Revealing the pressure pulsations that can cause water column separation in pump-turbine

Abstract

Water column separation (WCS) can lead to a lifting-up of the rotating parts of pump-turbine and even a catastrophe in pumped-storage hydropower stations. However, the fundamental question, which pressure pulsations can trigger WCS, has not been answered. To answer this problem, we quantitatively analyzed the responses of a cavitation bubble in compressible unbounded water and cavitation cavities in a prototype pump-turbine to different pulsating pressures, using the Keller-Miksis equation and three-dimensional computational fluid dynamics. The findings reveal that a single cavitation nucleus oscillates around its initial radius when the lowest pulsating pressure is higher than the saturated vapor pressure (pv); as numerous cavitation nuclei grow to the maximum radius, they will coalesce a cavitation cavity when the lowest pulsating pressure drops to the pv and maintains 0.5Tf; the cavitation cavity can develop into a WCS when the pulsating pressure continue to maintain at the pv for about 1.0TRmax. Specifically, the lowest pulsating pressure dropping to the pv is a crucial prerequisite for WCS, and the pressure pulsations with frequencies below 0.83 Hz can make the cavitation cavity becoming a WCS in the prototype pump-turbine. This finding provides the theoretical base for the revision of the WCS criterion for pump-turbines.