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Fluent works on an angle of Pi or 2Pi (it's in the manual and I can never remember) rather than 5 degrees in 2d axi-symmetric: that's CFX.
If you release a parcel from each facet (surface injection) you'll get equally weighted parcels which will mess with the DPM concentration. Partially because of the injection and partially because of the way Fluent works out DPM Concentration. The value is defined in the documentation. Using "scale by surface area" may even the injection out but may still give an uneven concentration due to the way the cell volume is calculated. Momentum transfer etc is correct, it's the Concentration that may be a bit misleading.
Going to 3d means you can have an (fairly) uniform facet size on the inlet which should mean the cell sizes are more uniform and therefore the concentration AND mass tie up. Using 30 degrees means you can have a decent cell shape at the axis and a sensible cell count.

I think we're both on the right track. DPM Concentration is an awkward one as it's cell volume based, so the result may be misleading.