[Stephen Waite]

Hello, I'm trying to simulate the evaporation of water from a thin film covering a wall boundary in CFX. The wall boundary represents biological tissue and the thin film of water represents the mucous layer above the tissue. A gas mixture of air and water vapor will flow over the (stationary) thin film of water and cause evaporation. The temperature and depth of the water film are to be held constant.

I'm unsure about how to specify this sort of boundary condition.
I've created two domains (one for the air flow and one for the film of water) and used a "Conservative Interface Flux" interface model for heat transfer.
The air domain contains both the air-water vapor gas mixture, the liquid water material and a homogeneous binary mixture containing liquid and gaseous water. The water film domain only contains the liquid water material but I'm not sure how to "fill" this domain so that it consists entirely of water that is stationary and constant in mass despite the evaporation.

Alternatively would a better approach be to specify a temperature and water source on the wall boundary?

Are there any tutorials with this sort of evaporation model?

Thanks in advance for any help.

[ghorrocks]

The best way to model this will depend on what you are trying to get out of the simulation. What is the thing you are trying to get out of this simulation? How the water vapour behaves as it leaves the area? Of the heat transfer involved in the evaporation? Or something else?

[Stephen Waite]

I guess the evaporation rate and the heat transfer involved in causing this evaporation. We know what the surface temperature is at a given time (I'm just working on steady state simulations at the moment) and what the inlet and outlet temperature and humidities are and we want to know what the total evaporation rate and heat flux to cause this evaporation is in order to achieve the outlet temperature and humidity. I hope that makes sense.

[ghorrocks]

So would a simple simulation of the air flow only, with a mass source at the wall to generate the water vapour be a useful starting point? You can make the local mass source a function of water vapour concentration to make it more realistic. This will give you an idea of the water vapour flow and it is a much simpler simulation (a single domain and a single phase, but multicomponent). But it does simplify the heat transfer side of things.

[Stephen Waite]

Thanks, so would I just need the gas mixture (air and water vapor) and have the source being a function of the water vapor partial pressure at the wall and wall temperature? Or just a function of the water vapor mass fraction?

[ghorrocks]

Yes, the source would be a function of the local humidity. But as for partial pressure or vapor mass fraction - you will have to look into this and find out what is the best way to describe the mass transfer for your situation.

[Stephen Waite]

Great thank you very much