The volume fraction of the secondar phase should decrease if you a sink source for that. If you do not have that then I expect the mass to be conserved and the number not to change. If you want to eliminate any particles settling at the wall or theier deposition at the wall (and then change the PSD) then you need to write some UDF to model the sink for volume fraction, as well as to model the PSD Size Change you want to achieve.
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