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-   -   Bubble column - Inlet boundary conditions (https://www.cfd-online.com/Forums/cfx/103519-bubble-column-inlet-boundary-conditions.html)

oj.bulmer June 19, 2012 20:56

Bubble column - Inlet boundary conditions
 
Hello

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

The options are:
  1. 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)

  2. 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

  3. 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 03:49

bubble column
 
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 (Post 680200)
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 (Post 680439)
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 (Post 680441)
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.


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