

th

phase: (17.336)

From the interface to the

th

phase: (17.337)

where th

and

are the

th

and

th

phase heat transfer coefficients, and

and

are the

th

and

phase enthalpies, respectively.

Since neither heat nor mass can be stored on the phase interface, the overall heat balance must be satisfied: (17.338) Therefore, in the absence of interphase mass transfer ( as follows:

) the interfacial temperature is determined (17.339)

and the interphase heat transfer is given by (17.340) Hence, in the absence of interphase mass transfer the two-resistance model works somewhat similar to coupled wall thermal boundaries, wherein the interface temperature and the overall heat transfer coefficient are determined by the heat transfer rates on the two sides. The phase heat transfer coefficients, and , can be computed using the same correlations that are available for computing the overall heat transfer coefficient, . In addition you can specify a zeroresistance condition on one side of the phase interface. This is equivalent to an infinite phase specific heat transfer coefficient. For example, if its effect is to force the interface temperature to be the same as the phase temperature, . You can also choose to use the constant time scale return to Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

587

Multiphase Flows saturation method proposed by Lavieville et al. [255] (p. 789) which is used by default for the interface to vapor heat transfer coefficient in the wall boiling models. Refer to Interface to Vapor Heat Transfer (p. 611) for details of the formulation of the Lavieville et al method.

17.5.15.1.8. Fixed To Saturation Temperature The fixed-to-sat-temp heat transfer model is intended to be used only when interphase mass transfer is being modeled. In this model, the following conditions are assumed and are applied to Equation 17.336 (p. 587), Equation 17.337 (p. 587), and Equation 17.338 (p. 587): • All of the heat transferred to a phase-to-phase interface goes into mass transfer. • The temperature at the To-Phase is equal to the saturation temperature. The volumetric mass transfer rate, , is determined from the selected mass transfer model (e.g. cavitation, evaporation-condensation Lee model, etc.). The energy relationships depending on the direction of mass transfer are: • For

(mass transfers from the pth phase to the qth phase): (17.341)

• For

(mass transfers from the qth phase to the pth phase): (17.342)

17.5.15.1.9. User Defined You can also specify the volumetric heat transfer coefficient, , as a user defined function using the DEFINE_EXCHANGE_PROPERTY UDF macro. See DEFINE_EXCHANGE_PROPERTY in the Fluent Customization Manual for details.

17.5.16. Turbulence Models To describe the effects of turbulent fluctuations of velocities and scalar quantities in a single phase, ANSYS Fluent uses various types of closure models, as described in Turbulence (p. 39). In comparison to single-phase flows, the number of terms to be modeled in the momentum equations in multiphase flows is large, and this makes the modeling of turbulence in multiphase simulations extremely complex. ANSYS Fluent provides three methods for modeling turbulence in multiphase flows within the context of the - and - models. In addition, ANSYS Fluent provides two turbulence options within the context of the Reynolds stress models (RSM). The -

and -

turbulence model options are:

• mixture turbulence model (the default) • dispersed turbulence model

588

Eulerian Model Theory • turbulence model for each phase

Important Note that the descriptions of each method below are presented based on the standard model. The multiphase modifications to the - , RNG and realizable - models are similar, and are therefore not presented explicitly. The RSM turbulence model options are: • mixture turbulence model (the default) • dispersed turbulence model For either category, the choice of model depends on the importance of the secondary-phase turbulence in your application.

17.5.16.1. k- ε Turbulence Models ANSYS Fluent provides three turbulence model options in the context of the - models: the mixture turbulence model (the default), the dispersed turbulence model, or a per-phase turbulence model.

17.5.16.1.1. k- ε Mixture Turbulence Model The mixture turbulence model is the default multiphase turbulence model. It represents the first extension of the single-phase - model, and it is applicable when phases separate, for stratified (or nearly stratified) multiphase flows, and when the density ratio between phases is close to 1. In these cases, using mixture properties and mixture velocities is sufficient to capture important features of the turbulent flow. The and equations describing this model (and without including buoyancy, dilation, and source terms) are as follows: (17.343) and (17.344) where the mixture density,

, molecular viscosity,

, and velocity,

, are computed from (17.345) (17.346)

and

589

Multiphase Flows

(17.347)

where phase.

,

,

, and

are, respectively, the volume fraction, density, viscosity, and velocity of the ith

The turbulent viscosity for the mixture,

, is computed from (17.348)

and the production of turbulence kinetic energy,

, is computed from (17.349)

The terms, and are source terms that can be included to model the turbulent interaction between the dispersed phases and the continuous phase (Turbulence Interaction Models (p. 595)). The turbulent viscosity for phase is computed from (17.350) The constants in these equations are the same as those described in Standard k-ε Model (p. 47) for the single-phase - model.

17.5.16.1.2. k- ε Dispersed Turbulence Model The dispersed turbulence model is the appropriate model when the concentrations of the secondary phases are dilute, or when using the granular model. Fluctuating quantities of the secondary phases can be given in terms of the mean characteristics of the primary phase and the ratio of the particle relaxation time and eddy-particle interaction time. The model is applicable when there is clearly one primary continuous phase and the rest are dispersed dilute secondary phases.

17.5.16.1.2.1. Assumptions The dispersed method for modeling turbulence in ANSYS Fluent assumes the following: • a modified -

model for the continuous phase

Turbulent predictions for the continuous phase are obtained using the standard - model supplemented with extra terms that include the interphase turbulent momentum transfer. • Tchen-theory correlations for the dispersed phases Predictions for turbulence quantities for the dispersed phases are obtained using the Tchen theory of dispersion of discrete particles by homogeneous turbulence [183] (p. 785). • interphase turbulent momentum transfer

590

Eulerian Model Theory In turbulent multiphase flows, the momentum exchange terms contain the correlation between the instantaneous distribution of the dispersed phases and the turbulent fluid motion. It is possible to take into account the dispersion of the dispersed phases transported by the turbulent fluid motion. • a phase-weighted averaging process The choice of averaging process has an impact on the modeling of dispersion in turbulent multiphase flows. A two-step averaging process leads to the appearance of fluctuations in the phase volume fractions. When the two-step averaging process is used with a phase-weighted average for the turbulence, however, turbulent fluctuations in the volume fractions do not appear. ANSYS Fluent uses phase-weighted averaging, so no volume fraction fluctuations are introduced into the continuity equations.

17.5.16.1.2.2. Turbulence in the Continuous Phase The eddy viscosity model is used to calculate averaged fluctuating quantities. The Reynolds stress tensor for continuous phase takes the following form: (17.351) where

is the phase-weighted velocity.

The turbulent viscosity

is written in terms of the turbulent kinetic energy of phase : (17.352)

and a characteristic time of the energetic turbulent eddies is defined as (17.353) where

is the dissipation rate and

.

The length scale of the turbulent eddies is (17.354) Turbulent predictions are obtained from the modified buoyancy, dilation, and user-defined source terms) are:

model. The transport equations (excluding

(17.355)

and

(17.356)

Here, the terms containing and are source terms that can be included to model the influence of the dispersed phases on the continuous phase (Turbulence Interaction Models (p. 595)). is the

591

Multiphase Flows production of turbulent kinetic energy, as defined in Modeling Turbulent Production in the k-ε Models (p. 54). All other terms have the same meaning as in the single-phase - model.

17.5.16.1.2.3. Turbulence in the Dispersed Phase The turbulence quantities for the dispersed phase are not obtained from transport equations. Time and length scales that characterize the motion are used to evaluate dispersion coefficients, correlation functions, and the turbulent kinetic energy of each dispersed phase.

17.5.16.1.3. k- ε Turbulence Model for Each Phase The most general multiphase turbulence model solves a set of and transport equations for each phase. This turbulence model is the appropriate choice when the turbulence transfer among the phases plays a dominant role. Note that, since ANSYS Fluent is solving two additional transport equations for each secondary phase, the per-phase turbulence model is more computationally intensive than the dispersed turbulence model.

17.5.16.1.3.1. Transport Equations The Reynolds stress tensor and turbulent viscosity are computed using Equation 17.351 (p. 591) and Equation 17.352 (p. 591). Turbulence predictions are obtained from

(17.357)

and

(17.358)

The terms

592

and

can be approximated as

Eulerian Model Theory

(17.359) where

is defined by Equation 17.380 (p. 597). The terms

and

are source terms which can be

included to model the influence of the inter-phase turbulence interaction (Turbulence Interaction Models (p. 595)).

17.5.16.2. RSM Turbulence Models Multiphase turbulence modeling typically involves two equation models that are based on single-phase models and often cannot accurately capture the underlying flow physics. Additional turbulence modeling for multiphase flows is diminished even more when the basic underlying single-phase model cannot capture the complex physics of the flow. In such situations, the logical next step is to combine the Reynolds stress model with the multiphase algorithm in order to handle challenging situations in which both factors, RSM for turbulence and the Eulerian multiphase formulation, are a precondition for accurate predictions [84] (p. 779). The phase-averaged continuity and momentum equations for a continuous phase are: (17.360) (17.361) For simplicity, the laminar stress-strain tensor and other body forces such as gravity have been omitted from Equation 17.360 (p. 593) - Equation 17.361 (p. 593). The tilde denotes phase-averaged variables while an overbar (for example, ) reflects time-averaged values. In general, any variable can have a phaseaverage value defined as (17.362) Considering only two phases for simplicity, the drag force between the continuous and the dispersed phases can be defined as: (17.363) where is the drag coefficient. Several terms in the Equation 17.363 (p. 593) need to be modeled in order to close the phase-averaged momentum equations. Full descriptions of all modeling assumptions can be found in [83] (p. 779). This section only describes the different modeling definition of the turbulent stresses that appears in Equation 17.361 (p. 593). The turbulent stress that appears in the momentum equations need to be defined on a per-phase basis and can be calculated as: (17.364) where the subscript is replaced by for the primary (that is, continuous) phase or by for any secondary (that is, dispersed) phases. As is the case for single-phase flows, the current multiphase Reynolds stress model (RSM) also solves the transport equations for Reynolds stresses . ANSYS Fluent includes two methods for modeling turbulence in multiphase flows within the context of the RSM model: the dispersed turbulence model, and the mixture turbulence model.

593

Multiphase Flows

17.5.16.2.1. RSM Dispersed Turbulence Model The dispersed turbulence model is used when the concentrations of the secondary phase are dilute and the primary phase turbulence is regarded as the dominant process. Consequently, the transport equations for turbulence quantities are only solved for the primary (continuous) phase, while the predictions of turbulence quantities for dispersed phases are obtained using the Tchen theory. The transport equation for the primary phase Reynolds stresses in the case of the dispersed model are:

(17.365)

The variables in Equation 17.365 (p. 594) are per continuous phase and the subscript is omitted for clarity. In general, the terms in Equation 17.365 (p. 594) are modeled in the same way as for the single phase case described in Reynolds Stress Model (RSM) (p. 82). The last term, , takes into account the interaction between the continuous and the dispersed phase turbulence. A general model for this term can be of the form: (17.366) where and are unknown coefficients, is the relative velocity, represents the drift or the relative velocity, and is the unknown particulate-fluid velocity correlation. To simplify this unknown term, the following assumption has been made: (17.367) where is the Kronecker delta, and [437] (p. 799).

represents the modified version of the original Simonin model (17.368)

where represents the turbulent kinetic energy of the continuous phase, is the continuous-dispersed phase velocity covariance and finally, and stand for the relative and the drift velocities, respectively. In order to achieve full closure, the transport equation for the turbulent kinetic energy dissipation rate ( ) is required. The modeling of together with all other unknown terms in Equation 17.368 (p. 594) are modeled in the same way as in [83] (p. 779).

17.5.16.2.2. RSM Mixture Turbulence Model The main assumption for the mixture model is that all phases share the same turbulence field which consequently means that the term in the Reynolds stress transport equations (Equation 17.365 (p. 594)) is neglected. Apart from that, the equations maintain the same form but with phase properties and phase velocities being replaced with mixture properties and mixture velocities. The mixture density, for example, can be expressed as (17.369) while mixture velocities can be expressed as

594

Eulerian Model Theory

(17.370)

where

is the number of species.

17.5.16.3. Turbulence Interaction Models When using a turbulence model in an Eulerian multiphase simulation, Fluent can optionally include the influence of the dispersed phase on the multiphase turbulence equations. The influence of the dispersed phases is represented by source terms ( and in Equation 17.355 (p. 591) and Equation 17.356 (p. 591)) whose form will depend on the model chosen. You can choose from the following models for turbulence interaction. 17.5.16.3.1. Simonin et al. 17.5.16.3.2.Troshko-Hassan 17.5.16.3.3. Sato 17.5.16.3.4. None Turbulence interaction can be included with any of the multiphase turbulence models and formulations in Fluent. See Including Turbulence Interaction Source Terms in the Fluent User's Guide for details about how to include turbulence interaction in your simulation.

17.5.16.3.1. Simonin et al. In the Simonin et al. model [437] (p. 799), the turbulence interaction is modeled by additional source terms in the turbulence transport equation(s). The Simonin model is only available with Dispersed and Per Phase turbulence models.

17.5.16.3.1.1. Formulation in Dispersed Turbulence Models 17.5.16.3.1.1.1. Continuous Phase The term is derived from the instantaneous equation of the continuous phase and takes the following form, where represents the number of secondary phases: (17.371) which can be simplified to (17.372)

where: is a user-modifiable model constant. By default,

.

595

Multiphase Flows is the covariance of the velocities of the continuous phase , calculated from Equation 17.381 (p. 597).

and the dispersed phase

is the relative velocity. is the drift velocity, calculated from Equation 17.279 (p. 576).

For granular flows,

.

Equation 17.372 (p. 595) can be split into two terms as follows: (17.373) The second term in Equation 17.373 (p. 596) is related to drift velocity and is referred to as the drift turbulent source. It is an option term in the turbulent kinetic energy source and can be included as described in Including Turbulence Interaction Source Terms in the Fluent User's Guide. is modeled according to Elgobashi et al. [120] (p. 781): (17.374) where

.

17.5.16.3.1.1.2. Dispersed Phases For the dispersed phases, the characteristic particle relaxation time connected with inertial effects acting on a dispersed phase is defined as: (17.375)

where

is the drag function described in Interphase Exchange Coefficients (p. 559).

The time scale of the energetic turbulent eddies is defined as: (17.376) The eddy particle interaction time is mainly affected by the crossing-trajectory effect [91] (p. 780), and is defined as (17.377)

where

, the parameter,

, is given by (17.378)

and (17.379)

596

Eulerian Model Theory where is the angle between the mean particle velocity and the mean relative velocity. The ratio between these two characteristic times is written as (17.380) Following Simonin [437] (p. 799), ANSYS Fluent writes the turbulence quantities for dispersed phase as follows:

(17.381)

and is the added-mass coefficient. For granular flows, secondary phase, for granular flows is approximated as:

is negligible. The viscosity for the (17.382)

while for bubbly flows it can be left as

.

17.5.16.3.1.2. Formulation in Per Phase Turbulence Models For Per Phase turbulence models, only the second term from Equation 17.373 (p. 596) (the drift turbulence source) is added in the phase turbulence modeling equations. For the turbulent kinetic energy equations: Continuous Phase (17.383)

Dispersed Phases (17.384) Turbulence dissipation sources for all phases are computed as: (17.385)

17.5.16.3.2. Troshko-Hassan Troshko and Hassan [488] (p. 801) proposed an alternative model to account for the turbulence of the dispersed phase in the - equations.

17.5.16.3.2.1. Troshko-Hassan Formulation in Mixture Turbulence Models In the Mixture turbulence models, the Troshko-Hassan turbulence interaction terms are:

597

Multiphase Flows

(17.386) (17.387) By default, and . These values are user-modifiable as described in Including Turbulence Interaction Source Terms in the Fluent User's Guide. is the characteristic time of the induced turbulence defined as (17.388) is the virtual mass coefficient and

is the drag coefficient.

17.5.16.3.2.2. Troshko-Hassan Formulation in Dispersed Turbulence Models 17.5.16.3.2.2.1. Continuous Phase In the Dispersed turbulence models, the term

is calculated as follows: (17.389)

and

is calculated as (17.390)

By default, and . These values are user-modifiable as described in Including Turbulence Interaction Source Terms in the Fluent User's Guide. is the characteristic time of the induced turbulence defined as in Equation 17.388 (p. 598). 17.5.16.3.2.2.2. Dispersed Phases For the Troshko-Hassan model, the kinematic turbulent viscosity of the dispersed phase is calculated as (17.391)

17.5.16.3.2.3. Troshko-Hassan Formulation in Per-Phase Turbulence Models 17.5.16.3.2.3.1. Continuous Phase In the Per-Phase turbulence models, the continuous phase equations are modified with the following terms: (17.392) (17.393) By default, and . These values are user-modifiable as described in Including Turbulence Interaction Source Terms in the Fluent User's Guide. is the characteristic time of the induced turbulence defined as in Equation 17.388 (p. 598).

598

Eulerian Model Theory 17.5.16.3.2.3.2. Dispersed Phases The dispersed phase equations are also modified with source terms defined as follows: (17.394) (17.395)

17.5.16.3.3. Sato Unlike the Simonin and Troshko-Hassan models, the Sato model [416] (p. 798) does not add explicit source terms to the turbulence equations. Instead, in an attempt to incorporate the effect of the random primary phase motion induced by the dispersed phase in bubbly flow, Sato et al. proposed the following relation: (17.396) where the relative velocity and the diameter of the dispersed phase represent velocity and time scales and . For the mixture model

where

is the primary phase turbulent viscosity before the Sato correction, which is calculated from

Equation 17.350 (p. 590). For the dispersed and per-phase turbulence models, and eddy dissipation rate, respectively.

and

are the primary phase turbulence intensity

17.5.16.3.4. None If None is selected, then no source terms are added to account for turbulent interaction. This is appropriate if you prefer to add your own source terms using the DEFINE_SOURCE UDF macro ( DEFINE_SOURCE in the Fluent Customization Manual).

17.5.17. Solution Method in ANSYS Fluent For Eulerian multiphase calculations, ANSYS Fluent can solve the phase momentum equations, the shared pressure, and phasic volume fraction equations in a coupled and segregated fashion. The coupled solution for multiphase flows is discussed in detail in Coupled Solution for Eulerian Multiphase Flows in the User's Guide. When solving the equations in a segregated manner, ANSYS Fluent uses the phase coupled SIMPLE (PC-SIMPLE) algorithm [496] (p. 802) for the pressure-velocity coupling. PC-SIMPLE is an extension of the SIMPLE algorithm [365] (p. 795) to multiphase flows. The velocities are solved coupled by phases, but in a segregated fashion. The block algebraic multigrid scheme used by the density-based solver described in [517] (p. 803) is used to solve a vector equation formed by the velocity components of all phases simultaneously. Then, a pressure correction equation is built based on total volume continuity rather than mass continuity. Pressure and velocities are then corrected so as to satisfy the continuity constraint.

17.5.17.1. The Pressure-Correction Equation For incompressible multiphase flow, the pressure-correction equation takes the form: Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

599

Multiphase Flows

(17.397)

where

is the phase reference density for the

phase (defined as the total volume average density

of phase ), is the velocity correction for the phase, and is the value of at the current iteration. The velocity corrections are themselves expressed as functions of the pressure corrections.

