I had a look at DPM and I tried it also. Some points are not clear to me: 

  1. the surface injection of bubbles from inlet. Are the particles considered as inert? 
  2. Point properties: how to specify velocity and mass flow rate? Is for example drift flux model a way to estimate their values? 
  3.    steady and unsteady tracking is not well explained in the manual. When can I use the unsteady one?