I think you can measure that by using some UDF.

The general idea for that UDF are:

1. Make the iso-surface for VOF cut-off value (about 0.95). You may adjust this value for better resolution of the droplet.

2. Search for cells that located inside the closed volume within the boundary of that iso-surface.

3. Sum up the volume of those cells for each closed volume (each droplet)

4. Report the number of closed volume (droplet number) and size of each volume (size distribution)

Although I have never tried that UDF but I think it is possible to measure that by DEFINE_ADJUST

The similar approach can be found here:

https://www.cfd-online.com/Forums/fluent-udf/152350-how-use-udf-set-volume-fraction-cells-next-wall.html

But your problem is more difficult because you need to make the children set of closed volume, then loop each volume. It is noted that you may try the code in serial first, since the UDF for parallel computing is more complex.

@ Son ich ngo, thank you so much for your answer. I have checked out the link you referred me to. However, I am new to Ansys fluent with just a little knowledge of programming. I will be very grateful if whatever modification that needs to be done on the UDF is done and sent to me as a screenshot with directives on how to implement. Thank you in advance

I'd normally not bother doing something like that in FLUENT, rather export data every few timesteps (under calculation activities), either as ASCII (to load & process in MATLAB) or some format for dedicated postprocessing software.

Anyhow, there seem to be few well-defined bubbles in the plot you are showing, is this representative of your problem?

@ Cees Haringa, Yes

Could you provide some more information on what you are trying to simulate and measure? Because in the current result, the dispersed phase seems to be very erratic at the top, with no clear bubble shapes to measure.

@ Cees Haringa, I am trying to simulate liquid film disintegration on a rotating wheel brush (bristles). I want to measure the resulting droplets size after disintegration

@ Onwuka Osinachi, You can find the direction through this example:

https://www.eureka.im/1249.html

It is very close to your problem. I did not try it but my idea is similar to that solution.