Rob
Ansys Employee

Volume injection is cell centre for every (most - look at the options) in the selected zone. It's for the fluid zones only, I have no idea what will happen if you inject particles into a solid: I also wonder if anyone thought to test it! 

With Eulerian Granular you'll see concentration contours. DDPM shows tracks (as it's using DPM formulations plus some extra bits). Ansys Rocky DEM shows particle positions (not tracks - it's transient) but models all contacts, so particle count becomes the limiter relative to compute. 

If you're using commercial licences by all means come on here, but please contact the local team. We can offer far more advice in private. 

Ansys Rocky can be downloaded from the Portal, and you'd then install once the Ansys software is ready. I tend to run Fluent from Rocky when I want coupled systems, and don't use Workbench. You will need a licence, so another reason to contact the local team.