THEORY VERSUS EXPERIMENT FOR THE ROTORDYNAMIC CHARACTERISTICS OF A SMOOTH ANNULAR GAS SEAL AT ECCENTRIC POSITIONS

Experimental results are presented for the rotordynamic coefficients o f a smooth gas seal at eccentricity ratios out to 0.5. The effects of speed, inlet pressure, pressure ratio, fluid prerotation, and eccentri city are investigated. The experimental results show that direct stiff ness K-XX decreases significantly, while direct damping and cross-coup led stiffness increase with increasing eccentricity. The whirl-frequen cy ratio, which is a measure of rotordynamic instability, increases wi th increasing eccentricity at 5000 rpm with fluid prerotation. At 16,0 00 rpm, the whirl-frequency ratio is insensitive to changes in the ecc entricity. Hence, the results show that eccentric operation of a gas s eal fends to destabilize a rotor operating at low speeds with preswirl ed flow. At higher speeds, eccentric operation has no significant impa ct on rotordynamic stability. The test results show that the customary , eccentricity-independent, model for rotordynamic coefficients is onl y valid out to an eccentricity ratio of 0.2 similar to 0.3. For larger eccentricity ratios, the dependency of rotordynamic coefficients on t he static eccentricity ratio needs to be accounted for. Experimental r esults are compared to predictions for static and dynamic characterist ics Based on an analysis by Yang (1993). In general, the theoretical r esults reasonably predict these results; however, theory overpredicts direct stiffness, fails to indicate the decrease in K-XX that occurs w ith increasing eccentricity, and incorrectly predicts the direction of change in K-XX with changing pressure ratio. Also, direct damping is substantially underpredicted for low preswirl values and low supply pr essures, but the predictions improve as either of these parameters inc rease.