Learning Forum

[Muhamad Eldebawy]

Hello,

I am wondering if I can use multiphase model VOF to simulate evaporation/condensation in a tube and put air gap "outside" the tube which does not interact with the evaporation and condensation process.

when I use VOF and define 3 phases the mass flow from air zone to water/steam zone and it should be sealed and the air is trapped

Thanks a lot

[Rob]

You want two phases, liquid & gas. Gas phase is a mixture of species. Three phases will work but depending on the model could be computationally more expensive.n

[Muhamad Eldebawy]

Hi Rob, nI really appreciate your help. nI did not understand well why it is better to put gas phase a mixture of species. I will try to illustrate more what is my issue nas shows in the photo below, I have tube that has a water and there is heat in and it is expected that the water will evaporate and get steam out of the tube. around this tube there is air gap works like thermal barrier to reduce the heat transfer to the other parts of the system and reduce the losses. what I understand is that I have to choose multiphase model VOF to simulate evaporation/condensation inside the tube. My issue is that if I chose two phases only ( water liquid + water vapor). I do not have an option to assign the zone of the airgap as a fluid (air), So I have to change the number of phases to three (air + water vapor + water liquid), although there is no air in the tube but I did that to put air in the air gap zone. after I did the simulation the mass transferred from water/steam tube to the air gap, this is not correct as the air gap in a separate zone ! nI hope you can help me nthanks a lot nn

[Muhamad Eldebawy]

Any recommendations please n

[Rob]

With VOF we solve one momentum field so using 3 phases or 2 phases & species is probably comparable in cpu cost. However, if you boil in a VOF model you also need to resolve the bubbles. nIn Eulerian we solve a momentum field for each phase. So using the above, 3 momentum fields is much more expensive than 2 + species. n

[Muhamad Eldebawy]

Thanks Rob for your reply it is clear now the difference between them. My concern now is using Eulerian model will solve 3 momentum equations and the gradient in pressure will be shared between the three phases right?nI need to solve a separate momentum (not shared gradient pressure), continuity and energy equations for air as it is not a phase in the multiphase zone it is a fluid in a separate zone. nthanks n

[Rob]

If there's a wall in the way it shouldn't be a problem. Other than energy (heat) transfer there should be no interaction between the air and water + vapour volumes. n

[Muhamad Eldebawy]

thank you so much Rob for your help n