Investigation of internal flow characteristics by a Thoma number in the turbine mode of a Pump–Turbine model under high flow rate

The pump–turbine unit used in pumped-storage power generation plays a role in backing up and adjusting the power system in a flexible power generation facility, providing stability in response to renewable energy power grids with intermittent power generation. Owing to changes in the operating conditions of flexible power generation facilities, pump–turbine units have been operated continuously in off-design conditions. At the high flow rates in off-design conditions, undesirable flow with vortex rope is caused in the draft tube, which leads to operating system instability. Meanwhile, when operating at high flow rates in the turbine mode of the pump–turbine unit, the Thoma number must be considered for relatively stable operations due to cavitation phenomena. Thus, to expand stable operating ranges through the understanding of high flow rates, unsteady internal flow and pressure phenomena were investigated depending on the Thoma number in the turbine mode of the pump–turbine model through analyses with laboratory-scale pump–turbine model experiments and unsteady-state numerical analyses via systematic quantitative and qualitative comparisons. There was no difference in flow characteristics at the runner and draft tube areas based on the Thoma number at the best efficiency point. Nevertheless, torch-shaped visible vortex ropes developed and differences in flow characteristics at the draft tube appeared based on Thoma number under high flow rates.