Ansys Employee

Hello Nabila,

I recommend you to set the boundary conditions at wall as mixed under thermal conditions and set the appropriate external emissivity.  Depending on whether it is gray body or non gray body , opaque or semi transparent go to radiation option and set the diffuse fraction and if it is semi transparent please set the internal emissivity.

For all translucent bodies, the sum of reflectivity , emissivity and transmissivity is equal to 1.

So accordingly if you know the transmissivity and reflectivity values , you can set the emissivity . 

For further understanding on how to create appropriate boundary conditions and cell zones for DO modelling . I am attaching a link for your reference.

5.3.6. Discrete Ordinates (DO) Radiation Model Theory (ansys.com)

Please refer Cell Zone and Boundary Condition Treatment at Semi-Transparent Walls

This will help you in understanding how ansys fluent assumes the transmissivity value for semi transparent case. Assuming your case involves semitransparent bodies.

As a workaround you can also enter specific profile of transmissivity through udf or expressions and perform the calculations.

I hope this helps you in your simulation.