Issues on Instability and Force Nonlinearity in Gas Foil Bearing Supported Rotors
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Current advancements in turbochargers and micro gas turbine engines (< 250 kW) implement advanced gas bearing technologies in oil-free compact units with improved efficiency. Gas foil bearings are a proven technology that enables systems with ultra-low emissions, reduced maintenance, and lower life cycle costs. The dominant challenges for gas bearing technology include intermittent contact and damaging wear at startup and shut down, temporary rubs during normal operating conditions, and the potential for large subsynchronous rotor motions at operating speeds above the rotor-bearing system (lowest) natural frequencies. Subsynchronous motions in gas foil bearings are common, though hastily attributed to a linear rotordynamic instability issue. In actuality, the load capacity of a gas foil bearing depends solely on its structural support which has a strong nonlinear (hardening) structural stiffness behavior. The paper presents predictions of the nonlinear forced performance of a rigid rotor supported on bump-type gas foil bearings and comparisons to rotor response measurements obtained in a laboratory test rig. The foil bearing force is modeled as a third order structural element with nonlinear stiffnesses derived from static load - deflection measurements conducted with a 2 nd generation foil bearing. The simple analysis predictions evidence a rotor-bearing behavior akin to that of a Duffing oscillator with multiple frequency responses, sub- and super-harmonic, within certain ranges of rotor speed. Predicted synchronous and subsynchronous amplitudes of motion reproduce with accuracy the measured responses, with a main subsynchronous frequency locked at the system natural frequency, and with a whirl frequency ratio bifurcating from 1/2 to 1/3 as shaft speed increases. The complexity of the system motions is due to the structural nonlinearity in the foil bearing, not related to hydrodynamic gas bearing rotordynamic instability. The predictions and measurements validate the major assumption for the simple foil bearing model, i.e. a minute gas film with effective infinite stiffness, with applicability to large amplitude rotordynamic motions. For the first time in the open literature, a simple physical model reproduces with exactness the richness and complexity of measured rotor-foil bearing system motions.
