| Topic: |
Science > Physics |
| User: |
"Dan Akers" |
| Date: |
28 Jul 2005 06:24:18 PM |
| Object: |
Help with the Physics of Evaporation |
I am trying to discern how one would go about calculating the
instantaneous net molecular evaporation rate from the surface of pure
water to the atmosphere given the temperature of the water's surface and
the partial pressure of the water vapor in the prevailing atmosphere
above the water. I have learned through my research so far that the net
evaporation rate is proportional to the difference in the vapor pressure
of the liquid phase and the atmospheric vapor phase. But that only gets
me so far; I want to determine the actual evaporation rate.
For example: What would be the net instantaneous molecular evaporation
rate (units such as: grams/m^2*hr) for pure water at 25C (vapor
pressure=23.749 mmHg) to an over-atmosphere with a water vapor partial
pressure of, say, 50% of that; 11.875 mmHg?
Any help or information would be greatly appreciated...
-Dan Akers
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| User: "Zigoteau" |
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| Title: Re: Help with the Physics of Evaporation |
29 Jul 2005 11:41:51 AM |
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Hi Dan,
Do you have access to the scientific literature? Geoff T. Barnes of
Queensland University did a lot of work on the rates of evaporation of
water surfaces with and without monolayer cover, e.g.:
J Colloid Interf Sci 207 (1998) 258-263
G. T. Barnes, Adv. Colloid Interface Sci. 25(1986) 89 .
and probably references therein.
Cheers,
Zigoteau.
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| User: "Dan Akers" |
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| Title: Re: Help with the Physics of Evaporation |
29 Jul 2005 09:06:18 PM |
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Zigoteau wrote:
"Do you have access to the scientific literature? Geoff T. Barnes of
Queensland University did a lot of work on the rates of evaporation of
water surfaces with and without monolayer cover, e.g.:
J Colloid Interf Sci 207 (1998) 258-263
=A0=A0G. T. Barnes, Adv. Colloid Interface Sci. 25(1986) 89 .
and probably references therein."
____________________________________
Re;
Thanks for the information, but no, I do not have access to references
that esoteric. I was looking for "first principles" sort of info. and I
found what I was looking for here:
http://van.hep.uiuc.edu/van/qa/section/States_of_Matter_and_Energy/Boiling=
_Evaporating_and_Condensing/20020321122324.htm
In any event, thank you for your consideration...
-Dan Akers
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| User: "Zigoteau" |
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| Title: Re: Help with the Physics of Evaporation |
30 Jul 2005 06:19:21 AM |
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Hi, Dan,
Thanks for the information, but no, I do not have access to references
that esoteric.
The scientific literature is not esoteric. It is serious. These days in
developed countries there is roughly one suitable library per 100,000
of population.
I was looking for "first principles" sort of info. and I
found what I was looking for here:
http://van.hep.uiuc.edu/van/qa/section/States_of_Matter_and_Energy/Boiling_Evaporating_and_Condensing/20020321122324.htm
With all due respect, does water at 310K in your vicinity evaporate at
the rate of 1.1 mm/s? However, if you are happy with what you have
found, good luck to you.
Cheers,
Zigoteau.
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| User: "Dan Akers" |
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| Title: Re: Help with the Physics of Evaporation |
31 Jul 2005 05:42:25 AM |
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Zigoteau wrote:
"The scientific literature is not esoteric. It is serious. These days in
developed countries there is roughly one suitable library per 100,000 of
population."
_____________________________________
Re;
I think you misunderstood. By "esoteric" I simply meant that I do not
have access to references that detailed in the subject matter I was
inquiring about. Perhaps I chose the wrong adjective...
_____________________________________
Zigoteau wrote:
"With all due respect, does water at 310K in your vicinity evaporate at
the rate of 1.1 mm/s? However, if you are happy with what you have
found, good luck to you.."
_____________________________________
Re;
Thank you. Yes, I was simply looking for the base mathematical
relationship that the differential vapor pressure played in the process
of evaporation. I realize that the evaporation process itself alters
both the over-atmosphere and the liquid in close proximity to the
liquid/gas interface, making the "actual" calculation of the evaporation
rate much more complex; not to mention other "outside" factors such as
convection and agitation of the liquid (wave action) or over-atmosphere
(wind). So, the answer is no; water in my vicinity does not, to my
knowledge, evaporate at 1.1mm/s @310K with an over-atmospheric vapor
partial pressure of "0.8 psia", as stated in the reference. As an
aside, I have found that the actual rate, at 25C and 55% RH, in still
air, is about 1/1000 of the rate calculated from the formula in the
reference I found and posted earlier.
-Dan Akers
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| User: "Zigoteau" |
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| Title: Re: Help with the Physics of Evaporation |
09 Aug 2005 02:26:44 AM |
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Hi, Dan,
If you're still watching this thread, I'm just back from my conference.
These days in developed countries there is roughly one
library [giving access to the serious scientific literature]
per 100,000 of population.
I think you misunderstood. By "esoteric" I simply meant that I do not
have access to references that detailed in the subject matter I was
inquiring about. Perhaps I chose the wrong adjective...
I think that, if you were really interested, it would not be impossible
for you to get some sort of access to the library of your nearest
university or college. Even if you are too shy to do that, most
journals now make their archives available on the web, sometimes at no
cost. Not Elsevier though. They are in it for the money, and charge
around $30 per paper.
As an aside, I have found that the actual rate, at 25C and 55% RH,
in still air, is about 1/1000 of the rate calculated from
the formula in the reference I found and posted earlier.
After having seen Barnes' work, I can believe that.
Cheers,
Zigoteau.
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| User: "AJW" |
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| Title: Re: Help with the Physics of Evaporation |
29 Jul 2005 10:18:12 PM |
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May I suggest you check out a Physical Chemistry text? It's 20 years or
more old, but Strong's P-Chem I remember as being pretty good.
About evaporation rates: what really goes on, of course, is that at a
given temperature and pressure, the number of molecules of water
leaving the surface and going into the gas phase is the same.
Increasing humidity simply means more molecules leave the gas phase and
enter the liquid. That is, if the surface is a plane. If the liquid
phase is in droplet form, the curve changes the surface tension and the
liquid volume is no longer in equilibrum (I seem to remember that, at
least).
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