Bubble column - Inlet boundary conditions

oj.bulmer · June 19, 2012, 21:56

Hello

I was wondering what is best way to specify the inlet boundary condition for bubble columns with ring sparger.

The options are:

Determine superficial air velocity by experimental measurement, assume a suitable volume fraction at sparger inlet (generally 0.3) and use this to calculate inlet gas velocity by following relation.

$V_{gas-inlet}A_{inlet}* \epsilon_{gas}=V_{gas-superficial} A_{column}$ ...........( $\epsilon_{gas}$ being inlet gas volume fraction)
Calculate ellispsoidal regime settling velocity $w_\infty=\sqrt{ \frac {2\sqrt{g \Delta \rho \sigma} } {\rho_{liquid}}}$ and use it as a superficial velocity, to calculate the inlet velocity as mentioned in point #1. This avoids the tedious and more-than-often impractical measurement of gas superficial velocity
Specify inlet volume fraction (~0.3) and mass flow rate that is known. This seems a lot easier, especially when it comes to mass imbalance calculations at degassing outlet at top.

In literature I have reviewed so far, option 1 seems mostly ( rather universally) popular, though difficult to implement in my case due to lack of availability of data. Option 2 is theoretical simplification and discards any geometrical influence and there is no evidence of this being appropriate in the literature. Option 3 seems so obvious choice, but I am yet to find a publication that mentions the use of mass flow rate at inlet. Quite surprising.

Any ideas about which may be the best way?

Regards
OJ

mohamad3564 · October 17, 2012, 04:49

I am using inlet same condition for bubble column and with try and error I found that 0.3 is suitable for my case.can we share our information?

jiejie · February 1, 2018, 21:49

Hi OJ

I am working on some validation of bubble column simulations. against Becker 1994, 1999. I am actually using option 1 at the moment.

I have a question about running a 2D case. Since there is no thickness of the bubble column, then how do you determine the inlet gas velocity in this case?

The second question I have is whether it is okay to use 100% air at the inlet?

Thanks

ghorrocks · February 2, 2018, 01:58

What do you mean by determine the inlet gas velocity? What do you know about the case you are trying to simulate?

Whether 100% air is OK will depend on what you are trying to do and how you are modelling it.

jiejie · February 4, 2018, 20:20

Quote:

Originally Posted by ghorrocks

What do you mean by determine the inlet gas velocity? What do you know about the case you are trying to simulate?

Whether 100% air is OK will depend on what you are trying to do and how you are modelling it.

Hi Glenn

Since I am using OpenFOAM twoPhaseEulerFoam for the calculation. If I would like to perform a 2D bubble column simulation, what is the inlet gas velocity I should impose at the inlet nozzle? Since OpenFOAM use 1 cell in the third dimension for the 2D case, the problem is whether the inlet gas velocity will be dependent on the length in third dimension, i.e. the depth of the bubble column? If it is not, then how the inlet gas velocity should be calculated for 2D bubble column?

Thanks
Jie

ghorrocks · February 4, 2018, 20:22

Try the OpenFOAM forum. This is the CFX forum.

jiejie · February 4, 2018, 20:24

Quote:

Originally Posted by ghorrocks

Try the OpenFOAM forum. This is the CFX forum.

Hi Glenn

I think this a general question, which is about how to calculate the correct inlet gas velocity that should be applied at the inlet nozzel for 2D bubble column based on the experimental superficial gas velocity.

Thanks
Jie

ghorrocks · February 4, 2018, 20:28

I do not know how OpenFOAM has handled this so I cannot say in your case. But I would have thought it very easy to just try it and find out - can't you just try a velocity and see what it does?

jiejie · February 4, 2018, 20:31

Quote:

Originally Posted by ghorrocks

I do not know how OpenFOAM has handled this so I cannot say in your case. But I would have thought it very easy to just try it and find out - can't you just try a velocity and see what it does?

Thanks away.

June 19, 2012, 21:56	Bubble column - Inlet boundary conditions	#1
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	Hello I was wondering what is best way to specify the inlet boundary condition for bubble columns with ring sparger. The options are: Determine superficial air velocity by experimental measurement, assume a suitable volume fraction at sparger inlet (generally 0.3) and use this to calculate inlet gas velocity by following relation. $V_{gas-inlet}A_{inlet}* \epsilon_{gas}=V_{gas-superficial} A_{column}$ ...........( $\epsilon_{gas}$ being inlet gas volume fraction) Calculate ellispsoidal regime settling velocity $w_\infty=\sqrt{ \frac {2\sqrt{g \Delta \rho \sigma} } {\rho_{liquid}}}$ and use it as a superficial velocity, to calculate the inlet velocity as mentioned in point #1. This avoids the tedious and more-than-often impractical measurement of gas superficial velocity Specify inlet volume fraction (~0.3) and mass flow rate that is known. This seems a lot easier, especially when it comes to mass imbalance calculations at degassing outlet at top. In literature I have reviewed so far, option 1 seems mostly ( rather universally) popular, though difficult to implement in my case due to lack of availability of data. Option 2 is theoretical simplification and discards any geometrical influence and there is no evidence of this being appropriate in the literature. Option 3 seems so obvious choice, but I am yet to find a publication that mentions the use of mass flow rate at inlet. Quite surprising. Any ideas about which may be the best way? Regards OJ

October 17, 2012, 04:49	bubble column	#2
mohamad3564 New Member mohamad Join Date: Jun 2012 Posts: 12 Rep Power: 14	I am using inlet same condition for bubble column and with try and error I found that 0.3 is suitable for my case.can we share our information?

February 2, 2018, 01:58		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	What do you mean by determine the inlet gas velocity? What do you know about the case you are trying to simulate? Whether 100% air is OK will depend on what you are trying to do and how you are modelling it. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

February 4, 2018, 20:22		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Try the OpenFOAM forum. This is the CFX forum. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

February 4, 2018, 20:28		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	I do not know how OpenFOAM has handled this so I cannot say in your case. But I would have thought it very easy to just try it and find out - can't you just try a velocity and see what it does? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

February 1, 2018, 21:49		#3
jiejie Senior Member Jie Join Date: Jan 2010 Location: Australia Posts: 134 Rep Power: 16	Hi OJ I am working on some validation of bubble column simulations. against Becker 1994, 1999. I am actually using option 1 at the moment. I have a question about running a 2D case. Since there is no thickness of the bubble column, then how do you determine the inlet gas velocity in this case? The second question I have is whether it is okay to use 100% air at the inlet? Thanks

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