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I’m having some problems to identify the type of heat transfer involved in a practical experiment I’ve carried out. In my experiment, I heat a metal rod by making water vapor flow inside the rod, then I let it cool down to room temperature.

On the one hand, I think it sounds reasonable to argue that both the heat transfer when heating and cooling occurs by convection, since a flow of moving air is involved (water vapor in the first case, room air in the second case). But at the same time, there is direct contact of hot air/cool air in both cases, especially when heating. So one could think heat transfer occurs by conduction.

Would you say both types of heat transfer are actually involved?

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It depends on the dimensions and thermal properties (thermal conductivity, density, heat capacity) of the rod. If the rod wall is very thin and has a high thermal conductivity, then the convective transport to the fluid stream will dominate. But if the rod wall is very thick and/or has a low thermal conductivity, then the conduction within the rod can be significant also.

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When only fluids are involved, then convection. Conduction happens between the material surface and an individual fluid particle, but then that particle is immediately replaced by another and a new convective transfer takes place. This repeating process is what we call convection.

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