Initial Moment Values for QMOM

jayorayo · July 9, 2018, 10:34

Hi,

I'm running a model of a bioreactor and have been using the Discrete method for tracking the bubble sizes and numbers within the vessel. There is a wide range of sizes (and therefore bins) so I want to switch to the QMOM which should be a more efficient calculation.

I was wondering how to calculate the initial moment values that are applied to the inlet of the model? I understand that the 0th moment is the total number of bubbles per m3 and 1st moment = total length/m3, 2nd = surface area/m3 etc but how are these calculate in the first place. Based on the literature around the subject the values of these moments are based on the form of the bubble (or particle) size distribution.

I've worked through the bioreactor tutorial from Ansys and they've supplied initial values for moments 0-5 at the boundary but don't give an indication as to where these originated. Any tips or insight would be much appreciated.

Thanks!!

Amir.11 · February 28, 2019, 05:10

Hi,

It's been a while but I was wondering if you still work on this issue and could help me out on definition of the initial moment values.

I've been told that the first and second moments are the mean and the variance but I don't know in this case whether the zero moment has to be normalized, or it should always be equal to the total number of particles.

So it would be great if you could share what you applied to initial moments?
Thanks
Amir

hfalleiro · December 31, 2019, 07:19

First you need to know the amount of gas flow inlet to the system. Based on the bubble density, and expected bubble size need to calculate the number of bubbles. once you get the number of bubbles the moments can be calculated as follows
mj = N x L^j
where j is 0, 1, 2,3 ....
L is bubble size in m
N is the number of bubbles based on the inlet flowrate

July 9, 2018, 10:34	Initial Moment Values for QMOM	#1
jayorayo New Member James Join Date: Jul 2009 Posts: 8 Rep Power: 17	Hi, I'm running a model of a bioreactor and have been using the Discrete method for tracking the bubble sizes and numbers within the vessel. There is a wide range of sizes (and therefore bins) so I want to switch to the QMOM which should be a more efficient calculation. I was wondering how to calculate the initial moment values that are applied to the inlet of the model? I understand that the 0th moment is the total number of bubbles per m3 and 1st moment = total length/m3, 2nd = surface area/m3 etc but how are these calculate in the first place. Based on the literature around the subject the values of these moments are based on the form of the bubble (or particle) size distribution. I've worked through the bioreactor tutorial from Ansys and they've supplied initial values for moments 0-5 at the boundary but don't give an indication as to where these originated. Any tips or insight would be much appreciated. Thanks!!

February 28, 2019, 05:10		#2
Amir.11 New Member Join Date: Aug 2018 Posts: 16 Rep Power: 8	Hi, It's been a while but I was wondering if you still work on this issue and could help me out on definition of the initial moment values. I've been told that the first and second moments are the mean and the variance but I don't know in this case whether the zero moment has to be normalized, or it should always be equal to the total number of particles. So it would be great if you could share what you applied to initial moments? Thanks Amir Last edited by Amir.11; February 28, 2019 at 07:58.

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December 31, 2019, 07:19		#3
hfalleiro New Member Lister Join Date: Dec 2015 Posts: 4 Rep Power: 10	First you need to know the amount of gas flow inlet to the system. Based on the bubble density, and expected bubble size need to calculate the number of bubbles. once you get the number of bubbles the moments can be calculated as follows mj = N x L^j where j is 0, 1, 2,3 .... L is bubble size in m N is the number of bubbles based on the inlet flowrate