Flow excitation mechanisms of unbalanced impeller forces after pump power-trip of ultra-high head pump-turbines

To elucidate the dynamic mechanisms of unbalanced impellers in ultra-high head pump-turbines (PTs), this study employed a one- and three-dimensional coupled method to simulate the pump power-trip (PPT) process of an ultra-high head PT. The investigation revealed two novel pulsation frequency components, denoted asf_DV andf_INFT, associated with impeller forces. The pulsation intensities of these components were markedly higher than those of rotor-stator interaction frequency components in ultra-high head PTs. Notably, thef_DV components exhibited pulsations at 1–2 times the rated rotation frequency of the impeller, spanning the entire transition period. Meanwhile, thef_INFT components constituted a complex frequency band with various frequency values, primarily occurring near conditions (Q = 0,n = 0,M = 0, and dM/dt = 0). These two pulsation frequency components were predominantly linked to the unsteady evolution of dean vortices inside the volute and complex transitions of the flow pattern within the impeller, respectively. It is crucial to note that these unbalanced flow-induced impeller axial forces can elevate the risk of accidents where the rotor is subjected to significant upwind axial forces. These findings offer valuable insights into mitigating the risk of rotor lifting due to axial forces during PT events in ultra-high head PTs.