Modeling the flow of solar radiation as the heat flux from the surface.
 

Please help!

Need to model the heating of the solid bodies in the solar radiation. Want to set the flux of solar radiation as the heat flux from the surface. Tell me please, what should be the boundary condition on this surface.

I need a narrow beam of radiation, which is heated to the body inside the computational domain, while not heating the air. The air does not absorb heat and dissipate.

And I get the hot wall and the convective heat transfer with the air flow.

p.s. I can not apply the model of Solar Ray Tracing because the need is a narrow beam of light, which will get only a few of the items.