Degassing boundary condition in Fluent?

srjp · March 24, 2009, 09:18

Hi, I am running a simulation of a bubble column with a free liquid surface on top, in Fluent.
I need to set a boundary condition for the top surface, where the air bubbles can escape, but the liquid cannot. In Fluent, currently all I can do is to set this surface as a pressure outlet with backflow volume fraction of air as zero. But this does not prevent the liquid from leaving the boundary.
Is it possible to set the boundary condition so that air can leave the boundary but liquid cannot? (Similar to the degassing B.C. in CFX)
I am running the mixture model, with 2D geometry, bubbles injected from bottom.

Tongji Environment · August 17, 2011, 23:26

The same question to you, Pray for reply

srjp · August 18, 2011, 09:46

Looks like there was no straight forward method to do this. I had taken a taller air section in the top, so that liquid does not leave the boundary. I had also patched the top region with air.

azna · November 23, 2015, 13:56

Hi,

I'm working on a bubble column. I was wondering that how can I modify settings in the degassing boundary condition ? close to the water surface, it under predicts velocity for both air and water. Is there any way that I can fix this problem near the water surface ? the flow pattern with degassing boundary is correct however, velocities near the water surface is very low comparing with experimental values.

Thanks a lot

Sijnhaven · June 12, 2018, 10:53

Dear all,

I am modelling a liquid vessel with side injection of gas. For the inlet I am using a pressure-inlet.

The liquid may not leave the domain, only the gas is allowed to leave the domain.

I believe the degassing is the appropriate choice, because it alows te gas phase to leave the domain, but not the liquid phase. However, this boundary conditions does not allow to specify a pressure (which should be atmosferic pressure) at the outlet.

This will also influence the pressure inside the liquid domain. The hydrostatic pressure will not be Patm + rho*g*h in this case?

How to handle this case?

yuvarajp · May 12, 2023, 05:32

hi, u raised the question 13years back, did you find the solution. ur suggestion would be appreciated

March 24, 2009, 09:18	Degassing boundary condition in Fluent?	#1
srjp Member JP Join Date: Mar 2009 Posts: 57 Rep Power: 18	Hi, I am running a simulation of a bubble column with a free liquid surface on top, in Fluent. I need to set a boundary condition for the top surface, where the air bubbles can escape, but the liquid cannot. In Fluent, currently all I can do is to set this surface as a pressure outlet with backflow volume fraction of air as zero. But this does not prevent the liquid from leaving the boundary. Is it possible to set the boundary condition so that air can leave the boundary but liquid cannot? (Similar to the degassing B.C. in CFX) I am running the mixture model, with 2D geometry, bubbles injected from bottom.

November 23, 2015, 13:56	degassing boundary condition	#4
azna Member azna Join Date: Nov 2012 Posts: 30 Rep Power: 14	Hi, I'm working on a bubble column. I was wondering that how can I modify settings in the degassing boundary condition ? close to the water surface, it under predicts velocity for both air and water. Is there any way that I can fix this problem near the water surface ? the flow pattern with degassing boundary is correct however, velocities near the water surface is very low comparing with experimental values. Thanks a lot

June 12, 2018, 10:53	Pressure at degassing boundary	#5
Sijnhaven New Member Lukas Join Date: Mar 2018 Posts: 6 Rep Power: 8	Dear all, I am modelling a liquid vessel with side injection of gas. For the inlet I am using a pressure-inlet. The liquid may not leave the domain, only the gas is allowed to leave the domain. I believe the degassing is the appropriate choice, because it alows te gas phase to leave the domain, but not the liquid phase. However, this boundary conditions does not allow to specify a pressure (which should be atmosferic pressure) at the outlet. This will also influence the pressure inside the liquid domain. The hydrostatic pressure will not be Patm + rhogh in this case? How to handle this case?

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August 17, 2011, 23:26		#2
Tongji Environment New Member Calvin Join Date: Aug 2011 Posts: 4 Rep Power: 15	The same question to you, Pray for reply

August 18, 2011, 09:46		#3
srjp Member JP Join Date: Mar 2009 Posts: 57 Rep Power: 18	Looks like there was no straight forward method to do this. I had taken a taller air section in the top, so that liquid does not leave the boundary. I had also patched the top region with air.

May 12, 2023, 05:32		#6
yuvarajp New Member Parthasarathy yuvaraj Join Date: Apr 2023 Posts: 6 Rep Power: 3	hi, u raised the question 13years back, did you find the solution. ur suggestion would be appreciated