Increasing grid does not converge
 

Hi,I have a problem which is about two-phase flow,solid-liquid (3D-model). I used Discrete Particle Modle(DPM)in my simulation because I want to find out a sigle particle trajectory comparing with lab-results. First I have to find out an appropriate number of cells,I incresed number of grid from GAMBIT and send to Fluent. I found that Number of grid about 400,000 converged but number of 600,000 and 800,000 don't converge.

Could anyone help me? There is another ways to find out an appropriate number of cells. And the size of gird must be larger than particle size or not(4mm). Thank you very much