Mixsim2: Heat exchanger coil or tube bundle as porous media zone?

A tank contains one or more helical heat exchanger coil(s), or bundles of vertical heat exchanger tubes in a circular arrangement. In both cases, resolving the geometry of these entities results in huge mesh cell counts. Approximating these entities by one or more cylindrical porous media zones is the appropriate way. How can this be done in MixSim??
- In MixSim, create a "Hex Region" of the type "Donut" (this can be a child either
of the tank or of the shaft). Give it the correct proportions so that it
represents the desired porous volume.

- Adjust the "Initial Mesh Size" value for the Hex Region to a sensible
value -- not larger than the "wall thickness" (i.e. half the difference
between inner and outer diameter), and somewhere between the impeller's
"Initial Mesh Size" and the Tank's "Initial Mesh Size". If meshing
fails, try a smaller value here.

- For each impeller that is inside or near this "Hex Region", please do
activate the "Fix Zone Dimensions" check button. (That is usually
located near the bottom end of the impeller's parameter list in MixSim's
"Model Management" panel.) Adjust the values if you feel that the MRF
zone should be larger than the (intentionally small) default values. But
make sure there's enough space for the transition from (large) pyramid
cells on the Hex Region to (small) tet cells on the MRF zone boundary.
(Activating the "Fix Zone Dimensions" button is necessary because
MixSim doesn't account for Hex Regions when it determines to which
extent MRF zones can be enlarged automatically.)
When you view the Hex Region and the visualized MRF zones in
the MixSim model setup preview graphics, you may find it advantageous
to use the {A,R}-{Hex,Tank,Zones} buttons in the preview graphics window.

- After meshing, go to "Solve---Iterate", UNcheck "save data" and "save
case", set the number of iterations to ZERO, and press "Iterate".

- When MixSim is finished setting up the FLUENT case, go to
"File---Control-Panel", and switch the GUI menu bar to the FLUENT GUI.

- In Define---Boundary-Conditions, find the zone "hex_region_X_fluid" (X
is some small, usually odd, integer number). This represents the cell
zone that has been meshed with hex cells. Edit the boundary conditions
for this zone, and add the "porous zone" property to it. Set the porous
zone parameters.

- Run iterations (using "Solve---Iterate" either in the FLUENT GUI or
after switching back to the MixSim GUI.

- Enjoy, including the MixSim postprocessing features!

If you find any problems or deficiencies with these instructions, please contact your technical support engineer or Fluent distributor, so that we can improve this.

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