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Porous Media Effective Thermal Conductivity

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Old   August 3, 2001, 00:43
Default Porous Media Effective Thermal Conductivity
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Greg Perkins
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Has anybody come across the limitation that Fluent requires a constant thermal conductivity for the solid in porous mediums?? If I try and specify a UDS or a polynomial function of temp. for the thermal cond., Fluent (5.5.14) reports an error stating that it needs to be a constant (god knows why!)

Going from there I thought I'd specify the effective conductivity for the medium using a UDS. But I can't work out how to connect a UDS for the effective therma conductivity for a porous medium to Fluent. The manual says you can do this (vol. 1, p6-107) but doesn't say how (I suspect the manual's wrong)??

I think the way around these Fluent 'features' is to specifiy the gas phase conductivity as the effective conductivity and set the solid conductivity to a very low value (like 1e-5).....If you've got a better idea pls let me know!

Greg

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Old   August 3, 2001, 12:45
Default Re: Porous Media Effective Thermal Conductivity
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Ugur
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I don't know have an exact answer for your question, but I guess that you can specify effective conductivity using the following array. C_K_EFF(c,t)

Ugur
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Old   August 3, 2001, 13:39
Default Re: Porous Media Effective Thermal Conductivity
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Evan Rosenbaum
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We've run into the same limitation. I seem to remember FLUENT Tech Support saying that a UDF was possible, but we've never gotten around to trying it yet. Before you change the gas conductivity, be sure that it won't affect any other portion of your solution. You don't need the set the conductivity for the solid portion very low, just set the porosity very high.
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Old   August 5, 2001, 22:53
Default Re: Porous Media Effective Thermal Conductivity
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Greg Perkins
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Thanks Evan,

I have tried a udf for the gas conductivity, tested it and it appears to be working fine. The thermal conductivity is only used in the energy equation so it doesn't effect other aspects - but you're right you need to be careful here - and what does Fluent do with these numbers...

I think setting the solid conductivity low is better than using a high porosity since the gas phase reaction rates are effected by the volume fraction of fluid phase.

Greg
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