What is courant number ?.what is the significance of courant number?Is it related to time step?

The courant number is the ratio of the timestep to the characteristic time of a control volume (i.e. the time required for fluid to travel through the control volume). Explicit solvers require that the maximum courant number is always less than 1 in order to remain numerically stable. CFX uses an implicit solver, so second order accuracy for transients can be acheived for courant numbers much larger than 1. With an implicit solver, the timestep is determined by the physics, not the numerics.

That said, for some applications it is necessary to keep the courant number low, but it is not strictly required for all transient runs.

Regards, Robin