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effective thermal conductivity and mass weighted thermal conductivity 

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June 18, 2020, 05:53 
effective thermal conductivity and mass weighted thermal conductivity

#1 
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Weiqiang Liu
Join Date: Feb 2018
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Hi all,
I am modeling a hydrogen combustion in porous zone with fluent. I used a UDS to solve solid energy equation and also the energy equation of fluid is modified. For fluid energy equation, I modified thermal conductivity of fluid to an effective thermal conductivity. Specificly, effective thermal conductivity is mass weighted thermal conductivity added with an additional term. Then I need to define source term for both solid and fluid energy equation and fluid thermal conductivity will be used in the source terms as well. My question is: for the thermal conductivity emerged in source term, should I should use the value return by DEFINE_PROPERTY which is effective thermal conductivity or the original mass weighted averaged velocity? According to the symbols emerged in paper, it seems like original mass weighted thermal conductivity should be used. However, I am not very sure. Can anybody give me some suggestions? Best regards Weiqiang 

June 18, 2020, 06:37 
Thermal Conductivity

#2 
Senior Member

DEFINE_PROPERTY returns value to Fluent, not to UDF. If you want to fetch value in UDF, use C_K_L(c,t). Which value to use depends on the source term. Fluent uses averaged properties only with equilibrium model. With nonequilibrium model, individual material properties are used. So, if you have different source terms for fluid and solid, you should use fluid and solid thermal conductivity, resp., not averaged. But, as mentioned above, it depends on source term formulation; the phenomenon which is the cause for the source.
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June 18, 2020, 06:42 

#3  
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Weiqiang Liu
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Quote:
I tried to compute non equilibrium phenomenon with my own UDF and still kept the in built non equilibrium model in fluent disabled 

June 18, 2020, 06:48 
Macro

#4 
Senior Member

C_K_L returns property being applied to the cell. If you have equilibrium model enabled, then it will return averaged property since there is only one cell zone. If you enable nonequilibrium model, then C_K_L will return property depending upon the cell zone that it addresses. Do note that by disabling the NET model, ET is still enabled and thermal equation is still based on porosity averaged properties of fluid and solid. So, your fluid thermal field is not really fluid thermal field, but thermal field based on fluidsolid averaged zone. If you really want only UDS to be active for solid, then set the porosity to 1. It will have no effect on momentum since porosity does not play a role there, but it will affect superficial velocities and reactions.
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June 18, 2020, 06:59 

#5  
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Weiqiang Liu
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Quote:


June 18, 2020, 07:30 
Fluid Thermal Conductivity

#6 
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You can do that; however, fluid's thermal conductivity is a userspecified property. So, until and unless this is a very complicated function requiring gradients of fields, you can directly calculate the thermal conductivity in your UDF.
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Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority. 

June 18, 2020, 08:32 

#7 
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Weiqiang Liu
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Do you mean I can just calculate thermal conductivity in DEFINE_SOURCE where I need to use fluid thermal conductivity?


June 18, 2020, 12:37 
Thermal Conductivity

#8 
Senior Member

If you have provided constant thermal conductivity for fluid, then you don't need to fetch or calculate. However, if you are using some profile or UDF, then you already know the equation and you can calculate it within UDF.
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Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority. 

June 19, 2020, 00:08 

#9 
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Weiqiang Liu
Join Date: Feb 2018
Posts: 278
Rep Power: 9 
yes, I know the equation. it's just a mass weighted average plus some additional terms. However, I think I can still use C_K_L to access thermal conductivity of cell and then use volume averaged equation to get gas conductivity. I mean I just need to calculate thermal conductivity in DEFINE_PROPERTY macro for one time. For other macros in which I need to use fluid thermal conductivity. I just need to use C_K_L(c,t) to fetch the value, right?


Tags 
define_property, define_source, effective conductivity, mass weighted 
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