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Using LogNormal distribution with DPM spray model

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Old   March 27, 2007, 14:39
Default Using LogNormal distribution with DPM spray model
  #1
joshkemp
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I am trying to model spray from a nozzle for which I have experimental data. The distribution is fit very well with a log normal curve, so I would like to use this to initialize the spray from the nozzle. I also would like to have to droplet size randomly distributed across the nozzle area.

What is the best way to do this? I know there is the read from file option or possibly a UDF, but what is the best approach?
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Old   March 28, 2007, 17:36
Default Re: Using LogNormal distribution with DPM spray mo
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Allan Walsh
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In my experience, it is better to write a stand alone program to make a file to read into Fluent. In version 4 of Fluent, we wrote our code in Fortran that was integrated into Fluent to generate the sprays. Incidentally, one of the reasons we wrote our spray routine was to integrate a lognormal diameter relationship and to have each diameter class represent an equal fraction of the spray.

In version 6, we have used Excel with a Visual Basic macro to generate the spray file. This has been much simpler as you don't have to do a lot of work generating on-the-fly forms to input spray parameters. And, you don't have to recompile all the time to debug your code.

Good luck.
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Old   March 28, 2007, 18:20
Default Re: Using LogNormal distribution with DPM spray mo
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joshkemp
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Thanks for the help Allan. I am new to spray modeling.

I have another question relating to mesh requirements for DPM. I recall reading somewhere that your mesh must be coarse enough so that your biggest droplet does not occupy more than one cell. I searched through the Fluent documentation and cannot find any mention of this. I think I may have read this in a multiphase book I have. Do you know if this is required for Fluent? If so, I may have a hard time keeping the correct y+ values near the wall for the k-epsilon turbulence model.
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Old   March 29, 2007, 13:23
Default Re: Using LogNormal distribution with DPM spray mo
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Allan Walsh
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I don't think it is a requirement - and after modeling hundreds of cases like this in Fluent - it hasn't been an issue. In a steady-state calculation, you can tweak the step size or length scale to get nominally one discrete exchange event in each cell for each time step. This would spread out the source exchange terms.

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