Wave Tank Evaporation

[cep]

Hi, I am currently doing tests in a wave tank to measure the wind and wave influences on evaporation. Obviously the wave roughness is closely related to the rate of evaporation, but anyway...

 

When I input large wave heights with high frequencies, there is a lot of splashing going on in the tank..and a lot of water adhering to the glass wall of the tank. Would the water adhering to the tank walls increase the evaporation rate and why?

 

*Note the tank is in a controlled environment with constant temperature and humidity parameters.

 

Thanks.

[Newbies_Kid]

I don't see any reason why evaporation rate should go faster. But this is interesting.. mind if i ask? How do you control the temperature of the tank?

Edited May 29, 2010 by Newbies_Kid

[InigoMontoya]

I don't see any reason why evaporation rate should go faster.

Two reasons pop into my mind....

 

1) More surface area.

2) Boundary layer disruption.

[CaptainPanic]

Hi, I am currently doing tests in a wave tank to measure the wind and wave influences on evaporation. Obviously the wave roughness is closely related to the rate of evaporation, but anyway...

 

When I input large wave heights with high frequencies, there is a lot of splashing going on in the tank..and a lot of water adhering to the glass wall of the tank. Would the water adhering to the tank walls increase the evaporation rate and why?

 

*Note the tank is in a controlled environment with constant temperature and humidity parameters.

 

Thanks.

 

Before going into some details, a word of warning: the field of mass and heat transfer is a b*tch. It'll take some time to get used to. However, this doesn't mean you cannot play with a tank of water. Measurements can be tricky though.

 

Evaporation is related to:

- Surface area

- Concentration gradient across the boundary

- Mass transfer coefficient across the liquid/gas boundary

 

Surface area

Waves and droplets obviously increase your surface area. So, this is relevant.

 

Concentration gradient

You need to know the vapor pressure of the water (depends on the temperature - so you need to measure that all the time). You need to know the moisture already present in the air, and the air temperature as well. That's the gradient across the two bulk phases.

 

Mass transfer coefficient

You need to find out of the wind is blowing, and if so, if it is turbulent. Maybe there is "artificial wind" in the form of ventilation?

If the water and air are not of the same temperature, then unfortunately for you, you are going to have heat transfer as well. It will cause the air to move faster, and therefore increasing the mass transfer too.

 

You see, at the part of the mass transfer coefficient it becomes really tricky. Unfortunately, a little wind can be just as relevant as millions of droplets on the wall of your tank.

 

At this point, I have talked enough. Time to listen again. Are you planning to do a measurement only - or do you need to develop a correlation or formula of some kind to describe what you measured?

[Newbies_Kid]

When he said about large wave length and high frequency, what comes in my mind is something like microwave oven. Makes water molecules in tank to vibrates billions of times a second. As they vibrate they bump into other molecules and generate heat which i think the evaporation process starts.

[cypress]

Water adhering to the glass wall will slightly increase evaporation due to the increased surface area with high waves vs lower waves. However the walls also represent a discontinuity that will influence convective mass transfer by altering localized concentration gradients which also are a function of wave amplitude and wind speed. This influence will reduce evaporation. Hard to say how significant these two influences are without putting a sharp pencil to it.