17.5.17.2. Volume Fractions The volume fractions are obtained from the phase continuity equations. In discretized form, the equation of the

volume fraction is (17.398)

In order to satisfy the condition that all the volume fractions sum to one, (17.399)

17.5.18. Dense Discrete Phase Model In the standard formulation of the Lagrangian multiphase model, described in Discrete Phase (p. 387), the assumption is that the volume fraction of the discrete phase is sufficiently low: it is not taken into account when assembling the continuous phase equations. The general form of the mass and momentum conservation equations in ANSYS Fluent is given in Equation 17.400 (p. 600) and Equation 17.401 (p. 600) (and also defined in Continuity and Momentum Equations (p. 2)). (17.400) (17.401) To overcome this limitation of the Lagrangian multiphase model, the volume fraction of the particulate phase is accounted for by extending Equation 17.400 (p. 600) and Equation 17.401 (p. 600) to the following set of equations (see also Conservation Equations (p. 556), written for phase ): (17.402)

(17.403)

Here, Equation 17.402 (p. 600) is the mass conservation equation for an individual phase and Equation 17.403 (p. 600) is the corresponding momentum conservation equation. The momentum exchange terms (denoted by ) are split into an explicit part, , and an implicit part, in which represents the particle averaged velocity of the considered discrete phase, and represents its

600

Eulerian Model Theory particle averaged interphase momentum exchange coefficient [386] (p. 796). Currently, these momentum exchange terms are considered only in the primary phase equations. In the resulting set of equations (one continuity and one momentum conservation equation per phase), those corresponding to a discrete phase are not solved. The solution, such as volume fraction or velocity field, is taken from the Lagrangian tracking solution. In versions prior to ANSYS Fluent 13, one drawback of the dense discrete phase model was that it did not prevent the actual concentration of particles from becoming unphysically high. Hence, the model had only a limited applicability to flows close to the packing regime, such as fluidized bed reactors. To overcome this, ANSYS Fluent now applies a special treatment to the particle momentum equation as soon as the particle volume fraction exceeds a certain user specified limit. Thus, the unlimited accumulation of particles is prevented. In turn, this will allow you to simulate suspensions and flows like bubbling fluidized bed reactors, operating at the packing limit conditions, allowing for polydispersed particle systems. However, no Discrete Element Method (DEM) type collision treatment is applied, which would otherwise allow the efficient simulation of systems with a large number of particles. In the context of the phase coupled SIMPLE algorithm (Solution Method in ANSYS Fluent (p. 599)) and the coupled algorithm for pressure-velocity coupling (Selecting the Pressure-Velocity Coupling Method in the User’s Guide), a higher degree of implicitness is achieved in the treatment of the drag coupling terms. All drag related terms appear as coefficients on the left hand side of the linear equation system.

17.5.18.1. Limitations Since the given approach makes use of the Eulerian multiphase model framework, all its limitations are adopted: • The turbulence models: LES, SAS, DES, SDES, and SBES turbulence models are not available. • The combustion models: PDF Transport model, Premixed, Non-premixed and partially premixed combustion models are not available. • The solidification and melting models are not available. • The Wet Steam model is not available. • The real gas model (pressure-based and density-based) is not available. • The density-based solver and models dependent on it are not available. • Parallel DPM with the shared memory option is disabled.

17.5.18.2. Granular Temperature The solids stress acting on particles in a dense flow situation is modeled via an additional acceleration in the particle force balance Equation 16.1 (p. 388). (17.404) The term models the additional acceleration acting on a particle, resulting from interparticle interaction. It is computed from the stress tensor given by the Kinetic Theory of Granular Flows as (17.405)

601

Multiphase Flows Following the same theory, the granular temperature is used in the model for the granular stress. For a more detailed description on the Kinetic Theory of Granular Flows refer to Solids Pressure (p. 578), Maximum Packing Limit in Binary Mixtures (p. 581), and Solids Shear Stresses (p. 581). The granular temperature can be estimated using any of the available options described in Granular Temperature (p. 583). By default, the algebraic formulation is used. The conservation equation for the granular temperature (kinetic energy of the fluctuating particle motion) is solved with the averaged particle velocity field. Therefore, a sufficient statistical representation of the particle phase is needed to ensure the stable behavior of the granular temperature equation. For details, refer to Conservation Equations (p. 556) – Granular Temperature (p. 583). The main advantage over the Eulerian model is that, there is no need to define classes to handle particle size distributions. This is done in a natural way in the Lagrangian formulation [386] (p. 796).

17.5.19. Multi-Fluid VOF Model The multi-fluid VOF model provides a framework to couple the VOF and Eulerian multiphase models. This allows the use of discretization schemes and options suited to both sharp and dispersed interface regimes while overcoming some limitations of the VOF model that arise due to the shared velocity and temperature formulation.

Modeling a Sharp Interface Regime For cases operating in a sharp interface regime, the Multi-Fluid VOF model provides interface sharpening schemes such as Geo-Reconstruct, CICSAM, and Compressive along with the symmetric and anisotropic drag laws. The symmetric drag law, which is used as the default in the sharp interface regime, is isotropic and tends to approach the behavior of the VOF model for high drag coefficients. The anisotropic drag law helps to overcome a limitation of the symmetric drag law by allowing higher drag in the normal direction to the interface and lower drag in the tangential direction. This facilities continuity of normal velocity across the interface while allowing different tangential velocities at the interface. The anisotropy is characterized by the anisotropy ratio, defined as:

Conceptually, this ratio can be very high, however higher values sometimes lead to numerical instability. Therefore, it is recommended that you use anisotropy ratios up to approximately 1000. Two types of drag formulations exist within the anisotropic drag law: one that is based on the symmetric drag law and the other is based on different viscosity options. Formulation 1 This is based on the symmetric drag law, where the effective drag coefficient in the principal direction is described as follows: (17.406) where is the friction factor vector in the principal direction. obtained from the symmetric drag law.

is the isotropic drag coefficient

Formulation 2 602

Eulerian Model Theory The effective drag coefficient in the principal direction

is described as follows: (17.407)

where

is the volume fraction for phase and

is the volume fraction for phase .

The viscous drag component in the principal direction

is (17.408)

where the viscosity options can be any one of the following: = = = = = = and

is the length scale.

Note Both the symmetric and anisotropic drag laws used in the sharp interface regime are numerical drags. Therefore, droplet diameter in the symmetric law and length scale in the anisotropic law can be considered as numerical means to provide the drag.

Modeling a Sharp/Dispersed Interface Regime For cases operating in the sharp/dispersed interface regime, the Multi-Fluid VOF model provides interface capturing schemes such as Compressive and Modified HRIC along with all options for all applicable drag laws. To learn how to use the multi-fluid VOF model and the two drag formulations, refer to Including the Multi-Fluid VOF Model in the User's Guide.

17.5.20. Wall Boiling Models 17.5.20.1. Overview The term “subcooled boiling” is used to describe the physical situation where the wall temperature is high enough to cause boiling to occur at the wall even though the bulk volume averaged liquid temperature is less than the saturation value. In such cases, the energy is transferred directly from the wall to the liquid. Part of this energy will cause the temperature of the liquid to increase and part will generate vapor. Interphase heat transfer will also cause the average liquid temperature to increase, however,

603

Multiphase Flows the saturated vapor will condense. Additionally, some of the energy may be transferred directly from the wall to the vapor. These basic mechanisms are the foundations of the so called Rensselaer Polytechnic Institute (RPI) models. In ANSYS Fluent, the wall boiling models are developed in the context of the Eulerian multiphase model. The multiphase flows are governed by the conservation equations for phase continuity (Equation 17.167 (p. 556)), momentum (Equation 17.168 (p. 556)), and energy (Equation 17.171 (p. 557)). The wall boiling phenomenon is modeled by the RPI nucleate boiling model of Kurual and Podowski [242] (p. 788) and an extended formulation for the departed nucleate boiling regime (DNB) by Lavieville et al [255] (p. 789). The wall boiling models are compatible with three different wall boundaries: isothermal wall, specified heat flux, and specified heat transfer coefficient (coupled wall boundary). Specific submodels have been considered to account for the interfacial transfers of momentum, mass, and heat, as well as turbulence models in boiling flows, as described below. To learn how to set up the boiling model, refer to Including the Boiling Model.

17.5.20.2. RPI Model According to the basic RPI model, the total heat flux from the wall to the liquid is partitioned into three components, namely the convective heat flux, the quenching heat flux, and the evaporative heat flux: (17.409) The heated wall surface is subdivided into area , which is covered by the fluid. • The convective heat flux

, which is covered by nucleating bubbles and a portion

is expressed as (17.410)

where is the single phase heat transfer coefficient, and respectively. • The quenching heat flux

and

are the wall and liquid temperatures,

models the cyclic averaged transient energy transfer related to liquid filling the

wall vicinity after bubble detachment, and is expressed as (17.411)

Where

is the conductivity,

• The evaporative flux

is the periodic time, and

is the diffusivity.

is given by (17.412)

Where is the volume of the bubble based on the bubble departure diameter, is the active nucleate site density, is the vapor density, and is the latent heat of evaporation, and is the bubble departure frequency. These equations need closure for the following parameters:

604

Eulerian Model Theory • Area of Influence Its definition is based on the departure diameter and the nucleate site density: (17.413) Note that in order to avoid numerical instabilities due to unbound empirical correlations for the nucleate site density, the area of influence has to be restricted. The area of influence is limited as follows: (17.414) The value of the empirical constant is usually set to 4, however it has been found that this value is not universal and may vary between 1.8 and 5. The following relation for this constant has also been implemented based on Del Valle and Kenning's findings [99] (p. 780): (17.415) and

is the subcooled Jacob number defined as: (17.416)

where • Frequency of Bubble Departure Implementation of the RPI model normally uses the frequency of bubble departure as the one based on inertia controlled growth (not really applicable to subcooled boiling). [85] (p. 779) (17.417) • Nucleate Site Density The nucleate site density is usually represented by a correlation based on the wall superheat. The general expression is of the form (17.418) Here the empirical parameters from Lemmert and Chawla [261] (p. 789) are used, where and . Other formulations are also available, such as Kocamustafaogullari and Ishii [235] (p. 788) where (17.419) Here

Where

is the bubble departure diameter and the density function is defined as Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

605

Multiphase Flows (17.420) • Bubble Departure Diameter The default bubble departure diameter for the RPI model is based on empirical correlations [484] (p. 801) and is calculated in meters as (17.421) while Kocamustafaogullari and Ishii [234] (p. 788) use (17.422) with

being the contact angle in degrees.

The bubble departure diameter in millimeters based on the Unal relationship [492] (p. 802) is calculated as (17.423)

(17.424)

(17.425)

(17.426) where is the flow pressure, wall bulk velocity, and vapor phase, respectively.

is the wall superheat, is latent heat, is the near m/s. The subscripts , , and denote the solid material, liquid, and

17.5.20.3. Non-equilibrium Subcooled Boiling When using the basic RPI model (RPI Model (p. 604)), the temperature of the vapor is not calculated, but instead is fixed at the saturation temperature. To model boiling departing from the nucleate boiling regime (DNB), or to model it up to the critical heat flux and post dry-out condition, it is necessary to include the vapor temperature in the solution process. The wall heat partition is now modified as follows: (17.427) Here

,

, and

are the liquid-phase convective heat flux, quenching heat flux, and evaporation

heat flux, respectively (described in detail in RPI Model (p. 604)). The extra heat fluxes are representing the convective heat flux of the vapor phase, and representing heat flux to any other possible gas phases in a system. These can be expressed as (17.428) (17.429)

606

Eulerian Model Theory Similar to the liquid phase , the convective heat transfer coefficients the wall function formulations.

and

are computed from

The function depends on the local liquid volume fraction with similar limiting values as the liquid volume fraction. Lavieville et al [255] (p. 789) proposed the following expression: (17.430)

Here, the critical value for the liquid fraction is

17.5.20.4. Critical Heat Flux In wall boiling, the critical heat flux (CHF) condition is characterized by a sharp reduction of local heat transfer coefficients and the excursion of wall surface temperatures. It occurs when heated surfaces are no longer wetted by boiling liquid with the increase of vapor content. At critical heat flux conditions, vapor replaces the liquid and occupies the space adjacent to heated walls. The energy is therefore directly transferred from the wall to the vapor. In turn, it results in rapid reduction of the heat removal ability and sharp rise of the vapor temperature, and most importantly, the wall temperatures. In addition, wall boiling departs from the nucleating boiling regime, and the multiphase flow regime changes from a bubbly flow to a mist flow. To model the critical heat flux conditions, the basic approach adopted in ANSYS Fluent is to extend the RPI model from the nucleate boiling regime to critical heat flux and post dry-out conditions, while considering the following: • The generalized and non-equilibrium wall heat flux partition • The flow regime transition from bubbly to mist flows

17.5.20.4.1. Wall Heat Flux Partition The wall heat partition is defined in the same way as Equation 17.427 (p. 606), with the exception of the function definition. Here, the function depends on the local liquid/vapor volume fraction with the same limiting values as the liquid volume fraction, that is, between zero and one. Lavieville et al. [255] (p. 789) proposed the following expression: (17.431)

The critical value for the liquid volume fraction is

, and for the vapor phase, it is

.

There are also some other functions available to define the wall heat flux partition. When defining wall boiling regimes, Tentner et al. [480] (p. 801) suggested the following expression based on the vapor volume fraction: (17.432)

607

Multiphase Flows Taking the thin film boiling into consideration in the wall heat flux partition, Ioilev et al. [194] (p. 785) used a linear function to extend Equation 17.432 (p. 607) to the critical heat flux condition: (17.433) Where the breakpoints have been set to

and

.

In ANSYS Fluent, Equation 17.432 (p. 607) is chosen as the default formulation for the wall heat flux partition.

17.5.20.4.2. Flow Regime Transition When wall boiling departs from the nucleate boiling regime and reaches the critical heat flux and post dry-out conditions, the multiphase flow regime changes from a bubbly flow to a mist flow. Consequently, the liquid phase switches from the continuous phase to the dispersed phase, while the vapor phase becomes the continuous phase from the originally dispersed phase in the bubbly flow regime. With the flow regime transition, the interfacial area, momentum transfer terms (drag, lift, turbulent dispersion, interfacial area, and so on), heat transfer and turbulence quantities will change accordingly. To mimic the change of the flow regime and compute the interfacial transfers, the so-called flow regime maps, based on cross-section averaged flow parameters, are traditionally used in sub-channel one-dimensional thermal-hydraulic codes. In CFD solvers, the concept of flow regime maps has been expanded into a local, cell-based interfacial surface topology to evaluate the flow regime transitions from local flow parameters. The ensemble of all the computational cells with their usually simple local interfacial surface topologies can provide complex global topologies to represent the different flow regimes as the traditional sub-channel flow regime maps. As a first step, this implementation adopts a simple local interfacial surface topology to control the transition smoothly from a continuous liquid bubbly flow to a continuous vapor droplet flow configuration [480] (p. 801), [194] (p. 785). It assumes that inside a computational cell, the local interfacial surface topology contains multi-connected interfaces, and the flow regimes are determined by a single local flow quantity — the vapor volume fraction : • Bubbly flow topology: the vapor phase is dispersed in the continuous liquid in the form of bubbles. Typically • Mist flow topology: the liquid phase is dispersed in the continuous vapor in the form of droplets. Typically • Churn flow: this is an intermediate topology between the bubbly and mist flow topology, where

The interfacial surface topologies are used to compute the interfacial area and interfacial transfers of momentum and heat. Introducing to represent interfacial quantities (interfacial area, drag, lift, turbulent drift force and heat transfer), then they are calculated using the following general form: (17.434) Here is computed using equation Equation 17.432 (p. 607) or Equation 17.433 (p. 608), but with different lower and upper limits of the breakpoints. Typically, the values of 0.3 and 0.7 are used and and are the interfacial quantities from bubbly flow and mist flow, respectively. They are calculated using the interfacial sub-models presented in Interfacial Momentum Transfer (p. 609) and Interfacial Heat Transfer (p. 611).

608

Eulerian Model Theory It may be noted that in the boiling models, the liquid is usually defined as the first phase, and the vapor as the second phase. Once this is defined, it remains unchanged with the flow regime transition. When and are calculated, however, the primary or secondary phases are switched. For , the liquid is treated as the primary phase, while the vapor is the secondary phase. Contrary to this, for , the vapor becomes the primary phase and the liquid is the secondary phase.

17.5.20.5. Interfacial Momentum Transfer The interfacial momentum transfer may include five parts: drag, lift, wall lubrication, turbulent drift forces, and virtual mass (described in Interphase Exchange Coefficients (p. 559)). Various models are available for each of these effects, some of which are specifically formulated for boiling flows. Also, user-defined options are available.

17.5.20.5.1. Interfacial Area The interfacial area can be calculated using either a transport equation or algebraic models as described in Interfacial Area Concentration (p. 558). For boiling flows, the algebraic formulations are typically chosen. See Interfacial Area Concentration (p. 558) for model details.

17.5.20.5.2. Bubble and Droplet Diameters 17.5.20.5.2.1. Bubble Diameter By default, Fluent uses the following model for bubble diameter as a function of the local subcooling, :

(17.435) where:

As an alternative, the bubble diameter,

, can be given by the Unal correlation [492] (p. 802) :

(17.436)

To use the Unal correlation, Equation 17.436 (p. 609), you can use the following scheme command: (rpsetvar 'mp/boiling/bubble-diameter-model 2)

To return to the default formulation, Equation 17.435 (p. 609), you can use the scheme command: (rpsetvar 'mp/boiling/bubble-diameter-model 1)

609

Multiphase Flows

17.5.20.5.2.2. Droplet Diameter When the flow regime transitions to mist flow, the droplet diameter can be assumed to be constant or estimated by the Kataoka-Ishii correlation [216] (p. 787): (17.437)

where:

is the vapor volumetric flux (superficial velocity) is the local liquid Reynolds number is the local vapor Reynolds number is the liquid viscosity is the vapor viscosity

17.5.20.5.3. Interfacial Drag Force The interfacial drag force is calculated using the standard model described in Interphase Exchange Coefficients (p. 559) (and defined in the context of the interfacial area from Interfacial Area Concentration (p. 558)). As described in Fluid-Fluid Exchange Coefficient (p. 560), ANSYS Fluent offers several options to calculate the drag force on dispersed phases. For boiling flows, the Ishii model (Ishii Model (p. 564)) is typically chosen, though any of the models listed in Fluid-Fluid Exchange Coefficient (p. 560) are available.

17.5.20.5.4. Interfacial Lift Force As described in Lift Force (p. 570), ANSYS Fluent offers several options to calculate the lift forces on the dispersed phases. For boiling flows, this force is important in the nucleating boiling regime. In the RPI model, the Tomiyama model (Tomiyama Lift Force Model (p. 572)) is usually chosen to account for the effects of the interfacial lift force.

17.5.20.5.5. Turbulent Dispersion Force As described in Turbulent Dispersion Force (p. 575), ANSYS Fluent offers several options to calculate the turbulent dispersion force. For boiling flows, this force is important in transporting the vapor from walls to the core fluid flow regions. In the RPI model, the Lopez de Bertodano model (Lopez de Bertodano Model (p. 576)) is usually chosen to account for the effects of the turbulent dispersion force.

17.5.20.5.6. Wall Lubrication Force As described in Wall Lubrication Force (p. 573), ANSYS Fluent offers several options to calculate the wall lubrication force on the dispersed phases. For boiling flows, this force can be important in the nucleating boiling regime. In the RPI model, the Antal et al. model (Antal et al. Model (p. 573)) is usually chosen to account for the effects of the wall lubrication force.

17.5.20.5.7. Virtual Mass Force In the wall boiling models, the virtual mass force can be modeled using the standard correlation implemented for the Eulerian multiphase model as described in Virtual Mass Force (p. 578). 610

Wet Steam Model Theory

17.5.20.6. Interfacial Heat Transfer 17.5.20.6.1. Interface to Liquid Heat Transfer As the bubbles depart from the wall and move towards the subcooled region, there is heat transfer from the bubble to the liquid, that is defined as (17.438) where is the volumetric heat transfer coefficient. The heat transfer coefficient can be computed using either the Ranz-Marshall or Tomiyama models described in The Heat Exchange Coefficient (p. 585).

