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March 17, 2013, 02:19 
Particle fate criteria

#1 
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S.Bogoda
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Dear All,
I have a doubt about particle fate criteria. I have a particle sample (010um) and I want to find the collection efficiency of those in a cyclone separator. Then CFX output file shows there are particles which exceeded time, integration and distance limits. My problem is how to know do I want to increase values of these criteria or not? Because some particles should endup with one of these fates but some particles should leave domain. Still nothing has leaved. Many thanks for helps.. 

March 17, 2013, 05:44 

#2 
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Glenn Horrocks
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If the particles have long residence times you might have to increases these particle termination parameters.


March 18, 2013, 00:28 

#3 
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S.Bogoda
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Dear Glenn,
Thanks a lot for the reply. Actually I don't know the residence time and have to find from this simulation. Total simulation time is 10s here and I am using 12m/s inlet velocity for particles. I guess particle residence time is less than 2min, but even after 5min nothing leaves in simulations. I can see there is a strong recirculating zone inside the cyclone. I have done experiments but there particles comes out from exit, after 1s. I cannot find the reason why simulations are going on like this? Do you have any idea? 

March 18, 2013, 05:14 

#4 
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OJ
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Are you sure you are injecting enough particles to do justice to probability of both occurrences? Why not do a bit literature search? Or, perhaps the particle number independence test etc?
OJ Last edited by oj.bulmer; March 18, 2013 at 05:38. 

March 18, 2013, 05:42 

#5 
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S.Bogoda
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Dear OJ, Thanks a lot for reply.
The particles having the range of 010um, so there should be particles trapped by geometry and leave the domain. If I understand you correctly, you are asking do I know whether particles exit or not? Then I know compared to assigned particle range at least 1/2 of particles should be leave the domain. And other 1/2 should be trapped inside. So, my problem is, still there are no leaving particles but particles terminated by exceeding time, distance and integration limit, so I have increased them. But I cannot find why particles are not leaving the domain? 

March 18, 2013, 06:31 

#6 
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OJ
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Okay, I would pitch it in different way. We say that if we toss a coin probability of it being heads or tails is 50%. But this argument is valid only if we toss the coin enough times! If we toss it only four times and result is heads for 3 times, it is not that probability of heads is 75%, but that we have not tossed coin enough times.
I was hinting at the fact that there has to be enough number of particles entering through the inlet so that some would be trapped and some would escape. If they aren't enough, then the result may not be ideal representation. How are you sure you have specified the appropriate number of particles in appropriate distribution (their spacing and position with respect to each other) at the inlet? If you aren't sure, doesn't it make sense to ensure this distribution as per the experimental data you have? If you can't determine the particle distribution and number, then try literature search to get some pointers that will guide you decide it. You can't expect the same results as experiment, without same/adequate boundary conditions. OJ 

March 18, 2013, 07:27 

#7 
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S.Bogoda
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Dear Oj,
Thanks a lot. I got your point. But I dont think it is going on my simulation. Please refer follows to get an idea on particles that I assigned. MATERIAL: Particles Material Group = Particle Solids Option = Pure Substance PROPERTIES: Option = General Material EQUATION OF STATE: Density = 500 [kg m^3] Molar Mass = 1.0 [kg kmol^1] Option = Value  FLUID: particles BOUNDARY CONDITIONS: MASS AND MOMENTUM: Normal Speed = 12 [m s^1] Option = Normal Speed END PARTICLE MASS FLOW RATE: Mass Flow Rate = 0.05 [g s^1] END PARTICLE POSITION: Option = Uniform Injection NUMBER OF POSITIONS: Number per Unit Time = 25 [s^1] Option = Direct Specification  FLUID DEFINITION: particles Material = Particles Option = Material Library MORPHOLOGY: Option = Dispersed Particle Transport Solid PARTICLE DIAMETER DISTRIBUTION: Maximum Diameter = 10.08 [micron] Mean Diameter = 3.78 [micron] Minimum Diameter = 0.647 [micron] Option = Normal in Diameter by Number Standard Deviation in Diameter = 2.44 [micron]  Could you pls tell me any thing wrong? I cannot find the mistake. Thank you. 

March 18, 2013, 09:59 

#8  
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OJ
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Quote:
Quote:
The answer to these questions may help you model your simulation more relevantly. From what I know, the separation efficiency curve slowly rises from smaller particles and gets flatter for larger particles with highest efficiency. Whilst, correct flow rate of particles and fluid is also relevant, since the efficiency decreases with higher flow rate. So your particle number rate and particle size distribution should greatly influence the efficiency. OJ 

March 18, 2013, 10:31 

#9 
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S.Bogoda
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NUMBER OF POSITIONS:
Number per Unit Time = 25 [s^1] This value is aasigned by me. As far as I understood, I can assign representative particles into any value? Am I wrong? PARTICLE DIAMETER DISTRIBUTION: Maximum Diameter = 10.08 [micron] Mean Diameter = 3.78 [micron] Minimum Diameter = 0.647 [micron] This normal distribution is relevent with particle sizes which I am going to use in experiments. Still I stuck on this and I would really greatful for advices. 

March 18, 2013, 10:54 

#10  
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OJ
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Quote:
Regarding distribution, you may want to check the standard deviation. The size distribution should greatly impact the results. If these basic things are right, then only you can move ahead and troubleshoot for other possibilities. OJ 

March 18, 2013, 13:54 

#11 
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S.Bogoda
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Dear OJ,
Thanks a lot for reply, but I have a doubt. My understanding in particle number rate is, Number rate= mass flow rate assigned/ mass of actual particle Here, if I find this for mean particle diameter (3.78um), number rate is 1.13E15/s. So if I assigned 25/s, then one particle position indicate 1.13E15/25 partilce. Am I wrong? Really appriciate your helps.. 

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