Measurement and prediction of nonlinear dynamics of a gas foil bearing supported rigid rotor system

Abstract

Gas foil bearings (GFBs) provide reliable supporting force with low drag in high speed microturbomachinery system. The physical parameters of GFBs are important to determine the rotor system performance. Three gas foil journal bearings (GFJBs), with nominal clearances of 30, 40, and 65 μm measured through static load-deflection test, are used in rotordynamic coast down tests. Two groups of tests are designed, 30 μm GFJB in drive end in Test (a), 65 μm GFJB in drive end in Test (b), and share one 40 μm GFJB in both test groups. The bearing dynamic force coefficients calculated using the perturbation method show clear correlation with given nominal clearance magnitudes. Coast down tests from 40 krpm to rest show synchronous motion and evident subsynchronous whip motion in the two test groups. Nonlinear rotordynamic prediction is established with a simplified foil structure stiffness and loss factor model. The predictions demonstrate that the whip motion is largely determined by the foil structure properties. Synchronous motions are also predicted in the two prediction models and show good agreement with the test data. Moreover, the recorded increase in the bearing temperature was correlated with the rotational speeds and operational conditions of GFJBs. Smaller nominal radial clearance results in higher temperature rise. The measurements and predictions obtained indicate that radial clearance and foil structure significantly affect the rotordynamic performance of rotor GFJBs system and provide further insight into the radial clearance of GFBs.