Secondary flow due to the tip clearance at the exit of centrifugal impellers.

The velocity distribution is measured at the exit of two different types of unshrouded centrifugal impellers under four different tip clearance conditions each, one with twenty radial blades and inducers and the other with sixteen backward-leaning blades. The secondary flow induced by the tip leakage is similar in both impellers, and the underturning near the blade tip due to the stationary shroud is reduced significantly with the increase of tip clearance. The local change of angular momentum due to the tip clearance is related to the flow rate ratio of leakage-flow and through-flow, and an almost proportional relationship between them is found. Despite a similar secondary flow in both impellers, the input power is hardly changed in the radial blades impeller and is decreased in the backward-leaning blades impeller by the increase of tip clearance.