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Two Dispersed Phases - Degassing Condition

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Old   April 19, 2022, 13:38
Default Two Dispersed Phases - Degassing Condition
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Richard M
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Hello -

I have a water, oil and air in a stirred tank bioreactor system. The oil is defined by a homogeneous MUSIG model. The air is a dispersed, fixed-diameter particle.

I would like to simulate mixing, but unfortunately the degassing condition allows the oil to escape the top (unrealistic). When an open boundary condition is used, the system overflows.

Is there a way to modify the degassing BC to allow only the air to escape?

Thanks!
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Old   April 19, 2022, 14:15
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I am not very familiar with multiphase models; however, how many continuous phases do you have? You stated Air is dispersed with a fixed size, what about oil?

As far as I understand the degassing condition allows continuous phases to leave at an outlet, but not dispersed/polydispersed phases.

Also, MUSIG models only support one continuous phase, so which one have you set as continuous?
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Old   April 19, 2022, 14:17
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There is 1 continuous phase - water.

Air is the dispersed phase (fixed diameter).

Oil is the polydispersed phase (homogeneous MUSIG).
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Old   April 19, 2022, 16:49
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Great!!

Have you tried setting oil as a dispersed phase, not polydispersed to see if it stays within the domain? It should since ALL dispersed phases should see a "wall at the outlet"

Perhaps the software is missing the "wall treatment" for the polydispersed phase at outlets?
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Old   April 19, 2022, 16:50
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Thank you, but I need a population balance to describe the oil particle size and morphology.
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Old   April 19, 2022, 17:05
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I understand that, but you need to know what the software is doing with either option. Otherwise, you are stuck, correct?

Reading the configuration files of ANSYS CFX I could see the Degassing Condition assumes that Continuous and Solid Dispersed Phases see a "free slip wall" at an outlet, but other Dispersed/Polydispersed Phases see a true "outlet treatment".

Modifying the configuration files can get tricky very quickly to access hidden behavior.
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Old   April 19, 2022, 17:09
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That is correct. The only other option would be to create some sort of expression as an "oil makeup." So when oil leaves the degassing outlet, you put back into the domain in a place that it would naturally occur due to splashing/fluid dynamics etc. Any thoughts?

I am not sure I am a capable enough user to modify the configuration file.
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Old   April 19, 2022, 17:20
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If you know how to search ("grep") files using the command line, you can find out where Degassing condition is defined

Go to your Ansys Inc installation directory, there should be an "etc" directory, and use the following command (assuming linux)

grep "Degassing Condition, OUTLET" ./*

you should find a file that contains the configuration for that Boundary Condition (BC
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Old   April 19, 2022, 18:10
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Thank you. I will give it a try.

Thanks you
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Old   April 19, 2022, 20:30
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How would I do this for a windows based system?

Thanks!
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Old   April 20, 2022, 07:01
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The simplest way is to load the files in a text editor and use search to look for the target string. Even notepad has a search function which would work for this.
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bioreactor, cfx, degassing, dispersed phase, multiphase flow

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