Julian is correct: you have two kinds of heat transfer going on in this case. One is direct, where one substance transfers energy to another – water to metal, metal to air – by molecular collision, the other is by radiant transfer, which occurs even in a vacuum. Too many variables and too complex a process to make generalizations about, but to generalize, the wider the difference in temperature between two objects in contact, the higher the heat "flow". Thats why a high air velocity through a heat exchanger is desirable (fans). But, having said that, for a given application, the guys to listen to are the ones in the field who deal with it every day, not academics like me.

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