Theory Versus Experiment for the Rotordynamic Impedances of Two Hole-Pattern-Stator Gas Annular Seals

Measured rotordynamic impedances are presented for two hole-pattern-stator seals and one smooth bore seal. These measured results are compared to predictions from a two-control-volume model and realized in the code ISOTSEAL (constant-temperature seal code). The hole-pattern seals have cell depths of 2.03 mm and 3.18 mm with a cell diameter of 1.59 mm. The hole-area density factor for both hole-pattern seals is 43 percent. The seal diameter is 114.71 mm with an L/D ratio of 0.75. Measured results for radial impedances and leakage were obtained. Test conditions involved three speeds out to 20,200 rpm, three inlet pressures out to 17.2 bar, and two exit-to-inlet pressure ratios of 40 percent and 54 percent. As predicted, the hole-pattern seals exhibit frequency-dependent rotordynamic coefficients K(Ω), k(Ω), C(Ω), c(Ω). Results of the tests show that the 3.18 mm hole-pattern seal has the highest average effective stiffness and lowest effective damping. Direct and effective stiffness were under-predicted in all cases; however, measured direct and effective damping are reasonably well predicted. Impedance predictions improve with increasing pressure ratio. Comparisons of leakage correlate extremely well with predictions; worse case deviations never exceed 10 percent. Results show that leakage decreases as cell depth increases. Results also show that the exit temperature increases substantially with increasing rotational speed.