Aeroelastic Instability in Labyrinth Seals

Labyrinth seals and associated structural elements have been noted to sustain fatigue failure in high powered, high speed rotating machinery under circumstances that preclude blaming standing wave resonances or “stick-slip” excitation from rotor/stator rubs. Alford [2] has hypothesized that such failures may be caused by self-excited aeroelastic vibration of the seal. A model is defined in which a pressure perturbation in the seal internal volume between the high and low pressure teeth can cause an elastic rotation of the seal rotor or stator element about a virtual pivot point located on the high or low pressure side of the seal. This rotation, in turn, causes a nonuniform opening and closing of the high and low pressure clearances which can give rise to the hypothesized pressure perturbation. A stability parameter is then derived in terms of the high pressure and low pressure tooth clearances, the supply pressure to the seal, the seal width and height, and two parameters indicative of the elastic properties of the seal—its stiffness to radial motion and the location of its virtual pivot point. The stability criterion is applied to a pair of actual labyrinth seal rotors which differ from each other only slightly and are known, from actual practice, to straddle the stability boundary. That is, the larger diameter unit is known to be unstable and the smaller diameter unit known to be stable. On the basis of an instability incident simulated on component test, the stiffness used in evaluating the stability parameter is modified to be representative of the least stiff vibratory mode, and the stability parameter is shown to give reasonable indication of the stability problem in labyrinth seals.