Yes, no problem, really. I obviously repsect your guidelines, so I'm trying to ask but not "going too far", let's say. If there is something at which you cannot give an answer, again, no problem, I fully understand :) .
To sum up what we have said until now:
Trying to adapt the mesh (in terms of equal surface for each cell in radial direction) and by applying the "scale flow rate by surface area" could give some imporvements to the result, but at the same time it could mess up other aspect of the simulation. Now i'm trying this, but just as a test.
The 3d "sector" method will reduce the dpm concentration problem but the number of cells will increase a lot.
Maybe, by removing the dependence of the dpm concentration on the area/cell volume we could solve the problem. So now I'll check if through a Custom Field Function I'm able to do so.
As soon as points 1. and 3. will be complete, I'll give you and update!