Hi Alireza,
Typically a convergence test is looking at the convergence of a value relative to the Mesh size. However, you must keep in mind that when you change the grid size you must also change the time step size in order to ensure stability. The condition for this is the Courant-Friedrichs-Levy condition and is dt <= (v*sqrt(dx^-2+dy^-2+dz^-2))^-1 for 3D and dt <= (v*sqrt(dx^-2+dz^-2))^-1 for 2D.
You can either manually calculate this or simply leave the dt and number of time steps checked to auto and the software will perform the calculation for you.
Scott