High-amplitude pressure pulsations induced by complex inter-blade flow during load rejection of ultrahigh-head prototype pump turbines
Xiaolong Fu, Deyou Li, Jingwei Lv, Biao Yang, Hongjie Wang, Xianzhu Wei
Abstract
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.