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It is a question that came to me, but evaporation doesn't seem a nice answer. Please help.

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  • $\begingroup$ Forced convection of heat from your body. $\endgroup$ – Deep Sep 23 '16 at 5:27
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    $\begingroup$ It depends on the temperature of the air whether the convection actually cools the skin. When I was a kid my dear Grandpa (RIP) demonstrated this by gently blowing towards my arm in a sauna. The temperature was about 80 degrees centigrade (175 Fahrenheit) - it was not a cooling effect. I recall feeling the same way the first time I visited NY (JFK actually) - the fan on the stationary airport bus idling on hot tarmac did not have a cooling effect :-) $\endgroup$ – Jyrki Lahtonen Sep 23 '16 at 5:38
  • $\begingroup$ In simple terms, your body heats the air around it. The fan moves that air away and replaces it with more cool air. $\endgroup$ – James Sep 23 '16 at 12:31
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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.

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  • $\begingroup$ Do you know offhand which mechanism is typically more significant? $\endgroup$ – aquirdturtle Sep 23 '16 at 7:23
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    $\begingroup$ @aquirdturtle: that will depend on conditions: high moisture content (near 100 %RH) in air will almost stop evaporative perspiration completely, something we've all experienced. High ambient temperature of course also makes cooling impossible: $T_{amb}>T$ would cause us to heat up! $\endgroup$ – Gert Sep 23 '16 at 7:26
  • $\begingroup$ Something to do with Joule Thomson effect, guys? $\endgroup$ – Swapnil Das Sep 23 '16 at 11:31
  • $\begingroup$ @SwapnilDas. No, I don't think so. Certain types of fan may be helped by the JTE but that's not the question here. :-) $\endgroup$ – Gert Sep 23 '16 at 15:33

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