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-   -   Particle fate criteria (http://www.cfd-online.com/Forums/cfx/114750-particle-fate-criteria.html)

sakurabogoda March 17, 2013 02:19

Particle fate criteria
 
Dear All,

I have a doubt about particle fate criteria.

I have a particle sample (0-10um) 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 end-up with one of these fates but some particles should leave domain. Still nothing has leaved.

Many thanks for helps..

ghorrocks March 17, 2013 05:44

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

sakurabogoda March 18, 2013 00:28

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?

oj.bulmer March 18, 2013 05:14

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

sakurabogoda March 18, 2013 05:42

Dear OJ, Thanks a lot for reply.

The particles having the range of 0-10um, 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?

oj.bulmer March 18, 2013 06:31

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

sakurabogoda March 18, 2013 07:27

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.

oj.bulmer March 18, 2013 09:59

Quote:

Number per Unit Time = 25 [s^-1]
Alright, here is the question. How did you arrive at 25 particles/s value?

Quote:

PARTICLE DIAMETER DISTRIBUTION:
Maximum Diameter = 10.08 [micron]
Mean Diameter = 3.78 [micron]
Minimum Diameter = 0.647 [micron]
How did you arrive at this particle size distribution? (I am assuming here that the particle density is right).

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

sakurabogoda March 18, 2013 10:31

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.

oj.bulmer March 18, 2013 10:54

Quote:

This value is aasigned by me. As far as I understood, I can assign representative particles into any value? Am I wrong?
In your experiment, if you put a palm at your inlet for 1 second and if you observe you have collected 25 particles, then the boundary condition you assigned is right. Otherwise it is not. You can not assign "any" value and still expect your results be same as experimental results. This is the point I am getting at.

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

sakurabogoda March 18, 2013 13:54

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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