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Old   March 12, 2004, 08:20
Default Gray Radiation
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Hi,

Can anyone give me a brief description of what gray radiation is and when should the discrete ordinates radiation model activate the non-gray model.

And is there a guideline for the number of bands that should be used if the gray modelling is necessary?

Thanx

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Old   December 20, 2012, 08:15
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Hi Community,

Requesting someone to shed light on a question that was previously asked. I am starting to learn radiation models in Fluent. While reading the user manual on Setting up of P-1 radiation model I came across the terms, "Non-gray model" and "Gray Model". If I increase the number of bands to any value greater than 0, it becomes a non-gray model which is expensive in terms of computation time.

Questions are:

What is the difference between non-gray and gray models OR Under what conditions would you go for a non-gray or a gray model?

How do you determine is a given problem can be considered as a gray radiation problem without losing significant accuracy.

Thanking you for sharing any thoughts or information,

Sincerely,

RSSLNT
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Old   December 22, 2012, 03:39
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Quote:
Originally Posted by rsslnt View Post
Hi Community,
What is the difference between non-gray and gray models OR Under what conditions would you go for a non-gray or a gray model?
A gray surface has a spectral emissivity and absorptivity that is not dependent on the wavelength of the radiation. Gray radiation refers to the type of radiation you would expect to see from a gray surface.

The gray model offers a huge simplification to real problems. The gray model should always be used first wherever possible. If more accurate solutions are desired, then it may be appropriate to use a non-gray model.

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Originally Posted by rsslnt View Post
How do you determine is a given problem can be considered as a gray radiation problem without losing significant accuracy.
This depends on the how complicated the system is. The spectral content usually will reveal fairly quickly whether a gray or non-gray model should be used. If a surface emits radiation at only 1 wavelength, then a gray model is not useful (because the gray surface emits radiation at all wavelengths). So only for cases where the emission bands are very few and very narrow will a gray model typically fail. On the otherhand, if the radiation being considered is broadband then a gray model may be useful. But if you are dealing with say, coherent laser light, then the gray model is not a very good one since there is only 1 spectral wavelength present.

Some care should be exercised, because many media being considered nowadays have narrow emission bands in their intended operating conditions but very broadband emission at "normal conditions".
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Old   December 31, 2012, 07:33
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Thank you very much. Appreciate your interest in clarifying the doubt.
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