several questions about fluent ddpm+dem
 

I'm trying to follow a tutorial to simulate a bubbling fluidized bed using ddpm+dem. In the tutorial, particles are injected with a injection file. The diameter of particle is 0.75mm, and density is 1550kg/m^3, parcel diameter is 3.7mm. Particles can be fluidized with gas velocity of 0.5m/s. However, when I change the particle diameter to 3.5mm, density to 2000kg/m^3 and parcel diameter to ~3.4mm, gas velocity to 2m/s, particles is hard to be fluidized and the flow pattern deviated from results obtained by TFM with same operating condition.
Here, I have several questions: 1. How to define a proper parcel diameter or other parameters in order to obtain a correct fluidizing parttern? From my inspection, the fluidizing characteristic is influenced by both particle diameter and parcel diameter.
2. Is it necessary to enable granular in the discrete panel? Now that particle collision is solved directly by soft sphere method, why should I take effort to solve granular equation.