In the context of DEM, the initial particle positions are adjusted to ensure that no particle overlap exists in order to prevent un-physical behavior. You can specify either the total flow rate or the total mass of particles injected and you can specify either the total number of parcels or the number of parcels per CFD cell. Note that for injections using DEM, the initial volume fraction is limited to 30%-40% depending on size distribution. Standard DPM injections do not have an initial volume fraction limitation.
Tip: If you are performing a DEM simulation and require an initial volume fraction higher than the 30%-40% guideline (for example, to initialize a packed bed), you can specify a larger volume for the initial seeding and run a preliminary calculation to allow the particles to settle.