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Emissivity and Bulk Heat Transfer Coefficient

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Old   June 19, 2007, 08:42
Default Emissivity and Bulk Heat Transfer Coefficient
  #1
Devolt
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Hi All,

I am giving emissivity of 0.3 and heat transfer coefficient of 500 at wall. And I am aslo specifying outside temperature. How emissivity will be taken into account to calculate heat flux( both convective and radiative). I tried to find in the manual, but I didn't get. Can any body explain the theory of this? If the explanation of the problem is not clear plz let me know. I am using DT model.

Thanks Devolt

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Old   June 19, 2007, 09:02
Default Re: Emissivity and Bulk Heat Transfer Coefficient
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opaque
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Dear Devolt,

Be careful here.. You have not specified with emissivity you are referring to: the outer wall emissivity, or the inner side one.

Assuming is the inner side emissivity, you set the HEAT TRANSFER boundary condition to Heat Transfer Coefficient and input your htc and external temperature. Then, you set the THERMAL RADIATION boundary condition to Opaque and input your inner side emissivity..

ANSYS CFX has no support for external radiation boundary condition.. You will have to do it by using a variable temperature heat transfer coefficient

Q_wall = htc * (T_wall - T_outer) + emiss_outer * (T_wall^4 - T_outer2^4)

and rewrite to something like

Q_wall = MyHTC * (T_wall - T_outer)

This approach may require smaller time steps because the boundary condition is not completely implicit.

Hope it helps,

Opaque

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Old   June 19, 2007, 09:27
Default Re: Emissivity and Bulk Heat Transfer Coefficient
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Devolt
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Hi Opaque,

Thanks for quick reply. I am not still clear. I am using heat transfer coefficient as function of temperature (T^3). But my surface is having some emmisivity. This emissivity is one of the most important properties. This i can't ignore. So, some how I want supply this factor directly or indirectly into the heat transfer coefficient.

The emmisivity, which I am giving at wall, is it taken by the solver to calculate effective heat transfer coefficient or not?

Is solver just ignores the emissivity value that what I am specifying?

Thanks in advance for ur answer. Devolt

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Old   June 19, 2007, 10:55
Default Re: Emissivity and Bulk Heat Transfer Coefficient
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opaque
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Dear Devolt,

I am sorry, but you have not clarified which emissivity you want to supply. As I said, ANSYS CFX only supports the internal (within the domain) value. Do we agree that there are two emissivities at a wall? One if for the external radiation heat transfer, and one if for the internal radiation modeling.

From you answer, it seems you are trying to model convective+radiation cooling at a wall. Again, ANSYS CFX does not support such boundary condition directly.. In this case, it is up to the user on how to introduce the effect of emissivity into the "effective heat transfer coefficient" into the supported boundary condition..

What effective heat transfer coefficient are you referring to? I assume you meant external HTC.. Am I correct?

Hope it helps,

Opaque

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