Learning Forum

[hurch346]

Hello,

I currently have a model which uses species transport and the lee evaporation-condensation model running consisting of a hot and cold air and water vapour mixture separated by a plate. The hot air condenses on the plate due to being cooled indirectly from the cold air. I am now trying to tune the "from" and "to" phase frequencies. My from phase is liquid water and my to phase is water vapour. I am confused on which one i alter to match my results, as it seems i have to alter both to get good matching with results. Moreover for some reason i am getting condensation within the cold air, which is physically impossible. Do i have the "from" and "to" frequencies equal? or do i just change one, or keep them in a certain ratio of eachother? 

Thank you

[Ahmed Hussien]

The first step is to determine which model best suits your needs. If your goal is to capture condensation on a surface, the Eulerian Wall Film model may be the more appropriate choice. You can find additional details on the theory behind this model, as well as instructions for setting it up, in the Ansys Theory and User Guides Chapter 18: Eulerian Wall Films (ansys.com), and Chapter 28: Modeling Eulerian Wall Films (ansys.com).

 

For the Lee models, A thumb rule to tune the evaporation condensation frequency is as follows:
• If more evaporation is required, increase the evaporation frequency and reduce the condensation frequency. 
• If more condensation is required, increase the condensation frequency and reduce the evaporation frequency. 
• Though default values are set to 0.1 & 0.1 for evaporation and condensation frequencies respectively, it is observed that they may vary from 1e-3 to 1.0e5. 
• The ratio of condensation freqeuency to evaporation frequency should be roughly equal to the ratio of density of liquid to density of vapor.

➢ For water at 1bar, you may want to use evaporation-condensation frequencies of 10 and 20000, because the density values of liquid and vapor are (roughly) 1000 kg/m3 & 0.5 kg/m3. 
➢ This ratio does not have to be exactly equal but the condensation frequency have to be set to higher value to get similar mass tranfer rates for similar driving force.

[hurch346]

Thank you for that. Would you have any idea why i am getting condensation on the cold side of my model? Experimental results find no condensation here, and this makes sense as the air mixture is being heated. The domain is shown below, the vertical stream is the hot moist air and the horizontal stream is the dry cold air. Temperature fields do show that the cold side is indeed heating up. Thank you

 

[Ahmed Hussien]

You may check the temperature in your domain. If the temperature of the cold air drops below the saturation temperature that you defined, you may see some condensation. I would suggest using the Eulerain Wall Film model as it aligns with what you are trying to do.