Question

How is the thermal energy of the radiator transferred to the surrounding air

Answer 1

Radiant energy. The heat in the radiator does not transfer from the radiator by convection because the heat in the radiator is inside the radiator and not a part of the surrounding air. There is a casing surrounding that heat source; the air cannot get to that heat source directly.

Here's what really happens. Radiant energy from the radiator honeycomb heats the surrounding air. And the heated surrounding air then carries off that heat by convection. My point is this... thermal energy is transferred from the radiator to the surrounding air as radiant energy. Only after that energy is in the air does convection play a role in the heat transfer.
oldprof