Energy loss characteristics of gas–liquid two-phase flow in pump-turbines under pumping mode

In gas–liquid two-phase flow within the pump mode of a pump-turbine, a trade-off was observed between the cavitation control and intensified energy dissipation due to gas-phase introduction. This study integrated experimental research with a Computational Fluid Dynamics-Population Balance Model (CFD-PBM) coupled model to investigate the influence mechanisms of air-injection concentration and flow conditions on energy loss. The reliability of the CFD-PBM numerical method for simulating gas–liquid two-phase flow in the pump mode of a pump-turbine was validated. Air-injection concentration was found to redistribute energy losses across flow passages. The primary energy dissipation zone, influenced by gas-phase accumulation, migrated from the double-row guide vanes to the runner passage as the air-injection concentration increased. Furthermore, due to gas-phase aggregation in low-pressure regions, energy dissipation on the low-pressure surface remained consistently higher than that on the high-pressure surface. Under constant air-injection conditions, flow rate magnitude affected entropy production migration. The runner passage persisted as the dominant energy dissipation zone across all flow rates, while the secondary dissipation zone shifted from the guide vane passage to the stay guide vane passage as the flow rate increased. Localized high entropy production regions induced by gas-phase accumulation in the runner blade channels were governed by bubble size, pressure gradient, liquid-phase backflow, and interphase interactions.