[IronLyon]

I'd like to ask about the radiation.



In the situation there is a chamber full filled with helium gas (room temperature about 25 degree), a 1500 degree metal ball is in the center of the chamber (quite small compared to the chamber). The radiation process should be simulated.


The helium gas is considered as transparent, so we choose the Discrete Transfer or Monte Carlo model as shown below
1.png
And for the case we know the emissivity of the metal is 0.2.


then there is two ways for the further options:
1. for the metal domain do not use the radiation option, then in the Interfaces there is only the option about the Heat Transfer, like this
1.1.PNG
1.2.PNG
in the Interface setting of the gas domain,
1.3.PNG
there is a option about the radiation Opaque (only one), I think I should change the emissivity to control the radiation.

1.4.PNG
But what does this one means?? Is it the emissivity of the gas, or something else related to the interaction?

(I have searched about the emissivity of the helium gas but did not get any useful infos. For example on the web of Engineering ToolBox, they do not list the emissivity coefficient about gas, that means for gas like helium the emissivity coefficient is just 1? I'm confused.

https://www.engineeringtoolbox.com/e...nts-d_447.html)

[IronLyon]

2. for the metal domain select the radiation option,
2.1.PNG
then in the Interfaces there is the option about the Heat Transfer also the Thermal radiation, like this
2.2.PNG
2.2.1.PNG
2.2.2.PNG
should I use the Opaque and set the Emissivity Coefficients, or should I choose other options?


#1 way or #2 way, which one do you think is right, or both not? Thank you!

[IronLyon]

Of course I have used the caculation to esstimete the radiation intensity of the ''hot'' metal, the links is below
https://www.engineeringtoolbox.com/r...er-d_431.html:)

[Opaque]

Though heat transfer textbooks focus heavily on conduction and convection, thermal radiation is not a trivial subject.

As you may have noticed, nearly all thermal radiation sections in textbooks focus on vocabulary before attempting to describe other details.

In thermal radiation parlance, it is very common to mix emittance with emissivity; therefore, the confusion.

Emissivity, Absorptivity, Reflectivity and Transmissivity are by definition a surface concept; therefore, there is no such a thing as gas emissivity and you will never find data for it in any site or manual.

Emittance is an engineering concept introduced to represent the effective emission of a medium (gas, liquid or solid). I usually avoid it unless I am doing rough estimations, or using a modeling approach based on emittances.

In ANSYS CFX, emissivity at a boundary represents the modeling value you expect at such boundary. Recall surface treatment/quality is what matters at the thermal radiation level, not the material the solid surface is made of, in particular for opaque surfaces. Say, you have a polished steel wall vs a heavily oxidized steel wall, the emissivity is not function of the material (steel) anymore, but the state of the surface: polished vs oxidized, or smooth vs rough.

For the medium of interest, say He, you need to provide the absorption coefficient (not absorptivity), refractive index, etc . See the material properties panel, Radiation Properties section and adjust as needed.

Hope the above helps,