Hello, I'm working on modelling the thermal behavour of a trapezoidal cavity, which is the receiver from a Fresnel Solar Collector. Inside the cavity there are 6 Aisi 304 steel tubes, which work with water inside, incoming at 180ºC, being heated by reflected solar radiation from below, to 210ºC aprox.

I have made an internal analysis of the fluid inside the tubes, by applying the radiation like a heat flux into the tubes surface on W/m2, and the results were like expected.

But now I need to make an external analysis of the air inside the cavity and it surroundings. For this, I need to consider just ambient air, with no specific mass flux or velocity, just the effect that the radiation going to the tubes makes on the air. The results I've been having are not the ones expected, so if anyone could help me please with this simulation it would be huge for me.

For the external analysis, I have selected gravity, radiation, heat conduction between solids, and applied the specific radiative surfaces and solid materials. I'm not sure what boundary conditions need to be applied here, and how to combine the results obtained on the Internal analysis. Or what I'm doing wrong.

You can see images of the model below. The cavity has a glass window below, and insulated walls for minimizing heat losses. Although the destribution of temperature on the results its fine, the values are way from what they should be, maximum 150-200 ºC near the tubes.