Experimental Study on Hysteresis during Cavitating Vortex Onset in Pump-as-Turbine Applications

Abstract

Centrifugal pumps operating in turbine mode (pump-as-turbine) have become a relatively common solution for small-scale hydropower plants and for energy recovery applications. One of the aspects of pump-as-turbine (PaT) operation is that the pump impeller is generally designed for exclusive operation in pump mode. Therefore, significant residual swirl exits the impeller in turbine mode and creates a dominant swirling motion in the discharge pipe. The magnitude of this swirling flow depends on the operating mode of the PaT. Due to inconvenient blade angles, the swirling motion with coherent vortices might be the dominant component even under the best efficiency operating conditions. The present study focuses on the experimental analysis of the cavitating vortex developed downstream in the discharge pipe. The effect of hysteresis on vortex dynamics is assessed with regard to increase and decrease of cavitation number. The primary concern is with the frequency and amplitude measured by the pressure sensor downstream of PaT. These results are correlated with the image analysis of the cavitating vortex shape recorded by the high-speed camera. The results indicated that the main source of hysteresis is caused by self-induced instabilities of the cavitating vortex. The description and understanding of this phenomenon is important for further correlation of CFD simulations with measured data and for reliable operation of PaTs.