Facing an issue with the discrete population balance model in a multiphase flow simul
 

I am working on a multiphase flow using population balance to track the development of a dispersed secondary phase, in a cylindrical pipe.

The geometry is 2D. It's a rectangle.

The inlet is split into 2: a water inlet and an air inlet. The inlet velocities are such that the inlet volume fraction of air is 10%.

I use the Eulerian multiphase model, and the Discrete population balance model.

I set my minimum diameter as 8e-06 m, and my maximum diameter as 8e-03 m. I set the bin number to 30. I can then print the bin sizes, which I use in MATLAB to obtain the bin fractions as a normal distribution, as shown in Plot 1.

I initialized the air inlet with these bin fractions.

The pipe is 2-m long, and so I defined a line every 0.1 m starting at the inlet and ending at the outlet. At each line I defined a report file for the number densities of each bins.

After running the simulation and processing the data, having the number densities of each bin at each line, I plot Number density vs Bin size, in Plot 2, at the whole inlet (the line at x=0).

Plot 3 shows Number densities vs Bin sizes at the air inlet only.

My issue is that the number densities, which are proportional to the bin fractions, should supposedly also show a normal distribution, since this is how I initialized them.

Can anyone knowledgeable on the matter provide me with any insights please?