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NEW Proposal to the TAMU Turbomachinery Research consortium

METAL MESH FOIL BEARINGS: OPERATION AT HIGH TEMPERATURE

Dr. Luis San Andrés, Mast-Childs Tribology Professor May 2011 SIGNIFICANCE AND JUSTIFICATION

Oil-free microturbomachinery (MTM < 400 kW) seeks reliable and cost effective rotor-support bearing elements for commercially viable mass-production and widespread usage. Presently, compliant (foil type) gas bearings (bump foil, overleaf foils and multi-wound foils) are custom designed for particular applications and demand of excessive costs from design to manufacturing to prototype testing to field deployment. Industry demands of an inexpensive and reliable bearing technology that uses common materials and processes, is easy to scale, and proven to deliver adequate load support with stable stiffness and lots of damping. NSF (2003-07), NASA GRC (2007-09), and the TRC (2004-2009) funded a research program on gas bearings at TAMU to advance experimentally benchmarked computational tools to design bump-type foil bearings and to predict their dynamic forced performance. The predictive program is presently used by a few TRC members and NASA GRC, and also licensed to several OEMs interested in developing oil-free TM applications (SNECMA-SEP, Barber-Nichols, Borg-Warner, Capstone Turbines, Air Products). The products of the research also include an extensive data base demonstrating the static and dynamic load performance of several commercial foil bearings operating to speeds as high as 50 krpm and at temperatures reaching 300C. The METAL MESH FOIL BEARING is an alternative bearing type for supporting oil-free MTM. MMFBs are inexpensive by employing commonly available materials and a simple manufacturing process. Ina MMFB, a ring shaped metal mesh provides a resilient support to a smooth top foil wrapped around a rotating journal; and a hydrodynamic film pressure builds up within the minute gap between the rotating journal and the top foil, thus providing a near frictionless load support. While mechanical energy dissipation (damping) in foil bearings is primarily due to Coulomb dry-friction, MMFBs offer much larger damping due to material hysteresis and dry-friction from Metal mesh foil bearing for the myriad of contacts within the metal mesh. oil-free turbomachinery

STATUS OF WORK 2008-2011

A concerted effort at TAMU (funded by TRC in 2008) has promoted the development of MMFBs by demonstrating reliable high speed operation (to 60 krpm) with a extremely low friction factor (f<0.03) and showing direct stiffness and damping coefficients that increase with excitation frequency. In 2008 [1], the first prototype of a MMFB (L=D=28.0 mm), constructed with a Copper mesh ring made of 0.3 mm wire and compactness of 20%, displayed an inherent ability to dissipate mechanical energy in the form of hysteresis. Experiments demonstrated that empirical formulas obtained for metal mesh dampers are accurate to predict the experimentally determined structural stiffness and damping of the MMFB. Adequate load capacity, low friction and reliable rotordynamic performance are important for the ready application of MMFBs into oil-free MTM. Further experiments on a turbocharger driven test rig demonstrate the readiness of the bearings by measuring break-away torque and rotor lift off and touchdown speeds during multiple start up and shutdown tests [2]. The recorded bearing load capacity and drag torque for rotor speeds up to 60 krpm demonstrate bearing performance on par with generation I BFBs [4]. During airborne operation, i.e., with a gas film separating the rotating journal from the bearing, the friction coefficient is two orders of magnitude smaller than that during dry-friction operation at start up (and shut down) where rubbing contact prevails.

METAL MESH FOIL BEARINGS: OPERATION AT HIGH TEMPERATURE

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In 2010 a test rig for dynamic shaker load excitations and identification of force coefficients was constructed [4] and measurements with a MMFB (L=38.0, D=36.5 mm and mesh thickness 2.6 mm) and with a similar size, generation I, bump-type foil bearing proceeded. The experiments reveal that MMFB has favorable stiffness and damping properties to support rotors in high speed, low load applications such as in turbochargers, turbo-compressors, etc. Additionally, the energy dissipation mechanism in MMFB is at least two fold better than that in typical generation I bump type foil bearing.

PROPOSED WORK 2011-12

The main objective of the proposed work is to demonstrate the reliable operation of MMFBs at elevated temperatures (max 200°C) and their ability to survive harsh environments with an adequate thermal management. A high temperature rotor-bearing test rig constructed with NASA funds for evaluation of bump type foil bearings is available, see Figure below. In the rig, an electric heater cartridge inside the hollow rotor warms the rotor to high temperature (max 300°C). The tasks for 2011/12 are: a) Construct two MMFBs fitting the existing bearing casing and shaft diameter, install the MMFBs in the test rig, align and balance the test rotor, and calibrate the high temperature instrumentation. b) Conduct experiments with rotor OD surface temperatures to 200°C and to a top rotor speed of 50 krpm. Measure bearing temperatures as the rotor (slowly) heats up and while providing increasing cooling streams. Assess the effectiveness of the cooling flow rates on managing the bearing and rotor temperatures. Rotor speed up and coast-down measurements will show rotor lift-off and touch down speeds, and the amplitude of motion data will serve to identify system damping ratios and effective bearing stiffnesses. c) Compare the thermal performance of the MMFBs with that recorded for generation I, bump-foil bearings. The results of the research will continue to characterize, both qualitatively and quantitatively, a novel (non proprietary) gas bearing type of low cost, simple in construction, and suitable for operation at high and low temperatures. The products of the research are important manufacturers of turbochargers, turboexpanders and micro gas turbines.

High temperature test rig for evaluation of foil bearing performance: thermal and rotordynamic (NASA funded)

METAL MESH FOIL BEARINGS: OPERATION AT HIGH TEMPERATURE

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BUDGET FROM TRC FOR 2011-2012

Support for graduate student (20 h/week) x $ 1,800 x 12 months Fringe benefits (0.6%) and medical insurance ($191/month) Travel to (US) technical conference Tuition three semesters ($3,802 x 3) Supplies for test rig and construction of test bearings

Total Cost:

Year I

$ $ $ $ $ 21,600 2,419 1,200 10,138 3,200

$ 38,608

References

[1] San Andrés, L., Chirathadam, T. A., and Kim, T.H., 2010, "Measurements of Structural Stiffness and Damping Coefficients in a Metal Mesh Foil Bearing," ASME J. Eng. Gas Turbines Power, Vol. 132(March), p. 032503 [2] San Andrés, L., and Chirathadam T.A., 2011, "Identification of Rotordynamic Force Coefficients of a Metal Mesh Foil Bearing Using Impact Load Excitations," ASME J. Eng. Gas Turbines Power, Vol. 133 (Nov), p. 112501. [3] San Andrés, L., Chirathadam, T., Ryu, K., and Kim, T.H., 2011, "Measurements of Drag Torque, Lift-Off Journal Speed and Temperature in a Metal Mesh Foil Bearing," ASME J. Eng. Gas Turbines Power, Vol. 132, p. 112503 (1-7) [4] San Andrés, L., and Chirathadam, T., 2011, "Metal Mesh Foil Bearings: Effect of Excitation Frequency on Rotordynamic Force Coefficients," ASME Paper GT2011-45257.

METAL MESH FOIL BEARINGS: OPERATION AT HIGH TEMPERATURE

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