Thanks for the reply, Rob. nYes, these terms are quite confusing. No of particles * volume of diam * density = mass for those particular diameter. Then we have a total mass for, and now can calculate mass fraction for each range.nBut in most literature the droplet size and count is given as pdf. so I was wondering if there is any way I can accommodate that directly. nIf i'm going to setup individual injection for each diameter.nI see this format for file injection in the manual n((x y z u v w diameter temperature mass-flow) name )nso if I write this line one time for particular diameter, then fluent will injected 1 diam at every timestep, right? Please correct me if I'm wrong. nFor example, if I have 21 no of particles for 50 um diameter, injected at origin with a velocity of 8.5 m/s, temp 307 K, mass flow rate of 1.02e-9 kg/s then my injection file will be n((0 0 0 8.5 0 0 50e-6 307 1.02e-9) fifty) ----> this will result in one 50 um diam per time step or anything else?nThanks a lot for your time.n