Dry vs. wet surface cooling

Ok, here is something we all know by experience. The heat loss by a surface can be increased if it is kept wet. For example, when one gets out of the shower, swimming pool or the ocean, our skin is wet and thus we instantly feel cold. That feeling of being cold is a direct measurement of the increase in heat loss through the skin. The question that I want to explore a little bit is, how much more cooling do you get if the surface is wet? The first thing to note is that the heat loss is proportional to the temperature difference between the surface and surroundings. For example, a dry surface at 30°C will lose heat faster if it is inside the refrigerator at 5°C as opposed to being outside in a room temperature of 24°C. If the dry surface and the surroundings are at equal temperature, no heat loss will occur because they are in a state of thermal equilibrium. The following picture illustrates the difference between dry and wet surface cooling for a sphere. Dimensions and conditions are specified in the picture and the calculations were performed under low mass transfer assumption and for temperature differences varying from 0-30°C.
The graph clearly shows than the amount of cooling from a wet surface is always grater than the amount of cooling from a dry surface. Two things to note are: a) for a zero temperature difference (300K surface temperature), the dry surface does not lose heat as previously mentioned, but the wet surface does (by mass transfer) and b) the increase in heat loss varies linearly with temperature difference for the dry surface while the heat loss by a wet surface varies exponentially with temperature difference. Note that at a surface temperature of 330K (30K temperature difference) the amount of heat lost by a wet surface is almost 9.5 times the amount of heat lost by the dry surface. The human body knows this, and thus anytime we need to cool faster, our body starts to sweat.

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