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-   -   wall emissivity for radiation problem: emitter is also receiving when it shouldn't (https://www.cfd-online.com/Forums/fluent/49750-wall-emissivity-radiation-problem-emitter-also-receiving-when-shouldnt.html)

 Young November 10, 2008 10:24

wall emissivity for radiation problem: emitter is also receiving when it shouldn't

Dear all,

I'm simulating a DO radiation model. It is simple, I just put a ball inside a cubic, set the ball temperature to 1000K and internal emissivity to 1. The cubic surfaces have a 0K temperature and internal emissivity also as 1. So according to the rules, all the radiation by the ball inside will be absorbed by the cubic surfaces. So the ball should only have a radiation out. However, after the simulation, I found ball also receive radiations. Not sure where the origin is. And as I changed the ball temperature, I found the received radiation is proportional to the radiation out. Could anyone help me out?

 Amod@Siemens November 11, 2008 13:11

By definition, black body (e = 1) imply that the surface will have 0 reflectivity. Hence, whatever radiation is falling over it, the surface will absorb them. However, any surface having temperature > 0 [K] will always radiate. Thus, ball surface will have both the incoming and outgoing radiation. There is nothing wrong with your simulation.

You can verify the result very easily since Shape Factor is 1 by virtue of being an enclosure.

Hope is clarifies!

Amod

 Young November 11, 2008 13:33

Amod, thanks very much for your reply. Yes, I understand why the ball surface have an outgoing radiation. But since I set the cubic surface (enclosure) to 0[k] so there shouldn't exist any radiation from the cubic surface. So how comes the incoming radiation for the ball?

Regards Young

 Amod November 11, 2008 13:35

Have you model the cubic surface as Adiabatic or Isothermal? In case adiabatic wall, its temperature will not remain 0 [K]. Probably this may be the reason!

Amod

 Young November 11, 2008 13:42

Yes, I set the cubic surface as isothermal (T=0K under wall boundary condition). And after the simulation, I check the wall temperature, it's 0[k]. In fact, I'm not sure whether the region outside my model will affect the simulation. I set up another model with additional cubic enclosure outside the original cubic. And link the two faces between two cubics, set the boundary conditions for them as "couple". After the simulation, the temperature of the wall increases as expected. However, there's no change in the results for the inside cubic walls. So confused.

Young

 Amod November 11, 2008 13:44

Mysterius! Let me try at my end and see it!

 Young November 11, 2008 13:46

Thanks Amod. Do tell me your results!

Young

 e0125583 November 12, 2008 11:34

I think you just have to set the internal emissivity of the cubic wall to 0. isothermal means that the temperature remains at 0K. So all the absorbed radiation must be emitted again to keep the temperature constant. if your internal emissivity is > 0, then a part of this radiation is emitted to the interior and therfore hits the ball. with internal emissivity = 0, radiation is only emitted to the exterior of the domain.

 Young November 13, 2008 03:06

Hi e0125583 when the internal emissivity is set to zero, the emittion of cubic wall is 0. However, within the boundary conditions, all incident light will be reflected back, either specular or diffused.

 e0125583 November 13, 2008 05:42

what are the material properties of your medium? do you use an absorbtion coefficient > 0? I set up the same case, and I dont have any incident radiation at the ball surface.

 Young November 13, 2008 06:32

I use air as the medium. Both the absorption coefficient and scattering coefficient are set as zero. Could you send your .cas file to me as a reference? Thanks. my E-mail address is: y04051246[at]gmail[dot]com.

Young

 e0125583 November 13, 2008 09:13

Hi! I just sent you the case-file. When I change the temperature of the ball, the received radiation remains the same (2.79e-8 W, which is negligible.

 Young November 13, 2008 09:18

Thanks : ) Let me see.

 gan1990 June 27, 2013 02:13

Hi Young,Amod,e0125583;
I know I am reviving a very old thread here,sorry for that.

Could you please let me know about the internal and external emissivity values you set for the boundary walls?

After following the above suggestions,did you get physically realistic solution?