Effect of Side Feed Pressurization on the Performance of Shimmed Foil Gas Bearings - Part II: Model, Predictions and Comparisons to Rotordynamic Measurements

Oil-free micro turbomachinery implementing gas foil bearings (GFBs) has improved mechanical efficiency and reliability. Adequate thermal management for operation in high temperature environments is an issue of importance in applications such as in gas turbines and turbochargers. GFBs often need a cooling gas flow, axially fed through one end of the bearings, to transport the heat conducted from a hot turbine, for example. Side gas pressurization has also a paramount effect on reducing amplitudes of motion, synchronous and subsynchronous. In a comparison 2007 TRC report, shaft motion measurements in a test rotor supported on GFBs show this remarkable effect. A computational gas film model implementing the evolution of gas circumferential velocity as a function of the imposed side pressure is advanced. For tested GFBs, predicted direct stiffnesses and damping coefficients increase as the magnitude of feed pressure raises, while the difference in cross-coupled stiffnesses, directly related to rotor-bearing system stability, decreases. Predictions of threshold speed of instability and whirl frequency ratio are in close agreement with laboratory measurements. A model for GFBs with shims is introduced. The shimmed GFB, whose film clearance resembles a three lobe bearing, generates significantly larger hydrodynamic pressures than those in a conventional GFB. Shimmed GFBs operate with a smaller journal eccentricity and attitude angle than conventional GFB, and with increased direct stiffness and damping coefficients. A linear finite element rotordynamic analysis (XLTRC 2) models the test rotor supported on GFBs and predicts the system rotordynamic stability and synchronous rotor responses, phase angle and amplitude. Predicted synchronous responses are in very good agreement with the test measurements, in particular for small to moderate imbalances. As a final observation, although external pressurization aids to improve GFB rotordynamic performance; a too large supplied gas flow rate for cooling an actual high temperature application may penalize sensibly the efficiency and performance of the turbomachinery.