Experimental and theoretical rotordynamic coefficients and leakage of straight smooth annular gas seals

by Kerr, Bradley Gray

Results are presented for experimental and theoretical rotordynamic coefficients

and leakage of straight smooth annular gas seals. Experimental rotordynamic

coefficients were measured and trends in changes of rotordynamic coefficients with

operating variables such as rotor speed, back-pressure, fluid preswirl, and seal clearance

are analyzed.

Experimental results show that cross-coupled stiffness coefficients are highly

influenced by fluid preswirl and only moderately influenced by other operating

parameters, whereas direct damping is nearly unaffected by changes in operating

parameters. Effective damping, a good indicator of stability, is highly affected by fluid

preswirl. Although rotordynamic coefficients of straight smooth annular gas seals are

assumed to be frequency independent, experimental results suggest a frequency

dependent nature at high back-pressures and high excitation frequencies.

Experimental results for rotordynamic coefficients and leakage are compared

with theoretical predictions of ISOTSEAL, an isothermal-flow, two-control-volume,

bulk-flow rotordynamic analysis program. All rotordynamic coefficients are underpredicted.

Direct stiffness is poorly predicted while cross-coupled stiffness and direct

damping are predicted reasonably well. Leakage is also consistently under-predicted.

Theory predicts a slight frequency dependent nature for a limited number of test

configurations.