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Particles in rotating frame

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Old   February 6, 2008, 07:16
Default Particles in rotating frame
  #1
skumanov
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Hallo! Sorry for my bad english! I have the following problem: i try to separated particles collectiv from water in rotating zentrifuge. I want to find out the separating curve. What i've done: 1. Flow field is already computed 2. Define ca. 150 particles (quarz with density=2700 kg/m3)with different diameters: 1e-09, 1e-07,1e-06,2e-06 etc. until 1e-05. 3. Use the uncoupled method to find out which are "escaped", "trap" or "incomplete". They all are traped or incomplete! And that cannot be the true. So small particles like 1e-06 should be not separated, what FLUENT shows!!! 4. With the coupled method ist the same thing. The separated curve ist always 1.

What do I wrong?
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Old   February 12, 2008, 05:27
Default Re: Particles in rotating frame
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randomwalker
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plotting the pathlines should give you a basic idea of what is wrong:

-I particles seem to bounce off the outlets, check boundary conditions for the discrete particles and make sure it is set to escape -If the paths never reach the outlet, increase the number of steps in the particle path (or take longer steps) -If particle are eaten by the grid interfaces, I don't know what to do. I seem to have that problem, but in that case no particles are trapped and all are incomplete (except one or two that manage to escape back into the inlet).
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Old   February 13, 2008, 03:08
Default Re: Particles in rotating frame
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skumanov
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Thanks for the answer! All boundary conditions are set to right: input and output are escape, two of the walls are trapped (separating effect in centrifuge), rest of them are reflect. Plotting the pathlines from fluid shows right computing of the flow. Plotting the particles pathlines shows me obviously something wrong - no separating effect. Increase the number of steps (try already with up to 10^8) had also no result. Apropo, I don't use the "Interaction with continous phase" function in DPM. Reason: the particles are ca. 200 and they have no bearing on the waterflow. It mean, I used the uncoupled method.
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Old   February 14, 2008, 08:26
Default Re: Particles in rotating frame
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randomwalker
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So the flow is correct, but the particles follow the fluid streamlines perfectly instead of being separated from it and each other. Your particles seem to behave like massless particles, in fact.

I don't have any good suggestions: assuming the material is defined correctly, the particle mass/density and drag coefficient (i assume spheres) should be computed correctly by the DPM model. I cannot find any relevant option you may have forgotten to enable.

But if the flow field is correct, then the error must be in the DPM model or particle definition. I would suggest tinkering with the model for one or two of the large particle until they start behaving more realistically (ie are drawn out to the wall).

final control question: you are of course sure that the velocity of the centrifuge is strong enough to actually separate the particles? Sometimes fluent gives the right answer, just to the wrong problem (such as when i specified a 10m pipe instead of a 10mm one).
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Old   February 14, 2008, 12:51
Default Re: Particles in rotating frame
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skumanov
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Hello!

The flow seems to be right - verification with the reality shows correct results for the tangential, axial and radial velocity. The particles cannot be massless, it is quartz with density 2.7x10^3 kg/m^3, and that is already defined in the Materials for inert particles. Perhaps I was in some way incorrect with my last comment - I have a total separating (which means that all of the particles go to the wall, and do not follow the flow in direction outlet - at least the small particles should do that). This happens with all of them, and that is the point - it must be wrong. The centrifuge exist in reality and results of experiments are already available. To the last question: I think, it is right. The velocity is computed from real experiments and must be enough. One remark to all, that I wrote: its relates to 2d model. Currently I compute the 3d model, at this point it is still at unsteady state (needs up to 5 sec simulating time), but there is already a separating effect - also it is working for 3d but not with the 2d model. The boundary conditions for the flow and the defined particles are the same. I know, the 3d model is much more accurate, but the 2d results for the rest of the variables were pretty good and I have tough, the particles will do this too. Do you have any ideas?
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Old   February 16, 2008, 20:43
Default Re: Particles in rotating frame
  #6
shehab abdelrahman
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hi there: it seems you used relative velocity with the rotating frame of reference. this will give you meaningless results with DPM. if yes, you should setup your simulation solver to solve for the absolute frame and this may olve you problem, see the documentation for DPM and Rotating frame of reference. hope that help. shehab
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Old   February 17, 2008, 06:37
Default Re: Particles in rotating frame
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skumanov
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hi, good idea!!! But it means, I must solve the problem again from the beginning with Relative Velocity Formulation in the Solver-menu, or it is enough to click in the Diskrete Phase Model under Numerics "Track In Absolute Frame"????
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Old   February 17, 2008, 06:49
Default Re: Particles in rotating frame
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skumanov
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Another posting: FLUENT'S UG describe the coupled case. I have always used the uncoupled method. When I use the coupled method, it is enough to iterate ca. 100 steps with the new formulation - this in case, that I don't calculate from the beginning, only make this 100 steps forward, or??? And when I compute all from the beginning, just define the particles too at this point and then iterate until steady state solution? Which is right?
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Old   February 18, 2008, 17:57
Default Re: Particles in rotating frame
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shehab
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hi again: actually I think you have to simulate all your work again with respect to the absolute frame. and also make use of the DPM absolute frame. about to use the coupled solver, what I know that there is some restriction and limitations about using the coupled solver with the DPM. you must be sure that your case study is free from these limitations. this may be done by using the help documentation for DPM modelling >>> limitations. you have to be awared that coupled solution is also computationally expensive, i.e. it will take a lot of time to converge. best regards. and by the way, I have a question to ask ..... how can I model a slip wall boundary condition ? ...do you know...and another one... do you know how to set up a droplet particle type material? thanks again.....bye
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Old   February 22, 2008, 07:47
Default Re: Particles in rotating frame
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skumanov
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Hi!

I'm not so sure, what exactly you mean. For slip you must go to the boundary condition for the wall from interest and just define the shear condition with the accordant coefficient. The second question... I don't know. I have worked only with solid particles. sorry.
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