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.