Abstract To study the hydraulic instability of a pumpturbine at the pump mode under different working conditions, the threedimensional, unsteady flow in the whole passage of the pumpturbine was conducted by using computational fluid dynamics softwareFluent under the optimal, low and high flow rate conditions. The pressure fluctuations were monitored respectively at the inlet of spiral casing, in the interface of runner and guide vanes, and in the space between head cover and runner, as well as in draft tube. The results show that a larger pressure fluctuation can be found in the interface of runner and guide vanes and in the space between the runner and head cover, while the pressure fluctuation is lower at the inlet of the spiral casing and in the draft tube. The pressure fluctuation is the smallest under the optimal condition; the larger the working condition deviates from the optimal one, the more considerable the fluctuation is. The dominant frequency of pressure fluctuation depends on the locations monitored. The dominant frequencies are integer multiples of the blade passing frequency and the runner rotational frequency in the interface of runner and guide vanes and in the space between runner and head cover, respectively. However, the dominant frequencies of the pressure fluctuation at the inlet of the spiral casing and in the draft tube are mainly related to lowfrequency components.
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