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Old   May 29, 2012, 03:39
Default free surface model
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Taleb Zarei
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Hi,
I am simulating a distillation tray of Air and water with free surface model and homogeneous model in CFX. System has spray regime so air is continues and water is dispersed. I have a problem. The most of water was exited from gas out boundary condition. Therefore, the liquid level in downcomer reduce significantly. How can I do to prevent of liquid exit from the gasout boundary condition? Gas out boundary condition is average pressure.

Thanks.
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Old   May 29, 2012, 07:40
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Glenn Horrocks
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You may be able to use a degassing boundary, but I suspect not.

But really you should look at why the flow is pushing all the liquid out the exit and fix that. That is the root cause of the problem.
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Old   May 30, 2012, 04:23
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Taleb Zarei
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Thank you
but, there is not degassing boundary condition for homogenous and free surface model.
Can I use from source or sink term for continuity term to solve this problem. For example, in gas out boundary condition introduce a sink for air mass flow.
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Old   May 30, 2012, 08:13
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Quote:
there is not degassing boundary condition for homogenous and free surface model.
OK, thanks for the clarification.

In that case then you have to look at your flow and work out why more liquid is going out the outlet then is physically the case. Fix that problem as it is the root cause. Source terms to magically make stuff disappear will just cover up a more serious problem.
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Old   June 11, 2012, 06:28
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Taleb Zarei
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Thanks a lot
As you know, in distillation tray, we have a cross contact between gas and liquid. I have some question.
1) Can I use from homogenous model for this system?
I have read your previous posts in homogeneous models that you said “If the phases never mix then use the homogenous model”.
But in my system, gas and liquid mixed together and a turbulent regime of gas and liquid and finally a spray regime was created in tray. Homogeneous model simulated the spray regime much better than inhomogeneous.
2) In gas out boundary condition because a same equation was solved for gas and liquid, the separation of gas and liquid in gas out boundary condition is not possible. Is it true?
Because in my simulations, most of water was exited from gas out in homogeneous model, In inhomogeneous model, this problem was not accrued but the mixing of water and gas is not good.
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