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Bubble mean diameter-volume fraction

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Old   January 8, 2008, 06:38
Default Bubble mean diameter-volume fraction
  #1
Nikolaos Kazakis
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I am trying to simulate a bubble column using constant bubble size inside the column. It is known that gas volume fraction is larger when the bubble size is smaller. However, when I simulate the column, using the same operating conditions, but with different mean bubble diameter (for example 5 and then 3mm) the gas holdup is exactly the same.

Does anyone have an idea?

Thanks in advance
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Old   January 8, 2008, 17:22
Default Re: Bubble mean diameter-volume fraction
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Glenn Horrocks
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Hi,

In a multiphase simulation with a complicated result output (gas holdup) there are no simple answers. To get accurate results you have to carefully validate all important physics in the simulation to get the correct final answer. Looks like you need to read the literature in the area (maybe somebody else had this problem in simulations?) and do careful validation.

Glenn Horrocks
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Old   January 9, 2008, 03:20
Default Re: Bubble mean diameter-volume fraction
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Nikolaos Kazakis
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I agree with you Glenn. The thing is that, in order to check this peculiar result, I made various runs by changing only the mean bubble diameter. Take a look at the results for d(mm)=1.0--> holdup(%)=3.70, for d(mm)=1.5--> holdup(%)=2.78, for d(mm)=2.0--> holdup(%)=2.30,

for d(mm)=2.5--> holdup(%)=2.16, for d(mm)=3.0--> holdup(%)=2.16, for d(mm)=3.5--> holdup(%)=2.16, for d(mm)=4.0--> holdup(%)=2.16, for d(mm)=4.5--> holdup(%)=2.16, for d(mm)=5.0--> holdup(%)=2.16,

Isn't it weird that after a specific bubble diameter the gas holdup doesn't change?

Thank you Glenn
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Old   January 9, 2008, 18:08
Default Re: Bubble mean diameter-volume fraction
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Glenn Horrocks
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Hi,

Are you correctly modelling surface tension and other interface physics? This will affect things.

Glenn Horrocks
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Old   January 10, 2008, 03:09
Default Re: Bubble mean diameter-volume fraction
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Nikolaos Kazakis
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I use the same liquid phase for all runs, meaning that all physical properties are the same (including the surface tension). The only force included is the drag force (Ishii-Zuber), same in all runs. As I said earlier, all conditions are the same except the bubble diameter. I also found something else more weird.

When I made graphs for the partial volume fraction in the middle of the column along the column diameter, the graphs were identical. It seems that all the runs are completely the same after a certain bubble diameter. It seems to me that there is a value (in an equation-model) at the beginning of the solver, which remains constant after a certain bubble diameter, leading to identical solution. I am simulating the simplest problem I could. I have no non-drag forces.

I also checked the grid, but if finer or coarser the results are the same. So, it is not a grid issue.

Thank you Glenn
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Old   January 13, 2008, 18:45
Default Re: Bubble mean diameter-volume fraction
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Glenn Horrocks
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Hi,

Can Lagragian particle tracking capture all the flow details of bubble hold up? I suspect you may need to model the flow going around the bubble to get it and that will need a eularian model - and 100x longer solve time.

Of course I don't know exactly what physics cause bubble hold up (In fact I don't even know what bubble hold up is) so don't take my word as Gospel.

Glenn Horrocks
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Old   January 15, 2008, 03:41
Default Re: Bubble mean diameter-volume fraction
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Nikolaos Kazakis
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Hi

the problem I am simulating is steady state Euler-Euler. The flow around a bubble is completely irrelevant with this problem.

I wonder, since you don't even know what gas hold up is, why you reply to my post. You have posted 2 replies to my post, recommending no solution, but just speaking vaguely and telling me that I should "carefully validate all important physics". I guess that is what CFD is about; trying to validate all important physics and all users are aware of that. And then in your third reply you admit that you don't even know what I am talking about.

I would appreciate it if users who have something to propose post an answer and not those who just want to increase their post number in this forum.

Thank you
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Old   January 15, 2008, 17:58
Default Re: Bubble mean diameter-volume fraction
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Glenn Horrocks
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If you don't find my comments useful then I think I shut up (on your threads anyway). Good luck in where ever your CFD adventure takes you as you won't be hearing from me again.

Glenn Horrocks
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