Gas foil bearings (GFBs) operating at high temperature rely on thermal management procedures that supply needed cooling flow streams to keep the bearing and rotor from overheating. Poor thermal management not only makes systems inefficient and costly to operate but could also cause bearing seizure and premature system destruction. To date, most of thermal management strategies rely on empirically based "make-and-break" techniques which are often inefficient. This dissertation presents comprehensive measurements of bearing temperatures and shaft dynamics conducted on a hollow rotor supported on two first generation GFBs. The hollow rotor (1.36 kg, 36.51 mm OD and 17.9 mm ID) is heated from inside to reach an outer surface temperature of 120 degrees C. Experiments are conducted with rotor speeds to 30 krpm and with forced streams of air cooling the bearings and rotor. Air pressurization in an enclosure at the rotor mid span forces cooling air through the test GFBs. The cooling effect of the forced external flows is most distinct when the rotor is hottest and operating at the highest speed. The temperature drop per unit cooling flow rate significantly decreases as the cooling flow rate increases. Further measurements at thermal steady state conditions and at constant rotor speeds show that the cooling flows do not affect the amplitude and frequency contents of the rotor motions. Other tests while the rotor decelerates from 30 krpm to rest show that the test system (rigid-mode) critical speeds and modal damping ratio remain nearly invariant for operation with increasing rotor temperatures and with increasing cooling flow rates. Computational model predictions reproduce with accuracy the test data. The work adds to the body of knowledge on GFB performance and operation and provides empirically derived guidance for successful integration of rotor-GFB systems.

Gas bearings enable successful applications in high speed oil-free microturbomachinery. This thesis presents analysis and experiments of the dynamic performance of a rotor supported on Rayleigh step gas bearings. Comprehensive experiments demonstrate that Rayleigh step hybrid gas bearings exhibit adequate stiffness and damping capability in a narrow range of shaft speeds, up to ~ 20 krpm. Rotor coastdown responses were performed for two test bearing sets with nominal radial clearance of 25.4 ?m and 38.1 ?m, respectively. A near-frictionless carbon (NFC) coating was applied on the rotor to reduce friction against its bearings at liftoff and touchdown. However, the rotor still experienced dry friction at low shaft speeds (below ~ 4,000 rpm). Experiments show that the supply pressure raises the rotor critical speed and decreases the system damping ratio. The geometry of the Rayleigh steps distributed on the rotor surface generates a time varying pressure field and results in a sizable 4X super synchronous component of bearing transmitted load. The external supply gas pressure affects slightly the onset speed of instability of the rotor-bearing system. The unstable whirl frequencies are nearly fixed at the system natural frequency (~ 120 Hz). Analysis with a finite element model predicts the stiffness and damping force coefficients for the bearing accounting for a purely hydrodynamic operation condition. Predictions show the synchronous stiffness and damping coefficients decrease with shaft speed. Predicted threshold speeds of instability are lower, ~ 50% or less than the measurement due to the analytical model limitations assuming a grooved stator. The predicted synchronous responses to imbalance correlate well with the measurements, however. The Rayleigh step gas bearing shows similar characteristics to the flexure pivot tilting pad bearing (FPTPB) tested in 2003. However, the test Rayleigh step gas bearings exhibit a much reduced stable operating speed range, below 20 krpm. The maximum speed achieved is much lower to that determined with an identical rotor supported on FPTPBs, i.e. rotor dynamically stable up to 100 krpm. The FPTPB is more reliable in high speed oil-free applications due to its excellent stability characteristics.

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