17.5.20.6.2. Interface to Vapor Heat Transfer The interface to vapor heat transfer is calculated using the constant time scale return to saturation method [255] (p. 789). It is assumed that the vapor retains the saturation temperature by rapid evaporation/condensation. The formulation is as follows: (17.439) Where

is the time scale set to a default value of 0.05 and

is the isobaric heat capacity.

17.5.20.7. Mass Transfer 17.5.20.7.1. Mass Transfer From the Wall to Vapor The evaporation mass flow is applied at the cell near the wall and it is derived from the evaporation heat flux, Equation 17.439 (p. 611): (17.440)

17.5.20.7.2. Interfacial Mass Transfer The interfacial mass transfer depends directly on the interfacial heat transfer. Assuming that all the heat transferred to the interface is used in mass transfer (that is, evaporation or condensation), the interfacial mass transfer rate can be written as: (17.441)

17.5.20.8. Turbulence Interactions To model boiling flows, turbulence interaction models are usually included in the multiphase turbulence models to describe additional bubble stirring and dissipation. As described in Turbulence Models (p. 588), three options are available for boiling flows: Troshko-Hassan (default), Simonin et al, and Sato.

17.6. Wet Steam Model Theory Information is organized into the following subsections: 17.6.1. Overview of the Wet Steam Model 17.6.2. Limitations of the Wet Steam Model 17.6.3. Wet Steam Flow Equations 17.6.4. Phase Change Model Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

611

Multiphase Flows 17.6.5. Built-in Thermodynamic Wet Steam Properties

17.6.1. Overview of the Wet Steam Model During the rapid expansion of steam, a condensation process will take place shortly after the state path crosses the vapor-saturation line. The expansion process causes the super-heated dry steam to first subcool and then nucleate to form a two-phase mixture of saturated vapor and fine liquid droplets known as wet steam. Modeling wet steam is very important in the analysis and design of steam turbines. The increase in steam turbine exit wetness can cause severe erosion to the turbine blades at the low-pressure stages, and a reduction in aerodynamic efficiency of the turbine stages operating in the wet steam region [330] (p. 793). ANSYS Fluent has adopted the Eulerian-Eulerian approach for modeling wet steam flow. The flow mixture is modeled using the compressible Navier-Stokes equations, in addition to two transport equations for the liquid-phase mass-fraction ( ), and the number of liquid-droplets per unit volume ( ). The phase change model, which involves the formation of liquid-droplets in a homogeneous nonequilibrium condensation process, is based on the classical non-isothermal nucleation theory. This section describes the theoretical aspects of the wet steam model. Information about enabling the model and using your own property functions and data with the wet steam model is provided in Setting Up the Wet Steam Model in the User's Guide. Solution settings and strategies for the wet steam model can be found in Wet Steam Model in the User’s Guide. Postprocessing variables are described in ModelSpecific Variables in the User's Guide.

17.6.2. Limitations of the Wet Steam Model The following restrictions and limitations currently apply to the wet steam model in ANSYS Fluent: • The wet steam model is available for the density-based solver only. • Pressure inlet, mass-flow inlet, and pressure outlet are the only inflow and outflow boundary conditions available. • When the wet steam model is active, the access to the Create/Edit Materials dialog box is restricted because the fluid mixture properties are determined from the built-in steam property functions or from the userdefined wet steam property functions. Therefore, if solid properties need to be set and adjusted, then it must be done in the Create/Edit Materials dialog box before activating the wet steam model.

17.6.3. Wet Steam Flow Equations The wet steam is a mixture of two-phases. The primary phase is the gaseous-phase consisting of watervapor (denoted by the subscript v) while the secondary phase is the liquid-phase consisting of condensedwater droplets (denoted by the subscript l). The following assumptions are made in this model: • The velocity slip between the droplets and gaseous-phase is negligible. • The interactions between droplets are neglected. • The mass fraction of the condensed phase,

612

(also known as wetness factor), is small (

).

Wet Steam Model Theory • Since droplet sizes are typically very small (from approximately 0.1 microns to approximately 100 microns), it is assumed that the volume of the condensed liquid phase is negligible. From the preceding assumptions, it follows that the mixture density ( ) can be related to the vapor density ( ) by the following equation: (17.442) In addition, the temperature and the pressure of the mixture will be equivalent to the temperature and pressure of the vapor-phase. The mixture flow is governed by the compressible Navier-Stokes equations given in vector form by Equation 21.70 (p. 705): (17.443) where =(P,u,v,w,T) are mixture quantities. The flow equations are solved using the same density-based solver algorithms employed for general compressible flows. To model wet steam, two additional transport equations are needed [195] (p. 785). The first transport equation governs the mass fraction of the condensed liquid phase ( ): (17.444) where is the mass generation rate due to condensation and evaporation (kg per unit volume per second). The second transport equation models the evolution of the number density of the droplets per unit volume: (17.445) where

is the nucleation rate (number of new droplets per unit volume per second).

To determine the number of droplets per unit volume, Equation 17.442 (p. 613) and the average droplet volume are combined in the following expression: (17.446) where

is the liquid density and the average droplet volume is defined as (17.447)

where

is the droplet radius.

Together, Equation 17.443 (p. 613), Equation 17.444 (p. 613), and Equation 17.445 (p. 613) form a closed system of equations that, along with Equation 17.442 (p. 613), permit the calculation of the wet steam flow field.

17.6.4. Phase Change Model The following is assumed in the phase change model:

613

Multiphase Flows • The condensation is homogeneous (that is, no impurities present to form nuclei). • The droplet growth is based on average representative mean radii. • The droplet is assumed to be spherical. • The droplet is surrounded by infinite vapor space. • The heat capacity of the fine droplet is negligible compared with the latent heat released in condensation. The mass generation rate in the classical nucleation theory during the nonequilibrium condensation process is given by the sum of mass increase due to nucleation (the formation of critically sized droplets) and also due to growth/demise of these droplets [195] (p. 785). Therefore,

is written as: (17.448)

where is the average radius of the droplet, and is the Kelvin-Helmholtz critical droplet radius, above which the droplet will grow and below which the droplet will evaporate. An expression for is given by [540] (p. 804). (17.449) where

is the liquid surface tension evaluated at temperature

,

is the condensed liquid density

(also evaluated at temperature ), and is the super saturation ratio defined as the ratio of vapor pressure to the equilibrium saturation pressure: (17.450) The expansion process is usually very rapid. Therefore, when the state path crosses the saturated-vapor line, the process will depart from equilibrium, and the supersaturation ratio can take on values greater than one. The condensation process involves two mechanisms, the transfer of mass from the vapor to the droplets and the transfer of heat from the droplets to the vapor in the form of latent heat. This energy transfer relation was presented in [538] (p. 804) and used in [195] (p. 785) and can be written as: (17.451)

where

is the droplet temperature.

The classical homogeneous nucleation theory describes the formation of a liquid-phase in the form of droplets from a supersaturated phase in the absence of impurities or foreign particles. The nucleation rate described by the steady-state classical homogeneous nucleation theory [540] (p. 804) and corrected for non-isothermal effects, is given by: (17.452) where is evaporation coefficient, is the Boltzmann constant, liquid surface tension, and is the liquid density at temperature .

614

is mass of one molecule,

is the

Wet Steam Model Theory A non-isothermal correction factor, , is given by: (17.453) where cities.

is the specific enthalpy of evaporation at pressure

and

is the ratio of specific heat capa-

17.6.5. Built-in Thermodynamic Wet Steam Properties There are many equations that describe the thermodynamic state and properties of steam. While some of these equations are accurate in generating property tables, they are not suitable for fast CFD computations. Therefore, ANSYS Fluent uses a simpler form of the thermodynamic state equations [539] (p. 804) for efficient CFD calculations that are accurate over a wide range of temperatures and pressures. These equations are described below.

17.6.5.1. Equation of State The steam equation of state used in the solver, which relates the pressure to the vapor density and the temperature, is given by [539] (p. 804): (17.454) where , and

are the second and the third virial coefficients given by the following empirical functions: (17.455)

where is given in m3/kg, = -0.0004882.

=

with

given in Kelvin,

= 10000.0,

= 0.0015,

= -0.000942, and (17.456)

where is given in m6/kg2, 1.5E-06.

=

with

given in Kelvin,

The two empirical functions that define the virial coefficients from 273 K to 1073 K. The vapor isobaric specific heat capacity

= 0.8978, =11.16, = 1.772, and =

and

cover the temperature range

is given by: (17.457)

The vapor specific enthalpy,

is given by: (17.458)

The vapor specific entropy,

is given by: (17.459)

The isobaric specific heat at zero pressure is defined by the following empirical equation:

615

Multiphase Flows

(17.460) where is in KJ/kg K, and = -9.70466E-14.

= 46.0,

= 1.47276,

= 8.38930E-04,

= -2.19989E-07,

= 2.46619E-10,

and = Both

,

= and

,

=

, and

=

.

are functions of temperature and they are defined by: (17.461) (17.462)

where

and

are arbitrary constants.

The vapor dynamic viscosity obtained from [538] (p. 804).

and thermal conductivity

are also functions of temperature and were

17.6.5.2. Saturated Vapor Line The saturation pressure equation as a function of temperature was obtained from [399] (p. 797). The example provided in UDWSPF Example in the User's Guide contains a function called wetst_satP() that represents the formulation for the saturation pressure.

17.6.5.3. Saturated Liquid Line At the saturated liquid-line, the liquid density, surface tension, specific heat , dynamic viscosity, and thermal conductivity must be defined. The equation for liquid density, , was obtained from [399] (p. 797). The liquid surface tension equation was obtained from [538] (p. 804). While the values of

,

and

were curve fit using published data from [115] (p. 781) and then written in polynomial forms. The example provided in UDWSPF Example in the User's Guide contains functions called wetst_cpl(), wetst_mul(), and wetst_ktl() that represent formulations for , and .

17.6.5.4. Mixture Properties The mixture properties are related to vapor and liquid properties via the wetness factor using the following mixing law: (17.463) where

represents any of the following thermodynamic properties: , ,

,

,

or

.

17.7. Modeling Mass Transfer in Multiphase Flows This section describes the modeling of mass transfer in the framework of ANSYS Fluent’s general multiphase models (that is, Eulerian multiphase, mixture multiphase, VOF multiphase). There are numerous kinds of mass transfer processes that can be modeled in ANSYS Fluent. You can use models available

616

Modeling Mass Transfer in Multiphase Flows in ANSYS Fluent (for example ANSYS Fluent’s cavitation model), or define your own mass transfer model via user-defined functions. See UDF-Prescribed Mass Transfer (p. 618) and the Fluent Customization Manual for more information about the modeling of mass transfer via user-defined functions. Information about mass transfer is presented in the following subsections: 17.7.1. Source Terms due to Mass Transfer 17.7.2. Unidirectional Constant Rate Mass Transfer 17.7.3. UDF-Prescribed Mass Transfer 17.7.4. Cavitation Models 17.7.5. Evaporation-Condensation Model 17.7.6. Interphase Species Mass Transfer

17.7.1. Source Terms due to Mass Transfer ANSYS Fluent adds contributions due to mass transfer only to the momentum, species, and energy equations. No source term is added for other scalars such as turbulence or user-defined scalars. Let be the mass transfer rate per unit volume from the species of phase to the species of phase . In case a particular phase does not have a mixture material associated with it, the mass transfer will be with the bulk phase.

17.7.1.1. Mass Equation The contribution to the mass source for phase

in a cell is (17.464)

and for phase

is (17.465)

17.7.1.2. Momentum Equation For VOF or mixture models, there is no momentum source. For the Eulerian model, the momentum source in a cell for phase

is (17.466)

and for phase

is (17.467)

17.7.1.3. Energy Equation For all multiphase models, the following energy sources are added. The energy source in a cell for phase

is (17.468)

and for phase

is (17.469)

617

Multiphase Flows where

and

are the formation enthalpies of species of phase

ively. The difference between

and

and species

of phase , respect-

is the latent heat.

17.7.1.4. Species Equation The species source in a cell for species of phase

is (17.470)

and for species

of phase

is (17.471)

17.7.1.5. Other Scalar Equations No source/sink terms are added for turbulence quantities and other scalars. The transfer of these scalar quantities due to mass transfer could be modeled using user-defined source terms.

17.7.2. Unidirectional Constant Rate Mass Transfer The unidirectional mass transfer model defines a positive mass flow rate per unit volume from phase to phase : (17.472) where (17.473) and is a constant rate of particle shrinking or swelling, such as the rate of burning of a liquid droplet (units of 1/ time unit). This is not available for the VOF model. If phase

is a mixture material and a mass transfer mechanism is defined for species of phase , then (17.474)

where

is the mass fraction of species in phase .

17.7.3. UDF-Prescribed Mass Transfer Because there is no universal model for mass transfer, ANSYS Fluent provides a UDF that you can use to input models for different types of mass transfer, for example, evaporation, condensation, boiling, and so on. Note that when using this UDF, ANSYS Fluent will automatically add the source contribution to all relevant momentum and scalar equations. This contribution is based on the assumption that the mass “created” or “destroyed” will have the same momentum and energy of the phase from which it was created or destroyed. If you would like to input your source terms directly into momentum, energy, or scalar equations, then the appropriate path is to use UDFs for user-defined sources for all equations, rather than the UDF for mass transfer. See the Fluent Customization Manual for more information about UDF-based mass transfer in multiphase.

17.7.4. Cavitation Models A liquid at constant temperature can be subjected to a decreasing pressure, which may fall below the saturated vapor pressure. The process of rupturing the liquid by a decrease of pressure at constant 618

Modeling Mass Transfer in Multiphase Flows temperature is called cavitation. The liquid also contains the micro-bubbles of noncondensable (dissolved or ingested) gases, or nuclei, which under decreasing pressure may grow and form cavities. In such processes, very large and steep density variations happen in the low-pressure/cavitating regions. This section provides information about the following three cavitation models used in ANSYS Fluent. • Singhal et al. model [438] (p. 799): You can use this model to include cavitation effects in two-phase flows when the mixture model is used. This is also known as the Full Cavitation Model, which has been implemented in ANSYS Fluent since Version 6.1. • Zwart-Gerber-Belamri model [549] (p. 805): You can use this model in both the mixture and Eulerian multiphase models. • Schnerr and Sauer model [426] (p. 798): This is the default model. You can use this model in both the mixture and Eulerian multiphase models. The following assumptions are made in the standard two-phase cavitation models: • The system under investigation must consist of a liquid and a vapor phase. • Mass transfer between the liquid and vapor phase is assumed to take place. Both bubble formation (evaporation) and collapse (condensation) are taken into account in the cavitation models. • For the cavitation model, ANSYS Fluent defines positive mass transfer as being from the liquid to the vapor. • The cavitation models are based on the Rayleigh-Plesset equation, describing the growth of a single vapor bubble in a liquid. • In the Singhal et al. model, noncondensable gases have been introduced into the system. The mass fraction of the noncondensable gases is assumed to be a known constant. • The input material properties used in the cavitation models can be constants, functions of temperature or user-defined. The cavitation models offer the following capabilities: • The Singhal et al. model can be used to account for the effect of noncondensable gases. The Zwart-GerberBelamri and Schnerr and Sauer models do not include the noncondensable gases in the basic model terms. • The Zwart-Gerber-Belamri and Schnerr and Sauer models are compatible with all the turbulence models available in ANSYS Fluent. • Both the pressure-based segregated and coupled solvers are available with the cavitation models. • They are all fully compatible with dynamic mesh and non-conformal interfaces. • Both liquid and vapor phases can be incompressible or compressible. For compressible liquids, the density is described using a user-defined function. See the Fluent Customization Manual for more information on user-defined density functions.

17.7.4.1. Limitations of the Cavitation Models The following limitations apply to the cavitation models in ANSYS Fluent:

619

Multiphase Flows • They can only be used for a single cavitation process. In other words, only a single liquid fluid is subjected to cavitation. Note that this liquid can be a phase or a species in a mixture phase. ANSYS Fluent cannot simulate multiple cavitation processes. • The Singhal et al. model requires the primary phase to be a liquid and the secondary phase to be a vapor. This model is only compatible with the multiphase mixture model. • The Singhal et al. model cannot be used with the Eulerian multiphase model. • The Singhal et al. model is not compatible with the LES turbulence model. • The Zwart-Gerber-Belamri and Schnerr and Sauer models do not take the effect of noncondensable gases into account by default.

17.7.4.1.1. Limitations of Cavitation with the VOF Model 1. Explicit VOF with cavitation is not recommended for the following reasons: • Explicit VOF is solved once per time step, therefore the mass transfer rate (which is a function of solution variables such as pressure, temperature, and volume fraction) is only evaluated once per time step. This may not give results that are consistent with those achieved using the implicit formulation. It is recommended that the volume fraction be solved every iteration, which can be enabled through the expert options in the Multiphase dialog box. • Explicit VOF may not be stable with sharpening schemes such as Geo-Reconstruct and CICSAM. You should use either a Sharp/Dispersed or Dispersed interface modeling option, which provide diffusive interface-capturing discretization schemes for volume fraction, such as QUICK, HRIC, and Compressive. • Explicit VOF does not model any cavitation-specific numerical treatments for added stability. 2. When using the implicit VOF formulation, numerical diffusion caused by turbulent effects is added by default. This added diffusion increases the solution stability, but has adverse effects on the interfacial accuracy. If interfacial sharpness is an area of concern, you can disable the diffusion by setting the rpvar (rpsetvar 'mp/turbulence-effect? #f).

17.7.4.2. Vapor Transport Equation With the multiphase cavitation modeling approach, a basic two-phase cavitation model consists of using the standard viscous flow equations governing the transport of mixture (Mixture model) or phases (Eulerian multiphase), and a conventional turbulence model (k- model). In cavitation, the liquid-vapor mass transfer (evaporation and condensation) is governed by the vapor transport equation: (17.475) where = vapor phase = vapor volume fraction = vapor density = vapor phase velocity , = mass transfer source terms connected to the growth and collapse of the vapor bubbles respectively

620

Modeling Mass Transfer in Multiphase Flows In Equation 17.475 (p. 620), the terms and account for the mass transfer between the liquid and vapor phases in cavitation. In ANSYS Fluent, they are modeled based on the Rayleigh-Plesset equation describing the growth of a single vapor bubble in a liquid.

17.7.4.3. Bubble Dynamics Consideration In most engineering situations we assume that there are plenty of nuclei for the inception of cavitation. Therefore, our primary focus is on proper accounting of bubble growth and collapse. In a flowing liquid with zero velocity slip between the fluid and bubbles, the bubble dynamics equation can be derived from the generalized Rayleigh-Plesset equation as [56] (p. 778) (17.476)

where, = bubble radius = liquid surface tension coefficient = liquid density = liquid kinematic viscosity = bubble surface pressure = local far-field pressure Neglecting the second-order terms and the surface tension force, Equation 17.476 (p. 621) is simplified to (17.477) This equation provides a physical approach to introduce the effects of bubble dynamics into the cavitation model. It can also be considered to be an equation for void propagation and, hence, mixture density.

