Abstract: This study develops a new three-dimensional multi-scale full-passage model of a pump-turbine that incorporates its millimeter-level sealing clearances, exploring the influence mechanism of its upper clearance width on operational stability. This study conducts numerical simulations of the flows for the five different dimensionless clearance widths for 50%, 80%, and 100% load conditions, focusing on comparison of the characteristics of flow structures, pressure pulsation, and hydrodynamic performance. It was found that wider clearances intensify secondary flow leakage vortices on the pressure suction side of the runner's upper crown. This leads to increasing fluctuations in the radial force on the top cover and higher pressure-pulsating amplitudes, which may rise by up to 15% in the extreme conditions. However, wider clearances reduce the axial thrust of the flow, with a reduction of 0.8% observed for high-load conditions. Conversely, narrower clearances help suppress leakage vortices and stabilize the flow, but result in an increase in the axial thrust at an minimal increment though－less than 0.1% for the full load conditions. Clearance adjustment significantly affects performance for high-load conditions, whereas for medium- or low-load conditions, the strengthening of blade passage vortices tends to induce hydraulic instability. This study has elucidated the mechanism of correlation between clearance widths and dynamic loads, and given a recommendation for engineering application－a moderate reduction of the upper seal clearance in the 0-20% range for enhanced operational stability under the rated conditions. This would lay a basis for optimizing sealing design, balancing axial thrust, and controlling vibration.

Key words: pump-turbine, upper seal clearance, operational stability, pressure pulsation, axial hydraulic thrust, leakage vortex