Clearance Effects on Rotordynamic Performance of a Long Smooth Seal With Two-Phase, Mainly-Air Mixtures

This paper experimentally studies the effects of changing radial clearance C_r on the performance of a long (length-to-diameter ratio L/D = 0.65) smooth seal under mainly-air (wet-gas) conditions. The test fluid is a mixture of air and silicone oil. Tests are conducted with C_r = 0.188, 0.163, and 0.140 mm, inlet pressure P_i = 62.1 bars, exit pressure P_e = 31 bars, inlet liquid volume fraction LVF = 0%, 2%, 5%, and 8%, and shaft speed ω = 10, 15, and 20 krpm. The seal's complex dynamic stiffness coefficients H_ij are measured. The real parts of H_ij cannot be fitted by frequency-independent stiffness and virtual-mass coefficients. Therefore, frequency-dependent direct K_Ω and cross-coupled k_Ω stiffness coefficients are used. The imaginary parts of direct H_ij produce frequency-independent direct damping C. Test results show that, for all pure- and mainly-air conditions, decreasing C_r decreases (as expected) the leakage mass flow rate $m˙$⁠. Under mainly-air conditions, decreasing C_r decreases K_Ω. This outcome is contrary to the test results at pure-air conditions, where K_Ω increases as C_r decreases. Since an unstable centrifugal compressor rotor may precess at approximately 0.5ω, the effective damping C_eff at about 0.5ω is used as an indicator of the impact a seal would have on its associated compressor. For pure-air conditions, when Ω ≈ 0.5ω, decreasing C_r increases C_eff and makes the seal more stabilizing. This trend continues after the oil is added. A bulk-flow model developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) produces predictions to compare with test results. $m˙$ predictions correlate with measurements. Under pure-air conditions, the model correctly predicts the effects of changing C_r on K_Ω and the C_eff value near 0.5ω. After the oil is added, as C_r decreases, predicted K_Ω increases while measured K_Ω decreases. Also, for mainly-air cases and Ω ≈ 0.5ω, decreasing C_r does not discernibly change predicted C_eff but increases the measured value.