Study on pressure fluctuation and rotating stall characteristics in the vaneless space of a pump-turbine in pump mode

With the rapid development of high-head, large-capacity and high-speed pump-turbines, the compressed vaneless space and frequent operating condition transitions exacerbate the instability of internal flow. Therefore, the pressure fluctuation characteristics in the vaneless space and rotating stall characteristics of a pump-turbine in pump mode were analyzed systematically through a combination of experimental and numerical methods. The entropy production theory was introduced to determine the energy loss characteristics of the pump-turbine with the whole passage. The results show that the pressure fluctuation amplitude in the vaneless space was highest and its maximum value was 4.4 times, 5.7 times and 7.5 times greater than that in the spiral casing inlet, draft tube cone and draft tube elbow, respectively. The characteristic frequencies in the vaneless space were mainly blade passing frequency and its harmonics caused by rotor-stator interaction, and low-frequency components caused by rotating stall. Particularly, rotating stall primarily occurred at the blade trailing edge of the guide vane at approximately 0.59D2 from the runner center, and displayed obvious vortex structures such as streamwise vortices, horseshoe vortices and spanwise vortices. Furthermore, the optimal operating condition OP1 exhibited the best internal flow state and the smallest distribution range of high entropy production rate (EPR), resulting in the lowest level of pressure fluctuations in the vaneless space. Under the conditions of OP2 and OP3, significant flow separations occurred on the top cover side of the guide-vane region, accompanied by a large distribution range of high EPR, leading to increased pressure fluctuations. Under the conditions of OP4 and OP5, the internal flow became highly disordered and a large distribution range of high EPR occurred in the guide-vane region, runner and vaneless space, resulting in further increases in pressure fluctuations.