If you are looking on dispersion, why did you enable growth in the PBM? I don't think that's relevant in this context. I suggest starting with constant-sized particles without using the PBM to see if this resolves the issue. This approach will help isolate whether the problem lies with the PBM model or not.
Additionally, the volume fraction may not accurately represent the mass transfer, if any occurs. The granular volume fraction could decrease due to dispersion rather than mass transfer. It would be worthwhile to generate a report on the total mass of each phase to determine whether the granular mass decreases significantly or not.
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