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I always thought that the random molecular motion was due to heat, not the
cause of heat. Using your example, and ignoring heat input from external sources
other than air temperature, can you tell me the surface temperature of an object
when the air temperature is an average 90* and with a constant wind velocity of
say 40 MPH?
Isn't the heating of an aircraft caused by the friction of the air
molecules against the surface of the plane?
John
----- Original Message -----
Sent: Monday, November 17, 2003 9:30
AM
Subject: Re: [smartBridges] Airpoint gone
mad and turned into AB
John, you're absolutely right! Moisture evaporation is
another method of cooling in addition to wind. But you are incorrect
regarding "no matter how much wind there is it can't be colder than the air
temperature."
There are several modes of cooling by motion of air
molecules over a surface -without the need for evaporation.
For
example, on a molecular level, all air within a given volume over, say Nevada,
is at the same temperature of 90*. This means that all the molecules
bouncing around in random directions collectively have an AVERAGE temperature
of 90*. The molecular speeds are actually very fast and taken together
have a skewed bell curve look. It is the speed of the molecules alone
that determines the temperature! If a wind blows, the random directions
of the molecules have a new resultant direction in the direction of the
wind. This new vector has on average a much lower speed than the other
random directions that the molecules are shooting off in. Given a test
subject also at the ambient temp of 90* within the path of the wind (and this
new vector), the molecules extract energy from the object in the direction
of the new vector (i.e. wind). This extraction of energy is cooling
the object.
In another application of this same principle, think what
happens to supersonic jet planes. The speed at which they strike
molecules -no matter what temperature they're at because they can only cool at
sonic or subsonic speeds- will heat up the normal surface (the flying edge) of
the plane.
Hope this clears it up. (yes, I'm a
physicist.)
Sevak
On Mon, 2003-11-17 at 11:53, John Banes wrote:
I think you guys are mixing up cooling and chill factor. Chill factor is the
result of moisture evaporation lowering the surface temperature below
ambient temperature.
In humans we always have moisture on our skin so the wind causes it to
evaporate faster lowering the temperature below the actual air temperature.
I would guess that on a rainy day your wet radio might also have a chill
factor, but if it is dry there is no evaporation therefore no chill factor.
No matter how much wind there is it can't be colder than the air
temperature.
John
----- Original Message -----
From: "Dimitrios Sidiropoulos" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, November 17, 2003 7:50 AM
Subject: Re: [smartBridges] Airpoint gone mad and turned into AB
> Eje,
>
> Do not forget that the unit radiates heat at all times.
> If it is 10*F and no wind the temp inside the unit will be 10+.
> If there is wind (still at 10F) it will absorbe more heat from the unit
and it will
> lower it's temp closer to the 10F.
>
> If you test the specs of a unit at -40F inside a freezer then the temp
inside the
> unit will higher then -40 (lets say -38) if there is wind at -40 then the
temp
> inside the unit is closer to -40 (although still not -40)The faster the
wind blows
> the closer it gets the ratior temp to the real temperature.
>
> So yes!! Wind does affect the temperature of the radiator all though it
has no
> effect on the radiating ellement, etc.
>
> Thank you
> Dimitri
>
> http://www.vernet.gr
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