Radiation Equilibrium Thermal Wall Condition

CFDGerCFDGer Member

Hello,

I want to model the wall in my simulation in the state of radiation equilibrium.

In a radiation cooled adiabatic wall, the heat flow of the air and the heat radiation of the wall cancel each other out and the heat flow into the wall is therefore zero. The heat radiation can be calculated according to the Stefan Bolzmann Law (see picture). In Fluent under Boundary Conditions -> Wall -> Thermal, it is possible to select Radiation. But there it is only possible to select the External Emissivity. But as I understand I need the internal Emissivity. Also, which value should the external radiation temperature be defined as? To my understanding this temperature is not known.

Is it possible to implement this thermal wall BC in Ansys Fluent easily or is it necessary to write a UDF?

Thank you very much for your support!

Answers

  • External radiation temperature sets the temperature of the external radiation source / sink to be used for the radiation condition. This is not meant solving internal radiation.

    If you would like to include internal radiation in your computational domain, I'd strongly recommend that you turn on the 'Radiation Models'. This will add another additional tab to your wall boundary condition selection, which will show the option of marking 'Internal Emissivity'. Please have a look at this link from the Fluent User's Guide.

    I hope this helps.

    Thanks.

    Karthik

  • CFDGerCFDGer Member

    Hello Karthik,


    thank you very much for your reply.

    So if I set the Radiation Model to gray P-1 (Number of bands is set to zero) and I choose an arbitrary value for my Internal Emissivity of my Wall (for example =0.8), then the radiation heat flux is in equilibrium with the heat flux to the wall of my object? (I am simulating hypersonic external flow around an object).

    And I saw that for the Inlet and Outlet BCs there is added the "Radiation" Tab in which I can choose the External Black Body Temperature Method and the Internal Emissivity. if I leave the default values for this BC, this doesn't affect the Inlet and Outlet BC?

    I just want to get the radiation equilibrium thermal BC for my wall and everything else should remain the same as if the Radiation Model was turned off.


    Thank you very much!

  • In order to have internal emissivity modeled at the walls, you will need to turn on Radiation. Turning on radiation will of course have its consequence as it will add another mode of heat transfer. So, you will need to worry about that.

    But, when you turn on the radiation, the radiation conditions at the wall will automatically ensure an equilibrium condition at the wall boundary.

    I'd carefully check how your radiation is interacting with other models.

    Thanks.

    Karthik

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