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Bubble breakup and coalescence (PBM)

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Old   February 10, 2020, 02:58
Default Bubble breakup and coalescence (PBM)
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I have several questions on Population Balance Modeling (PBM).

I have little understandings of boundary conditions with bins.

(Please refer to a tutorial, Fluent 14.0, Tutorial: Modeling Bubble Breakup and Coalescence in a Bubble Column Reactor.)

1. why is Bin-3-fraction the only variable set to 1 at both inlet and outlet, while the others are set to 0? (file attached.)
1. Bin-3-fraction value set to 1.PNG

2. Turbulent Kinetic Energy and Turbulent Dissipation Rate are set to 0.1 and .025, respectively. There are some standards saying how to set these values or just user-dependent?
2. TKE, TDR initializing values.PNG

3. Please let me know what is Number of Moments, and why this tutorial set the value to 4?
3. Number of Moments to 4.PNG

4. Volume- or surface-averaged report type means?

Thanks.
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Old   February 10, 2020, 05:47
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I would recommend going through the PBM manual or some general article or book on PBM. However, for your convenience, here are some quick replies

1. Bins are nothing but segregation based on some parameters, mostly, diameter but there could be other parameters, such as moments that could be used to define the bins. Just assume those as physical bins where you could put balls either of same size or of same color or material. In the tutorial, I haven't gone through it, most likely the bubble size is used as the parameter to define the bin; Fluent doesn't offer any other parameter as far as I know. And by one particular bin having volume fraction of 1 while others having 0 implies that at the boundary all the bubbles are of same size defined as bin-1 or bin-3, whichever has volume fraction of 1. Further downstream, size might change due to coalescence, breakup, etc. This can be different for your case.

2. Turbulence kinetic energy is kinetic energy due to the fluctuations in the velocity field. You have to assume the level of turbulence at the boundary. If you assume the intensity to be 5 %, and the mean inlet velocity is 10 m/s, this implies 0.5 m/s as fluctuations in the velocity. With this velocity, tke turns out to be 0.125 m^2/s^2. Formula is same as mean kinetic energy

3. Moment is a mathematical idea with a very close relation to the physical moment, i.e., force multiplied by its normal distance. Mathematics is little involved but can be understood with little help from various documents and articles available online, but most of the time it is not required. The approach is useful in solving Population Balance Equation by transformation of the variables and reduces the dimension a lot. These moments have a physical meaning, such as, 0th moment is the number density of bubbles, first moment is representative size or average size, second moment is area density, third is mass density, etc. Usually, first few moments are good enough to achieve the objective. Its like using a number of straight lines to describe a circle; more the merrier but beyond a certain limit, it is not justified. For PBM, 4-6 moments are good enough. Beyond that, better not to use moments method and use discrete approach.

4. Volume and Surface reports are averages of any field variable. However, averages are not arithmetic averages, rather weighted by either the volume or the surface area.
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Old   February 10, 2020, 22:27
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Thanks a lot!
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Old   October 22, 2020, 10:35
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Quote:
Originally Posted by starry09 View Post
I have several questions on Population Balance Modeling (PBM).

I have little understandings of boundary conditions with bins.

(Please refer to a tutorial, Fluent 14.0, Tutorial: Modeling Bubble Breakup and Coalescence in a Bubble Column Reactor.)

1. why is Bin-3-fraction the only variable set to 1 at both inlet and outlet, while the others are set to 0? (file attached.)
Attachment 74708

2. Turbulent Kinetic Energy and Turbulent Dissipation Rate are set to 0.1 and .025, respectively. There are some standards saying how to set these values or just user-dependent?
Attachment 74709

3. Please let me know what is Number of Moments, and why this tutorial set the value to 4?
Attachment 74710

4. Volume- or surface-averaged report type means?

Thanks.

Hello my Dears,
I am new in PBM coupling model , I would like to use this model to simulate my axial flow pump operating in two phase gas-liquid:

1) How can I know the bin that my inlet bubble diameter falls and is there any UDF which can predict the bin of my bubbles size ?

2) I have alpha1,2,3,4,5 and 6 volume fraction how can I implement the volume fraction in my inlet boundary conditions?
3)because the fraction of water, should the sum of all the bin fraction should be equal to 1?
4) should my outlet have same bin as the inlet with same volume fraction?
5) how can I calculate the totale bubbles ?
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