How do we run external flow simulation (with solar radiation and reflector material) for Parabolic Trough Collector (PTC) effectively?

I have designed a similar model while following the instructions shown in this link: Radiation in Flow Simulation - Part 1 : Reflection but I could not achieve similar results, could only reach about 65 degree Celsius.

Other than external flow, black body and symmetry, what else am I missing? Can someone kindly guide me on the details?

Thank you.