Right, technically the thermal energy has to conduct from the water, through the aluminum, then through the coating/paint before it can be "carried away" by the air. I was making the point that "radiators" work through heat being conducted into the air flowing through the core; if there's no forced air flow through the rad then the heat just hangs around and builds up. I remember working problems in my heat transfer class that dealt with combined conduction and convection, in those scenarios the fluid flowing over the outside has its own "convection coefficient" where the heat transfer equals the convection coefficient times the exposed area, times the difference in temperature between the surface and surroundings. IIRC in most cases the fluid was flowing continuously over the surface as if it was forced so convection could either be with forced fluid or fluid that moves 'naturally' due to the hotter parts closest to the surface being less dense. I must confess though that class was very difficult I'm by no means an expert on the subject lol.
