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Heat transfer within a porous domain

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Old   January 4, 2015, 09:09
Question Heat transfer within a porous domain
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Dear all,

Im having difficulties in simulating heat transfer within a porous medium in CFX. I tried to set up a simple case of a rectangular channel (600 x 1.5 x 0.3 mm). The top boundary (y = 1.5mm) is modelled as a wall with constant temperature of 80C, the bottom and side boundaries are symmetry.
The mesh has 1 element thickness in the Z-direction (~2D simulation), 15 elements in the Y-direction (biased, so that the region near the top wall has a better resolution) and element length of 0.5 mm in the X-direction.

The fluid (water) is entering at x=0 with a static temperature of 25C. For the outlet boundary (x = 600 mm) I have tried different settings: fixed temperature of 80C and adiabatic.

It seems to be a global convergence issue. I checked the residuals which are more or less spread throughout the whole domain. I reached after 1000 iterations a RMS for H-Energy of the magnitude 1E-2. I also monitored the global imbalance for H-Energy, which gave me the following terms:

Boundary: outlet -2.4882E-02
Boundary: wall 1.2982E+01
Domain Src (Neg) : Domain 1 -4.2079E+00
Domain Src (Pos) : Domain 1 4.2251E+00
Domain Imbalance : 1.2974E+01
Domain Imbalance, in %: 99.9405 %

Any ideas, as to why the simulation is not converging?
In the Ansys help I found an explanation for the term domain src (neg) as being the thermal energy that flows between fluid and solid, which makes sense. Why do I get a domain src (pos) term in the same order of magnitude? Why am I not getting a term for the inlet?

Might this be related to a limitation of CFX and/or thin slice domain?

Thanks for your help and happy new year!

Renato
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Old   January 4, 2015, 17:04
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Are you sure it is converged? 1E-2 is very loose convergence and rarely gives useable results. I think you just need to run longer.
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Old   January 7, 2015, 07:25
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Thanks for the reply.
I'm assuming the simulation has not converged.
But after about 300 iterations the residuals are not getting any lower, and I ran 1000 iterations, so I don't expect it to be an issue of slow convergence.
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Old   January 7, 2015, 09:38
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what flow boundary conditions are you using for the inlet ? You only described the heat transfer boundary conditions.

Have you tried the case without heat transfer first ? Does it converge ?

What timescale are you using for the fluid ? How about the solid timescale ?
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Old   January 7, 2015, 17:53
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But after about 300 iterations the residuals are not getting any lower
Sounds like an FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   January 9, 2015, 05:06
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Increasing the timescale helped in my case. Thanks for the replies!
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