About External Emissivity

Cloud · December 27, 2010, 05:15

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?

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.

December 27, 2010, 05:15	About External Emissivity	#1
Cloud New Member Cloud Zhang Join Date: Dec 2010 Posts: 8 Rep Power: 16	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!

December 29, 2010, 10:42		#3
ComputerGuy Senior Member Real Name :) Join Date: Jan 2010 Location: United States Posts: 192 Rep Power: 16	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 Oula likes this.

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December 29, 2010, 02:09		#2
Cloud New Member Cloud Zhang Join Date: Dec 2010 Posts: 8 Rep Power: 16	so no one knows about external emssivity?

January 3, 2011, 23:55		#4
Cloud New Member Cloud Zhang Join Date: Dec 2010 Posts: 8 Rep Power: 16	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.

June 27, 2013, 03:16		#5
gan1990 New Member Join Date: Jun 2013 Posts: 11 Rep Power: 13	Hi cloud, Could you please elaborate on the reply you got from ansys people? It would be of great help. Eagerly awaiting your response.