17.7.4.4. Singhal et al. Model This cavitation model is based on the “full cavitation model”, developed by Singhal et al. [438] (p. 799). It accounts for all first-order effects (that is, phase change, bubble dynamics, turbulent pressure fluctuations, and noncondensable gases). It has the capability to account for multiphase (N-phase) flows or flows with multiphase species transport, the effects of slip velocities between the liquid and gaseous phases, and the thermal effects and compressibility of both liquid and gas phases. The cavitation model can be used with the mixture multiphase model, with or without slip velocities. However, it is always preferable to solve for cavitation using the mixture model without slip velocity; slip velocities can be turned on if the problem suggests that there is significant slip between phases. To derive an expression of the net phase change rate, Singhal et al. [438] (p. 799) uses the following two-phase continuity equations: Liquid phase: (17.478) Vapor phase: Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

621

Multiphase Flows (17.479) Mixture: (17.480) where, = liquid phase = mixture density (function of phase volume fraction and density) Mixture density

is defined as (17.481)

Combining Equation 17.478 (p. 621), Equation 17.479 (p. 622), and Equation 17.480 (p. 622) yields a relationship between the mixture density and vapor volume fraction ( ): (17.482) The vapor volume fraction ( ) can be related to the bubble number density ( ) and the radius of bubble ( ) as (17.483) Substituting Equation 17.483 (p. 622) into Equation 17.482 (p. 622) gives the following: (17.484) Using the Equation 17.477 (p. 621), and combining Equation 17.478 (p. 621), Equation 17.479 (p. 622), Equation 17.482 (p. 622), and Equation 17.484 (p. 622), the expression for the net phase change rate ( ) is finally obtained as (17.485) Here represents the vapor generation or evaporation rate, that is, the source term in Equation 17.475 (p. 620). All terms, except , are either known constants or dependent variables. In the absence of a general model for estimation of the bubble number density, the phase change rate expression is rewritten in terms of bubble radius ( ), as follows: (17.486) Equation 17.486 (p. 622) indicates that the unit volume mass transfer rate is not only related to the vapor density ( ), but the function of the liquid density ( ), and the mixture density ( ) as well. Since Equation 17.486 (p. 622) is derived directly from phase volume fraction equations, it is exact and should accurately represent the mass transfer from liquid to vapor phase in cavitation (bubble growth or evaporation). Though the calculation of mass transfer for bubble collapse or condensation is expected to be different from that of bubble growth, Equation 17.486 (p. 622) is often used as a first approximation to model the bubble collapse. In this case, the right side of the equation is modified by using the absolute value of the pressure difference and is treated as a sink term.

622

Modeling Mass Transfer in Multiphase Flows It may be noted that in practical cavitation models, the local far-field pressure is usually taken to be the same as the cell center pressure. The bubble pressure is equal to the saturation vapor pressure in the absence of dissolved gases, mass transport and viscous damping, that is, . where, = bubble pressure = saturation vapor pressure Based on Equation 17.486 (p. 622), Singhal et al. [548] (p. 805) proposed a model where the vapor mass fraction is the dependent variable in the transport equation. This model accommodates also a single phase formulation where the governing equations is given by: (17.487) where, = vapor mass fraction = noncondensable gases = diffusion coefficient The rates of mass exchange are given by the following equations: If (17.488)

If (17.489) The saturation pressure is corrected by an estimation of the local values of the turbulent pressure fluctuations: (17.490) The constants have the values and . In this model, the liquid-vapor mixture is assumed to be compressible. Also, the effects of turbulence and the noncondensable gases have been taken into account.

17.7.4.5. Zwart-Gerber-Belamri Model Assuming that all the bubbles in a system have the same size, Zwart-Gerber-Belamri [549] (p. 805) proposed that the total interphase mass transfer rate per unit volume ( ) is calculated using the bubble number densities ( ), and the mass change rate of a single bubble: (17.491) Substituting the value of of the net mass transfer:

in Equation 17.491 (p. 623) into Equation 17.483 (p. 622), we have the expression

623

Multiphase Flows

(17.492) Comparing Equation 17.492 (p. 624) and Equation 17.486 (p. 622), you will notice that the difference is only in the density terms in the mass transfer rate. In Equation 17.492 (p. 624), the unit volume mass transfer rate is only related to the vapor phase density ( ). Unlike Equation 17.486 (p. 622), has no relation with the liquid phase and mixture densities in this model. As in Equation 17.486 (p. 622), Equation 17.492 (p. 624) is derived assuming bubble growth (evaporation). To apply it to the bubble collapse process (condensation), the following generalized formulation is used: (17.493) where is an empirical calibration coefficient. Though it is originally derived from evaporation, Equation 17.493 (p. 624) only works well for condensation. It is physically incorrect and numerically unstable if applied to evaporation. The fundamental reason is that one of the key assumptions is that the cavitation bubble does not interact with each other. This is plausible only during the earliest stage of cavitation when the cavitation bubble grows from the nucleation site. As the vapor volume fraction increases, the nucleation site density must decrease accordingly. To model this process, Zwart-Gerber-Belamri proposed to replace model is as follows:

with

in Equation 17.493 (p. 624). Then the final form of this cavitation

If (17.494) If (17.495) where, = bubble radius = 10–6m = nucleation site volume fraction = = evaporation coefficient = = condensation coefficient =

17.7.4.6. Schnerr and Sauer Model As in the Singhal et al. model, Schnerr and Sauer [426] (p. 798) follow a similar approach to derive the exact expression for the net mass transfer from liquid to vapor. The equation for the vapor volume fraction has the general form: (17.496) Here, the net mass source term is as follows: (17.497)

624

Modeling Mass Transfer in Multiphase Flows Unlike Zwart-Gerber-Belamri and Singhal et al., Schnerr and Sauer use the following expression to connect the vapor volume fraction to the number of bubbles per volume of liquid: (17.498) Following a similar approach to Singhal et al., they derived the following equation: (17.499) (17.500) where, = mass transfer rate = bubble radius Comparing Equation 17.499 (p. 625) with Equation 17.486 (p. 622) and Equation 17.492 (p. 624), it is obvious that unlike the two previous models, the mass transfer rate in the Schnerr and Sauer model is proportional to

. Moreover, the function

has the interesting property that

it approaches zero when and , and reaches the maximum in between. Also in this model, the only parameter that must be determined is the number of spherical bubbles per volume of liquid. If you assume that no bubbles are created or destroyed, the bubble number density would be constant. The initial conditions for the nucleation site volume fraction and the equilibrium bubble radius would therefore be sufficient to specify the bubble number density ( ) from Equation 17.498 (p. 625) and then the phase transition by Equation 17.499 (p. 625). As in the two other models, Equation 17.499 (p. 625) is also used to model the condensation process. The final form of the model is as follows: When

, (17.501)

When

, (17.502)

17.7.4.7. Turbulence Factor For the Schnerr-Sauer and Zwart-Gerber-Belamri models, the influence of turbulence on the threshold pressure can optionally be included. This is modeled in a similar way to that in Singhal et al. Model (p. 621). The threshold pressure is calculated from: (17.503) where

and

ded value for

are the liquid phase density and turbulence kinetic energy, respectively. The recommenis 0.39 and this value is used by default.

625

Multiphase Flows

17.7.4.8. Additional Guidelines for the Cavitation Models In practical applications of a cavitation model, several factors greatly influence numerical stability. For instance, the high pressure difference between the inlet and exit, large ratio of liquid to vapor density, and large phase change rates between the liquid and vapor all have unfavorable effects on solution convergence. In addition, poor initial conditions very often lead to an unrealistic pressure field and unexpected cavitating zones, which, once present, are usually difficult to correct. You may consider the following factors/tips when choosing a cavitation model and addressing potential numerical problems: • Choice of the cavitation models In ANSYS Fluent, there are three available cavitation models. The Zwart-Gerber-Belamri and the Schnerr and Sauer models have been implemented following an entirely different numerical procedure from the Singhal et al. model developed in ANSYS Fluent 6.1. Numerically, these two models are robust and converge quickly. It is therefore highly recommended that you should use the Schnerr and Sauer or the Zwart-Gerber-Bleamri model. The Singhal et al. model, though physically similar to the other two, is numerically less stable and more difficult to use. • Choice of the solvers In ANSYS Fluent, both the segregated (SIMPLE, SIMPLEC, and PISO) and coupled pressure-based solvers can be used in cavitation. As usual, the coupled solver is generally more robust and converges faster, particularly for cavitating flows in rotating machinery (liquid pumps, inducers, impellers, etc). For fuel injector applications, however, the segregated solver also performs very well with the Schnerr and Sauer and the Zwart-Gerber-Belamri models. As for the Singhal et al. model, since the coupled solver does not show any significant advantages, it is suggested that the segregated solver is used. • Initial conditions Though no special initial condition settings are required, we suggest that the vapor fraction is always set to inlet values. The pressure is set close to the highest pressure among the inlets and outlets to avoid unexpected low pressure and cavitating spots. In general, the Schnerr and Sauer and ZwartGerber-Belamri models are robust enough so that there is no need for specific initial conditions. But in some very complicated cases, it may be beneficial to obtain a realistic pressure field before substantial cavities are formed. This can be achieved by obtaining a converged/near-converged solution for a single phase liquid flow, and then enabling the cavitation model. Again, the Singhal et al.model is much more sensitive to initial conditions. The above mentioned treatments are generally required. • Pressure discretization schemes As for general multiphase flows, it is more desirable to use the following pressure discretization schemes in cavitation applications in this order: – PRESTO! – body force weighted – second order The standard and linear schemes generally are not very effective in complex cavitating flows and you should avoid using them. • Relaxation factors 626

Modeling Mass Transfer in Multiphase Flows – Schnerr and Sauer and Zwart-Gerber-Belamri models The default settings generally work well. To achieve numerical efficiency, however, the following values may be recommended: → The relaxation factor for vapor is 0.5 or higher unless the solution diverges or all the residuals oscillate excessively. → The density and the vaporization mass can be relaxed, but in general set them to 1. → For the segregated solver, the relaxation factor for pressure should be no less than the value for the momentum equations. → For the coupled solver, the default value for the Courant number (200) may need to be reduced to 2050 in some complex 3D cases. – Singhal et al. model In general, small relaxation factors are recommended for momentum equations, usually between 0.05 – 0.4. The relaxation factor for the pressure-correction equation should generally be larger than those for momentum equations, say in the range of 0.2 – 0.7. The density and the vaporization mass (source term in the vapor equation) can also be relaxed to improve convergence. Typically, the relaxation factor for density is set between the values of 0.3 and 1.0, while for the vaporization mass, values between 0.1 and 1.0 may be appropriate. For some extreme cases, even smaller relaxation factors may be required for all the equations. • Special tips for the Singal et al. model – Noncondensable gases Noncondensable gases are usually present in liquids. Even a small amount (for example, 15 ppm) of noncondensable gases can have significant effects on both the physical results and the convergence characteristics of the solution. A value of zero for the mass fraction of noncondensable gases should generally be avoided. In some cases, if the liquid is purified of noncondensable gases, a much smaller value (for example, ) may be used to replace the default value of 1.5 . In fact, higher mass fractions of the noncondensable gases may in many cases enhance numerical stability and lead to more realistic results. In particular, when the saturation pressure of a liquid at a certain temperature is zero or very small, noncondensable gases will play a crucial role both numerically and physically. – Limits for dependent variables In many cases, setting the pressure upper limit to a reasonable value can help convergence greatly at the early stage of the solution. It is advisable to always limit the maximum pressure when possible. By default, ANSYS Fluent sets the maximum pressure limit to 5.0

Pascal.

– Relaxation factor for the pressure correction equation For cavitating flows, a special relaxation factor is introduced for the pressure correction equation. By default, this factor is set to 0.7, which should work well for most of the cases. For some very complicated cases, however, you may experience the divergence of the AMG solver. Under those circumstances, this value may be reduced to no less than 0.4. You can set the value of this relaxation factor by typing a text command. For more information, contact your ANSYS Fluent support engineer.

627

Multiphase Flows

17.7.4.9. Extended Cavitation Model Capabilities When cavitation occurs, in many practical applications other gaseous species exist in the systems. For instance, in a ventilated supercavitating vehicle, air is injected into a liquid to stabilize or increase the cavitation along the vehicle surfaces. In some cases, the incoming flow is a mixture of a liquid and some gaseous species. To predict those types of cavitating flows, the basic two-phase cavitation model must be extended to a multiphase (N-phase) flow, or a multiphase species transport cavitation model.

17.7.4.9.1. Multiphase Cavitation Models The multiphase cavitation models are the extensions of the three basic two-phase cavitation models to multiphase flows. In addition to the primary liquid and secondary vapor phase, more secondary gaseous phases can be included into the computational system under the following assumptions: • Mass transfer (cavitation) only occurs between the liquid and vapor phase. • The basic two-phase cavitation models are still used to model the phase changes between the liquid and vapor. • Only one secondary phase can be defined as compressible gas phase, while a user-defined density may be applied to all the phases. • In the Singhal et al. model, the predescribed noncondensable gases can be included in the system. To exclude noncondensable gases from the system, the mass fraction must be set to 0, and the noncondensable gas must be modeled by a separate compressible gas phase. • For non-cavitating phase ( ), the general transport equation governing the vapor phase is the volume fraction equation in the Zwart-Gerber-Belamri and Schnerr and Sauer models, while in the Singhal et al. model, a mass transfer equation is solved and the vapor must be the second phase.

17.7.4.9.2. Multiphase Species Transport Cavitation Model In some cases, there are several gas phase components in a system that can be considered compressible. Since only one compressible gas phase is allowed in the general multiphase approach, the multiphase species transport approach offers an option to handle these types of applications by assuming that there is one compressible gas phase with multiple species. The detailed description of the multiphase species transport approach can be found in Modeling Species Transport in Multiphase Flows (p. 639). The multiphase species transport cavitation model can be summarized as follows: • All the assumptions/limitations for the multiphase cavitation model apply here. • The primary phase can only be a single-species liquid. • All the secondary phases allow more than one species. • The mass transfer between a liquid and a vapor phase/species is modeled by the basic cavitation models. • The mass transfer between other phases or species are modeled with the standard mass transfer approach. In the standard model, zero constant rates should be chosen. • For the phases with multiple species, the phase shares the same pressure as the other phases, but each species has its own pressure (that is, partial pressure). As a result, the vapor density and the pressure used in Equation 17.489 (p. 623) are the partial density and pressure of the vapor. 628

Modeling Mass Transfer in Multiphase Flows

17.7.5. Evaporation-Condensation Model ANSYS Fluent uses one of two models for interphase mass transfer through evaporation-condensation. With the VOF and mixture formulations the Lee model is used. With the Eulerian formulation, you can choose between the Lee model and the Thermal Phase Change model.

17.7.5.1. Lee Model The Lee model [258] (p. 789) is a mechanistic model with a physical basis. It is used with the mixture and VOF multiphase models and can be selected with the Eulerian multiphase model if one of the overall interfacial heat transfer coefficient models will be used (as opposed to the two-resistance model). In the Lee model, the liquid-vapor mass transfer (evaporation and condensation) is governed by the vapor transport equation: (17.504) where = vapor phase = vapor volume fraction = vapor density = vapor phase velocity ,

= the rates of mass transfer due to evaporation and condensation, respectively.

These rates use units of kg/s/m3 As shown in the right side of Equation 17.504 (p. 629), ANSYS Fluent defines positive mass transfer as being from the liquid to the vapor for evaporation-condensation problems. Based on the following temperature regimes, the mass transfer can be described as follows: If

(evaporation): (17.505)

If

(condensation): (17.506)

is a coefficient that must be fine tuned and can be interpreted as a relaxation time. and are the phase volume fraction and density, respectively. The source term for the energy equation can be obtained by multiplying the rate of mass transfer by the latent heat. Consider the Hertz Knudsen formula [178] (p. 785) [232] (p. 788), which gives the evaporation-condensation flux based on the kinetic theory for a flat interface: (17.507) The flux has units of kg/s/m2, is the pressure, is the temperature, and is the universal gas constant. The coefficient is the so-called accommodation coefficient that shows the portion of vapor molecules going into the liquid surface and adsorbed by this surface. represents the vapor partial pressure at Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

629

Multiphase Flows the interface on the vapor side. The Clapeyron-Clausius equation relates the pressure to the temperature for the saturation condition. (It is obtained by equating the vapor and liquid chemical potentials): (17.508) and are the inverse of the density for the vapor and liquid (volume per mass unit), respectively. is the latent heat (J/kg). Based on this differential expression, we can obtain variation of temperature from variation of pressure close to the saturation condition. Figure 17.6: The Stability Phase Diagram

The Clausius Clapeyron equation yields the following formula as long as saturation condition:

and

are close to the (17.509)

Using this relation in the above Hertz Knudsen equation yields [477] (p. 801) (17.510) The factor the vapor.

is defined by means of the accommodation coefficient and the physical characteristics of approaches 1.0 at near equilibrium conditions.

In the Eulerian and mixture multiphase models, the flow regime is assumed to be dispersed. If we assume that all vapor bubbles, for example, have the same diameter, then the interfacial area density is given by the following formula: (17.511) where

is the bubble diameter ( ) and the phase source term (kg/s/m3) should be of the form: (17.512)

From the above equation,

630

, which is the inverse of the relaxation time (1/s) is defined as

Modeling Mass Transfer in Multiphase Flows

(17.513) This leads to the final expression for the evaporation process defined in Equation 17.505 (p. 629). It can be treated implicitly as a source term in the phase conservation equation. A similar expression can be obtained for condensation. In this case, we consider small droplets in a continuous vapor phase even if your primary phase is a liquid. Note that the coefficient should theoretically be different for the condensation and evaporation expression. Furthermore, the theoretical expression is based on a few strong assumptions: • flat interface • dispersed regime with constant diameter • known The bubble diameter and accommodation coefficient are usually not very well known, which is why the coefficient must be fine tuned to match experimental data. By default, the coefficient for both evaporation and condensation is 0.1. However, in practical cases, the order of magnitude of can be as high 103.

17.7.5.2. Thermal Phase Change Model The thermal phase change model is available for evaporation-condensation when the Eulerian multiphase model is used with the two-resistance approach for computing phase heat transfer coefficients (TwoResistance Model (p. 587)). Applying Equation 17.336 (p. 587) and Equation 17.337 (p. 587) to a liquid-vapor pair gives: From the interface to the liquid phase: (17.514) From the interface to the vapor phase: (17.515) where and are the liquid and vapor phase heat transfer coefficients and and are the liquid and vapor phase enthalpies. The interfacial temperature, , is determined from consideration of thermodynamic equilibrium. Ignoring effects of surface tension on pressure, we can assume that , the saturation temperature. Because neither heat nor mass can be stored on the phase interface, the overall heat balance must be satisfied: (17.516) From the preceding equations, the mass transfer through evaporation from the liquid to the vapor phase can be expressed as: (17.517)

631

Multiphase Flows The phase enthalpies, and , need to be computed correctly to take account of the discontinuity in static enthalpy due to latent heat between the two phases as well as the heat transfer from either phase to the phase interface. Using the Prakash formulation [387] (p. 796) with the bulk fluid enthalpy carried out of the outgoing phase and the saturation enthalpy carried into the incoming phase we have: If

(evaporation, the liquid phase is the outgoing phase): (17.518)

If

(condensation, the liquid phase is the incoming phase): (17.519)

This leads to a formulation that is stable both physically and numerically. It implies that the denominator in Equation 17.517 (p. 631) is non-zero, being greater than or equal to the latent heat: (17.520) It may be noted that with the presence of mass transfer can be expressed in a general form:

and

are total phase enthalpies, which

(17.521) where is the reference temperature and is the standard state enthalpy at tion 17.521 (p. 632) applies for phases as well as species on each phase.

. Equa-

Note that in the thermal phase change model the evaporation-condensation mass transfer process is governed entirely by the interphase heat transfer processes and the overall heat balance. There is no calibration required for the mass transfer coefficients as there is in the Lee model. Therefore, it is generally recommended that you use the Eulerian multiphase formulation with the two-resistance heat transfer method when simulating evaporation-condensation processes.

