Fluids

Fluids

VOF, Lee model divergence

    • b404957074
      Subscriber

      Hi there,

      I am trying to simulate a natural evaporation process, as the pic shows. Thin liquid film on the left-hand side, heated by the wall with constant heat flux. Humid air flows from the top. Evaporation happens on the interface.

      I want the final solution of the simulation, hence I used a steady state simulation.

      (I know transient may be a better option, but it is really time-consuming.)

      When I specified the from phase/ to phase coefficients in the Lee model as both 0.1, everything works fine.

      However, when I tried to increase the from phase (evaporation) coefficient to 10, the results keep diverging.

      Could anybody please help? Thanks!

    • b404957074
      Subscriber
      Error message:
      Error at Node 5: floating point exception

      Error at
      Error at Node 1: fl
      Error at Node 2: floating point exception

      Error at Node 4: floating point exception
      oating point exception
      Node 0: floating point exception
      e correction to enhance linear solver robustness.
      Stabilizing pressure correction using GMRES to enhance linear solver robustness.

      Divergence detected in AMG solver: pressure correction Stabilizing pressure correction to enhance linear solver robustness.
      Stabilizing pressure correction using GMRES to enhance linear solver robustness.

      Divergence detected in
      Error at Node 3: floating point exception
      AMG solver: pressure correction Stabilizing temperature to enhance linear solver robustness.
      Stabilizing temperature using GMRES to enhance linear solver robustness.

      Divergence detected in AMG solver: temperature
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: temperature
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: temperature
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: temperature
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: temperature
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: pressure correction
      Divergence detected in AMG solver: temperature
      Error at host: floating point exception

      ===============Message from the Cortex Process================================

      Compute processes interrupted. Processing can be resumed.

      ==============================================================================

      Error: floating point exception
      Error Object: #f
      Registering UdfSource, ("H:/rail humid_files/dp4/FLU-4/Fluent/Saturation_temperature_print.c")
      Registering Udflib, ("H:/rail humid_files/dp4/FLU-4/Fluent/libudf")

      Calculation complete.
    • Rob
      Ansys Employee
      Those are rate coefficients, and what they do is explained in the documentation. If you increase them by two orders of magnitude you've very likely pushed so much mass to the "other" phase that the solver has become unstable. As an aside, how is the film being replenished as the steady result may be to have no film.
    • DrAmine
      Ansys Employee
      You want to account for evaporation but you need to know that Lee Model alone here is the wrong choice if you want that mass transfer occurs at gas/liquid interface. Just review how the model works and you will know why.
    • b404957074
      Subscriber
      Hi Rob, that is a good point! Do you think if I increase the coefficients step by step ( after each of them being steady) will work?
    • b404957074
      Subscriber
      Hi DrAmine I understand, I forgot to mention I have used a UDF which calculates the saturation temperature based on the water vapour pressure, and the results showed that mass transfer did happen on the interface.

    • Rob
      Ansys Employee
      Probably not, what data do you have to support increasing the coefficient value?
    • DrAmine
      Ansys Employee
      Decrease the time step or decrease the VOF cut-off.
    • b404957074
      Subscriber
      Hi Rob, I am comparing my results with the literature. It actually works when I lower down the interval of evaporation coefficients. But a new problem pops out, could you please see my new post?

    • b404957074
      Subscriber
      Hi DrAmine Thanks for your prompt reply, could you please see my new post?
    • DrAmine
      Ansys Employee
      Which new post?
    • Rob
      Ansys Employee
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