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-   -   Liquid Water Free Surface Evaporation (http://www.cfd-online.com/Forums/cfx/94359-liquid-water-free-surface-evaporation.html)

njw November 13, 2011 12:19

Liquid Water Free Surface Evaporation
 
I am trying to implement interfacial component transfer between liquid water and air.

I have a 50m high cylinder with a radius of 15m, where the bottom and side walls are modelled as no slip walls. The top is modelled as an opening with a 0atm relative pressure and a temperature of 298K.
I have succesfully been able to create two homogeneous phases with a clear free surface at the interface by simply creating two fluid types in the domain.

Buoyancy and thermal heat transfer is included, but I further wish to couple the two phases to allow water to evaporate into the air after placing an point energy souce by the surface. How can I go about coupling the phases?
Thanks

ghorrocks November 13, 2011 17:52

What does the free surface do? Is it always flat or close to it? Does it go up and down? Or does it have waves, splashing and/or foam?

Each of these different states should be modelled differently.

njw November 14, 2011 01:38

For now I just have it as a stationary free surface. Am I right in thinking that I will have to create 3 materials. The liquid water, Air- water vapour mixture and water liquid-vapour mixture?

I tried to go about this way, but I couldn't seem to find the option for component interfacial transfer. I wish to model it through Raoult's law.

njw November 14, 2011 01:38

For now I just have it as a stationary free surface. Am I right in thinking that I will have to create 3 materials. The liquid water, Air- water vapour mixture and water liquid-vapour mixture?

I tried to go about this way, but I couldn't seem to find the option for component interfacial transfer. I wish to model it through Raoult's law.

Thanks very much for replying

ghorrocks November 14, 2011 06:18

If the interface is stationary then I would consider having them as separate domains and not use a multiphase model at all. Then you can impose the free surface transfer functions as interface conditions.

Even if the interface moves up and down you can do this with moving mesh.

But if waves, churning, foaming or bubbles are important this approach will not work.

hello123 January 3, 2015 02:45

Hi all,

I am currently trying to simulate something extremely similar to njw's case. It is an evaporation from water in a open cylinder heated from the sides. The temperature of the bulk fluid does not exceed boiling point(saturated temperature), so there is only heat transfer by convection in the cylinder, with evaporation at the top open end of the pipe.

However, one difference from njw's case is that the free water surface will be gradually moving downwards as water evaporates from the cylinder. The cylinder is thin, around 5mmx0.5m. Hence, mass loss by evaporation will have a significant impact on the water level.

Does any one have any advice on how I can model the evaporation at the top surface?

Thank you so much in advance.

ghorrocks January 3, 2015 05:43

Why are you modelling this with CFD? Why can't you just model this with simple control volume approaches?

hello123 January 5, 2015 10:10

Hi Sir, thank you for you reply. Sorry but could you explain how I can go about solving this by control volume method? Or do you have any references that you would recommend? The heat transfer texts I can find at does not seem to have a method for solving this problem.

Thank you in advance!

JuPa January 5, 2015 12:09

Quote:

Originally Posted by hello123 (Post 525954)
Hi all,

I am currently trying to simulate something extremely similar to njw's case. It is an evaporation from water in a open cylinder heated from the sides. The temperature of the bulk fluid does not exceed boiling point(saturated temperature), so there is only heat transfer by convection in the cylinder, with evaporation at the top open end of the pipe.

However, one difference from njw's case is that the free water surface will be gradually moving downwards as water evaporates from the cylinder. The cylinder is thin, around 5mmx0.5m. Hence, mass loss by evaporation will have a significant impact on the water level.

Does any one have any advice on how I can model the evaporation at the top surface?

Thank you so much in advance.


I've done a simulation like this with success. The steady state solution is an empty tank. So you must be interested in the transient. What are you hoping to achieve from the simulation?

ghorrocks January 5, 2015 17:08

Treat the liquid domain as a control volume, with a specified volume and temperature. Work out the temperature change, the evaporation (mass loss) and volume change for that time step, and iterate over time. Simple(ish), and no CFD required.

farzaneh babaei March 20, 2015 05:41

Hi
My project is modeling of surface evaporation in dams, Can i model the evaporation by using CFD? Many people told me you can not do this. can you help me about this problem?

Thank you so much .

ghorrocks March 20, 2015 05:48

You can model evaporation in CFX. But the question is more: If you are modelling evaporation in a dam do you need a CFD solver to do it?

Why do you want to model evaporation in a dam? What will you learn other than that the water slowly moves from the dam into the air and the water level slowly goes down?

farzaneh babaei March 20, 2015 05:59

my aim is to show that water storage behind dams evaporate more quickly than when water flowing on the ground .

ghorrocks March 20, 2015 07:48

What physical process makes dams evaporate faster?

farzaneh babaei March 20, 2015 07:54

when the thickness of (height of ) water is too high

ghorrocks March 21, 2015 04:54

How does the water height increase the evaporation rate?

davide March 28, 2015 12:22

Evaporation from a free-surface
 
Hi everyone,

I do have the same problem (evaporation from a free-surface) which I would like to solve with CFX. This is a simple problem of a 2D domain with the domain occupied by water and air (water and air are separated with a clear interface). The water and air never mix, so the homogeneous free-surface model is enough. The interface will move due to evaporation (and mass conservation).

Please note I'm interested in CFD modeling of this problem not analytic solutions. I highly appreciate if anyone can let me know if they have ever solved this with CFX or are aware of any paper/article/video/tutorial that explains how to solve this with CFX.

Thanks :-)

ghorrocks March 29, 2015 05:22

I have never done this type of model. CFX should have the necessary models to do it. But do not be confused into thinking this is a simple simulation - multiphase with phase change mass transfer is never going to be straight forward.

Your best bet to get an example to start you off is to contact ANSYS support.

davide March 29, 2015 08:44

Thanks for the reply.

I have to agree with you. This problem, although incredibly easy looking, is actually quite complex. And we also have problems with spurious currents around the interface. And in agreement with you, I think mass-transfer in multiphase flows is still on its R&D phase and is not quite robust yet.

ghorrocks March 29, 2015 18:14

Spurious currents at the interface is a common problem with free surface models with surface tension. So only turn the surface tension model on if you really need it.

I strongly disagree that mass transfer in multiphase flows is R&D only. There are many industries which use is successfully and accurately. But it needs a skillful, experienced CFD person to drive it and requires careful derivation and validation. You cannot simply press a button and mass transfer "just works".


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