Fluids

Fluids

water-air interface is not dynamic equilibrium

    • b404957074
      Subscriber

      Hi experts

      I am simulating a falling liquid film evaporation, see the pic.

      Laminar, Mixture, Condensation-evaporation Lee model with a UDF defining the saturation temperature (based on the vapour partial pressure) was used. The mass transfer is indeed on the interface, which means my UDF work? It is a steady-state simulation. The settings are shown below. The water vapour- air mixture is incompressible.

      I expect the RH will be equal to 100% around the water-air interface, but in fact, the simulation results show that the RH near the interface is far lower than 100%.

      Does anyone have any idea about the reason?

      Regards

    • Karthik R
      Administrator
      Hello What is the value you are obtaining? Could you show us some plots?
      Also, did your model converge well?
      One thought - Instead of running a steady-state simulation directly, could you run a transient simulation and let it come to a natural steady state solution?
      Karthik
    • DrAmine
      Ansys Employee
      You might need to use Interfacial Area based mass transfer to limit the mass transfer at the interface. 100% safe if you code that via UDF for better handling.
    • b404957074
      Subscriber
      Hi there Thanks for your reply.
      This is the result after running 7s of transient simulation. LHS shows the interface temperature, RHS shows the mass fraction of water vapour. It shows that higher temperature, lower mass fraction of water vapour. That is against the physic?
      After 7s of transient simulation, I switched to steady-state simulation. Here is the result. Still, the higher temperature near the inlet on the interface, but a lower mass fraction. I do not think it is a transient or steady-state issue or convergence issue. What do you think? I also want to mention that the water liquid of the lower part of the geometry has already been totally evaporated. Maybe that is the reason why the mass fraction plot there shows a downward trend.



    • b404957074
      Subscriber
      Hi there Thanks for the reply. TBH I am not good at coding. Is there any easier way to simulate the natural evaporation of water. I just want to see how fast the water is evaporated...Is species mass transfer able to handle it without UDF? Cheers
    • DrAmine
      Ansys Employee
      It can but with UDF you have better control. The mass transfer should be driven by concentration gradient. You might try species mass transfer with mixture model and use Raoults law there.
      Are you verifying the saturation temperature returned from UDF ( store it in udm and post process it,)
      Can you plot the mass transfer rate? Your plot is along which line? You transient mass fraction profile looks more realistic. Run the whole transient please.
    • b404957074
      Subscriber
      I will give it a go on SMF. All plots are values from the water liquid-mixture gas interface, the X-axis of 1.875 m represents the inlet position. The transient results of MTR and saturation T are shown below. How come you say it is more realistic? Should not it be higher temperature lower mass fraction of water vapour? It is exactly the opposite.

      Issues of Species Mass Transfer are,
      what should I choose for the interfacial mass transfer coefficient of the water liquid face? Zero resistance or Ranz- Marshall?
      In the theory guide, it said that SMF is suitable for the mixture to mixture mass transfer, but in my case, one of the phases is just pure water liquid. I found setting the water liquid itself as a "mixture" works, but is it the right way?
    • DrAmine
      Ansys Employee
      I said that because you shared plots without providing any details where inlet is located etc...

      Mass transfer is largest at inlet then reduce to almost low but rather constant positive value: meaning always mass being transferred from liquid to gas phase. Please check the static temperature field and evaluate it.

      For SMT:
      1/Zero Resistance or high Sherwood number
      2/Mixture of Water and Dummy Water. For that reason I usually recommend using UDF and I have a feeling with UDF you will be more productive as your mass transfer is occurring at the interface! So the interface location is very important for the formulation of mass transfer: crisp interface is not always of big advantage!
    • b404957074
      Subscriber
      Thanks a lot for the reply, that means a lot to me.
      Okay, I understand now. For the Lee model, let's just put it aside since I am running a pure transient simulation at the moment, let's see what happens and I will keep you posted.
      For SMT:
      Understand, I will try Zero Resistance first.
      What do you mean the mixture of water and dummy water? Is it a mixture of water liquid and water liquid? What is the purpose of that UDF, defining the saturation pressure or?
    • DrAmine
      Ansys Employee
      Yes water and same component water-dummy. You set water mass fraction to one.
      With the UDF: you formulate the mass transfer based on species concentration difference: you calculate the saturation concentration / mass fraction and you have the difference and you multiply with interfacial area and you get so a rate of mass transfer.
    • b404957074
      Subscriber
      That sounds interesting, do you have any examples of the mass transfer UDF? I have never done that before.
      Cheers
    • Rob
      Ansys Employee
      We can only point at what's in the UDF manual. Other members of the community are free to show example code, staff are not permitted to ( https://forum.ansys.com/discussion/23093/why-ansys-employees-dont-download-attachments )
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