I always found warning#064 in the info file saying "Fixed mass flow rate outlet in compressible fluid 1 is allowed only if flow is subsonic in the whole stream" Does any body know the cause of it and how to fix that ?? Velocity vector is flowing outwards at the inlet which is set to be Pressure =0. Does that caused by the warning #064??

The reason is because of the physics of compressible flows. If the flow is chocked (which means that the flow speed is somewhere supersonic, hence the local Mach number is > 1), the mass flow rate cannot exceed a certain value (see also the theory on the critical section for Laval nozzles). As a matter of consequence, the statement "Fixed mass flow rate outlet in compressible fluid 1 is allowed only if flow is subsonic in the whole stream" makes perfectly sense and I suggest you should make use of a different boundary condition combination.

Hope this helps...