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Population Balance Modeling (PBM) - Ansys Fluent

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Old   October 22, 2014, 04:56
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Hi
I have a liquid-liquid-gas flow and I want to need about liquid and gas bubble distribution.I need help for getting this result.
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Old   April 15, 2015, 07:22
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is there any way to use population balance in order to find interfacial area in a two phase (gas-liquid) model in a UDF? ( I want to use it in order to find the interfacial area in a mass transfer UDF)
I didnt use this model at all and I dont want to study it unless it can be useful.
thanks in advance
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Old   June 8, 2015, 10:49
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Hi,

I am trying to simulate a biological cell suspension inside a bioreactor. I successfully obtained a multiphase model which does not take into account the fact that cells (10 microns in diameter) can aggregate and form aggregates up to 200 microns. I decided to apply to my model (2D-axisymmetric) a PBM model in which I am going to consider a gowth model and an aggregation kernel.
My problem is that my flow field is not stationary because there are modifications on the spacial distribution of the volume fraction of my dispersed phase which modify the velocity field of the primary phase as well (two-way phase interaction) . For the numerical stability I must use time steps of 2 to 10 ms. The characteristic time of aggregation and growth of the cells is unfortunately in the order of magnitude of hours. Do you have any suggestion to manage this? It may be impractical to simulate days since to simulate one hour I need one computational day.

Thank you for your attention

G
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Old   October 14, 2015, 09:55
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Hello everyone

I have a question about PBM in Fluent. I am working on a multiphase system and I have 6 different solid, so 6 secondary phases. Now I want to simulate the particle size distribution by PBM via inhomogeneous discrete method. But in this method I can defined just 2 secondary phase. How can I define 6 secondary phase?
If anyone know something, plz let me know. Thank you.
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Old   November 12, 2015, 07:47
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Muhammad Furqan Ali
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anyone can tell me what is Bin-0-fraction in PBM. how can we select its boundry conditions. I have simple cast of air in water bubble. then in last in want to know about patch for bin.
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Old   December 5, 2015, 01:09
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Quote:
Originally Posted by chittipo View Post
Dear Members,

If you have any specific questions on PBM implementation please post here. I will try to answer them, as I gained some experience in it.

on
- Existing module (or)
- Implementation through UDF
Hellow, I am writting an UDF about QMOM, have you meet the trouble about adding correction term into sorce term? I don't know how to add it reasonablely.
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Old   January 4, 2016, 11:11
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Yongfei Yang
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Hi,

I'm doing the simulation of oil-water seperation in a gravity seperator. Population balance model is used for the aqueous phase and the equation is solved using QMOM.

Only aggregation is considered for the drop size evolution. And as the flow is laminar, luo-model can not be used. So I set the aggregation kernel as constant(1e-19).

But the result of the drop is strange. The larger drop stays at the top and the finer drop settles faster. Also the number of the small drop inceases during the simulation. That is not resonable. Can you give me some advice?
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Old   January 4, 2016, 21:18
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Quote:
Originally Posted by Yongfei Yang View Post
Hi,

I'm doing the simulation of oil-water seperation in a gravity seperator. Population balance model is used for the aqueous phase and the equation is solved using QMOM.

Only aggregation is considered for the drop size evolution. And as the flow is laminar, luo-model can not be used. So I set the aggregation kernel as constant(1e-19).

But the result of the drop is strange. The larger drop stays at the top and the finer drop settles faster. Also the number of the small drop inceases during the simulation. That is not resonable. Can you give me some advice?
I think there maybe exist some problem in you definition on the aggregation term. wether the aggregation term of all moment bigger or smaller than zero?
that can lead to your problem.
Besides, can you show me a picture of your definition of correction term of the moment transport equition which is the second term on the right hand of the equation.
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Old   January 7, 2016, 07:23
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Yongfei Yang
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Quote:
Originally Posted by Athena View Post
I think there maybe exist some problem in you definition on the aggregation term. wether the aggregation term of all moment bigger or smaller than zero?
that can lead to your problem.
Besides, can you show me a picture of your definition of correction term of the moment transport equition which is the second term on the right hand of the equation.
Hi,thanks for your suggestion! I have icreased the number of moment. And the result is good now.
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Old   January 25, 2016, 10:39
Default how to define ni and nj in luo coalescence model via udf?
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could anyone tell me how to define ni and nj in luo coalescence model via udf?
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Old   March 4, 2016, 07:52
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Kristin Jansen
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Quote:
Originally Posted by chittipo View Post
Dear Members,

