DDPM parcel size
 

it is written in a best practice document for DDPM modelling that to avoid convergence issues parcel size must be maintained sufficiently smaller than cell size in the mesh. what does this sufficiently smaller mean in your opinion? should the parcel volume be 1/10 of cell volume? 1/20? 1/100. or what?

Parcel Size
 

1/10th in terms of volume is usually taken as upper limit. This directly comes from the assumption in DPM that particle volume fraction should not be more than 10%. Though DDPM is valid up to packing limit, the parcel size should always be smaller than cell size other it will be difficult for Fluent to identify the cell that currently contains the particle. Except VOF, all multiphase models require length scale to be smaller than cell size.