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-   -   About External Emissivity (http://www.cfd-online.com/Forums/fluent/83395-about-external-emissivity.html)

Cloud December 27, 2010 05:15

About External Emissivity
 
Hi everyone
I simulated a case about heat exchanger.I didn't select on radiation model. But about the wall boundary,I selected on mixed and entered 0.3 for External Emissivity,7 for HTC,293 for Free stream temperature,293 for External Radiation Temperature. I simulated another case same as case1 except the wall boundary. I selected on convection and 7 for HTC,293 for Free stream temperatue. I found the temperature of the wall was different.
I want to know the meaning of external emissivity and if I enter a number for external emissivity,the radiation model will be calculated? If the answer is yes, which radiation model will be calculated,RO,P1,S2S or else?

Thanks a lot for your time!

Cloud December 29, 2010 02:09

so no one knows about external emssivity?:confused:

ComputerGuy December 29, 2010 10:42

A thermal boundary condition with radiation does not necessarily require a radiation model to be implemented. Recall from heat transfer that the net loss of heat due to radiation is written as:
Q=\varepsilon A\sigma \left ( T_{hot}^{4}-T_{cold}^{4} \right )

where epsilon is the emissivity.

If your surface temperature is quite hot, radiation can dominate the heat loss out of the body, and therefore your choice of emissivity (or whether to account for radiative loss) can make a big difference in your wall temperature.

If you don't enable the radiation boundary condition, Fluent will not account for heat loss this way, and thus the wall temperature will be different. If you choose an emissivity of zero, the solution of wall temperature should be exactly the same as your convection-only boundary condition.

ComputerGuy

Cloud January 3, 2011 23:55

ComputerGuy:
Thanks for your reply. You are right about the wall boundary. And I asked Ansys about the external emissivity and internal emissivity. They said it depends on the adjacent medium.

gan1990 June 27, 2013 03:16

Hi cloud,
Could you please elaborate on the reply you got from ansys people?

It would be of great help.

Eagerly awaiting your response.


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