If you have any specific questions on PBM implementation please post here. I will try to answer them, as I gained some experience in it.

on
- Existing module (or)
- Implementation through UDF
Dear

for University I got a project to model a gas sparger system in a water flow. We determined tot take the Eulerian multiphase model. Now I was reading the Ansys Fluent user manual, I read some things about the model being only appropriate to simulate particle tracking with interaction with only one phase. So it is not clear for me if break up and coalescence can be modelled. Or do we have to addept the Eulerian model with the PBM model, if this is possible?

Thank you for your answer!
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Old   March 4, 2016, 08:59
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Cees Haringa
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Hi Kristin,

I am not exactly sure what you mean, can you link to the section of the manual that you are referring to?

In any case, normally the eulerian multiphase model bases the calculation of interphase forces (drag, lift,..) on a single bubble diameter that has to be supplied by the user. This may or may not be a reasonable approximation, depending on the specifics of your system. If breakup and coalescence are likely of importance (for example, at high flowrates), you should indeed include a population balance model.
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Old   March 4, 2016, 09:12
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This is the section from the 'ANSYS Fluent theory guide':

"All other features available in ANSYS Fluent can be used in conjunction with the Eulerian multiphase
model, except for the following limitations:
The Reynolds Stress turbulence model is not available on a per phase basis.
Particle tracking (using the Lagrangian dispersed phase model) interacts only with the primary phase.
..."

The goal of the simulation is to look at the coalescence or break up of the bubbles in the system, since they are defining the efficiency of the sparger system. The further the bubbles go from the sparger system, the smaller they should get. So we can' t input a fixed diameter of the bubbles. We know the input diameter, but the rest should be moddeled.

Together with the flow simulation we would like to model the diameters /break up. Is it possible to do this combined? By for example programming a UDF which contains the PBM. Or should we firsts make the Multi fluid VOF and later the PBM?

If the combined model is an option, we would prefer that one.

Thank you in advance for the answer!
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Old   March 4, 2016, 09:35
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Ah, i see what you mean. But that comment is only applicable in case you use the discrete particle model (DPM) next to Eulerian multiphase. if you do so, the movement of these particles only follow the movement of the carrier phase, and are not influenced by the gas phase. As you do not need the DPM model for your simulations, this notion does not apply for you.

Now related to what you are going to do, the VOF or Eulerian multiphase models are two very different things. In the VOF model, you explicitly model your bubbles, in Eulerian, you do not (i.e. your grid cells are bigger than your bubbles). So with VOF you can do very detailed studies, but of only a few, maybe tens of, bubbles. With Eulerian, you can do billions of bubbles but you only track averaged properties. So, for that, it depends on what you want to resolve.

For typical engineering applications, you will probably use the Eulerian model. In that case, you will indeed need to include PBM for the bubble diameter; VOF, since it explicitly resolves the bubbles, does not need such a thing. (but VOF is pretty bad at coalescence - better at breakup).

cheers!
Cees
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Old   March 4, 2016, 10:07
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Okay I think I get it, thank you for the answer!
So you study the break up/coalescence effects in Euler by looking at the diameter plot over the system? Is that right?

