I have a cube that has layers of different materials in it. The bottom layer is a heater defined as a volume heat source term. Then the heat travels through the cube, the only face it can exit is the side of the cube opposite the heater section, because all other walls are adiabatic.

My problem is with radiation. Of the different layers in the cube, all are different solids, except one is helium, which through the primary mode of heat transfer that I want is radiation. Also, at the face where heat exits the wall, I also want to only use radiation.

So the heat travels by conduction through the solid parts, then radiation across the gap, then conduction through more solid, and then finally radiation to exit the model.

And recommendation on what radiation model to use, my values are not consistent with analytical calculations. Thank you

Drop the conductivity of helium to a very low number ( that will rule out conductivity). Also model the density of helium as constsnt that will ensure natural convection doesnt take place ( that will rule out convection). Any of the radiation models will then do ( like DO)