High-amplitude pressure pulsations induced by complex inter-blade flow during load rejection of ultrahigh-head prototype pump turbines

The flow pattern evolution is particularly complicated in the low specific speed impeller of ultrahigh-head pump turbines. To reveal the mechanism of pressure pulsation induced by the inter-blade flow, the load rejection process of an ultrahigh-head prototype pump turbine was numerically calculated using one- and three-dimensional coupled compressible flow simulation methods. The mechanism of pressure pulsation was analyzed using time-frequency analysis and visualization method of flow field. The investigation captured two high-amplitude low-frequency components of the pressure pulsations during load rejection of the ultrahigh-head prototype pump turbine. The investigation suggested that the first frequency components of the pressure pulsations were unique to the ultrahigh-head pump turbines. They were approximately one- to sevenfold impeller rotation frequency, which are caused by inter-blade separation vortices in the impeller. Other low-frequency components of the pressure pulsations near the first speed-no-load condition were induced by the unstable transitions of the flow separation and backflow vortices close to the high-pressure entrance of the impeller. This study first found that unstable transitions of the flow patterns in the impeller of ultrahigh-head prototype pump turbines could cause high-amplitude pressure pulsations. However, stable flow-vortex structures cannot cause apparent pressure pulsations. The findings of this investigation provide a theoretical basis for suppressing pressure pulsations and controlling complex flows in ultrahigh-head prototype pump turbines.