Numerical investigation on fluid force acting on shaft seals in turbo-machines (Effect of seal length and gap)

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 shaft labyrinth seal of a steam turbine. The effects of different seal gap (0.42～0.85 mm) and seal length (47.6～357 mm) were systematically investigated. The predicted stiffness coefficients increased with increasing seal length as expected, but short seals (<100 mm) indicated a stronger dependence on seal length. Through the investigation of flow fields, it was found that this could be attributed to two causes: first the decay of circumferential velocity component along seal flow direction, and second lower fluid force in the cavities adjacent to seal entrance and outlet regions which greatly affects the fluid force of short seals. Stiffness coefficients of short seals indicated strong dependence on seal gap and increased inversely with decreasing gap while stiffness coefficients of long seals depend less on the gap. The seal fluid force increases for small gap seals, but the circumferential velocity decays faster, and this could offset the increase in the fluid force. This effect appears more pronounced in the long seal.

Keywords

• steam turbine
• vibration
• labyrinth seal
• dynamic characteristics
• cfd