Dynamic Response of a Rotor-Hybrid Gas Bearing System due to Base Induced Periodic MotionsKeun RyuResearch Assistant Luis San AndrsMast-Childs ProfessorFellow ASMETURBOMACHINERY LABORATORYTEXAS A&M UNIVERSITYSupported by TAMU Turbomachinery Research ConsortiumASME paper GT2010-22277Yaying NiuResearch Assistant ASME TURBO EXPO 2010, Glasgow, Scotland, UK

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Gas bearings for microturbomachinery Little friction and power losses Simple configuration High rotor speeds (DN value>4M) Operate at extreme temperaturesAdvantagesGas Foil BearingFlexure Pivot BearingMetal Mesh Foil BearingAIAA-2004-5720-984GT 2004-53621Issues Small damping Low static load capacity Prone to instabilityGT 2009-59315

Gas Bearing Research at TAMU2001/2 - Three Lobe Bearings

2003/4 - Rayleigh Step Bearings

2002-09 - Flexure Pivot Tilting Pad Bearings

2004-10: Bump-type Foil Bearings

2008-10: Metal Mesh Foil Bearings

Stability depends on feed pressure. Stable to 80 krpm with 5 bar pressure

Worst performance to date with grooved bearings

Stable to 93 krpm w/o feed pressure. Operation to 100 krpm w/o problems. Easy to install and align.

Industry standard. Reliable but costly.Models anchored to test data.

Cheap technology. Still infant. Users needed

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 predictions to test resultsObjective & tasksEvaluate the reliability of rotor-air bearing systems to withstanding periodic base or foundation excitations

Positioning BoltLOPRotor: 826 gramsBearings: L= 30 mm, D=29 mm190 mm, 29 mm diamGas Bearing Test Rig

Clearance ~42 mm, preload ~40%. Web rotational stiffness = 62 Nm/rad.Test rig tilted by 10. Flexure Pivot Hybrid Bearings: Improved stability, no pivot wearRotor and hybrid gas bearings 0.826 kg, 190 mm in length Location of sensors and bearings notedRotor

Rotor motion amplitudes increase with excitation of system natural frequency. NOT a rotordynamic instability!Intermittent base shock load excitationsPs=2.36 bar (ab)Previous work (GT 2009-59199)Drop induced shocks ~30 g. Full recovery within ~ 0.1 sec.

Gas bearing test rigFront and side views (not to scale)Base excitationShaker & rod push base of test rig

Hybrid gas bearing test rigRod pushes base plate!(no rigid connection)

Waterfalls in coast down Ps = 2.36 barNo base excitationSubsynchronous whirl > 30 krpm,fixed at system natural frequency = 193 Hz

Rotor speed coast down tests (35 krpm)Feed pressure increases natural frequency and lowers damping ratioNo base excitation1X responsePressure 2.36bar 3.72bar 5.08barNatural Freq 192Hz 217Hz 250Hz

Natural frequency whole test rig (5 Hz)Soft mounts (coils) produce low natural frequencyAcceleration (g)

Delivered excitations (6 Hz)Due to electric motorShaker transfers impacts to rig base! Super harmonic frequencies excited Rotor speed: 34 krpm (567 Hz) Acceleration (g)Acceleration (g)Acceleration (g)zoom

Waterfalls in coast down Ps = 2.36 bar (ab) Shaker frequency: 12Hz

Rotor speed coast down Ps = 2.36 bar (ab) Subsynchronous frequencies:24 Hz (2 x 12 Hz)Natural frequency 193 HzShaker frequency: 12HzSynchronous motion dominates! Excitation of system natural frequency does NOT mean instability!

Rotor motion amplitude at system naturalfrequency decreases as feed pressure increasesEffect of feed pressureShaker frequency: 12HzRotor speed: 34 krpm (567 Hz)193Hz215Hz243Hz12Hz, 24Hz, 36hz, etcNOT due to base motion!Offset by0.01 mmPs: 2.36, 3.72 & 5.08 barPressureincreases

SpeedincreasesShaker frequency: 12HzFeed pressure: 2.36 bar (ab)180Hz180Hz193Hz12Hz, 24Hz, 36hz, etcEffect of rotor speed26, 30 & 34 krpmRotor motion amplitude at system naturalfrequency increases as rotor speed increases

FrequencyincreasesRotor speed: 34 krpm (567Hz)Feed pressure: 2.36 bar (ab)193HzNOT due to base motion!Effect of base frequency0, 5, 6, 9, 12 HzRotor motion amplitude at natural frequency increases as excitation frequency increases

Rigid rotor modelSystem response = superposition of single frequency responsesRotor 1st elastic mode: 1,917 Hz (115 krpm)Equations of motion (linear system)K, C: bearing stiffness and damping from gas bearing model (San Andres, 2006)U, Ub: rotor and base (abs) motions, Z=U-UbM,G: rotor inertia and gyroscopic matricesW: rotor weight Fimb: imbalance force vectorRework equations in terms of measured variables:

Rigid rotor modelMeasured from 1X response testsGood agreement shows predicted bearing force coefficients are accurate Predicted natural frequenciesFor predictions: input RECORDED BASE accelerations (vertical)

Rotor speed26 krpm30 krpm34 krpmConical191 Hz 200 Hz 208 HzCylindrical184 Hz 192 Hz 200 Hz

Cylindrical180 Hz182 Hz193 Hz

Predictions in good agreement! Test rotor-bearing system shows good isolation. Predictions vs. measurementsShaker input frequency: 12HzFeed pressure: 2.36 bar (ab)Rotor speed: 34 krpm (567 Hz)Above natural frequency,RBS is isolated!Nat freq.1XExcitation freqs.

Rotor response contains 1X, excitation frequency (5-12 Hz) and its super harmonics and system natural frequency. Rotor motion amplitudes at natural frequency are smaller than synchronous amplitudes. Excited rotor motion amplitude at system natural frequency increases as gas bearing feed pressure (5.08~2.36bar) decreases, as rotor speed (26~34krpm) increases, and as the shaker input frequency (5~12 Hz) increases. Predicted rotor motion responses obtained from rigid rotor model show good correlation with test data.

ConclusionsDemonstrated isolation of rotor-air bearing system to withstand base excitations at low freqs.Base Excitations on Gas-Rotor Bearing Syst

AcknowledgmentsThanks support of TAMU Turbomachinery Research Consortium Bearings+ Co. (Houston)Learn morehttp://rotorlab.tamu.eduQuestions ?

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