The complex internal flow field of steam ejector was numerically simulated with commercial software FLUENT. The pressure and velocity variation of internal fluid in the flowing process were studied and analyzed. The influences of the mixing steam pressure and the outlet diameter of primary nozzle on the ejector performance of the steam ejector were discussed in a great length. It is found from the results that the entrainment ratio of the steam ejector remains constant first with the back pressure to its critical pressure and then decreases

nevertheless

the entrainment ratio initially increases sharply with the primary nozzle outlet diameter and then decreases after the outlet diameter exceeds a certain value. Therefore there exists a critical outlet pressure for the steam ejector that determines its operation states. The outlet diameter of main nozzle has an optimal range

in which the ejector obtains the best performance

and the dissipation loss of the shock wave is relatively small.