Duffing’s Vibration of a Rotor Supported on Gas Foil Bearings With Base Excitation: Measurements and Model Predictions

Abstract

The widespread application of gas foil bearings (GFBs) to high-performance microturbomachinery requires accurate predictions for their physical models based on experimental test data. This paper presents the experimental measurements and model predictions of Duffing’s vibration in a rotor supported on GFBs with base excitation, implemented for small oil-free turbomachinery. The rotor consisted of an impeller at one end and a thrust collar at the other end. Two gas foil journal bearings (GFJBs) located between the impeller and thrust collar supported the rotor, and one pair of gas foil thrust bearings (GFTBs) supported the thrust collar. A series of dynamic excitation tests on the rotor-GFBs system was conducted with increasing dynamic load and excitation frequency, with the rotor operating at 20,000 rpm. An electromagnetic shaker provided dynamic sine sweep loads at excitation frequencies of 10–200 Hz to the test rig base in the axial and horizontal directions. An accelerometer installed on the test rig measured the acceleration due to the dynamic loads and provided it to the shaker controller for use as a reference signal. The acceleration level was controlled to ensure a constant value, while the excitation frequency increased. During the excitation tests, two sets of orthogonally positioned eddy current sensors and one axially positioned eddy current sensor recorded the rotor’s horizontal, vertical, and axial vibrations. The test measurements demonstrated that the rotor’s vibrational motions synchronous to the shaker excitation were the most dominant. At a constant dynamic load, as the excitation frequency increased, the amplitude of the rotor motion gradually increased until it reached a certain frequency, after which it jumped down at the higher frequencies. This amplitude jump-down phenomenon became more pronounced as the dynamic load increased. In general, both the peak amplitude and jump-down frequency increased nonlinearly with the increasing dynamic loads, thus revealing the typical Duffing’s vibration. For benchmarking against the test measurements, a previously developed numerical integration of a nonlinear equation of motion (EOM) was modified to predict the rotor’s vibrational motions with base excitations to an acceleration of 9 G (gravity). This nonlinear equation uses a third-order polynomial equation that best fits the measured structural foil bearing deflection versus static load. Comparisons of the predicted synchronous amplitude and acceleration of the rotor for the increasing excitation frequencies and the predicted waterfall plot of the amplitude of the rotor motion with the test measurements showed excellent agreements, thus validating the predictive model of the rotor-GFB system with base excitation.