17.7.6. Interphase Species Mass Transfer You can use Fluent to model interphase species mass transfer. Interphase species mass transfer can occur across a phase interface (between a gas and a liquid, or between a liquid and a solid) depending directly on the concentration gradient of the transporting species in the phases. For example, • evaporation of a liquid into a gaseous mixture including its vapor, such as the evaporation of liquid water into a mixture of air and water vapor. • absorption/dissolution of a dissolved gas in a liquid from a gaseous mixture. For example, the absorption of oxygen by water from air. The interphase species mass transfer can be solved in either the Mixture model (with Slip Velocity enabled) or the Eulerian model subject to the following conditions: • Both phases consist of mixtures with at least two species, and at least one of the species is present in both phases. • The two mixture phases are in contact and separated by an interface.

632

Modeling Mass Transfer in Multiphase Flows • Species mass transfer can only occur between the same species from one phase to the other. For example, evaporation/condensation between water liquid and water vapor. • As in all interphase mass transfer models in Fluent, if a gas mixture is involved in an interphase species mass transfer process, it is always treated as a “to”-phase and the liquid mixture as a “from”-phase. • For the species involved in the mass transfer, the mass fractions in both phases must be determined by solving transport equations. For example, the mass fractions of water liquid and water vapor in an evaporation/condensation case must be solved directly from the governing equations, rather than algebraically or from the physical constraint relations. To model interphase species mass transfer, phase species transport equations are solved along with the phase mass, momentum and energy equations. The transport equation for , the local mass fraction of species in phase , is: (17.522)

where

denotes the pth phase, and

is the number of phases in the system. th

phase volume fraction, density, and velocity for the q phase.

,

, and

are the

is the net rate of production of homo-

geneous species through chemical reaction in phase . is the rate of production from external sources, and is the heterogeneous reaction rate. denotes the mass transfer source from species on phase to species on phase . Similarly, denotes the mass transfer from species on phase to species on phase . In practice, the source term for mass transfer of a species between phase and phase is treated a single term, either or depending on the direction of the mass transfer. The volumetric rate of species mass transfer is assumed to be a function of mass concentration gradient of the transported species. (17.523) where (= ) is the volumetric mass transfer coefficient between the pth phase and the qth phase, and is the interfacial area. is the mass concentration of species in phase , and is the equilibrium mass concentration of species in phase . In order to solve the species mass transfer it is necessary to determine appropriate values for and .

17.7.6.1. Modeling Approach You can use one of two modeling approaches for species mass transfer. These are the Equilibrium Model and the Two-Resistance Model. The Equilibrium Model enforces equilibrium between the two phases, while the Two-Resistance Model enforces equilibrium only at the interface.

17.7.6.1.1. Equilibrium Model The Equilibrium Model considers the case in which the species on the two phases are in dynamic equilibrium. The interphase mass transfer rate is determined from the relationship between the equilibrium species concentrations on the two phases. Typically at equilibrium the species concentrations on the two phases are not the same. However, there exists a well-defined equilibrium curve relating the two concentrations. For binary mixtures, the equilibRelease 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

633

Multiphase Flows rium curve depends on the temperature and pressure. For multi-component mixtures, it is also a function of the mixture composition. The equilibrium curve is usually monotonic and nonlinear, and is often expressed in terms of equilibrium mole fractions of species and on phases and : (17.524) The simplest curve or relationship is quasi-linear and assumes that at equilibrium the mole fractions of the species between the phases are in proportion: (17.525) where terms of

is the mole fraction equilibrium ratio. This relationship can, alternatively, be expressed in ,

, or

:

(17.526)

where , , and are the equilibrium ratios for molar concentration, mass concentration, and mass fraction, respectively. These equilibrium ratios are related by the following expression: (17.527) There are several well-known interphase equilibrium models to define or determine

for various

physical processes. ANSYS Fluent offers three ways to determine : Raoult’s Law, Henry’s Law, and Equilibrium Ratio. These are described in detail in Species Mass Transfer Models (p. 636).

17.7.6.1.2. Two-Resistance Model In situations where there are discontinuities in phase concentrations at dynamic equilibrium, it is in general not possible to simulate multi-component species mass transfer with the use of a single overall mass transfer coefficient. The two-resistance model, first proposed by Whitman [525] (p. 804), is a more general approach analogous to the two-resistance heat transfer models. It considers separate mass transfer processes with different mass transfer coefficients on either side of the phase interface. Again, consider a single species dissolved in the qth and pth phases with concentration of

and

th

,

th

respectively, and with a phase interface, . Interphase mass transfer from the q phase to the p phase involves three steps: • The transfer of species from the bulk qth phase to the phase interface, . • The transfer of species across the phase interface. It is identified as species in the pth phase. • The transfer of species from the interface, , to the bulk pth phase. The two-resistance model has two principal assumptions: • The rate of species transport between the phases is controlled by the rates of diffusion through the phases on each side of the interface. • The species transport across the interface is instantaneous (zero-resistance) and therefore equilibrium conditions prevail at the phase interface at all times.

634

Modeling Mass Transfer in Multiphase Flows In other words, there exist two “resistances” for species transport between two phases, and they are the diffusions of the species from the two bulk phases and to the phase interface, .This situation is illustrated graphically in Figure 17.7: Distribution of Molar Concentration in the Two-Resistance Model (p. 635). Figure 17.7: Distribution of Molar Concentration in the Two-Resistance Model

Using Equation 17.523 (p. 633) with the assumption that th

and

are the mass transfer coefficients for

th

the q and p phases, respectively, the volumetric rates of phase mass exchange can be expressed as follows: From the interface to the qth phase, (17.528) From the interface to the pth phase, (17.529) Under the dynamic equilibrium condition on the phase interface the overall mass balance must be satisfied: (17.530) The equilibrium relation in Equation 17.526 (p. 634) also applies at the interface: (17.531) From Equation 17.528 (p. 635) — Equation 17.531 (p. 635), one can determine the interface mass concentrations: (17.532) and then obtain the interface mass transfer rates: (17.533)

635

Multiphase Flows The phase-specific mass transfer coefficients, and , can each be determined from one of the methods described in Mass Transfer Coefficient Models (p. 638). It is also possible to specify a zero-resistance condition on one side of the interface. This is equivalent to an infinite phase-specific mass transfer coefficient. For example, if its effect is to force the interface concentration to be the same as the bulk concentration in phase .

17.7.6.2. Species Mass Transfer Models There are several different, but related, variables used to quantify the concentration of a species, , in phase . Molar concentration of species in phase Mass concentration of species in phase Molar fraction of species in phase Mass fraction of species in phase These four quantities are related as follows:

where

is the sum of the molar concentrations of all species in phase , and

is the phase density

of phase .

17.7.6.2.1. Raoult’s Law In gas-liquid systems, the equilibrium relations are most conveniently expressed in terms of the partial pressure of the species in the gas phase. It is well known that for a pure liquid in contact with a gas mixture containing its vapor dynamic equilibrium occurs when the partial pressure of the vapor species is equal to its saturation pressure (a function of temperature). Raoult’s law extends this statement to an ideal liquid mixture in contact with a gas. It states that the partial pressure of the vapor species in phase , phase,

, is equal to the product of its saturation pressure,

, and the molar fraction in the liquid

: (17.534)

If the gas phase behaves as an ideal gas, then Dalton’s law of partial pressure gives: (17.535) Using Equation 17.525 (p. 634), Equation 17.534 (p. 636), and Equation 17.535 (p. 636), Raoult’s law can be rewritten in terms of a molar fraction ratio: (17.536) where the equilibrium ratio,

. With Equation 17.536 (p. 636), the equilibrium mole fraction of

the liquid species can be computed which is then used to compute the mass concentration,

in

Equation 17.523 (p. 633) which can be solved with the models for interfacial area and volumetric mass 636

Modeling Mass Transfer in Multiphase Flows transfer coefficient. While Raoult’s law represents the simplest form of the vapor-liquid equilibrium (VLE) equation, keep in mind that it is of limited use, as the assumptions made for its derivation are usually unrealistic. The most critical assumption is that the liquid phase is an ideal solution. This is not likely to be valid, unless the system is made up of species of similar molecular sizes and chemical nature, such as in the case of benzene and toluene, or n-heptane and n-hexane (see Vapor Liquid Equilibrium Theory (p. 426)).

17.7.6.2.2. Henry’s Law Raoult’s Law applies only for a phase with an ideal liquid mixture. To handle the case of a gas species dissolved into a non-ideal liquid phase, Henry’s law provides a more generalized equilibrium relationship by replacing the saturation pressure, , in Equation 17.534 (p. 636) with a constant, species , referred to as Henry’s constant:

, for the liquid (17.537)

has units of pressure and is known empirically for a wide range of materials, in particular for common gases dissolved in water. It is usually strongly dependent on temperature. The above form of Henry’s laws is typically valid for a dilute mixture (mole fraction less than 0.1) and low pressures (less than 20 bar) [440] (p. 799). As with Raoult’s law, Henry’s law can be combined with Dalton’s law to yield an expression in terms of equilibrium ratio: (17.538) where the equilibrium ratio is

.

In addition to the molar fraction Henry’s constant, also commonly used:

, the molar concentration Henry’s constant,

is

(17.539) Fluent offers three methods for determining the Henry’s constants: constant, the Van’t Hoff correlation, and user-defined. In the Van’t Hoff correlation, the Henry’s constant is a function of temperature: (17.540) and (17.541) where is the enthalpy of solution and . The temperature dependence is:

is the Henry’s constant at the reference temperature,

(17.542) When using the Van’t Hoff correlation in Fluent you specify the value of the reference Henry’s constant, , and the temperature dependence defined in Equation 17.542 (p. 637). These are material-dependent

637

Multiphase Flows constants and can be found in [413] (p. 797). By default, the constants for oxygen are used. You can also choose user-defined constants.

Important Regardless of the units selected in the Fluent user interface, is always expressed in units of M/atm for consistency with the presentation in most reference data tables. Appropriate unit conversions are applied inside Fluent.

17.7.6.2.3. Equilibrium Ratio In situations where neither Raoult’s law nor Henry’s law apply, such as liquid-liquid extraction, or multiple species mass transfers, Fluent offers you the option to directly specify the equilibrium ratios. You may choose to specify the equilibrium ratio in the following forms: Molar concentration equilibrium ratio, Molar fraction equilibrium ratio, Mass fraction equilibrium ratio, These specify the ratio of the concentration variable for the From-phase divided by that for the To-phase as defined in Equation 17.525 (p. 634) and Equation 17.526 (p. 634). You can specify either a constant value or a user-defined function.

17.7.6.3. Mass Transfer Coefficient Models In Fluent, the mass transfer coefficient,

can be modeled as a constant, as a user-defined function,

or as a function of the Sherwood number of the qth phase,

: (17.543)

where is the diffusivity of the qth phase, is the bubble diameter and constant or determined from one of several empirical correlations.

is either specified as a

These methods are detailed in the following sections: 17.7.6.3.1. Constant 17.7.6.3.2. Sherwood Number 17.7.6.3.3. Ranz-Marshall Model 17.7.6.3.4. Hughmark Model 17.7.6.3.5. User-Defined

17.7.6.3.1. Constant When modeled as a constant the constant volumetric mass transfer coefficient, user.

is specified by the

17.7.6.3.2. Sherwood Number In the Sherwood number method the user specifies a constant value for Equation 17.543 (p. 638).

638

and

is computed from

Modeling Species Transport in Multiphase Flows

17.7.6.3.3. Ranz-Marshall Model The Ranz-Marshall model uses an analogous approach to that for the Ranz-Marshall heat transfer coefficient model. The expression for the Sherwood number for flow past a spherical particle has the same form as that for the Nusselt number in the context of heat transfer, with the Prandtl number replaced by the Schmidt number: (17.544) where

where

is the Schmidt number of the qth phase and

and

is the relative Reynolds number:

are, respectively, the dynamic viscosity and density of the qth phase.

is the

magnitude of the relative velocity between phases. The Ranz-Marshall model is based on boundary layer theory for steady flow past a spherical particle. It generally applies under the conditions:

17.7.6.3.4. Hughmark Model Like the Ranz-Marshall model, the Hughmark model [189] (p. 785) for mass transfer coefficient is also analogous to its heat transfer coefficient counterpart. The Ranz-Marshall model is extended to a wider range of Reynolds numbers by the Hughmark correction: (17.545) The Reynolds number crossover point is chosen to ensure continuity. The model should not be used outside of the recommended Schmidt number range.

17.7.6.3.5. User-Defined You can also use a user defined function for the mass transfer coefficient. To create a user-defined function, use the DEFINE_MASS_TRANSFER macro as you would in other mass transfer cases.

17.8. Modeling Species Transport in Multiphase Flows Species transport, as described in Species Transport and Finite-Rate Chemistry (p. 193), can also be applied to multiphase flows. You can choose to solve the conservation equations for chemical species in multiphase flows by having ANSYS Fluent, for each phase , predict the local mass fraction of each species, , through the solution of a convection-diffusion equation for the species. The generalized chemical species conservation equation (Equation 7.1 (p. 193)), when applied to a multiphase mixture can be represented in the following form: (17.546)

639

Multiphase Flows where is the net rate of production of homogeneous species by chemical reaction for phase , is the mass transfer source between species and from phase to , and is the heterogeneous reaction rate. In addition, is the volume fraction for phase from the dispersed phase plus any user-defined sources.

and

is the rate of creation by addition

ANSYS Fluent treats homogeneous gas phase chemical reactions the same as a single-phase chemical reaction. The reactants and the products belong to the same mixture material (set in the Species Model dialog box), and hence the same phase. The reaction rate is scaled by the volume fraction of the particular phase in the cell. The set-up of a homogeneous gas phase chemical reaction in ANSYS Fluent is the same as it is for a single phase. For more information, see Species Transport and Finite-Rate Chemistry (p. 193). For most multiphase species transport problems, boundary conditions for a particular species are set in the associated phase boundary condition dialog box (see Defining Multiphase Cell Zone and Boundary Conditions in the User's Guide), and postprocessing and reporting of results is performed on a per-phase basis (see Postprocessing for Multiphase Modeling in the User’s Guide). For multiphase species transport simulations, the Species Model dialog box allows you to include Volumetric, Wall Surface, and Particle Surface reactions. ANSYS Fluent treats multiphase surface reactions as it would a single-phase reaction. The reaction rate is scaled with the volume fraction of the particular phase in the cell. For more information, see Species Transport and Finite-Rate Chemistry (p. 193).

Important To turn off reactions for a particular phase, while keeping the reactions active for other phases, turn on Volumetric under Reactions in the Species Model dialog box. Then, in the Create/Edit Materials dialog box, select none from the Reactions drop-down list. The species of different phases is entirely independent. There is no implicit relationship between them even if they share the same name. Explicit relationships between species of different phases can be specified through mass transfer and heterogeneous reactions. For more information on mass transfer and heterogeneous reactions, see Including Mass Transfer Effects and Specifying Heterogeneous Reactions in the User's Guide, respectively. Some phases may have a fluid material associated with them instead of a mixture material. The species equations are solved in those phases that are assigned a mixture material. The species equation above is solved for the mass fraction of the species in a particular phase. The mass transfer and heterogeneous reactions will be associated with the bulk fluid for phases with a single fluid material. Additional information about modeling species transport is presented in the following subsections: 17.8.1. Limitations 17.8.2. Mass and Momentum Transfer with Multiphase Species Transport 17.8.3.The Stiff Chemistry Solver 17.8.4. Heterogeneous Phase Interaction

17.8.1. Limitations The following limitations exist for the modeling of species transport for multiphase flows: • The nonpremixed, premixed, partially-premixed combustion, or the composition PDF transport species transport models are not available for multiphase species reactions.

640

Modeling Species Transport in Multiphase Flows • Only the laminar finite-rate, finite-rate/eddy-dissipation, and eddy-dissipation turbulence-chemistry models of homogeneous reactions are available for multiphase species transport. • The discrete phase model (DPM) is not compatible with multiphase species transport.

17.8.2. Mass and Momentum Transfer with Multiphase Species Transport The ANSYS Fluent multiphase mass transfer model accommodates mass transfer between species belonging to different phases. Instead of a matrix-type input, multiple mass transfer mechanisms need to be input. Each mass transfer mechanism defines the mass transfer phenomenon from one entity to another entity. An entity is either a particular species in a phase, or the bulk phase itself if the phase does not have a mixture material associated with it. The mass transfer phenomenon could be specified either through the built-in unidirectional “constant-rate” mass transfer (Unidirectional Constant Rate Mass Transfer (p. 618)) or through user-defined functions. ANSYS Fluent loops through all the mass transfer mechanisms to compute the net mass source/sink of each species in each phase. The net mass source/sink of a species is used to compute species and mass source terms. ANSYS Fluent will also automatically add the source contribution to all relevant momentum and energy equations based on that assumption that the momentum and energy carried along with the transferred mass. For other equations, the transport due to mass transfer must be explicitly modeled by you.

17.8.2.1. Source Terms Due to Heterogeneous Reactions Consider the following reaction: (17.547) Let as assume that

and

belong to phase

and

and

to phase .

17.8.2.1.1. Mass Transfer Mass source for the phases are given by: (17.548) (17.549) where

is the mass source,

is the molecular weight, and

The general expression for the mass source for the

is the reaction rate.

phase is (17.550) (17.551) (17.552)

where

is the stoichiometric coefficient,

represents the product, and

represents the reactant.

17.8.2.1.2. Momentum Transfer Momentum transfer is more complicated, but we can assume that the reactants mix (conserving momentum) and the products take momentum in the ratio of the rate of their formation.

641

Multiphase Flows The net velocity,

, of the reactants is given by: (17.553)

The general expression for the net velocity of the reactants is given by: (17.554)

where

represents the

item (either a reactant or a product).

Momentum transfer for the phases is then given by: (17.555) (17.556) The general expression is (17.557) If we assume that there is no momentum transfer, then the above term will be zero.

17.8.2.1.3. Species Transfer The general expression for source for

species in the

phase is (17.558) (17.559) (17.560)

17.8.2.1.4. Heat Transfer For heat transfer, we need to consider the formation enthalpies of the reactants and products as well: The net enthalpy of the reactants is given by: (17.561)

where

represents the formation enthalpy, and

The general expression for

represents the enthalpy.

is: (17.562)

If we assume that this enthalpy gets distributed to the products in the ratio of their mass production rates, heat transfer for the phases are given by:

642

Modeling Species Transport in Multiphase Flows (17.563) (17.564) The last term in the above equations appears because our enthalpy is with reference to the formation enthalpy. The general expression for the heat source is: (17.565) If we assume that there is no heat transfer, we can assume that the different species only carry their formation enthalpies with them. Therefore the expression for will be: (17.566)

The expression

will be (17.567)

17.8.3. The Stiff Chemistry Solver ANSYS Fluent has the option of solving intraphase and interphase chemical reactions with a stiff chemistry solver. This option is only available for unsteady cases, where a fractional step scheme is applied. In the first fractional step, the multiphase species Equation 17.546 (p. 639) is solved spatially with the reaction term set to zero. In the second fractional step, the reaction term is integrated in every cell using a stiff ODE solver. To use this option, refer to Specifying Heterogeneous Reactions in the User's Guide.

17.8.4. Heterogeneous Phase Interaction To compute the heterogeneous phase interaction rates, a modified Arrhenius type rate expression is provided, which you can define using the graphical user interface (see Specifying Heterogeneous Reactions in the User's Guide), or a user-defined function (see DEFINE_HET_RXN_RATE in the Fluent Customization Manual). The rate expression is in the general form: (17.568)

where (17.569)

Here, is the species mass fraction, is the total number of reactants in a given inter-phase reaction, is the bulk density of the phase , is the volume fraction of the phase , the

643

Multiphase Flows molecular weight (kg/kmol) of the reactant species, in the given reaction, and is the rate constant.

is the stoichiometric coefficient of the reactant

The rate constant in the modified Arrhenius form is given by Equation 17.569 (p. 643). is the phase temperature (phase-1, phase-2, and so on) as required by the expression. Make sure you provide the correct phase for the temperature to be extracted. is the normalization temperature. Usually, is set to unity for most reactions, but certain reaction rate constants may have a given value, which is normally 298.15 K. The reaction rate given in Equation 17.569 (p. 643) will be valid above the kick-off temperature specified by you, where the temperature for the rate calculation is taken from the selected phase.

