Learning Forum

[Cathleen]

Hello,

 

I have another post on the forum regarding a moving interface problem with evaporation. I have a contact line between liquid water and a water-vapor/air vapor space. I want to model (track) the interface between these two areas as the liquid water evaporates into the vapor space. 

 

Modeling this with ANSYS multiphase VOF model does not capture the interface motion and it appears that the liquid water is dispersing into the vapor space instead of the interface receding as the water evaporates. 

 

From the forum I think I need to build a UDF for the moving interface with an interface temperature gradient boundary condition.

 

My question: how do I assign the UDF so the condition moves with the interface? I can designate a row of cells at the interface but once it moves out of them I don't think it will work.

[Rob]

If you want to move the interface like that you need to read up on moving mesh. You'll also need to provide heat transfer effects to get heat from the gas to liquid and account for latent heat.  I'd then be very wary of taking any data from the model as it'll do exactly what you code in: ie you may as well just write the report to suit the rate you want it to move at. 

Learning how the multiphase models work, and with that understanding doing it properly will give you more information. 

 

[Cathleen]

Ok, thank you. I will look into that. As far as the problem I am seeing, where the liquid disperses into the vapor space instead of the interface receding, do you have any insight as to why that is happening? Is it because of the sharp difference going from a VOF=0 to VOF =1 at the interface and that effectively getting spread out along the first few mesh cells?

[Rob]

If the phase interface is diffusing it can be convergence related. Not helped that phase change in VOF is currently (22R1) not really a good idea. Read the release notes when 22R2 come out, in the meantime decrease the time step and turn on interface anti-diffusion. 

[DrAmine]

I do not understand the goal of your run. Are you accounting for wall adhesion and different motion dependent contact angle? The mass transfer itself is probably wrong as you have another non-condensable gas component.  Evaporation is mainly driven by concentration gradient if you are below the boiling point of water.