Fluids

Fluids

air volume fraction in mixing tank

    • majam
      Subscriber

      I am simulating a multiphase mixing tank (air-water system).It is an Eulerian system, k-eps turbulence, drag, single bubble size model.

      My tank has a liquid volume of 87L and after 10 sec of simulation, I get 0.02 air volume fraction.

      I changed the liquid height slightly and increased the volume to 91L. with the same setting and after 10 sec I get a volume fraction of air equal to 0.06. The difference does not make sense.

      The simulations are in transient mode, sliding mesh model and 0.001 sec time step. simulations are converged after 10 sec (air volume fraction is stable).

      mesh is structured and no of mesh is slightly higher for 91L.

      Anyone can help me figure out what is wrong here?

       

      Thanks

    • Rob
      Ansys Employee

      Please post some screen shots, showing the flow field, boundary set up and results. 

      Typically, areas to look at are mesh & temporal resolution and convergence. 

    • majam
      Subscriber

      air inlet: mass flow inlet

      Liquid surface: degassing

      tank and impellers and baffles: no slip wall

    • majam
      Subscriber

    • Rob
      Ansys Employee

      I really don't like the way the top of the sliding zone seems to be blocking the gas flow. There's something really odd going on. 

    • DrAmine
      Ansys Employee

      Please recreate the interfaces, initialize and check if there are matching (list the interfaces and check the portion of overlapping). Looks like wall behavior.

       

    • majam
      Subscriber

      The interfaces are fine, the only problem with the interface could be the mesh motion and the fact that at some point during the simulations (and rotation of the impeller region) the locations of the nodes do not match. Could that be a problem?

      I use ICEM and structured meshing and some nodes in the rotating part do not match with the node of the interface in the non-rotating part. It is only for some nodes and it will be hard to make it so accurate that the node locations match in both interfaces.

    • Rob
      Ansys Employee

      The nodes shouldn't match if you have sliding mesh, but the interface zone should deal with that. 

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