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I understand that a domestic fan on a warm day will cool a person down (I know this has something to do with wicking the heat away though I still don't fully understand the physics of the process)

However, since speed makes particles more energetic, and the energy could be converted to heat, surely there would be a point where the air speed would create a heating effect (on a human subject) instead of a cooling effect? Is this right? How fast would a fan at room temperature need to be spinning if so?

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Yes, it is very possible for a heating effect to take place due to fast moving air.

Essentially, air cools down a subject by increasing the area over which the heat gets dissipated by passing more air per unit time across the subject.

In case of ideal gases, we have a formula that governs the temperature of the air with regards to its root mean squared velocity.

$V_{rms}=\sqrt{\frac{3RT}{M}}$

Where, $V_{rms}$ is the root mean squared velocity, R is the universal gas constant, T is the temperature in kelvins, and M is the molar mass in kg/mol.

A fan speeds up a tiny fraction of the total air in the room, this air then travels through the rest of the air present in a room, and collides with the slower (cooler) air and dissipates its energy significantly well before it reaches the subject. The increase in temperature of the overall air is negligible. But when dealing with fast moving air, this increase has to be taken into account, as faster moving air can cause a heating affect via the drag forces.

Let us consider the example of a supersonic jet, the air itself is not collectively moving at such high speeds, but we may still use this example as the relative velocity between the air and jet is still high. This fast moving air creates great drag (frictional) forces on the jet, causing it to heat up.

More information about this effect be found here

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    $\begingroup$ Thanks you - having it expressed of 'increasing the area over which the heat gets dissipated' really helped me visualise the physics of whats going on. However, given the average human skin temperature is 33 degrees c, if air temperature was 36 degrees c, does this mean that a fan moving this air more rapidly across skin would have a heating effect? $\endgroup$
    – Amphibio
    Commented Apr 4, 2021 at 9:15
  • $\begingroup$ ah yes, the New York City health department says as much! health.ny.gov/publications/6594 $\endgroup$
    – Amphibio
    Commented Apr 4, 2021 at 9:20
  • $\begingroup$ thanks = thank, of = as $\endgroup$
    – Amphibio
    Commented Apr 4, 2021 at 11:23
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    $\begingroup$ @Amphibio yes, if the temperature of the air itself is higher than the body temperature, then the fan blowing air will cause a heating effect. As an example, in India, during the summer season, there are strong gusts of hot and dry wind that blows across the northern subcontinent. These winds are known to cause heatstrokes and are a considerable problem during the summer. You may find more info about my example here: en.wikipedia.org/wiki/Loo_(wind) $\endgroup$
    – ravioli1369
    Commented Apr 7, 2021 at 14:14
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Yes sometimes there can be considerable heating. Most fans are in line and the heat comes from the motor. I suppose technically adding energy to the air movement would be adding some heat.

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However, since speed makes particles more energetic, and the energy could be converted to heat, surely there would be a point where the air speed would create a heating effect (on a human subject) instead of a cooling effect? Is this right? How fast would a fan at room temperature need to be spinning if so?

First of all, the fan increasing the kinetic energy of the gas particles is not energy "converted to heat". Heat is energy transfer due solely to temperature difference. What you are describing is the possibility of increasing the kinetic energy of the room air molecules which is reflected as an increase in the air temperature. In other words, the fan does not create heat.

So the real question should be whether or not the movement of the air by the fan will result in an increase in the temperature of the air to the point that it counters or negates the normal cooling effect of convective heat transfer.

While it is certainly possible that a fan could slightly increase the temperature of the air by "stirring up" the air molecules, for ordinary domestic fans this increase would probably be negligible, and certainly not enough to negate the cooling effect of the air movement. The fan motor coil, which gets hot when the motor is running, would probably have a greater effect on the air temperature than the movement of the air by the fan.

Hope this helps.

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