Theory Versus Experiment for the Rotordynamic Characteristics of a Smooth Annular Gas Seal at Eccentric Positions

[+] Author and Article Information
C. R. Alexander

Stress Engineering Services, Inc., Houston, TX 77041-1101

D. W. Childs

Turbomachinery Laboratory, Texas A&M University, College Station, TX 77843-3123

Z. Yang

Cummins Engine Co., Inc., Fuel Systems, Charleston, SC

J. Tribol 117(1), 148-152 (Jan 01, 1995) (5 pages) doi:10.1115/1.2830591 History: Received February 07, 1994; Revised June 17, 1994; Online January 24, 2008


Experimental results are presented for the rotordynamic coefficients of a smooth gas seal at eccentricity ratios out to 0.5. The effects of speed, inlet pressure, pressure ratio, fluid prerotation, and eccentricity are investigated. The experimental results show that direct stiffness KXX decreases significantly, while direct damping and cross-coupled stiffness increase with increasing eccentricity. The whirl-frequency ratio, which is a measure of rotordynamic instability, increases with increasing eccentricity at 5000 rpm with fluid prerotation. At 16,000 rpm, the whirl-frequency ratio is insensitive to changes in the eccentricity. Hence, the results show that eccentric operation of a gas seal tends to destabilize a rotor operating at low speeds with preswirled flow. At higher speeds, eccentric operation has no significant impact on rotordynamic stability. The test results show that the customary, eccentricity-independent, model for rotordynamic coefficients is only valid out to an eccentricity ratio of 0.2~0.3. For larger eccentricity ratios, the dependency of rotordynamic coefficients on the static eccentricity ratio needs to be accounted for. Experimental results are compared to predictions for static and dynamic characteristics based on an analysis by Yang (1993). In general, the theoretical results reasonably predict these results; however, theory overpredicts direct stiffness, fails to indicate the decrease in KXX that occurs with increasing eccentricity, and incorrectly predicts the direction of change in KXX with changing pressure ratio. Also, direct damping is substantially underpredicted for low preswirl values and low supply pressures, but the predictions improve as either of these parameters increase.

Copyright © 1995 by The American Society of Mechanical Engineers
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