If the air temperature were the same as the human body's....

[Nantes]

Damn title character limits.

I just had the weirdest thought while rearranging my computer's case fans. I put my hand in front of one of them and felt a light breeze of air, telling me it was working. This is due to the local cooling produced where the fan's airstream comes in contact with the skin. Since the air being blown is at a lower temperature than the human body, that local skin cooling triggers my sensory neurons, my skin feels slightly cold and thus I know there's air flowing against it. But what if the air were the exact same temperature as my skin? No heat transfer would occur and my neurons would not be activated. Thus, I can only think of two other mechanisms by which I could perceive the fan if I were not looking at it nor hearing it: perhaps the moving air slightly disturbs the hairs on my skin, which trigger sensory neurons of their own, OR the airstream would be strong enough to actually push my skin. The latter case is obviously not true with 120 mm case fans.
Assuming my hair sensitivity theory is correct, if the fan were functioning in a low enough setting so that the airstream's speed did not disturb the hairs, it stands to reason that the fan would be completely undetectable if the air temperature were the same as my body's. I would not be able to feel it blowing air at all.

It's quite a perturbing thought, no?

 

[mfb]

Since the air being blown is at a lower temperature than the human body

Not necessarily. Air flow increases evaporation, which can cool even if the temperature is higher than your body temperature.

perhaps the moving air slightly disturbs the hairs on my skin, which trigger sensory neurons of their own

That happens.

Assuming my hair sensitivity theory is correct, if the fan were functioning in a low enough setting so that the airstream's speed did not disturb the hairs, it stands to reason that the fan would be completely undetectable if the air temperature were the same as my body's. I would not be able to feel it blowing air at all.

If the airflow is too weak, you don't feel it. That is not very surprising I think.

 

[Nantes]

If the airflow is too weak, you don't feel it. That is not very surprising I think.

Yes, I'm more interested in whether equalizing air temperature would make undetectable a previously-detectable fan.
Your evaporation comment makes sense. I've noticed though that ceiling or table fans very rapidly lose their subjective cooling effect when the air temperature is very hot. I live in Brazil, and during heat waves I feel like using a fan is actually like blowing hot air at you, making the heat feel worse. Perhaps the evaporation effect only works to an appreciable degree if one is actually sweating.

 

[mfb]

Yes, I'm more interested in whether equalizing air temperature would make undetectable a previously-detectable fan.

I doubt that. Maybe some fan with extremely hot or cold air and very weak air flow.

I would assume the evaporation effect would only work if I were actually physically sweating, as opposed to just having microscopic water droplets on my skin which (I think) is usually the case.

This is easy to test - air from your lungs has body temperature to a good approximation. It also has a high humidity, so air from a fan at the same temperature feels colder.

 

[Nantes]

This is easy to test - air from your lungs has body temperature to a good approximation. It also has a high humidity, so air from a fan at the same temperature feels colder.

Since it feels slightly hot, I can presume the temperature of the skin is lower than average body temperature. Not a very valid test.