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 Jan January 18, 2005 18:43

Evaporation of droplet

Dear, Is it possible to calculate "analytically-or semi" the evaporation of a water droplet carried by steam. I would like to get a rough estimate of time before the droplet is evaporated and the diameter as function of time. I assume everything constant for the steam. I know I can just start a CFD calculation for finding the solution, but I would like to get an idea and a feeling with the problem before just doing some calculations Thank in advance Jan

 Dr Strangelove January 18, 2005 19:00

Re: Evaporation of droplet

Your entire problem will depend on the relative humidity of the atmosphere surrounding your droplet.

Secondly, on the temperature of the droplet and its relative velocity to the ambient.

Look up the heat transfer coefficient for spheres. Using that information and the latent heat of evaporization and the initial size of the droplet you should be able to get a rough idea of how long it will take for your droplet to disappear.

 Rami January 19, 2005 03:46

Re: Evaporation of droplet

Hi Jan,

I vaguely remember a similar problem was presented and its solution discussed in detail in an old textbook by Spalding. It might be difficult to obtain it, though.

Spalding, Dudley Brian, Combustion and mass transfer : a textbook with multiple-choice exercises for engineering students, Oxford, Pergamon, 1979

Good luck, Rami

 Chris Bailey January 19, 2005 12:39

Re: Evaporation of droplet

Droplet evaporation is a special and weird thing because the equilibration of humidity and evaporation / condensation at the droplet surface is a function of the curvature of the surface. That's why clouds are supersaturated. Since the curvature can also be negative, in a fine capillary, that's also why physical sorbents like silica gel work as dehumidifiers. This is called the "Kelvin effect". You can read about it in the book "Aerosol Technology" by W. C. Hinds. There are experts in the subject at the University of Minnesota, dep't of Mechanical Engineering, the aerosol science group of Dr. Ben Liu. Small droplets evaporate much faster than CFD would predict - if they are small enough they almost explode as they evaporate.

 Jan January 26, 2005 03:59

Re: Evaporation of droplet

Thankyou very much for all the valuable input. I do not understand the concern about the relative humidity. I want to evaporate water droplets in superheated steam, so I cant figure why the humidity is important.

Anyway I tried the suggestion from Dr. Strangelove and it results in timescales of the right decade, for a start that is sufficient, but I will try to investigate this further.

"I am a bit plesaed that it is difficult, it was a pain for me that I could not solve it"

Thanks Jan

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