Cases with Porous Zones & Energy equation yield unreasonable temperature distribution

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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