Gas Bearings for Oil-Free Turbomachinery 29th Turbomachinery Consortium Meeting Dynamic Response of a Rotor-Air Bearing System due to Base Induced Periodic Motions TRC-B&C TRC Project GAS BEARINGS FOR OIL-FREE TURBOMACHINERY Yaying Niu Research Assistant Luis San Andrés Mast-Childs Professor Principal Investigator Start date: Oct 1st, 2008

Gas Bearings for Oil-Free Turbomachinery Micro Turbomachinery (< 0.5 MW) Compact and fewer parts Portable and easily sized High energy density Lower pollutant emissions Low operation cost Advantages Turbo Compressor 100 krpm, 10 kW Micro Turbo 500 krpm, 0.1~0.5 kW Oil-free turbocharger 120 krpm, 110 kW

Gas Bearings for Oil-Free Turbomachinery Gas Bearings for MTM Small friction and power losses Less heat generation Simple configuration High rotating speed (DN value>4M) Operate at extreme temperatures Advantages Gas Foil Bearing Flexure Pivot Bearing Metal Mesh Foil Bearing AIAA GT Issues Small damping Low load carrying capacity Prone to instability GT

Gas Bearings for Oil-Free Turbomachinery Rotor motion amplitude increases largely. Subsynchronous amplitudes larger than synchronous. Excitation of system natural frequency. NOT a rotordynamic instability! Gas Bearings for MTM Ps=2.36 bar (ab) GT : Intermittent base shock load excitations Drop induced shocks ~30 g. Full recovery within ~ 0.1 sec.

Gas Bearings for Oil-Free Turbomachinery Set up an electromagnetic shaker under the base of test rig to deliver periodic load excitations Measure the rig acceleration and rotordynamic responses due to shaker induced excitations Model the rotor-air bearing system subject to base motions and compare the predictions to test results Objectives Evaluate the reliability of rotor-air bearing systems to withstanding base load excitations

Gas Bearings for Oil-Free Turbomachinery 2009 Gas bearing test rig The rod merely pushes on the base plate!

Gas Bearings for Oil-Free Turbomachinery Clearances ~42  m, Preload ~40%. Web rotational stiffness = 62 Nm/rad. Test rig tilts by 10°. NOT Load-on-Pad (LOP) ! Flexure Pivot Hybrid Bearings: Improved stability, no pivot wear. Test rotor and gas bearings kg in weight 190 mm in length Location of sensors and bearings noted Rotor Gas bearing

Gas Bearings for Oil-Free Turbomachinery Shaker delivered accelerations Due to electric motor Shaker transfers impacts to the rig base! Super harmonic frequencies are excited. Rotor speed: 34 krpm (567 Hz)

Gas Bearings for Oil-Free Turbomachinery Rotor speed coast down tests Ps = 2.36 bar (ab) Subsynchronous whirling starts beyond 30 krpm, fixed at system natural frequency 193 Hz No base excitation Slow roll compensated Synchronous response Rotor coasts down from 35 krpm No added imbalance Pressure 2.36bar 3.72bar 5.08bar Natural Freq 192Hz 217Hz 250Hz

Gas Bearings for Oil-Free Turbomachinery Rotor speed coast down tests Ps = 2.36 bar (ab) Subsynchronous response: 1)24 Hz (Harmonic of 12 Hz) 2)Natural frequency 193 Hz Shaker input frequency: 12Hz Synchronous Dominant! Excitation of system natural frequency does NOT mean instability!

Gas Bearings for Oil-Free Turbomachinery Rotor response amplitude at the system natural frequency decreases, as the feed pressure increases. Fixed speed, increasing pressures Pressure increases Pressure increases Shaker input frequency: 12Hz Rotor speed: 34 krpm (567 Hz) 193Hz 215Hz 243Hz 12Hz, 24Hz, 36hz, etc NOT due to base motion! Offset by 0.01 mm

Gas Bearings for Oil-Free Turbomachinery Fixed pressure, increasing speeds Rotor response amplitude at the system natural frequency increases, as the rotor speed increases. Speed increases Speed increases Shaker input frequency: 12Hz Feed pressure: 2.36 bar (ab) 180Hz 193Hz 12Hz, 24Hz, 36hz, etc

Gas Bearings for Oil-Free Turbomachinery Fixed speed and pressure, increasing input frequency Rotor response amplitude at the system natural frequency increases, as the input frequency increases. Frequency increases Frequency increases Rotor speed: 34 krpm (567Hz) Feed pressure: 2.36 bar (ab) 193Hz NOT due to base motion! Same excitation magnitude for 6, 9, and 12 Hz

Gas Bearings for Oil-Free Turbomachinery Prediction uses synchronous speed bearing force coefficients. In actuality, gas bearing force coefficients are frequency dependent! XLTRC 2 prediction Rotor speed 26 krpm30 krpm34 krpm Conical202 Hz212 Hz222 Hz Cylindrical185 Hz193 Hz201 Hz Rotor speed 26 krpm30 krpm34 krpm Cylindrical 180 Hz 193 Hz XLTRC 2 Measured FE rotor model Conical Cylindrical System Natural Freq Input acceleration in XLTRC 2, simulate actual acceleration. Shaker input frequency: 12Hz Feed pressure: 2.36 bar (ab) XLTiltPadHGB™

Gas Bearings for Oil-Free Turbomachinery XLTRC 2 prediction Shaker input frequency: 12Hz Feed pressure: 2.36 bar (ab) Rotor speed: 34 krpm (567 Hz) Input acceleration only on VERTICAL direction XLTRC 2 predicts absolute rotor motions! Measured rotor response is relative to bearing housing. Predicted natural frequency component Prediction frequency step: 1.25 Hz. Measured N.F. component

Gas Bearings for Oil-Free Turbomachinery Rigid rotor model prediction Rotor speed 26 krpm30 krpm34 krpm Conical191 Hz 200 Hz 208 Hz Cylindrical184 Hz 192 Hz 200 Hz XLTRC 2 Conical 202 Hz 212 Hz 222 Hz Cylindrical 185 Hz 193 Hz 201 Hz Measured Cylindrical180 Hz 193 Hz Rigid rotor model System response equals to the superposition of unique single frequency responses. System Natural Frequency: Input acceleration in rigid rotor model, VERTICAL direction only! Equations of Motion: Shaker input frequency: 12Hz Feed pressure: 2.36 bar (ab) Steady-State! Absolute rotor response Relative rotor response Rotor 1 st bending mode: 1,917 Hz (115 krpm)

Gas Bearings for Oil-Free Turbomachinery Rigid rotor model predicts relative rotor motions! The test rotor-bearing system shows good isolation. Rigid rotor model prediction Shaker input frequency: 12Hz Feed pressure: 2.36 bar (ab) Rotor speed: 34 krpm (567 Hz) Relative rotor motion Predicted natural frequency component Above the natural frequency, the system is isolated! Above the natural frequency, the system is isolated! Measured N.F. component

Gas Bearings for Oil-Free Turbomachinery The recorded rotor response contains the main input frequency (5-12 Hz) and its super harmonics, and the rotor-bearing system natural frequency. The motion amplitudes at the natural frequency are smaller than the components synchronous with rotor speed. The rotor motion amplitude at the system natural frequency increases as the gas bearing feed pressure (5.08~2.36bar) decreases, as the rotor speed (26~34krpm) increases, and as the shaker input frequency (5~12 Hz) increases. Predicted rotor motion responses obtained from XLTRC 2 ® and an analytical rigid rotor model show good correlation with the measurements. The system shows reliable isolation. Conclusions Reliability of rotor-air bearing system to withstanding base load excitations demonstrated