Compressor performance influenced by distribution of tip clearance of centrifugal impeller

The tip-clearance effects of centrifugal impellers are usually related to the clearance/bladewidth ratio at the impeller exit, and the relationship has been experimentally examined. In some cases the clearance was changed by the axial movement of the shroud casing relative to the impeller, and as a result the radial clearance in the inducer was hardly changed. If the tip clearance is changed uniformly along the shroud, the effects of tip clearance must be larger than predicted based on those experiments. In the case of a centrifugal compressor with 6 : 1 pressure ratio, the decrement of impeller efficiency due to tip clearance is theoretically examined for five types of clearance-distribution along the shroud, and significant influences of the radial clerance in the inducer are demonstrated.