Performance characteristics of a contra-rotating pump-turbine in turbine and pump modes under cavitating flow conditions

Recent studies have indicated the potential of a contra-rotating pump-turbine (CRPT) as a low-head design that enables pumped hydro storage in regions with flat topography. However, the effects of cavitation have scarcely been investigated for the CRPT. The current paper utilises computational fluid dynamics simulations to study a model scale CRPT subjected to cavitating flow conditions to determine how cavitation affects the machine’s operating performance in both pump and turbine modes. In total, eight operating conditions have been evaluated for each mode. The inlet flow rate is considered fixed at 0.27 m3/s, while the outlet pressure is gradually changed to induce cavitating conditions. The study demonstrates that the pump mode operation of the CRPT is more sensitive to cavitation compared to the turbine mode. The pump mode operation shows a steady decline in efficiency with decreasing inlet pressure, whereas in turbine mode the efficiency settles at a lower level. The 3% head drop occurs at a Thoma number of 1.0 in pump mode and at 0.6 in turbine mode. At the 3% head drop, a large cavitating region is already present at the runner blades’ suction side of the runner closest to the lower reservoir in both modes. The large cavitating region causes the flow to separate from the runner blade surfaces, which explains the reduced operating performance. To ensure an almost cavitating-free operating condition and unaffected performance, the Thoma number needs to be above 1.0 in turbine mode and at least 1.5 in pump mode. A frequency analysis reveals that the presence of cavitation affects the dominant pressure pulsations in the system. A dynamic mode decomposition analysis is carried out, demonstrating that the non-trivial pressure pulsations are connected to the support struts.