Equivalent Evaporative Heat Transfer Coefficient

[tkunke]

Hello,

I have created an ANSYS transient thermal model for hopefully validating and characterizing the heat flux through a test instrument design to study microchannel evaporation. In general, after a thorough literature review, evaporative heat transfer is still a fairly vague subject, so I was hoping for some help.

I need to calculate an average heat transfer coefficient for the thin film evaporation of a fluid. I know the volume, molar density, enthalphy change of vaporization, etc. of the fluid droplet that spreads over a horizontal surface. What I need are an equivalent heat transfer coefficient, and appropriate reference temperature for the ANSYS transient thermal analysis.

If anyone could direct me in the proper direction, that would be excellent. Thank you all in advance for your help.

Taylor

 

[chicopee]

It would be a good idea to give the readers an idea as to the medium involved.

 

[tkunke]

Sorry about that, my fault. Evaporation takes place on a metallic surface, the thin film is denatured alcohol evaporating into the atmosphere.

 

[IRstuff]

That's pretty much it, other than the equivalent mass flow. Ultimately, you need to wind up with something that presents W/m²-K, which will be determined by your ability to move the alcohol into the location, having it evaporate, removing the vapor, and replenishing the fluid.

TTFN
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[chicopee]

Get a book on heat transfer and study the section on heat transfer with change in phase since your inquiry deals with a liquid changing into a vapor. My H.T. book by Keith is a 1967 edition and discusses pool boiling and presents a modified equation by investigator Rohsenow. His investigation came up with a whole bunch coefficients labeled Csf some of which are for isopropyl alcohol on copper surface and ethyl alcohol on chromium surface. His equation is too detailed for me to type. So since we are looking at over 40 years of time elapsing, I would not be surprise if updated details were available in more recent editions of heat transfer books.
which has

 

[IRstuff]

Lienhard's free HT text discusses pool boiling in Chapter 9:

TTFN
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[davefitz]

More relevant for such modeling is the combination of the surface oxidation fouling in series with the boiling liquid. Although theoretical correlations for boiling heat transfer coefficients may lead you to a very high value for h ( and consequently low value of R), ignoring the resistance to heat transfer due to the surface oxidation will lead to an erroneous conclusion in many cases.

Older correlations by Bergles, Whalley, Rosenhow, Groeneveld may be used, but I prefer the ones by KWU authors Kiefer, Kohler and Hein

"Whom the gods would destroy, they first make mad "

 

[racookpe1978]

Davefitz:

Is that reference by the KWU authors valid for open (calm water) oceans or lakes evaporating into the sky under light or medium winds?

Seems all I can find references for heat loss from open waters evaporating is the "loss of water level" method - invalid for oceans or sea water when water level really doesn't change with time. It would appear necessary to account for delta T between water surface and air of course, but also relative humidity in the air above the lake.

 

[davefitz]

racoope1978:
No, I had thought the interest was in cooling equipment, and not bodies of water. Papers on the topic of cooling ponds and environmental energy balances would be the likely source for the correlation you are after.

"Whom the gods would destroy, they first make mad "