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-   -   Temperature Dependant Porosity in Combustion (http://www.cfd-online.com/Forums/fluent/112418-temperature-dependant-porosity-combustion.html)

NormalVector January 28, 2013 11:48

Temperature Dependant Porosity in Combustion
 
Hello,

I'm trying to model the temperature dependent porosity of a porous medium in a combustion application. I'm using a UDF to loop over the cells in the porous medium and assign a porosity value based on the cell temperature. I don't believe my UDF is wrong so I'm not posting in the UDF section of the forums but I'm including it here for completeness:

__________________________________________________ __________
#include "udf.h"

DEFINE_PROFILE(porosity_udf_test,t,nv)
{
real temperature;
real a;
cell_t c;

begin_c_loop(c,t)
{
temperature = C_T(c,t);
a = 0.5*((exp(2*temperature - 1000)-1)/(exp(2*temperature - 1000)+1)) + 0.5;

C_PROFILE(c,t,nv) = a;
}
end_c_loop(c,t)
}
__________________________________________________ __________

If you notice, I'm trying to approximate a step in porosity (ie, below one temperature the porous medium is there and above it it is not) using a hyperbolic tangent function centered around 500. When I run my simulation, it's fine until the porous media heats up and then suddenly Pressure Correction diverges. Here are my relevant solver settings:
  • Pressure Based, Transient
  • Realizeable k-e
  • Finite Rate/Eddy Dissipation combustion
  • PISO pressure-velocity coupling (I've tried all of the other P-V couplings)
  • Standard pressure discretization (I've tried all other pressure discretizations)
  • the other discretizations are 1st order
Any ideas? I was originally using a full tetrahedral mesh but after I made the porous region structured it seemed to help. Not sure what the problem is.


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