your case study depends on the boundary conditions of the top and bottom surfaces and the limitation of heating and cooling and the type of gas in order to determine the expected heat transfer types conduction , convection or radiation and which model to be used.
also your case study unclear maybe due to high flux lead to high pressure.
In this case the heat transfer is mainly by conduction. The contribution of the radiation depends on the thermal difference between the two horizontal walls. It also depends on the nature of the gas in the cavity (if it is transparent or participates in the radiative exchanges, air is practically transparent). On the other hand, convection is absent in this configuration