Unstable Performance of a Low Specific Speed Mixed Flow Pump (1st Report, a Diffuser Rotating Stall Behavior)

This paper treats the flow instabilities in a mixed flow pump with a vaned diffuser. Tested pump has a positive slope of the head-flow performance curve at about 65% flow rate of BEP (Best Efficiency Point). In order to investigate this phenomenon, pressure fluctuation measurements and dynamic PIV (Particle Image Velocimetry) measurements have been performed. As a result, it is found that unstable performance is caused by a rotating stall in the vaned diffuser. The flow from impeller exit is influenced on the large scaled backflow generated periodically from the vaned diffuser. Moreover, the backflow is induced by the strong vortex developed at the leading edge of diffuser vane. Based on PIV pictures, the relationship between the static pressure at the inlet of diffuser vane and the internal flow conditions is clarified.