Cases with Porous Zones & Energy equation yield unreasonable temperature distribution
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Users have reported unreasonable temperature distributions in cases where
-porous fluid zones are present -heat exchanger model is used The symptoms which are reported are -excessive temperature stratification on the inlet / outlet from various porous zones, especially from those which are setup as heat exchangers -heat source from heat exchanger zone is reaching too far upstream of it The issue with the temperature distribution is caused by the very large effective thermal conductivity values in the areas where turbulent viscosity is very large. (See 10.4.7 User's Guide for the math). These areas of very large turbulent viscosity may appear because of 1. flow separation 2. large momentum sources in the fluid porous media zones. In order to improve the temperature solution, the flow solution needs to be improved. Here are couple of things which would be recommendable: a) Turn "Laminar Zone" button ON in Define>Boundary Conditions for Fluid zones which are Porous. (See 6.19.4 User's Guide for details), including the Heat Exchanger zone(s). b) Run the simulation with RNG k-e after starting it with Standard k-e. SKE is robust but it shows up larger than real values of turbulent viscosity and they lead to accuracy problems (heat transfer predictions are one of them). c) Reduce the under-relaxation for k, epsilon, and turbulent viscosity, especially when start running the case. Starting from some initial guesses which far away from the solution will lead to oscillations and that may make the solution process longer. Start a run with 0.4, 0.4 for k and epsilon, especially if the mesh quality features cells above the recommended skewness. |
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