The flow phenomena around the positive slope region of the head-flow characteristic were investigated experimentally on a mixed-flow pump impeller at various tip clearances for both shrouded and unshrouded cases. A positively-sloped head-flow characteristic (abrupt decrease in pressure head) was caused by the onset of extensive flow separation in the impeller at the casing-suction surface corner. The corner separation in unshrouded cases appeared at much lower flow rate than the shrouded case due to the favorable effect of the tip leakage flow which displaced the wake region away from the corner. The interaction between the tip leakage flows and secondary flows and the formation of the wake regions in shrouded and unshrouded cases were explained based on experimental observation and computations by the Dawes’ 3-D Navier-Stokes code. In the shrouded case, the flow rate, at which an abrupt decrease in pressure head appeared, was lowered substantially by introducing a leakage flow through a slit made between the shroud and the blade tip. Inlet recirculation was triggered by the corner separation and developed more gradually for larger tip clearances. Both the increased loss, due to the extensive flow separation, and the decreased Euler’s head, due to the abrupt change in flow pattern caused by the inlet recirculation, were responsible for the generation of positively-sloped head-flow characteristic in the unshrouded case when the tip clearance was small, while the increased loss was the primary factor in the shrouded case.

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