Effect of frequency and design parameters on pocket damper seal performance

Pocket damper seals can produce large amounts of damping in turbomachines where the working fluid is compressible. Even with non-optimum design configurations, they have solved rotordynamic instability problems in high-pressure compressors. They do not rely on viscous effects, so they are ideal for oil-free applications. It is shown here that proper sizing of the seal pockets can maximize the damping. Significant parameters and variables have been identified, both experimentally and theoretically, that reflect the fundamental effects of the pocket damper seal to produce rotordynamic damping. Dimensionless analysis was performed, resulting in significant Pi groups involving damping, stiffness and leakage parameters. Of particular interest is a Helmholtz Pi group that shows 1) the benefit from optimizing the volume of the pockets around the seal, and 2) the frequency dependence of the seal. Dimensionless prediction equations are shown graphically. The graphs were generated by a computer code that uses a single control volume model. The code has been verified experimentally on several different seal test rigs with data taken at different vibration frequencies spaced an octave apart.