Effects of static and imbalance loads on nonlinear response of rigid rotor supported on gas foil bearings

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Abstract This paper presents an experimental and numerical investigation into the effects of static and imbalance loads on the nonlinear rotordynamic performance of a rigid rotor supported on gas foil bearings (GFBs). The interesting phenomenon that static loads can lead to high sub-synchronous motions in rotor-GFB systems is reported for the first time in this paper. The effects of static and imbalance loads on coast-down tests, synchronous response amplitudes, peak response frequency, appearance of sub-synchronous vibrations and time extent of coast-down responses are measured and discussed. A rotordynamic model based on FEM and unsteady Reynolds equation is derived to predict the nonlinear response of the system. The foil structure and gas film in-series are considered to derive the unsteady bearing force. The predictions are consistent with the test data. The relationships between static or imbalance loads and nonlinear response, especially the vibrations in sub-synchronous frequencies, are discussed. In the numerical simulations, as the static and imbalance loads increase, the vibration component changes from mainly synchronous to sub-synchronous and then back to synchronous vibrations. When the static and imbalance loads change, the natural frequency of the rotor-GFB system could be shifted into or away from the threshold of the whirl vibration with a typical factor, λ = 0.5, and lead to serious sub-synchronous vibrations. The experimental data and theoretical predictions help with the engineering design and implementation of GFBs in high-performance micro turbomachineries.