Numerical comparisons of rotordynamic characteristics for three types of labyrinth gas seals with inlet preswirl

Abstract

Numerical results of frequency-dependent rotordynamic force coefficients and leakage flow rates are presented and compared with three types of labyrinth gas seals, which include a tooth-on-stator (TOS) labyrinth seal, a tooth-on-rotor (TOR) labyrinth seal, and an interlocking-tooth (INT) labyrinth seal. These three labyrinth seals represent the typical labyrinth seal designs used in rotating machinery as shaft seals to limit leakage and ensure a robust rotordynamic design. The three labyrinth seals have the identical rotor diameter, sealing clearance, tooth number, and profile. Using a proposed transient computational fluid dynamics (CFD) method based on the multi-frequency elliptical orbit whirling model, transient CFD solutions were conducted at rotational speed of 7000 r/min, inlet pressure of 6.9 bar, ambient out pressure, and three inlet preswirl ratios up to 1.0. The accuracy and availability of the present transient CFD method were demonstrated with the experiment data of frequency-dependent rotordynamic coefficients of the TOS labyrinth seal with different rotational speeds and inlet preswirl ratios. The rotordynamic coefficients were generally well-predicted by the present numerical method, with direct stiffness and direct damping modestly under predicted (∼67% and ∼34%, respectively). Numerical results compared leakage flow rates and rotordynamic coefficients for three labyrinth seals, while paying special attention to the cross-coupling stiffness, effective damping, crossover frequency, and swirl velocity development along the flow direction in seal. The INT and TOS labyrinth seals consistently leaks less than the TOR labyrinth seal across all preswirl ratios, respectively, by factors of 27% and 5%. The TOS labyrinth seal is relatively most stable, followed by the INT labyrinth seal and then the TOR labyrinth seal. The stability of labyrinth seals can be improved by reducing the swirl velocity at the seal entrance (installing anti-swirl devices) and in the seal cavity (applying “textured” or “pocket” stator surfaces).