Performance discontinuity of a shrouded centrifugal pump impeller: Part 2 - stabilizing geometrical modifications

The discontinuity observed in the performance characteristic of a shrouded centrifugal pump impeller is induced by a boundary layer separation in the centre part of the impeller passage acid is accompanied by an abrupt inception of backflow and prerotation at the impeller inlet. The near-shroud flow field at the throat region is blocked. The knowledge of the physical mechanism of the flow instability in the impeller passage allows numerical investigations of geometrical modifications with the goal to stabilize the characteristic and to avoid the associated flow field hysteresis. Three modified impellers with identical blade geometries and numbers as the reference impeller are numerically found to have a stable characteristic, The geometrical modifications consist of an unshrouded impeller, an impeller with modified shroud and a shrouded impeller with a slit at the tip of the blades. The disappearance of the discontinuity and the hysteresis is validated by experiments.