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UDF bubble diameter

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Old   July 28, 2014, 16:17
Default UDF bubble diameter
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Cees Haringa
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Hello all,

I'm writing a UDF for mass transfer between the gas and liquid phase in the eulerian multiphase model. One of the parameters in the Kla is of course the bubble size (or sauter mean diameter).

Now, I was looking for a macro to read this diameter from FLUENT but could not find one. For regular calculations, this is not extremely problematic as the bubble diameter can be fixed in the UDF. But as I want to move towards population balance models this becomes somewhat more problematic - I need the local bubble diameter and bin distribution of sizes.

I know people used kla models with population balances - at least they published work on it - but could not find a UDF code that does something like that, or a way to load the local bubble distribution into my UDF. Could anyone help me out on this?

Thanks in advance,
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Old   August 1, 2014, 04:51
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Hi,
You can use
Code:
C_PHASE_DIAMETER(c,t)
to get the diameter of the dispersed phase. But somehow I think this would only solve the first part of your problem (say you have either a constant bubble size or a homogeneous size distribution which is only varying by time not space.)
For the second part, however, when you are solving a PBE, and I am assuming here you are solving it via tracking moments of an NDF, you might be able to get away by storing the moments into UDS(s) and then use the stored moments to build the size distribution function, or at least get its mean and variance.
cheers!
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Old   August 1, 2014, 15:45
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Hi Sun,

Thanks! I didn't stumble across that one yet.

I did find C_PB_DISCI(c,t,i) which should give the volume fraction per bin i in a pop. balance model; so combining that with knowing the bin size should get me there, I think.

Another issue I'm now facing (for the single bubble size) is that FLUENT does not seem to recognize my mass transfer UDF, also when I turn `use linearized m.t. udf' off. It simply says no m.t. UDF is loaded. Did anyone experience this problem?

Maybe using a linearized version solves it, but haven't worked on that yet.

Best,
Cees
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Old   August 1, 2014, 16:01
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nvm, fixed it. it seems to matter if you switch between m.t. and linearized m.t. before or after compiling...
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Old   April 16, 2015, 06:47
Default a littie more explanation
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could you please give me more details?
could you find bubble diameter in population balance model?
how?
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Old   April 16, 2015, 11:49
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I never worked with the discrete population balance models in the end, switched to QMOM. There, there is no bubble diameter that you can extract as you are tracking purely statistical characteristics. You can however use the definition of the sauter mean diameter: moment 3/moment 2. These you can extract and use, if sauter is the applicable diameter to represent in your case.
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Old   April 18, 2015, 08:53
Default macros?
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sauter mean is applicable.is there any specific macros that can be used in order to attain d3/d2?
how did you defined it? sorry I am new to PBM.
I cant use QMOM because I only have two phases.
thanks in advance
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Old   April 19, 2015, 06:06
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You can use QMOM with 2 phases - QMOM uses moments to describe the population distribution in the 2nd phase rather than a discrete-bin approach.

It could be there is a direct way to call the sauter diameter as FLUENT also uses this for drag calculations when using qmom. In any way, you can use the moment macros to calculate the sauter diameter:

Code:
S_DIAM = C_PB_QMOMI(c,t,3)/C_PB_QMOMI(c,t,2);
should do the trick. Don't forget to include sg_pb.h

More information can be found in the manual - advance addon modules/population balance
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Old   April 19, 2015, 06:33
Default thanks a lot
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thanks a lot for the details.
I hope I can do something in return
best wishes
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