Abstract
Fluid-induced forces generated by annular gas seals have aroused considerable attention owing to their substantial impact on the rotordynamic stability of turbomachinery. Positive preswirl at the seal entrance has been identified as the dominant factor in generating destabilizing forces. In this paper, a rotordynamic test rig was developed to measure the leakage and frequency-dependent dynamic coefficients of annular gas seals based on the mechanical impedance method. A tooth-on-stator labyrinth seal with a rotor diameter of 170.0 mm, a radial clearance of 0.3 mm, and a length-to-diameter ratio of 0.38 was tested under zero, positive, and negative preswirl conditions. Tests were conducted at three pressure ratios (PR = 0.20, 0.25, and 0.33) and three rotor speeds (ω = 3, 6, and 8.5 krpm) in an atmospheric backpressure environment. Test results indicate that both positive and negative inlet preswirl has little effect on the sealing capability of the labyrinth seal, whereas significantly influencing its rotordynamic performance. Compared to positive preswirl cases, the negative inlet preswirl produces negative cross-coupled stiffness and positive effective damping. The stabilizing forces are more pronounced at higher inlet pressure. The sensitivity of negative direct stiffness to variations in inlet pressure and rotor speed is also reduced under negative preswirl conditions, suggesting a smaller effect on rotor modes under variable operating conditions. Furthermore, a computational fluid dynamics analysis based on the frequency-sweep whirling orbit model is conducted for the test labyrinth seal to investigate the influence of inlet preswirl on local effective stiffness and damping distributions along seal axial positions. The accuracy and reliability of the present numerical method are demonstrated by comparing predictions to test data presented in this paper. Numerical results show that upstream cavities play a dominant role in generating effective stiffness and effective damping. The inlet preswirl has little effect on the local effective stiffness but determines the local effective damping, which is more pronounced in upstream cavities.
