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Trouble with population balance for bubble column simulation

    • Trygve Staupe
      Subscriber

      Hello!

      I am trying to simulate a bubble column in Fluent with the following geometry. 

      However, I run into problems when I try to use the QMOM to account for the bubbles' polydispersity, coalescence, and breakage phenomena. The problematic result I am getting can be seen in the following figure:

      What is problematic is the larger bubbles. They just keep slowly increasing during the simulation and don't stop. And to my understanding, it doesn't make sense for larger bubbles to be located in these areas. That is, between the bubble plumes at the inlet and in the recirculating liquid flows.

      I have tried several things to try and fix this, such as:

      • Different types of grids
        • Poly-hexcore (Fluent meshing)
        • Polyhedral (Fluent meshing)
        • Hexahedral (ICEM blocking)
      • Refining the meshes
        • I have tried having from 20 000 to 130 000 cells in the mesh
      • Adjusting the timestep
        • Fixed timestep between 1e-5 and 0.005 [s]
      • Playing with Under relaxation factors
      • Tried 4 and 6 moments
      • Tried different spatial discretization schemes
        • All schemes QUICK
        • All schemes are QUICK, except moments, which is 1st order upwind
        • All schemes are QUICK, except moments and turbulence, which is 1st order upwind
      • Tried the different time schemes
        • 1st order, 2nd order, 2nd order bounded
      • Tried enabling warped-face gradient correction and higher-order term relaxation.

      I'm honestly out of ideas on what might be wrong. Any help would be very appreciated!

       

      Some extra info:

      The poly-hex-core and polyhedral meshes are just standard meshes made with fluent meshing and using the improve orthogonality and skewness option. A mesh made by using  blocking in ICEM can be seen below:

      I have used the following for boundary conditions:

    • Rob
      Forum Moderator

      Please check the mesh aspect ratio, and change in cell size around the inlets. I suspect the cells are OK for single phase, but you need a fine mesh with very low aspect ratio and growth rate for most multiphase applications. 

      If the population balance allows bubbles to grow in regions of low shear, how does that plot tie up with the bubble size? How is the bulk flow behaving? 

    • Trygve Staupe
      Subscriber

      Thank you for the answer! I was away traveling, so please excuse my late reply. 

      Before I try to make a more refined mesh, I wonder if you think a polyhedral or a poly-hexcore created using fluent meshing should be sufficient for the simulation, or if I should try to improve the hex mesh I created using ICEM? 

       

      I think the bulk flow is behaving as expected. The water is flowing upwards above the spargers and is flowing downward/recirculated between the spargers. 

      Figure 1: Velocity of the column in the crosssection that goes through one sparger (see original post)

      Figure 2: Velocity of the column in the crosssection that goes through the two spargers (see original post)

      Figure 3: Velocity of water at a crosssection of the column at h = 10.5 cm

       

       

    • Rob
      Forum Moderator

      I would tend to favour pure poly mesh and in general don't recommend ICEM CFD other than in very specific cases, this isn't one of those.  

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