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 March 12, 2004, 07:20 Gray Radiation #1 Newbie Guest   Posts: n/a 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 Newbie openfoammaofnepo, BhushanM. and sepideh.bz like this.

 December 20, 2012, 07:15 #2 New Member   Odakkattuvalasu Join Date: Jan 2012 Posts: 17 Rep Power: 11 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 Ind88 likes this.

December 22, 2012, 02:39
#3
Senior Member

Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 4,123
Rep Power: 50
Quote:
 Originally Posted by rsslnt 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.

Quote:
 Originally Posted by rsslnt 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".

 December 31, 2012, 06:33 #4 New Member   Odakkattuvalasu Join Date: Jan 2012 Posts: 17 Rep Power: 11 Thank you very much. Appreciate your interest in clarifying the doubt.

 March 23, 2016, 06:18 #5 New Member   Bhushan Join Date: Jan 2016 Posts: 1 Rep Power: 0 Hi, I am dealing with same radiation problem as yours. While doing radiation modelling by D.O. method. Can you tell me how should I proceed??? Thanks and regards.

 March 23, 2016, 10:53 #6 New Member     Suraji Join Date: Mar 2016 Posts: 17 Rep Power: 7 Some modelling to anlyze radiation case in fluent, how is better modelling to analyze furnace radiation?

May 15, 2020, 03:45
#7
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noorlina
Join Date: Apr 2020
Location: malaysia
Posts: 7
Rep Power: 3
Dear lucy tan, when you said about absoprtivity, is there refering to absoprtion of coeffiecient air? because when used the radiation model, i found the material properties of air need to insert in some value.

Quote:
 Originally Posted by LuckyTran 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. 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".