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"radiation model" VS "radoation loss"

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Old   August 13, 2012, 02:16
Default "radiation model" VS "radoation loss"
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Hi all,
One thing I need to be clarified for my self is the difference between the "Radiation model" in the fluent (say DO, DTRM S2S etc) with the available radiation option under the thermal conditions dialog box for a wall boundary condition. Lets I try to explain with example which I understand. Then I will request you to amend me it if wrong.

#Radiation model is OFF: Let liquid coolant is passing through a very hot bare tube. If I wanna calculate radiation loss from the tube outer surface to the environment, I don't require to activate Radiation model. Instead I should just provide the external emissivity of the tube surface and the temperature of the external environment.

#Radiation model is ON: Now lets consider the same hot tube is enclosed by an evacuated glass tube. If I wanna calculate heat loss from the tube surface through the glass tube to the environment and my model also include the glass tube, then I need to activate any of the radiation models (s2s for instance).
However, if my model does not include the glass tube, I don't need to activate any of the radiation models. Instead, I can set an operating condition which will satisfy the evacuated glass tube environment: (for instance, operating temperature approximately 50C (prospective glass tube temperature of my model), Operating pressure 0 pascal and operating density 0 kg/m3; since evacuated).

Am I right? If not please help me to be clear the concept. I'll be grateful to you.
Thanks
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Old   August 13, 2012, 02:35
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You got this correctly, just one thing:
In your second case, if you want to model it without the glass tube and thus without a radiation model, the operating temperature is not the temperature of the glass tube.

Consider the glass tube fully transparent (which you have to do if you do not model it explicitly), the temperature the tube "sees" for radiation is still the temperature of the environment, not the glass tube.
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Old   August 13, 2012, 19:27
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Quote:
Originally Posted by flotus1 View Post
You got this correctly, just one thing:
In your second case, if you want to model it without the glass tube and thus without a radiation model, the operating temperature is not the temperature of the glass tube.

Consider the glass tube fully transparent (which you have to do if you do not model it explicitly), the temperature the tube "sees" for radiation is still the temperature of the environment, not the glass tube.
Hi Alex,
That means even if I wanna simulate the condition for evacuated glass tube, I don't actually need to include glass tube physically in my design model. What I need is, to assume the glass is so transparent that the radiation is exchanged between the environment and the metal tube surface with the temperature of the tube surface and the environment. But the operating pressure and density must be the values for those of evacuation (i.e both values are zero) so that I can calculate convection loss if I want for (though theoretically no advection).
Is this the concept, dear Alex?
Many many thanks for your response.
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Old   August 14, 2012, 02:17
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I doubt that the solver will run with both operating pressure and density set to zero.
If you want to neglect conductive and convective heat losses, I would not model the glass tube around the collector tube. Just set the boundary contition of the collector wall to radiation with the ambient temperature.

The thing is that neither glass is fully transparent nor is there an absolute vacuum in the glass tube. It would be quite interesting to see by what extent these factors alter the result of the simulation.
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Old   August 14, 2012, 02:36
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Quote:
Originally Posted by flotus1 View Post

The thing is that neither glass is fully transparent nor is there an absolute vacuum in the glass tube. It would be quite interesting to see by what extent these factors alter the result of the simulation.
Thanks Alex,
Most probably the operating density and pressures are misunderstood by me as the properties of the environment, where a system is being modeled, as in case of evacuated tube it is vacuum. Most probably they are actually some reference properties of the working domain itself. Could you have any suggestion to model convection loss for the evacuated tube considering a minimul pressure and density of air for the evacuation?

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