Experimental Study of the Leakage and Rotordynamic Coefficients of a Long-Smooth Seal With Two-Phase, Mainly Oil Mixtures

This paper experimentally studies the leakage and rotordynamic performance of a long-smooth seal with air-oil mixtures. Tests are performed with inlet gas-volume-fraction gas volume fraction (GVF) = 0%, 2%, 4%, 6%, and 10%, rotor speed omega = 5, 7.5, 10, and 15 krpm, inlet temperature T-i = 39.4 degrees C, exit pressure P-e = 6.9 bars, and pressure drop (PD) = 31, 37.9, and 48.3 bars. Test results show that adding air into the oil flow does not change the seal's mass flow leakage (m) over dot discernibly but significantly impacts the seal's rotordynamic characteristics. For all PDs and speeds, K increases as inlet GVF increases from zero to 10% except for 6% <= inlet GVF <= 10% when PD = 48.3 bars, where K decreases as inlet GVF increases. The K increment will increase a pump rotor's natural frequency and critical speed. Increasing the rotor's natural frequency would also increase the onset speed of instability (OSI) and improve the stability of the rotor. Adding air into the oil flow has little impact on cross-coupled stiffness k, direct damping C, and effective damping C-eff. C-eff = C - k/omega + m(q)omega, where m(q) is the cross-coupled virtual-mass. Test results are compared to predictions from San Andres's (San Andres, 2011, "Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals," ASME J. Eng. Gas Turbines Power, 134(2), p. 022503.) bulk-flow model, which assumes that the liquid-gas mixture is isothermal and homogenous. The model reasonably predicts (m) over dot, C, and C-eff. All predicted K values are positive, while measured K values are negative for some test cases. Predicted k values are close to measurements when omega = 5 krpm and are larger than measurements when 7.5 < omega < 15 krpm.