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Research Papers: Gas Turbines: Structures and Dynamics

Experimental Identification of Dynamic Force Coefficients for a 110 MM Compliantly Damped Hybrid Gas Bearing

[+] Author and Article Information
Adolfo Delgado

GE Global Research Center,
One Research Circle,
Niskayuna, NY 12309
e-mail: delgadoa@ge.com

Hydrostatic films including supply recess geometries are prone to pneumatic hammer instabilities [22]. There is typically a tradeoff between film stiffness and damping coefficients. These instabilities can be avoided with proper design of supply geometry and selection of operating conditions.

The current identification methodology cannot yield synchronous coefficients since it is not possible to discern the contributions from the shaker and unbalance excitation forces to the housing relative motion at the rotor running speed. However, synchronous response can be inferred from excitations at frequencies close to the running speed.

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received September 3, 2014; final manuscript received November 12, 2014; published online December 30, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(7), 072502 (Jul 01, 2015) (8 pages) Paper No: GTP-14-1526; doi: 10.1115/1.4029203 History: Received September 03, 2014; Revised November 12, 2014; Online December 30, 2014

Compliant hybrid gas bearings (HGBs) combine key enabling features from both fixed geometry externally pressurized gas bearings and compliant foil bearings. The compliant hybrid bearing relies on both hydrostatic and hydrodynamic film pressures to generate load capacity and stiffness to the rotor system, while providing damping through integrally mounted metal mesh bearing support dampers. This paper presents experimentally identified force coefficients for a 110 mm compliantly damped gas bearing using a controlled-motion test rig. Test parameters include hydrostatic inlet pressure, excitation frequency, and rotor speed. The experiments were structured to evaluate the feasibility of implementing these bearings in large size turbomachinery. Dynamic test results indicate weak dependency of equivalent direct stiffness coefficients to most test parameters except for frequency and speed, where higher speeds and excitation frequency decreased equivalent bearing stiffness values. The bearing system equivalent direct damping was negatively impacted by increased inlet pressure and excitation frequency, while the cross-coupled force coefficients showed values an order of magnitude lower than the direct coefficients. The experiments also include orbital excitations to simulate unbalance response representative of a target machine while synchronously traversing a critical speed. The results indicate the gas bearing can accommodate vibration levels larger than the set bore clearance while maintaining satisfactory damping levels.

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References

Figures

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Fig. 1

Compliant HGB using integral wire mesh bearing support dampers [16,17]

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Fig. 2

View of the test rig and test assembly, depicting coordinate system instrumentation and main components

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Fig. 3

Physical (linearized) representation of test bearing system force coefficients as a two-degree-of-freedom system

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Fig. 4

Synchronous stiffness and damping coefficients at 7500 rpm for three supply pressures

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Fig. 5

Film thickness of loaded pads from pad probes versus speed for low and high supply pressures (unit load: 140 kPa)

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Fig. 6

HGB stiffness and damping coefficients at two speeds (7.5 and 12.5 krpm) from 25 μm pk-pk linear excitation tests. Applied load: 1335 N.

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Fig. 7

Excitation force and ensuing bearing displacement orbits (solid line: bearing motion, and dashed line: pad motion) for tests at 5000 rpm

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Fig. 8

Synchronous stiffness and damping coefficients versus speed identified from large orbit excitations (circle) and linear excitation (rhombus) extracted from Fig. 6

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Fig. 9

Asynchronous (70 Hz) stiffness and damping coefficients versus speed identified from large orbit excitations

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