Numerical Investigation on Fluid Force Acting on Rotor Blade Shroud Labyrinth Seals

Abstract

Labyrinth seals have the potential to cause rotordynamic instability induced by the fluid force of seal flows. We conducted a computational fluid dynamics (CFD) study to investigate the rotordynamic characteristics of the shroud labyrinth seal of a high-pressure steam turbine. We constructed a CFD model consisting of a 1-stage stator/rotor cascade and a labyrinth seal over the rotor shroud to properly evaluate the seal force of an actual steam turbine and numerically investigated the effects of different seal gaps (0.2-0.8mm) and fin configurations (stator fin/rotor fin). The predicted stiffness coefficients increased inversely with decreasing seal gaps up to a gap of 0.4mm, but a seal gap smaller than 0.4 mm resulted in a slight increase in stiffness coefficients. Through investigation of flow fields, it was found that this could be attributed to the difference in the distribution of circumferential velocity component inside seals and the corresponding fluid force distribution. A comparison between different fin configurations showed that the stator fin indicated a higher stiffness coefficient, but the rotor fin became unstable at a higher whirl frequency due to its lower damping characteristic.