Why the mass fraction is fluctuating during the simulation?


    • abdul.zahir

      I have been trying to study the rate of mass transfer in rotating packed bed reaction. Following are the models I am using for the CFD study 

      1. VOF (multiphase model)
      2. SST k omega model 
      3. Reference fram model
      4. specie transport model 

      Following are the mixture template

      1. primary phase (O2, water vapor); Fluids are defind in the same order
      2. Secondary phase (O2, water liquid)

      The mechanism is that the O2 from the secondary phase will go to the primary phase 

      the boundary conditions for the phases are as follow

      primary phase (mass fraction of water vapor=1 and O2=0

      secondary phase: mass fraction of water liquid = 0.9 and O2= 0.1


      Till 6000 itterations, everything remains normal. By normal i mean the mass fraction of O2 in the secondary phase remain 0.1 in interior of the CFD geometry. However, after 6000 itterations, the mass fraction suddendly increases to 0.5-0.8 (O2). which can not be possible pricipally as the boundary condition is constant.



      why is this happening with the simulation?

      Morover, I am using the mass transfer UDF for this purpose to make sure everything is fine in the UDF or not I tried to run the same case without the UDF and constract rate of mass transfer but the problem is still there. Which means that the problem is not due to the UDF.


      Please guide in this regard.

      Your kind advice/suggestion in this regard will be highly appreciated. 
      Thank you

    • Rob
      Ansys Employee

      Have a very careful read up on the VOF model and how boundary conditions are set. Then explain why you chose VOF for a packed bed system. How are you representing the packed section? 

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