Research Papers: Micro-Nano Tribology

High-Speed Operation of a Gas-Bearing Supported MEMS-Air Turbine

[+] Author and Article Information
C. J. Teo, L. X. Liu, H. Q. Li, L. C. Ho, S. A. Jacobson, F. F. Ehrich, A. H. Epstein, Z. S. Spakovszky

Department of Aeronautics and Astronautics, Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139

An isotropic journal bearing has a circumferentially uniform direct-coupled radial stiffness.

Although two peaks may be discernible in Fig. 6, they are significantly closer together than those in Fig. 6. Strictly speaking, the journal bearing in Fig. 6 operates in a nearly (albeit not perfectly) isotropic mode.

A rotor failure corresponds to physical contact between the high-speed spinning silicon rotor and stator, causing the brittle rotor to fracture, thus rendering it no longer operable.

J. Tribol 131(3), 032001 (May 27, 2009) (9 pages) doi:10.1115/1.3123343 History: Received April 15, 2006; Revised February 27, 2009; Published May 27, 2009

Silicon based power micro-electro-mechanical system (MEMS) applications require high-speed microrotating machinery operating stably over a large range of operating conditions. The technical barriers to achieving stable high-speed operation with micro-gas-bearings are governed by (1) stringent fabrication tolerance requirements and manufacturing repeatability, (2) structural integrity of the silicon rotors, (3) rotordynamic coupling effects due to leakage flows, (4) bearing losses and power requirements, and (5) transcritical operation and whirl instability issues. To enable high-power density the micro-turbomachinery must be run at tip speeds comparable to conventional scale turbomachinery. The rotors of the micro-gas turbines are supported by hydrostatic gas journal and hydrostatic gas thrust bearings. Dictated by fabrication constraints the location of the gas journal bearings is at the outer periphery of the rotor. The high bearing surface speeds (target nearly 10×106mmrpm), the very low bearing aspect ratios (L/D<0.1), and the laminar flow regime in the bearing gap (Re<500) place these micro-bearing designs into unexplored regimes in the parameter space. A gas-bearing supported micro-air turbine was developed with the objectives of demonstrating repeatable, stable high-speed gas-bearing operation and verifying the previously developed micro-gas-bearing analytical models. The paper synthesizes and integrates the established micro-gas-bearing theories and insight gained from extensive experimental work. The characteristics of the new micro-air turbine include a four-chamber journal bearing feed system to introduce stiffness anisotropy, labyrinth seals to avoid rotordynamic coupling effects of leakage flows, a reinforced thrust bearing structural design, a redesigned turbine rotor to increase power, a symmetric feed system to avoid flow and force nonuniformity, and a new rotor micro-fabrication methodology for reduced rotor imbalance. A large number of test devices were successfully manufactured demonstrating repeatable bearing geometry. More specifically, three sets of devices with different journal bearing clearances were produced to investigate the dynamic behavior as a function of bearing geometry. Experiments were conducted to characterize the “as-fabricated” bearing geometry, the damping ratio, and the natural frequencies. Repeatable high-speed bearing operation was demonstrated using isotropic and anisotropic bearing settings reaching whirl-ratios between 20 and 40. A rotor speed of 1.7×106rpm (equivalent to 370 m/s blade tip speed or a bearing DN number of 7×106mmrpm) was achieved demonstrating the feasibility of MEMS-based micro-scale rotating machinery and validating key aspects of the micro-gas-bearing theory.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Cross sectional view of the gas-bearing supported MEMS-air turbine

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Figure 2

Four-chamber journal bearing feed system: (a) isotropic operation and (b) anisotropic operation

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Figure 3

Key structural features of the MEMS-air turbine

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Figure 4

Exchange of neighboring rotors prior to the fusion bonding process results in devices with different journal bearing clearances

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Figure 5

Journal bearing flow characteristics. The symbols correspond to experimental measurements in devices with target journal bearing clearance of 16 μm.

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Figure 6

Measured transcritical journal bearing response for (a) anisotropic journal bearing operation and (b) isotropic journal bearing operation in the same device

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Figure 7

Measured top rotor speeds for different devices as a function of journal differential pressure ΔP

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Figure 8

Comparison of experimental and analytical-CFD predictions of isotropic journal bearing stability boundary as a function of hydrostatic differential pressure ΔP

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Figure 9

Comparison of typical operating protocols for previous and current-generation micro-air turbines. The top rotor speed for current-generation devices demonstrated at 1.7×106 rpm.

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Figure 10

Whirl-ratio for different devices as a function of journal bearing damping ratio

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Figure 11

SEM pictures of fillet at the root of a turbine blade




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