Gas Bearings for OilFree Turbomachinery 29 th Turbomachinery
Gas Bearings for Oil-Free Turbomachinery 29 th Turbomachinery Consortium Meeting Dynamic Response of a Rotor-Air Bearing System due to Base Induced Periodic Motions Yaying Niu Research Assistant Luis San Andrés Mast-Childs Professor Principal Investigator TRC-B&C-1 -09 Start date: Oct 1 st, 2008 2009 TRC Project GAS BEARINGS FOR OIL-FREE TURBOMACHINERY
Micro Turbomachinery (< 0. 5 MW) Gas Bearings for Oil-Free Turbomachinery Turbo Compressor 100 krpm, 10 k. W Advantages • Compact and fewer parts • Portable and easily sized • High energy density • Lower pollutant emissions • Low operation cost http: //www. hsturbo. de/en/produkte/turboverdichter. html Micro Turbo 500 krpm, 0. 1~0. 5 k. W http: //www. hsturbo. de/en/produkte/micro-turbo. html Oil-free turbocharger 120 krpm, 110 k. W http: //www. miti. cc/new_products. html
Gas Bearings for Oil-Free Turbomachinery Gas Bearings for MTM Metal Mesh Foil Bearing Advantages • Small friction and power losses • Less heat generation • Simple configuration • High rotating speed (DN value>4 M) • Operate at extreme temperatures Issues GT 2009 -59315 Gas Foil Bearing AIAA-2004 -5720 -984 • Small damping • Low load carrying capacity • Prone to instability Flexure Pivot Bearing GT 2004 -53621
Gas Bearings for MTM Gas Bearings for Oil-Free Turbomachinery. GT 2009 -59199 2008: Intermittent base shock load excitations Drop induced shocks ~30 g. Full recovery within ~ 0. 1 sec. Ps=2. 36 bar (ab) Rotor motion amplitude increases largely. Subsynchronous amplitudes larger than synchronous. Excitation of system natural frequency. NOT a rotordynamic instability!
2008 -2009 Objectives Gas Bearings for Oil-Free Turbomachinery Evaluate the reliability of rotor-air bearing systems to withstanding base load excitations • 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
2009 Gas bearing test rig Gas Bearings for Oil-Free Turbomachinery The rod merely pushes on the base plate!
Gas Bearings for Oil-Free Turbomachinery Test rotor and gas bearings Flexure Pivot Hybrid Bearings: Improved stability, no pivot wear. Rotor • 0. 825 kg in weight • 190 mm in length • Location of sensors and bearings noted Gas bearing Clearances ~42 mm, Preload ~40%. Web rotational stiffness = 62 Nm/rad. Test rig tilts by 10°. NOT Load-on-Pad (LOP) !
Shaker delivered accelerations Gas Bearings for Oil-Free Turbomachinery Rotor speed: 34 krpm (567 Hz) Due to electric motor Shaker transfers impacts to the rig base! Super harmonic frequencies are excited.
Rotor speed coast down tests Gas Bearings for Oil-Free Turbomachinery No base excitation Ps = 2. 36 bar (ab) Rotor coasts down from 35 krpm No added imbalance Subsynchronous whirling starts beyond 30 krpm, fixed at system natural frequency 193 Hz Pressure 2. 36 bar 3. 72 bar 5. 08 bar Natural Freq 192 Hz 217 Hz 250 Hz Slow roll compensated Synchronous response
Rotor speed coast down tests Gas Bearings for Oil-Free Turbomachinery Ps = 2. 36 bar (ab) Shaker input frequency: 12 Hz Subsynchronous response: 1) 24 Hz (Harmonic of 12 Hz) 2) Natural frequency 193 Hz Synchronous Dominant! Excitation of system natural frequency does NOT mean instability!
Fixed speed, increasing pressures Gas Bearings for Oil-Free Turbomachinery Shaker input frequency: 12 Hz Rotor speed: 34 krpm (567 Hz) 12 Hz, 24 Hz, 36 hz, etc NOT due to base motion! Pressure increases 243 Hz Offset by 0. 01 mm 215 Hz 193 Hz Rotor response amplitude at the system natural frequency decreases, as the feed pressure increases.
Gas Bearings for Oil-Free Turbomachinery Fixed pressure, increasing speeds Shaker input frequency: 12 Hz Feed pressure: 2. 36 bar (ab) 12 Hz, 24 Hz, 36 hz, etc Speed increases 193 Hz 180 Hz Rotor response amplitude at the system natural frequency increases, as the rotor speed increases.
Gas Bearings Oil-Free Turbomachinery Fixed speed for and pressure, increasing input frequency Rotor speed: 34 krpm (567 Hz) Feed pressure: 2. 36 bar (ab) 193 Hz Frequency increases NOT due to base motion! Same excitation magnitude for 6, 9, and 12 Hz Rotor response amplitude at the system natural frequency increases, as the input frequency increases.
XLTRC prediction Gas Bearings 2 for Oil-Free Turbomachinery FE rotor model System Natural Freq Shaker input frequency: 12 Hz Feed pressure: 2. 36 bar (ab) Conical XLTRC 2 26 krpm 30 krpm 34 krpm Conical 202 Hz 212 Hz 222 Hz Cylindrical 185 Hz 193 Hz 201 Hz 26 krpm 30 krpm 34 krpm 180 Hz 193 Hz Rotor speed Measured Rotor speed Cylindrical Input acceleration in XLTRC 2, simulate actual acceleration. XLTilt. Pad. HGB™ Prediction uses synchronous speed bearing force coefficients. In actuality, gas bearing force coefficients are frequency dependent!
XLTRC prediction Gas Bearings 2 for Oil-Free Turbomachinery Shaker input frequency: 12 Hz Input acceleration Feed pressure: 2. 36 bar (ab) only on VERTICAL Rotor speed: 34 krpm (567 Hz) direction Measured N. F. component Predicted natural frequency component Prediction frequency step: 1. 25 Hz. XLTRC 2 predicts absolute rotor motions! Measured rotor response is relative to bearing housing.
Rigid rotor model prediction Gas Bearings for Oil-Free Turbomachinery Equations of Motion: Rotor 1 st bending mode: 1, 917 Hz (115 krpm) Shaker input frequency: 12 Hz Feed pressure: 2. 36 bar (ab) Steady-State! Absolute rotor response Relative rotor response System Natural Frequency: Input acceleration in rigid rotor model, VERTICAL direction only! Rigid rotor model 26 krpm 30 krpm 34 krpm Conical 191 Hz 200 Hz 208 Hz Cylindrical 184 Hz 192 Hz 200 Hz Conical 202 Hz 212 Hz 222 Hz Cylindrical 185 Hz 193 Hz 201 Hz 180 Hz 193 Hz Rotor speed XLTRC 2 Measured Cylindrical System response equals to the superposition of unique single frequency responses.
Gas Bearings for Oil-Free Turbomachinery Rigid rotor model prediction Shaker input frequency: 12 Hz Feed pressure: 2. 36 bar (ab) Rotor speed: 34 krpm (567 Hz) Measured N. F. component Above the natural frequency, Predicted natural frequency the system is isolated! component Relative rotor motion Rigid rotor model predicts relative rotor motions! The test rotor-bearing system shows good isolation.
Conclusions 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. 36 bar) decreases, as the rotor speed (26~34 krpm) 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. Reliability of rotor-air bearing system to withstanding base load excitations demonstrated
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