Unsteady blade-passage flows of a mixed-flow pump at performance-curve instability

Abstract

For designing a high-performance mixed-flow pump, control and suppression of performance-curve instability is crucial. For doing so, clarification of the essential flow physics that leads to the performance-curve instability is crucial in addition to an accurate prediction of the performance curve. We investigated internal flows of a mixed-flow pump, with performance-curve instability at round 60% of the designed flow rate, by using Large Eddy Simulation (LES). In the previous report, we clarified that drop of the Euler's head caused by stall near the tip of the impeller's blades is the major cause of the performance-curve instability. In the present report, we investigated in detail unsteady internal flows of individual blade passage near the head-drop point. As a result, we clarified the followings. (1) When the flow rate through a blade passage is low, the Euler's head is high for that particular passage, and this condition propagates in the direction of the impeller rotation for the whole impeller while the impeller makes approximately 20 revolutions. (2) Upstream flow of the stalled blade passage is deviated to the downward neighboring blade passage, resulting in increase in the load of the neighboring blade near its trailing edge. The neighboring blade passage will then be stalled. (3) When the flow rate in a stalled blade passage reaches the minimum, the passage starts to recover from stall while the pressure gradient in the streamwise direction will decrease.