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

Abstract

To design a high-performance mixed-flow pump, performance-curve instability must be controlled and suppressed. To do this, the essential flow physics that leads to the performance-curve instability must be clarified in addition to an accurate prediction of the performance curve. We analyzed internal flows of a test mixed-flow pump, with performance-curve instability by using Large Eddy Simulation (LES). In particular, we investigated in detail unsteady internal flows of individual blade passages near the head-drop point (60% ~ 61% of the designed flow rate). As a result, we clarified the following. (a) 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 opposite direction of the impeller rotation for the whole impeller while the impeller makes approximately 20 revolutions. (b) The upstream flow of the stalled blade passage deviates to the downward neighboring blade passage, resulting in an increase in the load of the neighboring blade near its trailing edge. When the load of the blade increases, the separation of flow occurs from the trailing edge of the blade, and the separation point gradually moves toward the leading edge of the blade. The neighboring blade passage is then stalled similarly because the angle of attack becomes large. (c) When the flow rate in a stalled blade passage minimizes, the passage starts to recover from the stall, whereas the pressure gradient in the streamwise direction decreases and the flow rate in the passage increases.