Abstract: This study addresses unclear energy dissipation mechanisms in complicated flows in the runner labyrinth ring section of a pump-turbine under the runaway condition, and examines how these flows affect the unit’s overall hydraulic performance. We compare three seal types of crown labyrinth rings: ladder, serrated, and inclined-tooth, using a combined method of the high-resolution three-dimensional (3D) computational fluid dynamics (CFD) and the entropy production theory. The results indicate that of the three types, the inclined-tooth seal, through inducing a circumferential swirl, manifests the highest total entropy production and superior sealing performance. The serrated seal suppresses the leakage flows effectively and reduces the axial hydraulic thrust significantly, whereas the inclined-tooth seal tends to increase radial forces and cause pronounced fluctuations in the axial thrust. The performance of the ladder seal falls between the two. We have observed the labyrinth structure indirectly affects entropy production and vortex development in the runner and guide vane sections. The design of the upper crown labyrinth seal should balance local dissipation and overall stability－ since the serrated type is suitable for controlling axial thrust, while the inclined-tooth type is ideal for applications requiring high sealing performance. This study lays a theoretical basis for optimizing the labyrinth seal design in pump-turbine runners.

Key words: pump-turbine, reversible runner, runner labyrinth seal clearance, CFD simulation, entropy production theory