644

Chapter 18: Solidification and Melting This chapter describes how you can model solidification and melting in ANSYS Fluent. For information about using the model, see Modeling Solidification and Melting of the User's Guide. Information about the theory behind the model is organized into the following sections: 18.1. Overview 18.2. Limitations 18.3. Introduction 18.4. Energy Equation 18.5. Momentum Equations 18.6.Turbulence Equations 18.7. Species Equations 18.8. Back Diffusion 18.9. Pull Velocity for Continuous Casting 18.10. Contact Resistance at Walls 18.11.Thermal and Solutal Buoyancy

18.1. Overview ANSYS Fluent can be used to solve fluid flow problems involving solidification and/or melting taking place at one temperature (for example, in pure metals) or over a range of temperatures (for example, in binary alloys). Instead of tracking the liquid-solid front explicitly, ANSYS Fluent uses an enthalpyporosity formulation. The liquid-solid mushy zone is treated as a porous zone with porosity equal to the liquid fraction, and appropriate momentum sink terms are added to the momentum equations to account for the pressure drop caused by the presence of solid material. Sinks are also added to the turbulence equations to account for reduced porosity in the solid regions. ANSYS Fluent provides the following capabilities for modeling solidification and melting: • calculation of liquid-solid solidification/melting in pure metals as well as in binary alloys • modeling of continuous casting processes (that is,“pulling” of solid material out of the domain) • modeling of the thermal contact resistance between solidified material and walls (for example, due to the presence of an air gap) • modeling of species transport with solidification/melting • postprocessing of quantities related to solidification/melting (that is, liquid fraction and pull velocities) These modeling capabilities allow ANSYS Fluent to simulate a wide range of solidification/melting problems, including melting, freezing, crystal growth, and continuous casting. The physical equations used for these calculations are described in Introduction (p. 646), and instructions for setting up and solving a solidification/melting problem are provided in Modeling Solidification and Melting of the User's Guide.

645

Solidification and Melting

18.2. Limitations As mentioned in Overview (p. 645), the formulation in ANSYS Fluent can be used to model the solidification/melting of pure materials, as well as alloys. In the case of alloys, ANSYS Fluent offers two rules to determine the liquid fraction versus temperature relationship, namely the linear Lever rule and the nonlinear Scheil rule. The following limitations apply to the solidification/melting model in ANSYS Fluent: • The solidification/melting model can be used only with the pressure-based solver; it is not available with the density-based solvers. • The solidification/melting model cannot be used for compressible flows. • Of the general multiphase models (VOF, mixture, and Eulerian), only the VOF model can be used with the solidification/melting model. • With the exception of species diffusivities, you cannot specify separate material properties for the solid and liquid materials through the user interface. However, if needed, separate solid and liquid properties can be specified with user-defined functions using the DEFINE_PROPERTY macro. For details on using the DEFINE_PROPERTY macro, refer to DEFINE_PROPERTY UDFs in the Fluent Customization Manual. • When using the solidification/melting model in conjunction with modeling species transport with reactions, there is no mechanism to restrict the reactions to only the liquid region; that is, the reactions are solved everywhere.

18.3. Introduction An enthalpy-porosity technique [503] (p. 802), [505] (p. 803), [506] (p. 803) is used in ANSYS Fluent for modeling the solidification/melting process. In this technique, the melt interface is not tracked explicitly. Instead, a quantity called the liquid fraction, which indicates the fraction of the cell volume that is in liquid form, is associated with each cell in the domain. The liquid fraction is computed at each iteration, based on an enthalpy balance. The mushy zone is a region in which the liquid fraction lies between 0 and 1. The mushy zone is modeled as a “pseudo” porous medium in which the porosity decreases from 1 to 0 as the material solidifies. When the material has fully solidified in a cell, the porosity becomes zero and hence the velocities also drop to zero. In this section, an overview of the solidification/melting theory is given. Refer to Voller and Prakash [506] (p. 803) for details on the enthalpy-porosity method.

18.4. Energy Equation The enthalpy of the material is computed as the sum of the sensible enthalpy, , and the latent heat, : (18.1) where (18.2)

646

Momentum Equations and = reference enthalpy = reference temperature = specific heat at constant pressure The liquid fraction, , can be defined as

(18.3)

The latent heat content can now be written in terms of the latent heat of the material, : (18.4) The latent heat content can vary between zero (for a solid) and

(for a liquid).

For solidification/melting problems, the energy equation is written as (18.5) where = enthalpy (see Equation 18.1 (p. 646)) = density = fluid velocity = source term The solution for temperature is essentially an iteration between the energy equation (Equation 18.5 (p. 647)) and the liquid fraction equation (Equation 18.3 (p. 647)). Directly using Equation 18.3 (p. 647) [507] (p. 803) to update the liquid fraction usually results in poor convergence of the energy equation. In ANSYS Fluent, the method suggested by Voller and Swaminathan is used to update the liquid fraction. For pure metals, where and are equal, a method based on specific heat, given by Voller and Prakash [506] (p. 803), is used instead.

18.5. Momentum Equations The enthalpy-porosity technique treats the mushy region (partially solidified region) as a porous medium. The porosity in each cell is set equal to the liquid fraction in that cell. In fully solidified regions, the porosity is equal to zero, which extinguishes the velocities in these regions. The momentum sink due to the reduced porosity in the mushy zone takes the following form: (18.6)

where is the liquid volume fraction, is a small number (0.001) to prevent division by zero, is the mushy zone constant, and is the solid velocity due to the pulling of solidified material out of the domain (also referred to as the pull velocity).

647

Solidification and Melting The mushy zone constant measures the amplitude of the damping; the higher this value, the steeper the transition of the velocity of the material to zero as it solidifies. Very large values may cause the solution to oscillate. The pull velocity is included to account for the movement of the solidified material as it is continuously withdrawn from the domain in continuous casting processes. The presence of this term in Equation 18.6 (p. 647) allows newly solidified material to move at the pull velocity. If solidified material is not being pulled from the domain, . More details about the pull velocity are provided in Pull Velocity for Continuous Casting (p. 650).

18.6. Turbulence Equations Sinks are added to all of the turbulence equations in the mushy and solidified zones to account for the presence of solid matter. The sink term is very similar to the momentum sink term (Equation 18.6 (p. 647)): (18.7) where represents the turbulence quantity being solved ( , , , and so on), and the mushy zone constant, , is the same as the one used in Equation 18.6 (p. 647).

18.7. Species Equations For solidification and melting of a pure substance, phase change occurs at a distinct melting temperature, . For a multicomponent mixture, however, a mushy freeze/melt zone exists between a lower solidus and an upper liquidus temperature. When a multicomponent liquid solidifies, solutes diffuse from the solid phase into the liquid phase. This effect is quantified by the partition coefficient of solute , denoted , which is the ratio of the mass fraction in the solid to that in the liquid at the interface. ANSYS Fluent computes the solidus and liquidus temperatures in a species mixture as, (18.8) (18.9) where is the partition coefficient of solute , is the mass fraction of solute , and is the slope of the liquidus surface with respect to . If the value of the mass fraction exceeds the value of the eutectic mass fraction , then is clipped to when calculating the liquidus and solidus temperatures. It is assumed that the last species material of the mixture is the solvent and that the other species are the solutes. ANSYS Fluent expects that you will input a negative value for the liquidus slope of species ( ). If you input a positive slope for , ANSYS Fluent will disregard your input and instead calculate it using the Eutectic temperature and the Eutectic mass fraction : (18.10) Updating the liquid fraction via Equation 18.3 (p. 647) can cause numerical errors and convergence difficulties in multicomponent mixtures. Instead, the liquid fraction is updated as,

648

Species Equations

(18.11)

where the superscript indicates the iteration number, is a relaxation factor with a default value of 0.9, is the cell matrix coefficient, is the time step, is the current density, is the cell volume, is the current cell temperature and is the interface temperature. ANSYS Fluent offers two models for species segregation at the micro-scale, namely the Lever rule and the Scheil rule. The former assumes infinite diffusion of the solute species in the solid, and the latter assumes zero diffusion. For the Lever rule, the interface temperature , is calculated as: (18.12)

where

is the number of species.

The Scheil rule evaluates

as: (18.13)

For information about how back diffusion (that is, a finite amount of diffusion of the solute species) can be incorporated into the formulation, see the section that follows. For the Lever rule, species transport equations are solved for the total mass fraction of species ,

: (18.14)

where

is the reaction rate and

is given by (18.15)

is the velocity of the liquid and is the solid (pull) velocity. is set to zero if pull velocities are not included in the solution. The liquid velocity can be found from the average velocity (as determined by the flow equation) as (18.16) The liquid ( :

) and solid (

) mass fractions are related to each other by the partition coefficient (18.17)

When the Scheil model is selected, ANSYS Fluent solves for

as the dependent variable [504] (p. 802):

(18.18)

649

Solidification and Melting

18.8. Back Diffusion The Scheil rule assumes zero solute diffusion in the solid zone, while the Lever rule assumes infinite solute diffusion in the solid. Back diffusion models a finite solid diffusion at the micro level using a nondimensional parameter (denoted by ) between 0 (no diffusion, as in the Scheil rule) and 1 (infinite diffusion, as in the Lever Rule). typically depends on the solidification conditions such as the solid diffusivity, the local solidification time, and the secondary dendritic arm spacing. The interface temperature is calculated as follows: (18.19) When back diffusion is included with the Scheil rule formulation, the species transport equation is solved with the liquid species mass fraction as the dependent variable:

(18.20)

The total species mass fraction and liquid species mass fraction are related as (18.21)

18.9. Pull Velocity for Continuous Casting In continuous casting processes, the solidified matter is usually continuously pulled out from the computational domain, as shown in Figure 18.1: “Pulling” a Solid in Continuous Casting (p. 651). Consequently, the solid material will have a finite velocity that must be accounted for in the enthalpy-porosity technique.

650

Pull Velocity for Continuous Casting Figure 18.1: “Pulling” a Solid in Continuous Casting

As mentioned in Momentum Equations (p. 647), the enthalpy-porosity approach treats the solid-liquid mushy zone as a porous medium with porosity equal to the liquid fraction. A suitable sink term is added in the momentum equation to account for the pressure drop due to the porous structure of the mushy zone. For continuous casting applications, the relative velocity between the molten liquid and the solid is used in the momentum sink term (Equation 18.6 (p. 647)) rather than the absolute velocity of the liquid. The exact computation of the pull velocity for the solid material is dependent on Young’s modulus and Poisson’s ratio of the solid and the forces acting on it. ANSYS Fluent uses a Laplacian equation to approximate the pull velocities in the solid region based on the velocities at the boundaries of the solidified region: (18.22) ANSYS Fluent uses the following boundary conditions when computing the pull velocities: • At a velocity inlet, a stationary wall, or a moving wall, the specified velocity is used. • At all other boundaries (including the liquid-solid interface between the liquid and solidified material), a zero-gradient velocity is used. The pull velocities are computed only in the solid region. Note that ANSYS Fluent can also use a specified constant value or custom field function for the pull velocity, instead of computing it. See Procedures for Modeling Continuous Casting of the User's Guide for details.

651

Solidification and Melting

18.10. Contact Resistance at Walls ANSYS Fluent’s solidification/melting model can account for the presence of an air gap between the walls and the solidified material, using an additional heat transfer resistance between walls and cells with liquid fractions less than 1. This contact resistance is accounted for by modifying the conductivity of the fluid near the wall. Thus, the wall heat flux, as shown in Figure 18.2: Circuit for Contact Resistance (p. 652), is written as (18.23) where , , and are defined in Figure 18.2: Circuit for Contact Resistance (p. 652), is the thermal conductivity of the fluid, is the liquid volume fraction, and is the contact resistance, which has the same units as the inverse of the heat transfer coefficient. Figure 18.2: Circuit for Contact Resistance

18.11. Thermal and Solutal Buoyancy When multiple species are involved in the solidification/melting process, the density of the liquid pool varies with temperature as well as species composition. In the presence of a gravitational field, buoyancy will be induced by two mechanisms: • Thermal buoyancy: is the gravitational force due to the variation of density with temperature. • Solutal buoyancy: is the gravitational force due to the variation of density with the change in the species composition of the melt. In multi-component solidification problems, the solutes are continuously rejected near the liquid solid interface, leading to the enrichment of the liquid pool with the solutes. This enrichment of the solutes causes a concentration gradient in the immediate vicinity of the solidliquid interface, and consequently a density gradient.

652

Thermal and Solutal Buoyancy Modeling of thermal and solutal buoyancy is often important for the accurate prediction of the overall solidification behavior. The flow due to thermal buoyancy tends to promote mixing and smooths out temperature gradients. Therefore, excluding this term can lead to inaccurate predictions of solidification time. Solutal buoyancy moves the enriched liquid away from the liquid-solid interface and replaces it with the far field nominal composition liquid. Excluding solutal buoyancy can lead to an over prediction of segregation patterns for multi-component materials. For multi-component solidification problems, the buoyancy induced flows are modeled in ANSYS Fluent using the Boussinesq approach. The thermal buoyancy is calculated as described in Natural Convection and Buoyancy-Driven Flows Theory (p. 143). The body forces due to solutal buoyancy are calculated using a similar approach: (18.24)

where

is the mass fraction of the th solute in the liquid phase and

fraction of the

th

species.

number of solute species,

is the solutal expansion coefficient of the is the gravity vector, and

is the reference mass th

species and

is the total

is the reference density.

The total body force is the sum of the solutal and thermal buoyancy, which can have similar or opposite signs, and their relative importance can change significantly during the course of the simulation. Refer to Modeling Thermal and Solutal Buoyancy in the User's Guide to learn how to include thermal and solutal buoyancy.

653

654

Chapter 19: Eulerian Wall Films The Eulerian Wall Film (EWF) model can be used to predict the creation and flow of thin liquid films on the surface of walls. This chapter presents an overview of the theory and the governing equations for the methods used in the Eulerian Wall Film (EWF) model. 19.1. Introduction 19.2. Mass, Momentum, and Energy Conservation Equations for Wall Film 19.3. Passive Scalar Equation for Wall Film 19.4. Numerical Schemes and Solution Algorithm For more information about using the Eulerian Wall Film model, see Modeling Eulerian Wall Films in the User's Guide.

19.1. Introduction The fundamental picture of a film model consists of a two dimensional thin film of liquid on a wall surface, as shown in Figure 19.1: Subgrid Processes That Require a Wall Film Model (p. 655). A thin liquid film forms as liquid drops impinge on a solid surface in the domain. There are several possible outcomes from this impingement: stick, where the droplet impacts the wall with little energy and remains nearly spherical; rebound, where the drop leaves the surface relatively intact but with changed velocity; spread, where the drop hits the wall with moderate energy and spreads out into the film; and splash, where part of the impinging drop joins the film and another part of the drop leaves the wall in several other smaller drops. The thin film assumption is one usually made in both Eulerian and Lagrangian film modeling approaches, specifically that the thickness of the film is small compared to the radius of curvature of the surface so that the properties do not vary across the thickness of the film and that films formed are thin enough so that the liquid flow in the film can be considered parallel to the wall, with an assumed quadratic shape. Further modeling assumptions are needed to close the problem. Figure 19.1: Subgrid Processes That Require a Wall Film Model

655

Eulerian Wall Films

19.2. Mass, Momentum, and Energy Conservation Equations for Wall Film Conservation of mass for a two dimensional film in a three dimensional domain is: (19.1) where

is the liquid density, h the film height,

the surface gradient operator,

the mean film

velocity and the mass source per unit wall area due to droplet collection, film separation, film stripping, and phase change. Conservation of film momentum is given, (19.2) where

The terms on the left hand side of Equation 19.2 (p. 656) represent transient and convection effects, respectively, with tensor denoting the differential advection term computed on the bases of the quadratic film velocity profile representation. On the right hand side, the first term includes the effects of gas-flow pressure, the gravity component normal to the wall surface (known as spreading), and surface tension; the second term represents the effect of gravity in the direction parallel to the film; the third and fourth terms represent the net viscous shear force on the gas-film and film-wall interfaces, based on the quadratic film velocity profile representation; and the last term is associated with droplet collection or separation. Note that in arriving at the shear and viscous terms on the RHS, a parabolic film velocity profile has been assumed. Conservation of film energy is given as: (19.3)

In the above equation, is the average film temperature, and vector is the differential advection term computed using the representation of quadratic film velocity and temperature profiles. On the right hand side, the first term inside the bracket represents the net heat flux on the gas-film and filmwall interfaces, with and as the film surface and wall temperatures, respectively, and as the film half depth temperature, all computed from the film temperature profile representation and thermal boundary conditions at gas-film and film-wall interfaces. is the source term due to liquid impingement from the bulk flow to the wall. is the mass vaporization or condensation rate and latent heat associated with the phase change.

is the

Equation 19.1 (p. 656) and Equation 19.2 (p. 656) form the foundation of EWF modeling, with the solution of Equation 19.3 (p. 656) being optional only when thermal modeling is desired. These equations are solved on the surface of a wall boundary. Since the film considered here is thin, the lubrication approximation (parallel flow) is valid and therefore these equations are solved in local coordinates that are parallel to the surface.

656

Mass, Momentum, and Energy Conservation Equations for Wall Film Subsequent sections present the EWF sub-models and numerical solution procedures adopted in ANSYS Fluent.

19.2.1. Film Sub-Models The Eulerian wall film model can interact with the discrete phase model (DPM) and Eulerian multiphase model through source terms to the film equations. During the interaction with the DPM model, the discrete particles are collected to form a wall film. The discrete particles can splash when interacting with a film boundary, creating additional particles in the same way as described in the Lagrangian film model. Additional particles can be created when the film separates from the wall, or when the shear stress is sufficient that large particles can be stripped from the film. Mass leaving the film surface by separation or stripping is accounted for through source terms to the film equations. In the Eulerian multiphase interaction, a secondary phase in the multiphase flow is captured on solid surfaces, forming liquid films. Collection efficiency can be computed for the solid surfaces. Mass and momentum leaving the liquid film due to separation and stripping is added to the secondary phase in the multiphase flow.

19.2.1.1. DPM Collection Discrete particle streams or discrete particles hitting a face on a wall boundary are absorbed into the film. When particles are absorbed, their mass and momentum are added to the source terms in Equation 19.1 (p. 656) and Equation 19.2 (p. 656), continuity and momentum equations, respectively. The mass source term is given by (19.4) where

is the flow rate of the particle stream impinging on the face. The momentum source term is (19.5)

where

denotes the velocity of the particle stream and

denotes the film velocity.

19.2.1.2. Particle-Wall Interaction Impingement of particles on the film wall boundary may result in particle rebounding, splashing or being absorbed to form wall film. Details of the particle interaction with the wall boundary is described in the Lagrangian film model (Interaction During Impact with a Boundary (p. 436)).

19.2.1.3. Film Separation The film can separate from an edge if two criteria are met – first that the angle between faces is sufficiently large and second if the film inertia is above a critical value (defined by you). If separation occurs, a source term in the film equation is used to remove mass and momentum from the face corresponding to the edge upstream of where the separation occurs. This source term is accumulated at each film time step, and particle streams are created at DPM iterations using this source. New particle streams are created only during the DPM iteration, however the source terms due to film separating from an edge are updated each film step.

