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define boundary-condition in two-phase-flow

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Old   December 30, 2004, 05:20
Default define boundary-condition in two-phase-flow
  #1
Johannes Radke
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Hi!

In a gas-liquid-flow I am trying to define a boundary condition for each fluid at a single surface.

1. case is where the gas shall not leave the domain whereas the liquid flows right through it.

2. case is a countercurrent flow.

I do not know how to manage that. Is it possible to define a single surface for example as gas-inlet and liquid-outlet?

Thanks in advance. Johannes
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Old   December 30, 2004, 11:51
Default Re: define boundary-condition in two-phase-flow
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Rui
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Hi,

To set different boundary conditions to each fluid you obviously have to chose the inhomogeneous model (leave the Homogeneous Model box unchecked). As with the homogeneous model the velocity field will be the same for both phases.

However, even with the inhomogeneous model you cannot define a boundary as inlet for a fluid and outlet for the other. But defining it as Opening, and chosing Fluid Velocity at the Mass and Momentum option, you can define the velocity at the boundary for each fluid. This shall work for your case 2. For case 1, I suppose you can do the same but setting the velocity to zero for the gas.

Regards,

Rui

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Old   December 30, 2004, 12:04
Default Re: define boundary-condition in two-phase-flow
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Johannes Radke
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Thanks for your proposals.

About the Opening BC: What shall I do if I do not know the fluid velocities at the surfaces where the fluids leave the domain? I only know the inlet velocities.

Best regards, Johannes
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Old   December 30, 2004, 23:47
Default Re: define boundary-condition in two-phase-flow
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test
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Hi,

One of the work arounds is to define inlets for both surfaces and using an expert parameter which will not create artificial walls on the inlet when th flow is going out from it.

The you can use a step function to define the inlet flow condition.
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Old   January 3, 2005, 15:19
Default Re: define boundary-condition in two-phase-flow
  #5
Rui
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Hi,

I'm sorry but I don't know how to handle it. Perhaps test or someone else has some sugestions.

Could you descrive what you're trying to model. A transient or steady-state situation? How many inlets and outlets are there for each fluid?

Regards,

Rui
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Old   January 4, 2005, 04:58
Default Re: define boundary-condition in two-phase-flow
  #6
Johannes Radke
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I am trying to model a column containing structured packings without modeling the geometry of the packing.

So there is the gas-inlet and liquid-outlet at the bottom and gas-outlet and liquid-inlet on top of the geometry. I am using a simple cube as geometry for testing purposes. That is why I have only 2 surfaces to model 4 boundary conditions.

Has anyone another suggestion?

Best regards, Johannes
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Old   January 4, 2005, 19:52
Default Re: define boundary-condition in two-phase-flow
  #7
Rui
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Hi,

If you are dealing with a steady-state situation and there are only 2 inlet/outlet boundaries, the mass flows of either liquid and gas at their outlet boundaries are the same as their mass flows at their inlet boundaries.

I would sugest you to ask help from CFX support and to take a look at Tutorial 16 (Gas-Liquid Flow in an Airlift Reactor).

Regards,

Rui
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