Evolutions of flow patterns and pressure fluctuations in a prototype pump-turbine during the runaway transient process after pump-trip

Abstract The runaway after pump-trip is a fast transient process, during which unit vibrations, water-hammer fluctuations, pressure pulsation changes and flow pattern transitions happen violently, endangering pump-turbine units of pumped-storage power stations. In this paper, based on validated CFD data from 1D-3D coupled simulation, we describe the evolutions of flow patterns and pressure fluctuations in a prototype pump-turbine during this working point fast sliding process, and try to reveal the influence mechanism of flow pattern transitions on pressure fluctuation changes. The results show that the general evolutions of flow patterns are dominated by the changes of the runner velocity triangles, which are controlled by the changes of the discharge and rotational speed. Inhomogeneous local flow also contributes to abundant local vortex structures, such as rotating-stall vortices in the vane and runner regions, skirt and helical vortices in the draft-tube, separation vortices in the blade channels, and the impinging jets on the blade surfaces. These unstable vortices appear, change, and vanish in a fast transitional and uneven manner, leading to great differences in pressure and runner force fluctuations in different working regions.