3-D internal flow of a centrifugal compressor with complementary gas added in space between stages was studied to improve the uniformity of the flow at the inlet of the next stage so that the performance of the stages thereafter and thus the efficiency of the whole unit were not affected adversely. Blade profiles

blade numbers

cord of the blade

and the straight line extension of the trailing edge were simulated numerically for the return channel blades in an attempt to avoid flow separation. The twist blades were designed and the meridian flow path was modified to reach a uniform flow field at the end of the mixing of the complementary gas with the main flow. The variation of the flow angles at the inlet of the next stage was reduced by nearly 20 degrees. Numerical results indicate that the final structure design can effectively solve the problem of the next stage inlet distortion.