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fluid specific heat and thermal conductivity in porous zone

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Old   June 15, 2020, 04:39
Default fluid specific heat and thermal conductivity in porous zone
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Hi all,

I am modeling a hydrogen combustion in porous media. Actually, I am trying to reproduce results in literature. In literature, Specific heat and thermal conductivity of gas mixture are used to calculate Pr number.

In my udf code , I used C_CP(c,t) and C_K_L(c,t) to access cell specific heat and thermal conductivity. However, I know cell specific heat and cell thermal conductivity are volume averaged in porous zones. Therefore, my question is:

Should I access cell value of specific heat and thermal conductivity values first and then use volume averaged method to get gas mixture specific heat and thermal conductivity. These are the right values I should use in UDF instead of directly accessed cell values?

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Weiqiang
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Old   June 15, 2020, 06:00
Default Averaged Values
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What do you mean by

Quote:
cell specific heat and cell thermal conductivity are volume averaged in porous zones
If you're implying that Fluent uses averaged values, then that's incorrect. However, you are applying averaged values, then it is a different matter. Otherwise, each cell has its own value.
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Old   June 15, 2020, 06:11
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Quote:
Originally Posted by vinerm View Post
What do you mean by



If you're implying that Fluent uses averaged values, then that's incorrect. However, you are applying averaged values, then it is a different matter. Otherwise, each cell has its own value.
Hi Vinerm,

I can't really understand your answer. I am not a native English speaker. Actually, I used UDS to solve porous solid matrix energy equation and the porous thermal model is kept as thermal equilibrium.

My confusion is if I keep the default thermal equilibrium model, then fluent will use volume averaged method to solve energy equation for fluid. However, I also defined enthalpy source term for fluid and solid as well. Am I solving energy equation of fluid twice?

Should I just select non equilibrium thermal model and then set all heat transfer coefficients in fluent panel to zero. Then defined my own enthalpy source for both fluid and solid.

I think this is reasonable. Am I right?

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Weiqiang
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Old   June 15, 2020, 06:21
Default Energy Equation in Solid
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Equilibrium model assumes that the temperature of solid and fluid in each cell is same. That does not mean the properties are homogeneous or same. Just for the heat conduction part, weighted-average properties are used. If you use non-equilibrium model with HTC 0, that implies adiabatic boundary condition. Do you want no heat transfer between the solid matrix and the fluid? Then, you can do that. Otherwise, enable non-equilibrium model. But just check for the compatibility of the non-equilibrium model with combustion models; it may not be available for all of those.

Don't worry, I'm not a native English speaker as well.
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Old   June 15, 2020, 06:41
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Quote:
Originally Posted by vinerm View Post
Equilibrium model assumes that the temperature of solid and fluid in each cell is same. That does not mean the properties are homogeneous or same. Just for the heat conduction part, weighted-average properties are used. If you use non-equilibrium model with HTC 0, that implies adiabatic boundary condition. Do you want no heat transfer between the solid matrix and the fluid? Then, you can do that. Otherwise, enable non-equilibrium model. But just check for the compatibility of the non-equilibrium model with combustion models; it may not be available for all of those.

Don't worry, I'm not a native English speaker as well.
Hi, Vinerm,

I do want to model heat transfer between fluid and solid. However, if I select non equilibrium with HTC 0, I just disable the heat transfer defined by fluent solver. I can still use enthalpy source term to model heat transfer between fluid and solid.

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Weiqiang
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