Comparative research on the performance of three impeller seal clearance structures of pumped-storage units based on improved entropy production theory

The narrow seal clearance between the rotor and stationary casing is a critical area affecting the performance of pump-turbines. However, systematic studies on the influence of different seal structures remain limited. To fill this research gap, three typical impeller crown seal structures (ladder, serrated, and adjustable types) were selected to investigate their effects on the energy loss characteristics, operating stability, and structural safety of a high-head pump-turbine systematically. Three-dimensional flow simulations, validated by experiments, were conducted to compare various performance indicators under typical operating conditions on turbine mode. The results showed that the ladder type exhibited the lowest energy dissipation but had poor stability. The large-scale vortices within its cavity acted as a strong excitation source, causing unstable flow-induced vibrations. These vortices adversely coupled with the main flow, worsening undesirable flow patterns in the impeller and vaneless space. In contrast, the adjustable type had the best stability and caused minimal disturbance to the main flow channel. However, it involved higher energy dissipation and experienced severe axial hydraulic thrust fluctuations, which may lead to unit lifting under unstable conditions. The serrated type exhibited balanced performance. It exhibited moderate energy dissipation while most effectively suppressing axial hydraulic thrust fluctuations, making it the safest option for pump-turbines. This study provides an important theoretical basis for the selection and optimal design of seal structures in pump-turbines.