19.2.1.3.1. Separation Criteria Based on the work by Foucart, separation can occur at an edge if a critical angle, θ, is exceeded and a Weber number based on the film, , is above a minimum value. The Weber number is defined as,

657

Eulerian Wall Films

(19.6) and

is surface tension of the film. The separation criteria become, (19.7)

Figure 19.2: Separation Criteria

Once separation occurs, you can specify three different models to calculate the number and diameter of the shed particle stream at an edge, based on work by Foucart ([140] (p. 782)), O’Rourke ([356] (p. 794)) and Friedrich ([145] (p. 783)).

19.2.1.3.1.1. Foucart Separation The Foucart model for edge separation assumes that the drop diameter is given by (19.8) where is the length of the edge and the height of the film. The flow rate film is equal to the mass flux of the film across the edge

of the particles from the (19.9)

where is the inward pointing normal from the edge centroid to the face center. The particles separated from an edge using Foucart’s criterion are given the velocity of the film at the edge where the separation occurs.

19.2.1.3.1.2. O’Rourke Separation The O’Rourke model for edge separation assumes that the drop diameter is equal to the height of the film at the edge, and the flow rate of the particles from the film is equal to the mass flux of the film across the edge.

19.2.1.3.1.3. Friedrich Separation Friedrich’s separation model assumes that the drop diameter is equal to the height of the film at the edge, but the flow rate of the particles from the film is calculated from an experimentally derived force ratio between the inertia of the film and the surface tension forces. The force ratio is given by

658

Mass, Momentum, and Energy Conservation Equations for Wall Film

(19.10)

where

and the characteristic breakup length

is given by: (19.11)

and the relative Weber number

and the film flow Reynolds number are defined as follows, (19.12) (19.13)

Unlike the O’Rourke or Foucart separation methods, the mass flux into the particle stream is an experimentally derived percentage of the flux across the edge as given in Equation 19.9 (p. 658). This percentage is assumed to be a function of the force ratio given by: (19.14) which is a curve fit to the data in Friedrich, et.al. ([145] (p. 783)) and is valid for force ratios greater than one (that is, separated flows). If the force ratio in the Friedrich correlation is less than one, the curve fit is clipped to 0.05, implying that 5% of the film mass flux crossing the edge is shed into the particle phase if separation does take place.

19.2.1.4. Film Stripping Film stripping occurs when high relative velocities exist between the gas phase and the liquid film on a wall surface. At sufficiently high shear rates, Kelvin-Helmholtz waves form on the surface of the film and grow, eventually stripping off droplets from the surface. The model for this behavior in the current implementation of the wall film model in ANSYS Fluent is based on work by Lopez de Bertodano, et al. ([287] (p. 791)) and Mayer ([305] (p. 792)). As described in Mayer, in a thin liquid sheet in a shear flow, waves of length λ are formed due a KelvinHelmholtz type of instability, but the waves are damped out by viscous forces in the film. The balance of wave growth and damping provides a term for the frequency ω (19.15) where (19.16)

with

as a sheltering parameter with a value of 0.3, and (19.17)

For waves to grow and eventually break off from the film, ω must be greater than zero which implies that the minimum wave length for growing waves should be: Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

659

Eulerian Wall Films

(19.18)

An expression for frequency from Taylor's theory can be derived using linear stability analysis, and is given by Lopez de Bertodano et al [287] (p. 791) (19.19) Following Mayer, the average droplet size of the stripped droplets is (19.20) where (with a default value of 0.14) is a numerical factor that is equivalent to the Diameter Coefficient in the graphical user interface. To obtain a mass flow rate at the surface of the liquid, it is assumed that erage wavelength of the disturbance. Mayer uses the expression expression for the flow rate of drops stripped from the surface:

where

is the av-

which results in the following (19.21)

where (with a default value of 0.5) is a numerical factor that is equivalent to the Mass Coefficient in the graphical user interface. Mass sources are accumulated during the course of the film calculation according to the above expression, and are injected into the free stream with the velocity of the film and the calculated mass flow rate from the face during the DPM iteration. No mass will be taken from the film unless a minimum (critical) shear rate (defined in the graphical user interface) is exceeded on the face where liquid film exists.

19.2.1.5. Secondary Phase Accretion In an Eulerian multiphase flow, the secondary phase to be collected on a solid surface must have the same material as defined in the Eulerian wall film model. When the secondary phase is captured by the wall surface, its mass and momentum are removed from the multiphase flow, and added as source terms to the continuity and momentum equations, Equation 19.1 (p. 656) and Equation 19.2 (p. 656) respectively, of the wall film. The mass source term is given by (19.22) where

is the secondary phase volume fraction,

velocity normal to the wall surface, and

is the secondary phase density,

is the phase

is the wall surface area.

The momentum source is give by (19.23) where

is the secondary phase velocity vector.

The collection efficiency of the secondary phase is computed as 660

Mass, Momentum, and Energy Conservation Equations for Wall Film

(19.24) where

and

are the reference (far field) secondary phase concentration and velocity, respectively.

19.2.1.6. Coupling of Wall Film with Mixture Species Transport The Eulerian wall film model can also be coupled with the mixture species transport model to consider phase changes between film material (liquid) and the gas species (vapor). The rate of phase change is governed by (19.25) where is the density of the gas mixture, is the mass diffusivity of the vapor species, is the cellcenter-to-wall distance, is the phase change constant, and represents the cell-center mass fraction of the vapor species. The saturation species mass fraction is computed as (19.26) where is the absolute pressure of the gas mixture and, and is the molecular weight of the vapor species and the mixture, respectively. The saturation pressure is a function of temperature only. By default, water vapor is assumed to be the vapor species and its saturation pressure is computed, following Springer et al ([461] (p. 800)), as (19.27)

However, you can replace the above default formulation by using a user-defined property when defining the vapor material in the Materials task page. It is clear from Equation 19.25 (p. 661) that when the vapor mass fraction exceeds the saturation mass fraction, condensation takes place (that is, negative ) instead, when the vapor mass fraction is less than the saturation mass fraction, the liquid film vaporizes. The phase change constant in Equation 19.25 (p. 661) takes different values for condensation and vaporization, respectively. (19.28) where the film height

is used to prevent generating vapor without the presence of liquid.

User-defined phase change rates can be specified through user-defined profile UDFs. Note that such UDFs must return the relevant rates in kg/s. The UDF returned rate will be converted into kg/s/m2 internally in the code.

19.2.2. Boundary Conditions Boundary conditions for Equation 19.1 (p. 656) and Equation 19.2 (p. 656), film mass and momentum equations, are specified only at the wall on which film modeling is enabled. There are two types of conditions: one is the fixed mass and momentum fluxes and the other specified initial film height and velocity. When the fluxes are specified, they are added to the equations as source terms. When the initial film height and velocity are specified, these values are enforced only at the first film time step. Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

661

Eulerian Wall Films

19.2.3. Obtaining Film Velocity Without Solving the Momentum Equations Equation 19.2 (p. 656) is used to obtain film average velocity through momentum conservation. However, one can estimate the film velocity without solving Equation 19.2 (p. 656). In this case, the film velocity is assumed to have a component that is driven by the shear due to the external flow and a component that is driven by gravity.

19.2.3.1. Shear-Driven Film Velocity As shown in Figure 19.3: Shear-Driven Film Velocity (p. 662), it is assumed that the film velocity varies linearly from the wall to the air-film interface. Figure 19.3: Shear-Driven Film Velocity

By equating the two shear forces at the film-air interface, one from the air side and one from the film side, we have, (19.29)

where

is the surface velocity. The film average velocity is therefore, (19.30)

19.2.3.2. Gravity-Driven Film Velocity It is assumed that the film movement is entirely controlled by the balance of gravity force and the wall shear force, and that the film velocity has a parabolic profile with a zero-gradient at the air-film interface, as shown in Figure 19.4: Gravity-Driven Film Velocity (p. 663).

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Passive Scalar Equation for Wall Film Figure 19.4: Gravity-Driven Film Velocity

Assuming no-slip at the wall, the parabolic film velocity becomes

.

Balancing the weight of the film and the wall shear force, we have, (19.31) therefore, (19.32) Applying the zero-gradient assumption at the air-film interface, we have, (19.33) Thus the gravity-induced film velocity is, (19.34) The film average velocity is, (19.35)

19.3. Passive Scalar Equation for Wall Film Transport equation for a passive scalar in wall film is given by: (19.36)

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Eulerian Wall Films where, = arbitrary passive scalar = film height = mean film velocity = liquid density = scalar related mass source term per unit area = diffusivity for the scalar If you select the Passive Scalar option in the Eulerian Wall Film dialog box, the Fluent solver uses Equation 19.36 (p. 663) to solve the transportation of a passive scalar due to film convection and scalar diffusion. Note that the solution of the passive scalar transport has no influence on the solution of wall film equations. For more details, see Setting Eulerian Wall Film Model Options in the Fluent User's Guide.

19.4. Numerical Schemes and Solution Algorithm This section describes the temporal and spatial differencing schemes, as well as the solution algorithms used with the Eulerian Wall Film (EWF) model. 19.4.1.Temporal Differencing Schemes 19.4.2. Spatial Differencing Schemes 19.4.3. Solution Algorithm

19.4.1. Temporal Differencing Schemes The following temporal differencing schemes are available for the Eulerian Wall Film (EWF) model. 19.4.1.1. First-Order Explicit Method 19.4.1.2. First-Order Implicit Method 19.4.1.3. Second-Order Implicit Method

19.4.1.1. First-Order Explicit Method Defining F and G as follows, with the subscript indicates the previous time-step values: (19.37)

We have the following discretized film mass and momentum equations, with subscript +1 represents the current time-step values, and the time step used for film computations.

(19.38)

Film height and velocity are then computed as follows,

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Numerical Schemes and Solution Algorithm

(19.39)

The above set of equations complete the explicit differencing scheme in which film height is computed first, based upon values of evaluated at the previous film time step; then film velocity is calculated using the latest film height and values of evaluated at the previous film time step.

19.4.1.2. First-Order Implicit Method With the explicit method, evaluations of and are done based upon the previous time step film height and velocity vector. In order to improve accuracy of this explicit method, a first-order implicit method is introduced in which and values are updated during an iterative loop within a film time step. This new method can be described as a predictor-corrector procedure. At the beginning, that is, the predictor step, the explicit scheme is used to compute film height and velocity vector, Predictor:

(19.40)

The superscript

indicates the first step in the iteration loop.

Corrector: The latest film height and velocity vector are used to update are recomputed,

and ; then film height and velocity

(19.41)

The superscript +1 and represent the current and the previous iterations, respectively. The iteration procedure ends with the following convergence criteria: (19.42)

where

represent each component of the velocity vector.

19.4.1.3. Second-Order Implicit Method With the above discussed explicit and first-order implicit methods, time differencing is only first-order accurate. A second-order implicit method is introduced below. The iterative procedure is very similar

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Eulerian Wall Films to that used in the first-order implicit method, but two time-steps ( and –1) are used for time differencing in the predictor step. Predictor:

(19.43)

Corrector:

(19.44)

The iteration procedure ends with the following convergence criteria: (19.45)

where

represent each component of the velocity vector.

19.4.2. Spatial Differencing Schemes Computing and , defined in Equation 19.37 (p. 664), involves evaluations of spatial gradients for various quantities at face centers. Applying Green-Gauss theorem the gradient of a scalar at face center is computed as, (19.46) where the subscript represents the center of each edge of a film face, the area of the face and the length vector (whose magnitude is the length of the edge and whose direction is normal to the edge), as in the sketch below, Figure 19.5: Spatial Gradient

Clearly, at the center of the gradient calculation is how to obtain taken in the current implementation.

. The following describes the steps

(1) Primary gradient computation

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Numerical Schemes and Solution Algorithm A primary gradient is calculated using Green-Gauss theorem by simply estimating edge center value as, (19.47) (19.48) (2) Reconstruction of edge-center values The edge-center values are then computed using either the first-order upwind or the second-order upwind schemes, 1st-order upwind: 2nd-order upwind: is the value of at the UPWIND face center; center to the edge center.

is the distance vector from the UPWIND face

(3) Final gradient calculation Finally the gradient at face center is computed as, (19.49)

19.4.3. Solution Algorithm While film movement is always tracked with time, the EWF model can be run with both steady and transient flow simulations.

19.4.3.1. Steady Flow It is assumed that while flow affects its movement, the thin film does not, however, affect the flow field. In this case the flow field has converged and is kept unchanged (frozen flow field). The elapsed time for film development is increased by the film time step, . This time step can either be a user-specified constant or can be computed automatically using an adaptive time-stepping method depending upon the maximum Courant number, described as follows: 1. When the computed maximum Courant number exceeds the user-specified value, the film time step is reduced by a factor of 2. 2. When the computed maximum Courant number is less than half the user-specified value, the film time step is increased by a factor of 1.5.

19.4.3.2. Transient Flow Within each flow time step, , a number of film sub-time steps are used to advance the film time to the same physical time of flow. The film sub-time step is determined as follows, (19.50)

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Eulerian Wall Films is the number of film time steps;

is the flow time step.

Since film computation starts at the end of each flow time step, the effect of the film on flow is lagged by one flow time step.

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Chapter 20: Electric Potential This chapter outlines the basis of the implementation of the electric potential model in ANSYS Fluent. For information about using the model, see Modeling Electric Potential Field in the Fluent User's Guide. The information in this chapter is organized into the following sections: 20.1. Overview and Limitations 20.2. Electric Potential Equation 20.3. Energy Equation Source Term

20.1. Overview and Limitations ANSYS Fluent can model problems involving the electric potential field by solving the electric potential equation, which can be solved in both fluid and solid zones. The electric potential solver can be used alone or in conjunction with other ANSYS Fluent models, except for multiphase flow models. One example where the electric potential model could be coupled with other ANSYS Fluent models is a discrete phase simulation of an electrostatic precipitator. To compute the electrostatic force of the charged particles, the electric potential equation must be solved. The electric potential solver is automatically used with the built-in electrochemical reaction model allowing for the simulation of chemical and electrochemical reactions. For details on how the electrochemical reaction model is implemented in ANSYS Fluent, refer to Electrochemical Reactions (p. 213). If you want to use the electric potential solver together with other ANSYS Fluent CFD solvers, you need to manually enable the potential equation as described in Using the Electric Potential Model in the Fluent User's Guide. The electric potential modeling capabilities allow ANSYS Fluent to simulate a wide variety of phenomena associated with electric potential fields, such as electro-plating, corrosion, flow battery, and others. The physical equation used in this model is described in Electric Potential Equation (p. 669). Instructions for setting up and solving an electric potential field problem can be found in Using the Electric Potential Model in the Fluent User's Guide. Details about using the electric potential solver within the electrochemical reaction model are provided in Electrochemical Reactions in the Fluent User's Guide.

20.2. Electric Potential Equation When the electric potential solver is enabled, ANSYS Fluent solves the following electric potential equation: (20.1) where = electric potential = electric conductivity in a solid zone or ionic conductivity in a fluid zone = source term

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Electric Potential Note that Equation 20.1 (p. 669) does not have a transient term. To solve Equation 20.1 (p. 669), either electric potential or electric current needs to be specified at all external boundaries as described in Using the Electric Potential Model in the Fluent User's Guide.

20.3. Energy Equation Source Term When electric current flows through a medium, it produces heat. This phenomenon is called Joule heating. Joule heating generated by electric current, , can be computed as: (20.2) When solving the potential equation (Equation 20.1 (p. 669)), you can add the Joule heating to the energy equation (Equation 5.1 (p. 140)).

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Chapter 21: Solver Theory This chapter describes the ANSYS Fluent solver theory. Details about the solver algorithms used by ANSYS Fluent are provided in Overview of Flow Solvers (p. 671) – Multigrid Method (p. 715). For more information about using the solver, see Using the Solver in the User's Guide. 21.1. Overview of Flow Solvers 21.2. General Scalar Transport Equation: Discretization and Solution 21.3. Discretization 21.4. Pressure-Based Solver 21.5. Density-Based Solver 21.6. Pseudo Transient Under-Relaxation 21.7. Multigrid Method 21.8. Hybrid Initialization 21.9. Full Multigrid (FMG) Initialization

21.1. Overview of Flow Solvers ANSYS Fluent allows you to choose one of the two numerical methods: • pressure-based solver (see Pressure-Based Solver (p. 672)) • density-based solver (see Density-Based Solver (p. 674)) Historically speaking, the pressure-based approach was developed for low-speed incompressible flows, while the density-based approach was mainly used for high-speed compressible flows. However, recently both methods have been extended and reformulated to solve and operate for a wide range of flow conditions beyond their traditional or original intent. In both methods the velocity field is obtained from the momentum equations. In the density-based approach, the continuity equation is used to obtain the density field while the pressure field is determined from the equation of state. On the other hand, in the pressure-based approach, the pressure field is extracted by solving a pressure or pressure correction equation which is obtained by manipulating continuity and momentum equations. Using either method, ANSYS Fluent will solve the governing integral equations for the conservation of mass and momentum, and (when appropriate) for energy and other scalars such as turbulence and chemical species. In both cases a control-volume-based technique is used that consists of: • Division of the domain into discrete control volumes using a computational grid. • Integration of the governing equations on the individual control volumes to construct algebraic equations for the discrete dependent variables (“unknowns”) such as velocities, pressure, temperature, and conserved scalars. • Linearization of the discretized equations and solution of the resultant linear equation system to yield updated values of the dependent variables.

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Solver Theory The two numerical methods employ a similar discretization process (finite-volume), but the approach used to linearize and solve the discretized equations is different. The general solution methods are described in Pressure-Based Solver (p. 672) and Density-Based Solver (p. 674). To learn how to apply the solvers, see Choosing the Solver in the User's Guide.

21.1.1. Pressure-Based Solver The pressure-based solver employs an algorithm which belongs to a general class of methods called the projection method [77] (p. 779). In the projection method, wherein the constraint of mass conservation (continuity) of the velocity field is achieved by solving a pressure (or pressure correction) equation. The pressure equation is derived from the continuity and the momentum equations in such away that the velocity field, corrected by the pressure, satisfies the continuity. Since the governing equations are nonlinear and coupled to one another, the solution process involves iterations wherein the entire set of governing equations is solved repeatedly until the solution converges. Two pressure-based solver algorithms are available in ANSYS Fluent. A segregated algorithm, and a coupled algorithm. These two approaches are discussed in the sections below.

21.1.1.1. The Pressure-Based Segregated Algorithm The pressure-based solver uses a solution algorithm where the governing equations are solved sequentially (that is, segregated from one another). Because the governing equations are nonlinear and coupled, the solution loop must be carried out iteratively in order to obtain a converged numerical solution. In the segregated algorithm, the individual governing equations for the solution variables (for example, , , , , , , , and so on) are solved one after another. Each governing equation, while being solved, is “decoupled” or “segregated” from other equations, hence its name. The segregated algorithm is memory-efficient, since the discretized equations need only be stored in the memory one at a time. However, the solution convergence is relatively slow, inasmuch as the equations are solved in a decoupled manner. With the segregated algorithm, each iteration consists of the steps illustrated in Figure 21.1: Overview of the Pressure-Based Solution Methods (p. 673) and outlined below: 1. Update fluid properties (for example, density, viscosity, specific heat) including turbulent viscosity (diffusivity) based on the current solution. 2. Solve the momentum equations, one after another, using the recently updated values of pressure and face mass fluxes. 3. Solve the pressure correction equation using the recently obtained velocity field and the mass-flux. 4. Correct face mass fluxes, pressure, and the velocity field using the pressure correction obtained from Step 3. 5. Solve the equations for additional scalars, if any, such as turbulent quantities, energy, species, and radiation intensity using the current values of the solution variables. 6. Update the source terms arising from the interactions among different phases (for example, source term for the carrier phase due to discrete particles). 7. Check for the convergence of the equations.

