A Study on the Leakage and Rotordynamic Performance of a Long Labyrinth Seal Under Mainly Air Conditions
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AbstractThis paper investigates the impact of liquid presence in air on the leakage and rotordynamic coefficients of a long (length-to-diameter ratio L/D=0.747) teeth-on-stator labyrinth seal. The test fluid is a mixture of air and silicone oil (PSF-5cSt). Tests are carried out at inlet pressure Pi=62.1 bars, three pressure ratios from 0.21 to 0.46, three speeds from 10 to 20 krpm, and six inlet liquid volume fractions (LVFs) from 0% to 15%. Complex dynamic-stiffness coefficients Hij are measured. The real parts of Hij are too frequency dependent to be fitted by frequency-independent stiffness and virtual-mass coefficients. Therefore, this paper presents frequency-dependent direct stiffness K and cross-coupled stiffness k. The imaginary parts of Hij produce frequency-independent direct damping C. Test results show that, under both pure- and mainly air conditions, the leakage mass flowrate m of the test seal steadily increases as inlet LVF increases. K is negative under all test conditions, and the magnitude of K increases as inlet LVF increases, leading to a larger negative centering force on the associated compressor rotor. Under pure-air conditions, k is a small negative value. Injecting oil into the air increases k slightly and make the magnitude of k closer to zero. Under mainly air conditions, increasing inlet LVF from 2% to 15% has little impact on k. C normally increases as inlet LVF increases. The value of the effective damping Ceff=Ck/ near 0.5 is of significant interest to the system stability since an unstable centrifugal compressor may precess at approximately 0.5. denotes the excitation frequency. The oil presence in the air has little impact on the value of Ceff near 0.5. Also, the liquid presence does not change the insensitiveness of m, K, k, C, and Ceff to change in ; i.e., under both pure- and mainly air conditions, changes in has little impact on m, K, k, C, and Ceff.
