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Old   June 5, 2001, 11:40
Default Gas-liquid reactor
  #1
Tiger
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Hi, everyone,

I am solving the mixing of gas-liquid system in a small reactor. In my experimental work, I found the diameter of some gas bubbles was about 1.5 mm, however, I divided my reactor into thousands of cells, whose dimension was 1.0 mm, which means that the diameter of gas bubbles from CFD calculation are different from my experiments. I think the gas volume fraction will be different too.

Anyone has some ideas about how to solve this problem?

thanks in advance.
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Old   June 5, 2001, 16:03
Default Re: Gas-liquid reactor
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Danny Tan
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I am just curious, could you tell me what cfd code are you using ?

Thank you

Danny
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Old   June 6, 2001, 07:03
Default Re: Gas-liquid reactor
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Tiger
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Hi,

I am using CFX4.1. You know, I intended to solve the problem with a small reactor ( the diameter is 0.008 m ) and I had divided the reactor into thousands of cells ( the dimension is about 0.0002 m), however, the gas bubble diameter may be greater than the size of a cell, which means you can not solve the gas volume fraction in a so small cell. That is my problem.

Tiger
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Old   June 6, 2001, 12:37
Default Re: Gas-liquid reactor
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Danny Tan
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Hi,

May I suggest you to use CFX discussion forum. I believe someone over there can help you.

Danny
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Old   June 6, 2001, 23:46
Default Re: Gas-liquid reactor
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Dan Williams
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The disperse phase models in CFX-4 are based on a source term in the momentum equation which couples the two phases and represents the drag force on the bubbles at the slip velocity (Continuous Phase Velocity-Disperse Phase Velocity).

There is no need to resolve individual bubbles because the model inherently assumes that you are not, and that the bubbles themselves behave as if they are a single fluid. The only reason you should need to worry about the bubble diameter is it's effect on the source term coefficient, or drag coefficient in this case.
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Old   June 7, 2001, 08:05
Default Re: Gas-liquid reactor
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Tiger
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Hi, Dan Williams,

You are right, in my case, how to calculate the drag cofficient is my problem. Do you have any idea about it?

Tiger
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Old   June 7, 2001, 23:34
Default Re: Gas-liquid reactor
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Dan Williams
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If you enter the particle diameter and you are running the continuous-disperse model, then you have a selection of models for the drag coefficient. I can't remember the exact formulation for the source term off the top of my head but it is something like: 1/2*bulkdensity*Cd*particlediameter^2*Uslip^2/(ralpha*rbeta) (this is just a guess). There are several correleations for Cd in this case. One is Schiller Naumann I beleive. There are others that might be more appropriate to your application. Read the section on multiphase flow in the CFX-4 manual.

Dan.
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Old   June 11, 2001, 05:31
Default Re: Gas-liquid reactor
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Tiger
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Hello, Dan,

I hope my problem is that the diameter is changing from point to point, so I have to calculate the bubble diameter at first, however, the diameter of the bubble mayber greater than the dimension of the control cell. If I can get a reasonable bubble diameter, so all of the problem will be solved.
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