Experimental response of gas hybrid bearings for high speed oil-free turbomachinery

Abstract

The present investigation advances the analysis and experimental validation of simple gas bearing configurations with static and dynamic force characteristics desirable in high-speed turbomachinery. Comprehensive experiments and analysis were conducted on a small rotor supported on three lobed hybrid (hydrostatic/hydrodynamic) rigid gas bearings. The rigid bearings are made up of preloaded 120⁰ lobes with minute feed holes for external pressurization that are fed, for example, by bleed off from a turbocharger compressor outlet. The bearing nominal clearance and dimensionless preload are 66 microns and 0.33, respectively. The test rotor, weighing 827 grams, integrates a DC motor and can achieve speeds as large as 100,000 rpm. For various imbalance conditions, coast down tests from 60,000 rpm characterize the rotor response on its bearings. As the supply pressure rises, the rotor response shows an increase in critical speed and a noticeable reduction in damping ratio. Threshold speeds of instability also increase with increasing supply pressures, and whirl frequency ratios range from nearly 50% of rotor speed for a purely hydrodynamic condition to 25% for a pressure supply five times ambient conditions. Bearing transmitted loads closely follow the rotor imbalance responses with large peak values while traversing the critical speeds. Similar imbalance response measurements were conducted with the test rotor supported on hybrid pressure dam gas bearings and on HyPad® tilting pad gas bearings. A linear rotordynamics model accounts for the rotor and gas hybrid bearings. A FE laminar flow model for hybrid gas bearing analysis predicts frequency dependent stiffness and damping force coefficients at the operating speeds and levels of feed pressure tested. The eigenvalue analysis forwards natural frequencies in agreement with the measurements. Estimated whirl frequency ratios are typically 50% of rotor speed, thus predicting sub synchronous instabilities at lower rotor speeds than found experimentally when increasing the magnitude of feed pressurization.