continuity convergence problems in stirred tank

nycera

Hi everyone. I'm trying to simulate a stirred tank with a rushton turbine (H=0.5m Diam=0.5m). I adapted the mesh many times, and now it's composed by almost 2M cells. I'm using steady MRF simulation with k-eps-rng turbolence model, with two zones, one cylindrical and moving, surrounding the turbine, and the outer one stationary. I'm using SIMPLE algorithm for pressure-velocity coupling, presto! for pressure discretization, and 2nd order upwind for the rest, with the default underelaxation values. I started off with 100 rpm and I'm trying to increase the rotational speed.

However I'm experiencing difficulties in continuity convergence. The residuals get stuck on about 5e-02 after 11000 iteration while velocity components k and epsilon drop well under 1e-03. The flow field looks very good though. Also the velocity magnitudes are consistent with the 100 rpm.

What should I do?. Help plz.