# Particle diameter calculation

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 February 20, 2019, 20:22 Particle diameter calculation #1 New Member   Mia Join Date: Apr 2013 Posts: 3 Rep Power: 13 Hi I'm trying to simulate the evaporation phenomenon of a multi-component single droplet(urea-water solution) over time. The droplet is at the center of the domain, surrounding with hot and dry air. (droplet and the air are stationery.) To simulate the single droplet, I set the droplet as Particle Transport Fluid. And during the simulation, only one particle is in the domain by injecting a mass corresponding to the diameter of the droplet in the first time step. So I think Mean Particle Diameter and Total Particle Mass are the actual values of the droplet. However, I found something weird. The Mean Particle Diameter changes depending on when I stop the simulation. For example, when the time duration is 0.1[s] and 0.02[s], the Mean Particle Diameter at 0.02[s] are diffeerent. So I wonder what I am missing. Is it right to find Mean Particle Diameter of Res particle track in CFD-Post to know the diameter of the particle? Thanks in advance.

 February 20, 2019, 20:42 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,729 Rep Power: 143 I think you are saying that if you use 0.1[s] and 0.2[s] time step size you get different results. If this is what you are saying, it means that your simulation is using too large a time step size and you need to do a time step size sensitivity analysis to determine what time step size you need for accurate results. To do a time step sensitivity analysis: Try a few time step sizes, maybe 0.1[s] and 0.05[s]. If these simulations give the same results within a tolerance you are happy with then your time step size is OK. If not, then halve the time step size again and repeat. Keep halving the time step size until the results do converge to an accuracy you are happy with. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 February 20, 2019, 20:55 #3 New Member   Mia Join Date: Apr 2013 Posts: 3 Rep Power: 13 Thank you for your answer, Glenn. The difference between the two simulations is only the time duration. Same time step, and same condition. Even I didn't change the Time Duration, just stopped the simulation before the simulation ends. I am monitoring Total Particle Mass in Transient Particle Diagnostics. It is the same in both simulation. Mean Particle Diameter is differnet in CFD-Post. CEL is "minVal(droplet.Mean particel Diameter@Res PT for droplet)" What do you think is the problem?

February 21, 2019, 02:30
#4
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Frank Weise
Join Date: Mar 2009
Location: Germany
Posts: 55
Rep Power: 17
Quote:
 Originally Posted by hebe2u Hi For example, when the time duration is 0.1[s] and 0.02[s], the Mean Particle Diameter at 0.02[s] are diffeerent.
Does the drop shrink over time? Then the drop just evaporates.

 February 21, 2019, 04:43 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,729 Rep Power: 143 As Frank said - isn't that a transient effect? Also be aware that your function minVal(droplet.Mean particel Diameter@Res PT for droplet) is going to be a noisy signal as you are taking the minimum value of a distribution. This is going to bounce around as the drops come and go. Better to take an average or integrated quantity where the variations in individual drops are smoothed out across a large population. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.