It is a question that came to me, but evaporation doesn't seem a nice answer. Please help.
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Two main mechanisms help cool your body when a fan blows on it:
1. Forced convection:
Newton's cooling law tells us that an object at temperature $T$ surrounded by a cooling medium at $T_{amb}$ will lose heat at a rate of:
$$\dot{q}=hA(T-T_{amb})$$
Where $h$ is the heat transfer coefficient and $A$ the surface area between object and cooling medium.
As long as $T>T_{amb}$ heat will be carried off the object by the medium (air, in this case).
Conversely, the medium will heat up (conservation of energy principle). But by providing a constant flow of air at $T_{amb}$, as a fan does, $\dot{q}$ is maximised.
The air speed also has the effect of increasing $h$ somewhat, causing greater values of $\dot{q}$ and thus better cooling.
2. Perspiration:
We sweat because evaporative cooling helps keep us cool: evaporating water costs heat, the so-called Latent Heat of Evaporation.
Fanning also enhances evaporative perspiration because the fresh air is low in moisture, which speeds up the mass transfer of water from skin to air, increasing the amount of heat needed to achieve this.
Gert
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