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-   -   Particle Size Distribution with R-R fails? (http://www.cfd-online.com/Forums/fluent/36678-particle-size-distribution-r-r-fails.html)

HP May 17, 2005 07:50

Particle Size Distribution with R-R fails?
 
Hi! I would like to ask a Particle Size Distribution related question.

I inject particles from a surface declaring a R-R type particle size distribution, and I use 15 diameters with a spread factor and a mean diameter. But when I check the particles' diameters in the particle tracks panel (and in the sample tranjectory histograms panel as well), I get a uniform distribution for the 15 diameters, which take place between the max and min values that I defined (uniformly). When I try then to change the spread factor or the mean diameter of the R-R distribution nothing changes. Imagine that when I define number of diameters =3, the two diameters take the value of dmax and dmin, and the third the value(dmax+dmin)/2! Isn't this linear?

Can somebody tell me what the problem is?

Thank you Ilias

Allan Walsh May 17, 2005 13:02

Re: Particle Size Distribution with R-R fails?
 
I believe FLUENT will use even spacing on the particle diameter, but the mass fraction represented by each particle will be representative of RR distribution. This is okay since normally you want to see effects of particle phase on continuous phase, and not just look at particles.

If you want each particle to represent the same mass fraction, you need to generate particles in a separate program and then read in the file.

HP May 18, 2005 06:28

Re: Particle Size Distribution with R-R fails?
 
You are right! After I turned my particles into inert type (to keep their mass constant), I checked the trajectory sample histogram at the outlet of my domain and the total mass flow had been distributed in a RR way (as I assume while the masses were not equal).

But how FLUENT is calculating the corresponding mass flow for each particle diameter (if dmin, dmax, dmean, n, #diams are given)? Or the opposite; how can someone calculate dmean and n (spread factor) if the diameters and the corresponding masses of the particles are given? (and I do not mean ranges of diameters as the user's guide describes).

The existance of minimum and maximum limits, made things even more complex, as RR (Weibull) distribution ranges from 0->inf. So my efforts to divide the space into classes of the same possibility with the corresponding particle diameters possibilities (mass fractions), stopped quickly...

The reason I want to understand completely the mechanism, with which FLUENT converts the RR distribution into discrete particle diameters and corresponding mass flows, is that I already have this data (particle diameters and mass flows) from an older simulation (open code). In order to compare the results of the open code with FLUENT's I have to keep all the parameters unchanged. So I must find the exact RR distributions, or I make a file with the injections' data (25 injections with 3 different psd's). And if I create the injections file and want to increase the number of the representative diameters then what? Create another file?

I hope that I was clear enough. Thank you in advance for your time.

Best regards,

Ilias

rom May 18, 2005 08:18

Re: Particle Size Distribution with R-R fails?
 
You can list the diameters and corresponding mass-flow rates with the tui. Type "//define/injections/lp" and enter the name of your injection. Maybe this will help you to track down the problem.

good luck

rom

HP May 18, 2005 08:31

Re: Particle Size Distribution with R-R fails?
 
Yes of course! To track down the problem I created an injection with some typical R-R parameters (dmin, dmax, dmean, n, number of diams=5) and then I listed the initial conditions of the injection. From the list I got the five diameters and the corresponding mass flows. But the problem is that I didn't manage to correlate them. Namely I didn't manage to calculate the distribution parameters starting from the particle diameters and mass flows.

Thanks anyway for time.

Regards,

Ilias

Allan Walsh May 18, 2005 13:37

Re: Particle Size Distribution with R-R fails?
 
Not exactly sure of your question, but if I understand that you have data (i.e. 25 particles, each with unique diameter and mass fraction, then from my experience, it would be quite simple to make a little spread sheet with these parameters and the initial location (x,y,z) and velocities (u,v,w) for the particles. In a spread sheet, expanding to 3 distributions for 25 particles, is also quite simple, as is creating the text file to read into FLUENT.

Calculating your own particle size distribution from the RR parameters is more work, but shouldn't take more than a week. Have you actually graphed up what FLUENT calculates (diameter vs. mass fraction) to see if it actually is a RR distribution? And, how does the FLUENT calculated distribution compare with what you would calculate for the same parmaters?


HP May 19, 2005 06:35

Re: Particle Size Distribution with R-R fails?
 
Yes of course it is simple to make files for the injections. But consider that in my case (pf boiler) there are 25 surface injections (on for each inlet). In order to create these files, I must find the centroid of each triangular cell surface for each inlet, and from there inject a particle for each diameter. But for each facet I must calculate the area in order to find out the corresponding area-weighted mass flow!

So temporarily I created for each surface-inlet (25 inlets), one surface injection with uniform distribution for each diameter (I remind you that given for me are 5 diams and their mass flows). Namely for the 5 diameters I created 25 X 5 =125 surface injections.

Concerning FLUENT calculations. Yes I have already graphed up diameters vs mass flows and seem to follow RR distribution. But how can I tell for sure without finding the RR parameters? I mean this is the same problem: I cannot make any calculations because I cannot figure out the exact mechanism with which FLUENT transforms the RR parameters to discrete diams and mass flows!

Thank you for your help.

Best regards,

Ilias


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