Mitigating pump-turbine hump instabilities via high-pressure edge optimization: Vortex dynamics and pressure fluctuation control

To assess the influence of various high-pressure edge shapes on hydraulic performance and instability features in a pump-turbine under pumping conditions, three designs—concave, slanted, and convex—were analyzed using the SST k-ω turbulence model. Results revealed flow separation near the guide vane inlet and backflow near the runner outlet under low-flow hump conditions. Different schemes result in varying forms of outlet flow angles, which in turn generate vortices of different intensities in the guide vanes, leading to differences in hump characteristics. Notably, under small flow rates (0.63QBEP – 0.69QBEP), the convex design excelled, enhancing head and efficiency, boosting hump safety margins, reducing outlet flow angles, and optimizing loss distribution to mitigate excessive hydraulic losses in the hump zone. An evaluation of pressure fluctuations across the flow passage showed that, excluding the vaneless region, slanted and convex designs markedly decreased characteristic frequency amplitudes compared to the concave design. In the vaneless region, these designs also reduced and more evenly distributed amplitudes of mixed-frequency and characteristic frequency. Specifically, guide vane passage pressure fluctuations decreased by 14.6 % and 5.3 % for slanted and convex designs, respectively. These findings offer valuable insights for predicting hump characteristics and optimizing designs.