[dadash]

hello.
i want to simulate radiation heat transfer in the enclosure, but i don't (what is the difference between non-gray and gray radiation?).
please help me.
thanks,
dadash.

[LuckyTran]

Originally Posted by dadash

hello.
i want to simulate radiation heat transfer in the enclosure, but i don't (what is the difference between non-gray and gray radiation?).
please help me.
thanks,
dadash.

gray radiation is non-spectral (absorptivitiy, emissivity, transmissivity do not depend on wavelength). It is called gray because it is similar to ideal blackbody radiation but with emissivity less than unity.

Note, in case you are not aware: for diffuse radiation in a closed container (an enclosure) with non-participating media and occurring at thermal equilibrium, the surface to surface radiation can be treated as black body radiation. It sounds like a lot of conditions but it's a fairly practical result.

[dadash]

thanks dear LuckyTran.

I have another question for you. what is the difference between emissivity and scattering coefficient in medium?

[LuckyTran]

They belong to two different physical phenomena.

Emission is spontaneous (also called spontaneous emission, as opposed to stimulated emission). An object at a finite temperature will emit a certain amount of energy in the form of radiation spontaneously.

As light passes through a media it can be transmitted, redirected/scattered (or in the extreme case, reflected), or it can be absorbed.

The scattering coefficient has to do with the amount of incident light that is scattered (or redirected) by the media as the light passes through the media. Obviously scattering cannot occur if there is no radiation/light incident or traveling through the media.

These two processes can occur simultaneously. An object can emit its own radiation while also interacting with the radiation it receives from its environment.

[dadash]

Thank you for your help dear LuckyTran.

[dadash]

hi LuckyTran.
excuse me.
in FLUENT,why the dimension of absorption coefficient a medium is (1/m)?
absorption coefficient should be dimensionless.
what is difference between absorption coefficient(1/m) and dimensionless absorption coefficient?

[LuckyTran]

You are probably confusing absorption coefficient with absorptivity (molar absorption coefficient). Absorption coefficient has units 1/m, which together with the path length yields the actual absorbance (Beer-Lambert Law).

[mehdiyeganeh]

thanks.
In my simulation, the air temperature varies approximately from 300 to 400 degrees Kelvin. I need the values of the absorption and scattering coefficient in mentioned temperature ranges. Can you help me to find the coefficients?