FLUENT unreasonable gas holdup

- May 12, 2022 at 7:45 am
  
  jiangtao.lu
  Subscriber
  
  I am using Euler-Euler model to simulate stirred tank reactors.
  At low viscosities (~0.02 [Pa m]), gas holdup is about 0.15, which is normal.
  At increased viscosities (~0.08 [Pa m]), with other things unchanged, gas holdup increases to ~0.5, which is quite unreasonable.
  Anyone has a clue? Thanks.
- May 12, 2022 at 8:58 am
  
  Rob
  Forum Moderator
  
  What's the bubble diameter?
- May 12, 2022 at 9:21 am
  
  jiangtao.lu
  Subscriber
  
  Thank you Rob for replying. The bubble diameter is 5.5 mm. I expected some increase, but not that large
- May 12, 2022 at 10:48 am
  
  Rob
  Forum Moderator
  
  Can you post some plots? Are you modelling a free surface or using degassing?
- May 12, 2022 at 1:39 pm
  
  jiangtao.lu
  Subscriber
  
  I used pressure-outlet. Due to confidentiality, I blacked some parts in the figure, but I guess enough to tell the difference
- May 12, 2022 at 1:47 pm
  
  Rob
  Forum Moderator
  
  You've lost the free surface too. How does the convergence look as the system looks like it's trying to foam.
- May 12, 2022 at 2:02 pm
  
  jiangtao.lu
  Subscriber
  
  yes, I have the same suspicion as you: foaming. The convergence has no problem, energy dissipation is oscillating as expected.
- May 12, 2022 at 3:15 pm
  
  Rob
  Forum Moderator
  
  Hmm. Can you run a couple of time steps with no gas injection and then switch on the gas?
- May 13, 2022 at 7:19 am
  
  jiangtao.lu
  Subscriber
  
  It is a transient simulation. At beginning it is ok, clear free surface, reasonable gas holdup, just like the left panel. Gradually, it develops into the right panel.
- May 13, 2022 at 7:23 am
  
  jiangtao.lu
  Subscriber
  
  Do you know if FLUENT tested increased viscosities? to which value?
- May 13, 2022 at 8:50 am
  
  Rob
  Forum Moderator
  
  We did, but not sure on the values - we do run with oil as the carrier phase for pipe flows.
- May 13, 2022 at 9:36 am
  
  jiangtao.lu
  Subscriber
  
  well, horizontal flow will behave differently from my case for free surface, mixture expansion, etc

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