Eventually, all the assumptions and theories will have to be tested, and the tests determine whether to validate or discard that assumption or theory.
A couple of years after the fall of the USSR, my company was able to hire a pair of Russian scientists with PhDs whose experience was thermal management. They would make these incredibly complex theoretical analysis, using some mathematical operators I had never seen before. Yet, they always validated them with actual testing.
The problem with heat transfer is that it is a three-dimensional field, and has to be analyzed accordingly.
One of them had actually worked for spacecraft electronics. He had a saying: here in earth it is simple, one can always rely on dumping the heat to the air. In space, one only has radiation...but sometimes one has positive radiation gain, for instance surfaces pointing to the sun. Very interesting.