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Old   November 23, 2014, 14:12
Default population balance model
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Hi to all

Could anyone help me to find out "population balance model" applications?
In which kind of problems, we could use this model!?

I hope somebody will help me...
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Old   November 24, 2014, 07:33
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Hello Amin,

Population balance models can be used in problems where the behavior of a (discontinuous) phase cannot be accurately described by assuming all particles have the same properties (typically equal to the population-based mean of that property).

The most common application is in modeling of gas in a continuous liquid. Since several forces acting on bubbles depend on the diameter of the bubble, choosing the mean diameter can result in an erroneous estimation of the gas holdup, mass transfer, and so on.

Using a population balance model, the distribution of bubble sizes over the population can be accounted for. Breakup and coalescence of bubbles can be included, meaning that (provided the mechanisms of breakup and coalescence are properly captured by the model) the gas behavior can be described with higher accuracy; at the cost of additional computation time. Similarly, solid particles with a size distribution, where breakup/coalescence is important, can be described using a population balance approach.
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Old   November 24, 2014, 09:17
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Quote:
Originally Posted by CeesH View Post
Hello Amin,

Population balance models can be used in problems where the behavior of a (discontinuous) phase cannot be accurately described by assuming all particles have the same properties (typically equal to the population-based mean of that property).

The most common application is in modeling of gas in a continuous liquid. Since several forces acting on bubbles depend on the diameter of the bubble, choosing the mean diameter can result in an erroneous estimation of the gas holdup, mass transfer, and so on.

Using a population balance model, the distribution of bubble sizes over the population can be accounted for. Breakup and coalescence of bubbles can be included, meaning that (provided the mechanisms of breakup and coalescence are properly captured by the model) the gas behavior can be described with higher accuracy; at the cost of additional computation time. Similarly, solid particles with a size distribution, where breakup/coalescence is important, can be described using a population balance approach.
Tnx for your help

As you know, I have a problem about air- water simulation
In my model most important result for me is air bubbles speed relative to water speed!
And I guess the bubble speed isn't low so break up simulation isn't important for me!
And the bubbles diameter is about 0.3 mm
In this condition, do you suggest me using DPM?
Tnx
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Old   November 25, 2014, 06:53
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In that case I have a few questions:

- why do you need to model the spray section? I guess the contribution of momentum to the water pool of that is rather small? or am I wrong there?
- if it small, you can just model a bubble column. In first approximation, you can of course simply calculate the rise velocity of a 0.3mm bubble in water. (quite a small bubble by the way, are you sure of the size?). You can use DPM, as small bubbles act rather rigid, but Eulerian 2-phase with a steady bubble size would work as well.
- If the momentum contribution by the water in and outflow is not small, you can include the water in/outflow as mass & momentum source and sinks, or maybe simple in and outlets, as long as you know the stationary water level. This may be easier than simulating the whole system
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Old   November 28, 2014, 10:10
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Tnx deer friend!

1. In my case, water doesn't sprayed ! water passes through the orifice in order to rising the speed and it stays liquid!
2. yes, at first step i want to model only bubble column!
3. The bubbles size is almost this amount
4. So are you suggesting using the Eulerian model!?

very Tnx friend!
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Last edited by amin.z; November 28, 2014 at 16:37.
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Old   March 15, 2016, 11:38
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Quote:
Originally Posted by CeesH View Post
In that case I have a few questions:

- why do you need to model the spray section? I guess the contribution of momentum to the water pool of that is rather small? or am I wrong there?
- if it small, you can just model a bubble column. In first approximation, you can of course simply calculate the rise velocity of a 0.3mm bubble in water. (quite a small bubble by the way, are you sure of the size?). You can use DPM, as small bubbles act rather rigid, but Eulerian 2-phase with a steady bubble size would work as well.
- If the momentum contribution by the water in and outflow is not small, you can include the water in/outflow as mass & momentum source and sinks, or maybe simple in and outlets, as long as you know the stationary water level. This may be easier than simulating the whole system
Respected experts,

When applying Discrete*Population Balance Model in ANSYS Fluent we have to put predefine boundary conditions values. For Example,

phase.. Air.. Bins...7 Ratio Exponent...1.7 Minimum Dia...0.05cm then in boundary conditions... Velocity Inlet...Boundary Values*for setting*are..

Bin-0-fraction________ * Constant

Bin-1-fraction_________ Constant

Bin-2-fraction_________Constant

Bin-3-fraction_________Constant

Bin-4-fraction_________Constant

Bin-5-fraction_________Constant

Bin-6-fraction_________Constant

How to set these values to get appropriate gas distribution ?

How to set boundary condition values in Fluent Population Balance model ? - ResearchGate. Available from: https://www.researchgate.net/post/Ho..._Balance_model [accessed Mar 15, 2016].
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Old   March 15, 2016, 11:44
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If you use the discrete model, easiest in my opinion is to use the log-normal distribution and give in some parameters on the mean and deviation of the bubble size distribution there. This will suffice for many applications. Base the numbers on experimental values if you have any, or calculate it based on engineering correlations (see for example pettiti 2011)

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Old   March 15, 2016, 11:56
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Quote:
Originally Posted by CeesH View Post
If you use the discrete model, easiest in my opinion is to use the log-normal distribution and give in some parameters on the mean and deviation of the bubble size distribution there. This will suffice for many applications. Base the numbers on experimental values if you have any, or calculate it based on engineering correlations (see for example pettiti 2011)

Best,
Cees
but after clicking log-normal, and putting Mean with St. Dev... these boundary conditions still blank to fill
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Old   March 15, 2016, 12:43
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you do need to click initialize or generate or so after filling in the values
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Old   March 16, 2016, 05:11
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Originally Posted by CeesH View Post
you do need to click initialize or generate or so after filling in the values
thank you very much, your suggestion is really working and informative..
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Old   March 16, 2016, 05:13
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Good to hear! Good luck on your simulations
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Old   March 17, 2016, 04:01
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Quote:
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Good to hear! Good luck on your simulations
Respected sir,
please please guide me one thing more. In log-normal window, there are two options MEAN and St. Dv. , I have to put MEAN of what ??
MEAN of BUBBLE DIAMETER( like bubble dia 3mm or 5mm etc.. ) or MEAN of FRACTIONS. because, below are options like this ( bin-0-fraction , bin-1-fraction) , so, kindly suggest me your expert opinion. I am really waiting for this answer.. ( I would highly appreciate you if you give me any example please..)
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Old   March 17, 2016, 04:15
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mean of the bubble diameter; so if you have experimental data, give the mean and deviation of the measured distribution. Otherwise, estimate the bubble size from some correlation (or a force balance)
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Old   March 17, 2016, 04:27
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Quote:
Originally Posted by CeesH View Post
mean of the bubble diameter; so if you have experimental data, give the mean and deviation of the measured distribution. Otherwise, estimate the bubble size from some correlation (or a force balance)
Thank you very much sir..
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