for compressible flow. can it lead to error that specify the inlet with vilocity inlet? i have tried a case,the residual of mass rise very fast.then disconvegence

For compressible flow a "Mass Flow Inlet" - boundary will give you better results. For the outlet boundary use "Pressure Outlet" , but you have to be careful to set proper values for the backflow values (only needed if reversed flow occurs at the outlet).

in my case,oulet is a mechanic fan. i only know the mass flow rate, not the pressure jump. how can i specify the out boundary condition thanks