The static and dynamic characteristics of divergent tapered-bore hole-pattern gas seals

Abstract

Experimental data is presented for three hole-pattern annular gas seals with differing diverging tapers machined in them. The holes are approximately 3.12 mm (.123 in) deep on average with a hole area to surface area ratio of 43%. The L/d ratio of the seals is approximately 0.75. Taper designs for the three seals include clearance ratios (CR, exit clearance / inlet clearance) of 1.00 (straight-bore), 1.42, and 2.15. Data is presented for tests taken at 10,200 rpm, 15,200 rpm, and 20,200 rpm and at exit-to-inlet pressure ratios of 40% and approximately 53%. Comparisons are given between the three seals for direct and cross-coupled impedances; mass flow rate and effective stiffness and damping coefficients. Results show that an optimum effective damping is obtained with the medium taper geometry. However, the direct damping values are relatively unchanged. Furthermore, a decrease in effective stiffness with increasing taper angle is observed. The tapered seals displayed significantly higher leakage values than the straight seals, with the 2.15 CR seal showing the highest leakage under all test conditions. Measurement data is compared with theoretical predictions generated with the ISOTSEAL modeling code, using the two-control-volume model developed by Kleynhans and Childs (1997). Impedance data is approximated moderately well by the analytical prediction. Effective stiffness and damping values are closely predicted by theory. Although, peak damping values occur at higher frequencies than predicted. This is likely due to varying hole depths. The 1.42 CR seal performance is better predicted by theory than the 2.15 CR seal.