Study on the Internal Flow Field in the Tip Clearance of a Kaplan Turbine

As a key type of hydro-turbine unit in China, the Kaplan turbine plays a significant role in the national energy portfolio. Nevertheless, being a rotating machine, it unavoidably leaves a tip clearance between the runner blade tip and the chamber wall, which strongly disturbs the internal flow field. Based on experimental data, this study conducts numerical simulations to investigate how the blade-tip clearance affects the runner flow under various guide-vane openings and operating conditions. The results reveal that the leakage flow markedly modifies both the pressure and velocity distributions on the blades; however, its influence is weakest at the on-cam (combined) operating point. Frequency analysis shows that the pressure pulsation in the clearance region is dominated by 1 fn and 2 fn components, indicating that leakage vortices and runner rotation are the main sources of blade-tip pressure fluctuations. The findings provide valuable guidance for improving the economic efficiency, safety, and stable operation of Kaplan turbine units.