Rob
Forum Moderator

DPM particles have mass, just not volume. The diameter is used to calculate drag and in the collision checks. 

Euler is a scalar based model which then includes terms to account for granular viscosity & temperature (friction & bounce) as well as a packing limit. 

The button you're looking for is "Help" on the multiphase panel, or the "?" top right of the Fluent main window.