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Overview of Flow Solvers These steps are continued until the convergence criteria are met. Figure 21.1: Overview of the Pressure-Based Solution Methods

21.1.1.2. The Pressure-Based Coupled Algorithm Unlike the segregated algorithm described above, the pressure-based coupled algorithm solves a coupled system of equations comprising the momentum equations and the pressure-based continuity equation. Thus, in the coupled algorithm, Steps 2 and 3 in the segregated solution algorithm are replaced by a single step in which the coupled system of equations are solved. The remaining equations are solved in a decoupled fashion as in the segregated algorithm. Since the momentum and continuity equations are solved in a closely coupled manner, the rate of solution convergence significantly improves when compared to the segregated algorithm. However, the memory requirement increases by 1.5 – 2 times that of the segregated algorithm since the discrete system of all momentum and pressure-based continuity equations must be stored in the memory when solving for the velocity and pressure fields (rather than just a single equation, as is the case with the segregated algorithm).

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Solver Theory

21.1.2. Density-Based Solver The density-based solver solves the governing equations of continuity, momentum, and (where appropriate) energy and species transport simultaneously (that is, coupled together). Governing equations for additional scalars will be solved afterward and sequentially (that is, segregated from one another and from the coupled set) using the procedure described in General Scalar Transport Equation: Discretization and Solution (p. 676). Because the governing equations are nonlinear (and coupled), several iterations of the solution loop must be performed before a converged solution is obtained. Each iteration consists of the steps illustrated in Figure 21.2: Overview of the Density-Based Solution Method (p. 675) and outlined below: 1. Update the fluid properties based on the current solution. (If the calculation has just begun, the fluid properties will be updated based on the initialized solution.) 2. Solve the continuity, momentum, and (where appropriate) energy and species equations simultaneously. 3. Where appropriate, solve equations for scalars such as turbulence and radiation using the previously updated values of the other variables. 4. When interphase coupling is to be included, update the source terms in the appropriate continuous phase equations with a discrete phase trajectory calculation. 5. Check for convergence of the equation set. These steps are continued until the convergence criteria are met.

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Overview of Flow Solvers Figure 21.2: Overview of the Density-Based Solution Method

In the density-based solution method, you can solve the coupled system of equations (continuity, momentum, energy and species equations if available) using, either the coupled-explicit formulation or the coupled-implicit formulation. The main distinction between the density-based explicit and implicit formulations is described next. In the density-based solution methods, the discrete, nonlinear governing equations are linearized to produce a system of equations for the dependent variables in every computational cell. The resultant linear system is then solved to yield an updated flow-field solution. The manner in which the governing equations are linearized may take an “implicit” or “explicit” form with respect to the dependent variable (or set of variables) of interest. By implicit or explicit we mean the following: • implicit: For a given variable, the unknown value in each cell is computed using a relation that includes both existing and unknown values from neighboring cells. Therefore each unknown will appear in more than one equation in the system, and these equations must be solved simultaneously to give the unknown quantities.

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Solver Theory • explicit: For a given variable, the unknown value in each cell is computed using a relation that includes only existing values. Therefore each unknown will appear in only one equation in the system and the equations for the unknown value in each cell can be solved one at a time to give the unknown quantities. In the density-based solution method you have a choice of using either an implicit or explicit linearization of the governing equations. This choice applies only to the coupled set of governing equations. Transport equations for additional scalars are solved segregated from the coupled set (such as turbulence, radiation, and so on). The transport equations are linearized and solved implicitly using the method described in General Scalar Transport Equation: Discretization and Solution (p. 676). Regardless of whether you choose the implicit or explicit methods, the solution procedure shown in Figure 21.2: Overview of the DensityBased Solution Method (p. 675) is followed. If you choose the implicit option of the density-based solver, each equation in the coupled set of governing equations is linearized implicitly with respect to all dependent variables in the set. This will result in a system of linear equations with equations for each cell in the domain, where is the number of coupled equations in the set. Because there are equations per cell, this is sometimes called a “block” system of equations. A point implicit linear equation solver (Incomplete Lower Upper (ILU) factorization scheme or a symmetric block Gauss-Seidel) is used in conjunction with an algebraic multigrid (AMG) method to solve the resultant block system of equations for all dependent variables in each cell. For example, linearization of the coupled continuity, -, -, -momentum, and energy equation set will produce a system of equations in which , , , , and are the unknowns. Simultaneous solution of this equation system (using the block AMG solver) yields at once updated pressure, -, -, -velocity, and temperature fields. In summary, the coupled implicit approach solves for all variables ( , , , time.

, ) in all cells at the same

If you choose the explicit option of the density-based solver, each equation in the coupled set of governing equations is linearized explicitly. As in the implicit option, this too will result in a system of equations with equations for each cell in the domain and likewise, all dependent variables in the set will be updated at once. However, this system of equations is explicit in the unknown dependent variables. For example, the -momentum equation is written such that the updated velocity is a function of existing values of the field variables. Because of this, a linear equation solver is not needed. Instead, the solution is updated using a multi-stage (Runge-Kutta) solver. Here you have the additional option of employing a full approximation storage (FAS) multigrid scheme to accelerate the multi-stage solver. In summary, the density-based explicit approach solves for all variables ( , , ,

, ) one cell at a time.

Note that the FAS multigrid is an optional component of the explicit approach, while the AMG is a required element in both the pressure-based and density-based implicit approaches.

21.2. General Scalar Transport Equation: Discretization and Solution ANSYS Fluent uses a control-volume-based technique to convert a general scalar transport equation to an algebraic equation that can be solved numerically. This control volume technique consists of integrating the transport equation about each control volume, yielding a discrete equation that expresses the conservation law on a control-volume basis. Discretization of the governing equations can be illustrated most easily by considering the unsteady conservation equation for transport of a scalar quantity . This is demonstrated by the following equation written in integral form for an arbitrary control volume as follows:

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General Scalar Transport Equation: Discretization and Solution

(21.1) where = density = velocity vector (=

in 2D)

= surface area vector = diffusion coefficient for = gradient of = source of

in 2D)

per unit volume

Equation 21.1 (p. 677) is applied to each control volume, or cell, in the computational domain. The twodimensional, triangular cell shown in Figure 21.3: Control Volume Used to Illustrate Discretization of a Scalar Transport Equation (p. 678) is an example of such a control volume. Discretization of Equation 21.1 (p. 677) on a given cell yields (21.2)

where = number of faces enclosing cell = value of

convected through face

= mass flux through the face = area of face , = gradient of

(=

in 2D)

at face

= cell volume Where is defined in Temporal Discretization (p. 685). The equations solved by ANSYS Fluent take the same general form as the one given above and apply readily to multi-dimensional, unstructured meshes composed of arbitrary polyhedra.

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Solver Theory Figure 21.3: Control Volume Used to Illustrate Discretization of a Scalar Transport Equation

For more information, see the following section: 21.2.1. Solving the Linear System

21.2.1. Solving the Linear System The discretized scalar transport equation (Equation 21.2 (p. 677)) contains the unknown scalar variable at the cell center as well as the unknown values in surrounding neighbor cells. This equation will, in general, be nonlinear with respect to these variables. A linearized form of Equation 21.2 (p. 677) can be written as (21.3) where the subscript .

refers to neighbor cells, and

and

are the linearized coefficients for

and

The number of neighbors for each cell depends on the mesh topology, but will typically equal the number of faces enclosing the cell (boundary cells being the exception). Similar equations can be written for each cell in the mesh. This results in a set of algebraic equations with a sparse coefficient matrix. For scalar equations, ANSYS Fluent solves this linear system using a point implicit (Gauss-Seidel) linear equation solver in conjunction with an algebraic multigrid (AMG) method that is described in Algebraic Multigrid (AMG) (p. 721).

21.3. Discretization Information is organized into the following subsections: 21.3.1. Spatial Discretization 21.3.2.Temporal Discretization 21.3.3. Evaluation of Gradients and Derivatives 21.3.4. Gradient Limiters

21.3.1. Spatial Discretization By default, ANSYS Fluent stores discrete values of the scalar at the cell centers ( and in Figure 21.3: Control Volume Used to Illustrate Discretization of a Scalar Transport Equation (p. 678)). However,

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Discretization face values

are required for the convection terms in Equation 21.2 (p. 677) and must be interpolated

from the cell center values. This is accomplished using an upwind scheme. Upwinding means that the face value

is derived from quantities in the cell upstream, or “upwind,”

relative to the direction of the normal velocity in Equation 21.2 (p. 677). ANSYS Fluent allows you to choose from several upwind schemes: first-order upwind, second-order upwind, power law, and QUICK. These schemes are described in First-Order Upwind Scheme (p. 679) – QUICK Scheme (p. 682). The diffusion terms in Equation 21.2 (p. 677) are central-differenced and are always second-order accurate. For information on how to use the various spatial discretization schemes, see Choosing the Spatial Discretization Scheme in the User’s Guide.

21.3.1.1. First-Order Upwind Scheme When first-order accuracy is desired, quantities at cell faces are determined by assuming that the cellcenter values of any field variable represent a cell-average value and hold throughout the entire cell; the face quantities are identical to the cell quantities. Thus when first-order upwinding is selected, the face value is set equal to the cell-center value of in the upstream cell.

Important First-order upwind is available in the pressure-based and density-based solvers.

21.3.1.2. Power-Law Scheme The power-law discretization scheme interpolates the face value of a variable, , using the exact solution to a one-dimensional convection-diffusion equation (21.4) where and are constant across the interval . Equation 21.4 (p. 679) can be integrated to yield the following solution describing how varies with : (21.5)

where = = and

is the Peclet number: (21.6)

The variation of between and is depicted in Figure 21.4: Variation of a Variable Phi Between x=0 and x=L (p. 680) (Equation 21.4 (p. 679)) for a range of values of the Peclet number. Figure 21.4: Variation of a Variable Phi Between x=0 and x=L (p. 680) shows that for large , the value of at is approximately equal to the upstream value. This implies that when the flow is dominated Release 18.1 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

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Solver Theory by convection, interpolation can be accomplished by simply letting the face value of a variable be set equal to its “upwind” or upstream value. This is the standard first-order scheme for ANSYS Fluent. Figure 21.4: Variation of a Variable Phi Between x=0 and x=L

If the power-law scheme is selected, ANSYS Fluent uses Equation 21.5 (p. 679) in an equivalent “power law” format [365] (p. 795), as its interpolation scheme. As discussed in First-Order Upwind Scheme (p. 679), Figure 21.4: Variation of a Variable Phi Between x=0 and x=L (p. 680) shows that for large Pe, the value of at is approximately equal to the upstream value. When = 0 (no flow, or pure diffusion), Figure 21.4: Variation of a Variable Phi Between x=0 and x=L (p. 680) shows that may be interpolated using a simple linear average between the values at and . When the Peclet number has an intermediate value, the interpolated value for at must be derived by applying the “power law” equivalent of Equation 21.5 (p. 679).

Important The power-law scheme is available in the pressure-based solver and when solving additional scalar equations in the density-based solver.

21.3.1.3. Second-Order Upwind Scheme When second-order accuracy is desired, quantities at cell faces are computed using a multidimensional linear reconstruction approach [27] (p. 776). In this approach, higher-order accuracy is achieved at cell faces through a Taylor series expansion of the cell-centered solution about the cell centroid. Thus when second-order upwinding is selected, the face value is computed using the following expression: (21.7)

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Discretization where and are the cell-centered value and its gradient in the upstream cell, and is the displacement vector from the upstream cell centroid to the face centroid. This formulation requires the determination of the gradient in each cell, as discussed in Evaluation of Gradients and Derivatives (p. 687). Finally, the gradient is limited so that no new maxima or minima are introduced.

Important Second-order upwind is available in the pressure-based and density-based solvers.

21.3.1.4. First-to-Higher Order Blending In some instances, and at certain flow conditions, a converged solution to steady-state may not be possible with the use of higher-order discretization schemes due to local flow fluctuations (physical or numerical). On the other hand, a converged solution for the same flow conditions maybe possible with a first-order discretization scheme. For this type of flow and situation, if a better than first-order accurate solution is desired, then first-to-higher-order blending can be used to obtain a converged steady-state solution. The first-order to higher-order blending is applicable only when higher-order discretization is used. It is applicable with the following discretization schemes: second-order upwinding, central-differencing schemes, QUICK, and third-order MUSCL. The blending is not applicable to first-order, power-law, modified HRIC schemes, or the Geo-reconstruct and CICSAM schemes. In the density-based solver, the blending is applied as a scaling factor to the reconstruction gradients. While in the pressure-based solver, the blending is applied to the higher-order terms for the convective transport variable. To learn how to apply this option, refer to First-to-Higher Order Blending in the User's Guide.

21.3.1.5. Central-Differencing Scheme A second-order-accurate central-differencing discretization scheme is available for the momentum equations when you are using the LES turbulence model. This scheme provides improved accuracy for LES calculations. The central-differencing scheme calculates the face value for a variable (

) as follows: (21.8)

where the indices 0 and 1 refer to the cells that share face , gradients at cells 0 and 1, respectively, and face centroid.

and

are the reconstructed

is the vector directed from the cell centroid toward the

It is well known that central-differencing schemes can produce unbounded solutions and non-physical wiggles, which can lead to stability problems for the numerical procedure. These stability problems can often be avoided if a deferred correction is used for the central-differencing scheme. In this approach, the face value is calculated as follows: (21.9)

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Solver Theory where UP stands for upwind. As indicated, the upwind part is treated implicitly while the difference between the central-difference and upwind values is treated explicitly. Provided that the numerical solution converges, this approach leads to pure second-order differencing.

Important The central differencing scheme is available only in the pressure-based solver.

21.3.1.6. Bounded Central Differencing Scheme The central differencing scheme described in Central-Differencing Scheme (p. 681) is an ideal choice for LES in view of its meritoriously low numerical diffusion. However, it often leads to unphysical oscillations in the solution fields. In LES, the situation is exacerbated by usually very low subgrid-scale turbulent diffusivity. The bounded central differencing scheme is essentially based on the normalized variable diagram (NVD) approach [262] (p. 789) together with the convection boundedness criterion (CBC). The bounded central differencing scheme is a composite NVD-scheme that consists of a pure central differencing, a blended scheme of the central differencing and the second-order upwind scheme, and the first-order upwind scheme. It should be noted that the first-order scheme is used only when the CBC is violated.

Important The bounded central differencing scheme is the default convection scheme for LES. When you select LES, the convection discretization schemes for all transport equations are automatically switched to the bounded central differencing scheme.

Important The bounded central differencing scheme is available only in the pressure-based solver.

21.3.1.7. QUICK Scheme For quadrilateral and hexahedral meshes, where unique upstream and downstream faces and cells can be identified, ANSYS Fluent also provides the QUICK scheme for computing a higher-order value of the convected variable at a face. QUICK-type schemes [263] (p. 789) are based on a weighted average of second-order-upwind and central interpolations of the variable. For the face in Figure 21.5: One-Dimensional Control Volume (p. 682), if the flow is from left to right, such a value can be written as (21.10) Figure 21.5: One-Dimensional Control Volume

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Discretization in the above equation results in a central second-order interpolation while yields a secondorder upwind value. The traditional QUICK scheme is obtained by setting . The implementation in ANSYS Fluent uses a variable, solution-dependent value of , chosen so as to avoid introducing new solution extrema. The QUICK scheme will typically be more accurate on structured meshes aligned with the flow direction. Note that ANSYS Fluent allows the use of the QUICK scheme for unstructured or hybrid meshes as well; in such cases the usual second-order upwind discretization scheme (described in Second-Order Upwind Scheme (p. 680)) will be used at the faces of non-hexahedral (or non-quadrilateral, in 2D) cells. The second-order upwind scheme will also be used at partition boundaries when the parallel solver is used.

Important The QUICK scheme is available in the pressure-based solver and when solving additional scalar equations in the density-based solver.

21.3.1.8. Third-Order MUSCL Scheme This third-order convection scheme was conceived from the original MUSCL (Monotone UpstreamCentered Schemes for Conservation Laws) [493] (p. 802) by blending a central differencing scheme and second-order upwind scheme as (21.11) where

is defined in Equation 21.8 (p. 681), and

is computed using the second-order upwind

scheme as described in Second-Order Upwind Scheme (p. 680). Unlike the QUICK scheme, which is applicable to structured hex meshes only, the MUSCL scheme is applicable to arbitrary meshes. Compared to the second-order upwind scheme, the third-order MUSCL has a potential to improve spatial accuracy for all types of meshes by reducing numerical diffusion, most significantly for complex three-dimensional flows, and it is available for all transport equations.

Important The third-order MUSCL currently implemented in ANSYS Fluent does not contain any Gradient limiter. As a result, it can produce undershoots and overshoots when the flow-field under consideration has discontinuities such as shock waves.

Important The MUSCL scheme is available in the pressure-based and density-based solvers.

21.3.1.9. Modified HRIC Scheme For simulations using the VOF multiphase model, upwind schemes are generally unsuitable for interface tracking because of their overly diffusive nature. Central differencing schemes, while generally able to retain the sharpness of the interface, are unbounded and often give unphysical results. In order to overcome these deficiencies, ANSYS Fluent uses a modified version of the High Resolution Interface Capturing (HRIC) scheme. The modified HRIC scheme is a composite NVD scheme that consists of a nonlinear blend of upwind and downwind differencing [337] (p. 793).

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Solver Theory First, the normalized cell value of volume fraction, face value, , as follows:

, is computed and is used to find the normalized

(21.12) Figure 21.6: Cell Representation for Modified HRIC Scheme

where

is the acceptor cell,

is the donor cell, and

is the upwind cell, and (21.13)

Here, if the upwind cell is not available (for example, unstructured mesh), an extrapolated value is used for . Directly using this value of causes wrinkles in the interface, if the flow is parallel to the interface. So, ANSYS Fluent switches to the ULTIMATE QUICKEST scheme (the one-dimensional bounded version of the QUICK scheme [262] (p. 789)) based on the angle between the face normal and interface normal: (21.14)

This leads to a corrected version of the face volume fraction,

: (21.15)

where (21.16)

and

is a vector connecting cell centers adjacent to the face .

The face volume fraction is now obtained from the normalized value computed above as follows: (21.17)

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Discretization The modified HRIC scheme provides improved accuracy for VOF calculations when compared to QUICK and second-order schemes, and is less computationally expensive than the Geo-Reconstruct scheme.

21.3.1.10. High Order Term Relaxation Higher order schemes can be written as a first-order scheme plus additional terms for the higher-order scheme. The higher-order relaxation can be applied to these additional terms. The under-relaxation of high order terms follows the standard formulation for any generic property (21.18) Where is the under-relaxation factor. Note that the default value of for steady-state cases is 0.25 and for transient cases is 0.75. The same factor is applied to all equations solved. For information about this solver option, see High Order Term Relaxation (HOTR) in the User's Guide.

21.3.2. Temporal Discretization For transient simulations, the governing equations must be discretized in both space and time. The spatial discretization for the time-dependent equations is identical to the steady-state case. Temporal discretization involves the integration of every term in the differential equations over a time step . The integration of the transient terms is straightforward, as shown below. A generic expression for the time evolution of a variable

is given by (21.19)

where the function incorporates any spatial discretization. If the time derivative is discretized using backward differences, the first-order accurate temporal discretization is given by (21.20) and the second-order discretization is given by (21.21) where = a scalar quantity = value at the next time level, = value at the current time level, = value at the previous time level, Once the time derivative has been discretized, a choice remains for evaluating time level values of should be used in evaluating .

: in particular, which

Important Do not vary the timestep size during a calculation run when using second-order discretization. Doing so creates an error that reduces with a reduction of the timestep jump.

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Solve