Study on the influence of air-injection on cavitation characteristics of pump-turbine under turbine condition

Abstract

The occurrence of cavitation not only reduces the hydraulic performance of the unit but also generates vibrations and noise. In severe cases, cavitation can cause erosion damage to the flow components, significantly shortening the operational lifespan of the unit. Previous studies on air injection in pump-turbine experiments have mainly focused on vibration reduction or improvements in various unstable flow conditions, with limited research on air injection for Cavitation improvement. In this study, short air injection pipe were installed at the adjustable guide vane region and the main shaft center to inject air into the fluid domain. The effects of different air injection locations and concentrations on cavitation improvement in key flow components were analyzed. The results show that at cavitation coefficient $\sigma =0.176$ (mild cavitation) and an air injection volume fraction (IAVF) of 4 % in the adjustable guide vane region, the cavitation bubbles attached to the turbine blades and the runner cone surface were significantly reduced. At cavitation coefficient $\sigma =0.02$ (severe cavitation) and an IAVF of 6 % in the adjustable guide vane region, the maximum pressure values at the chord line of the blade and the draft tube region increased by 0.9 kPa and 1.8 kPa, respectively. When the IAVF at the main shaft center was 5 %, the maximum pressure in the draft tube region increased by 0.8 kPa. Both air injection schemes significantly reduced the cavitation volume in the draft tube region. It is evident that the increase in pressure reduced the extent of cavitation, with air injection at the main shaft center showing a more significant effect on cavitation improvement in the draft tube region. With the increase of IAVF (Injected Air Volume Fraction), the efficiency of the pump-turbine gradually rises. However, the injected air introduces additional turbulent kinetic energy, resulting in energy loss. The study reveals that the optimal aeration rate is IAVF = 6 % for air injection in the guide vane region, while IAVF ≈ 3.3 % achieves optimal performance for air injection at the main shaft center.