Re: fanning yourself - can it be effective?

On Apr 2, 6:46 am, rich...@xxxxxxxxxxxxxxx (Richard Tobin) wrote:
In article <9df1bef6-9712-4732-9fd1-540bea372...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,

RichD <r_delaney2...@xxxxxxxxx> wrote:
I watched someone doing that yesterday, and said
"That's an illusion, it can't really cool you, that would
be a violation of the 2nd law of thermodynamics.

Unless the air temperature is above 37C, the air around you is cooler
than you are. You are constantly losing heat to the air, and fanning
increases the cooling by convection.

-- Richard
--
:wq

What's this convection everyone is talking about? I thought
convection is the transport of a fluid, usually because of bouyancy.
It is how hot air is usually transported upward. However, it has
nothing to do with fanning, which is often pictured as moving the air
laterally.
If the air is dry, fanning increases the rate of evaporation
by increasing the turbulence. The turbulence created by the moving air
causes the humid boundary layer near the skin to diffuse away.
Therefore, the air touching the wet surface is dry. The liquid water
evaporates into the dry air near the surface, turning into water
vapor, which again leaves the boundary layer due to turbulent
diffusion.
The transition of liquid water to water vapor requires both
energy and entropy to be drawn from the skin. Therefore, both the
temperature and the entropy of the skin decrease. The entropy of the
surrounding air increases, as it is receiving the water vapor with the
extra entropy.
If the was no fanning, there would be a boundary layer of moist,
humid air close to the skin. The liquid water can't evaporate into
humid air. Therefore, there is no evaporation. The energy and entropy
stay on the skin. The skin remains hot.
Where is the convection? I see turbulent diffusion of water vapor,
I see mean flow advection, but I don't see any convection at all.
Tadchem was close when he was talking about wind chill factor.
Unfortunately, his sarcasm was a little wrong. Maybe not totally
wrong, but his analogy was incomplete.
Another process occurs at the colder outside temperatures. That
process is turbulent diffusion of heat energy. If the process was
entirely that of evaporation, as is the case at higher temperatures,
wearing a rain coat would be completely sufficient for keeping warm.
By stopping the evaporation, the person would be stopping the wind
chill factor.
The wind chill factor is caused by the combination of turbulent
heat diffusion and evaporation.
.

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