I understand the difference between the Euler and the VOF model. But in the ANSYS Fluent user guide, it shows a couple of models under the heading of Euler model:
*Dense dicrete phase model: we can not use this since bubbles do not interact with eachother. (correct me if I' m wrong)
*Multi-fluid VOF model: The multi-fluid VOF model for Eulerian multiphase allows you to use the Geo-Reconstruct, compressive,
and CICSAM sharpening schemes with the explicit VOF option.
*Some boiling, condensing or combustion models

Are these models to be considered as additional to the Euler model or just different ways to solve the Eulerian model. I am quite confused how you can have a VOF AND Eulerian model at once.

Thank you for your time, we are only starting to work with cfd/Ansys so everything is a bit complicated so far. I will look further for a way to implement PBM into our Eulerian model.

Kind regards
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Old   March 4, 2016, 10:55
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I have to admit that I have no practical experience with any of these particular models, only with 'regular Euler Euler' and the 'true' VOF method, but from what I read:

- The dense discrete phase model treats the gas phase itself as particles; in contrast to the regular DPM model (which I referred to before), collisions between the gas bubbles (or other particles) are accounted for, as is the physical space taken up by the particles. I have no experience with how breakup and coalescence are treated in this option. But, in this model bubbles do interact with eachother - the comment before was related to a 3-phase model, with gas liquid and solid, were the gas and solid don't interact.

- The VOF model under the Eulerian tab, us, as Prof. Martin v. Sint Annaland explained it to me boldly, just Eulerian multiphase with interface sharpening. It's not real VOF. (but again, I have no practical experience with this model, so i can't verify this claim).

For your situations, if indeed the number of bubbles you want to model is large (which i understand it is), the Eulerian multiphase itself should suffice. Only, you will need to include the population balance model for the interaction, else the model assumes all bubbles have the same size, which you have to give in yourself.

Best,
Cees
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Old   March 10, 2016, 10:08
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Dear Cees

as I investegated more in the PBM, I have some remaining questions.

I don' t know whether to imply the PBM with the Discrete method or with the QMOM. As I understood, it is impossible to model break up phenomena with the SMM, so this one is no option.

Secondly, on which base can you decide which model to use for the break up/coalescence or to define a UDF. Is there an overview somewhere with the properties of a whole number of models?

Thank you!
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Old   March 10, 2016, 10:36
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Hi Kristin,

Although I can't speak in general, my personal experience is that the discrete method is more stable, but also more computationally intensive. To attain representative results, you need 10 - 20 size bins (depending on the situation), while for QMOM typically 6 bins suffice; so, less additional equations and less computational Burden.

Regarding the models; well, it's a mess. There are a lot of (frequently phenomological) models out there. The ones by Luo that are default in FLUENT give reasonable predictions, but may be slightly over-predictive in size. I noticed the same for Prince and Blanch. I've also used Laakkonen, but it has a scale-dependent fitting parameter, making it non ideal (see Petitti et al.). A good overview of frequent used models (Luo, Prince-blanch, Lehr) is given by Wang et al., together with their own model. Of course, their own model (which is an extension of Prince) comes out best, for bubble columns. Luo come is probably worst off in their prediction, but what I recall as the most striking message is the large discrepancy in breakup and coalescence rates between the model.

The best advice I can give you is to first check if there is a specific model available for the type of situation you are considering. If not, it may be a reasonable start to just try Luo (as it is available) and see how it compares to experimental data. If it doesn't give proper results, you can look at UDFs (if you make a UDF for Wang's model, I'd love to test it for stirred tanks as well - so far I haven't gotten around to implementing it because it's a bit less straightforward than some others). Based on the comparison of Wang, showing the largely different rates between the models, I would advice not to mix different breakup and coalescence models.

hope that helps!
Cees
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Old   March 10, 2016, 11:05
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Thank you! I will look into this.

Further, with the PBM, can you make a graphical representation of the diameter distribution over the bubble column? This is quite essential to our approach. I thought I saw it somewhere, but now I can' t find it anymore, so I m starting to doubt it.

thank you
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Old   March 10, 2016, 11:09
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Yes, FLUENT gives the sauter mean diameter under phase 2 > properties > diameter. In QMOM it's calculated by dividing moment3/moment2
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