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Old   February 20, 2019, 20:22
Default Particle diameter calculation
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Mia
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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.
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Old   February 20, 2019, 20:42
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Glenn Horrocks
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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.
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Old   February 20, 2019, 20:55
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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?
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Old   February 21, 2019, 02:30
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Quote:
Originally Posted by hebe2u View Post
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.
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Old   February 21, 2019, 04:43
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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.
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