Rotordynamic evaluation of frequency dependent impedances of hole-pattern gas damper seals

Abstract

Two hole-pattern seals are compared with one smooth bore seal. The two 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%. The L/D ratio of the seals is 0.75. Parametric identification tests were performed on all three seals for comparison, and results for impedance and leakage were obtained. Test conditions involved three speeds out to 20,200 rpm and three inlet pressures out to 17.2 bar with an exit-to-inlet pressure ratio of 40 % and 54 %. Results of the tests show that the 3.18 mm hole-pattern seal has the highest average effective stiffness and lowest effective damping of all three seals. Results show that as cell depth increases, leakage decreases. Results also show that with increasing rotational speed, the exit temperature increases substantially. The two hole-pattern seals exhibit frequency dependent rotordynamic coefficients K([]), k([]), C([]), c([]). Measurements were compared with the ISTOSEAL program developed by Kleynhans and Childs (1997). Comparisons of leakage correlate extremely well with worse case conditions never exceeding 10 % between measurement and code prediction. Direct and effective stiffnesses were under-predicted in all cases; however, direct and effective damping correlated relatively well with measured results. Impedance predictions versus pressure ratio improve with increasing pressure ratio; however, leakage predictions are less accurate.