Effect of Blade Tip Configuration on Tip Clearance Loss of a Centrifugal Impeller

According to the theory presented by the authors, the tip clearance loss of an unshrouded centrifugal impeller is attributed to two parts; one is the drag due to the leakage flow through the blade tip clearance and the other is the pressure loss to support the fluid in the thin annular clearance space between the shroud and the blade tip against the pressure gradient in the meridional plane without blades. The former is proportional to the leakage flow or the contraction coefficient a of leakage flow. The authors have conducted performance tests of an impeller with sixteen backward leaning blades in three configurations of the blade tip: round edge, sharp square edge and edge with an end plate. The experimental tip clearance effects can be predicted by the theory assuming α=0.91, 0.73 and 0.53, respectively, and if the tip clearance ratio at the impeller exit is 0.1, the impeller efficiency is improved by 1.5 percent by reducing a from 0.91 to 0.53.