F. F. Ehrich

Power MEMS and microengines

MIT is developing a MEMS-based gas turbine generator. Based on high speed rotating machinery, this 1 cm diameter by 3 mm thick SiC heat engine is designed to produce 10-20 W of electric power while consuming 10 grams/hr of H/sub 2/. Later versions may produce up to 100 W using hydrocarbon fuels. The combustor is now operating and an 80 W micro-turbine has been fabricated and is being tested. This engine can be considered the first of a new class of MEMS device, power MEMS, which are heat engines operating at power densities similar to those of the best large scale devices made today.

Micro - Heat Engines, Gas Turbines, and Rocket Engines - The MIT Microengine Project -

This is a report on work in progress on microelectrical and mechanical systems (MEMS)-based gas turbine engines, turbogenerators, and rocket engines currently under development at MIT. Fabricated in large numbers in parallel using semiconductor manufacturing techniques, these engines are based on micro-high speed rotating machinery with the same power density as that achieved in their more familiar, full-sized brethren. The micro-gas turbine is a 1 cm diameter by 3 mm thick SiC heat engine designed to produce 10-20 W of electric power or 0.050.1 Nt of thrust while consuming under 10 grams/hr of H 2 . Later versions may produce up to 100 W using hydrocarbon fuels. A liquid fuel, bi-propellant rocket motor of similar size could develop over 3 lb of thrust. The rocket motor would be complete with turbopumps and control valves on the same chip. These devices may enable new concepts in propulsion, fluid control, and por table power generation.

High-speed microfabricated silicon turbomachinery and fluid film bearings

A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels approaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic journal and thrust bearings. This work presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbomachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling.

Low-Order Models for Very Short Hybrid Gas Bearings

A low-order model was created to analyze a small-scale gas bearing with a diameter of 4.1 mm, designed to spin at 2.4 million rpm. Due to microfabrication constraints, the bearing lies outside the standard operating space and stable operation is a challenge. The model is constructed by reference to Newtons second law for the rotor and employs stiffness and damping coefficients predicted by other models. At any operating point it is able to predict (1) whether the journal can sustain stable operation, and (2) the whirling frequency of the journal. Analysis shows that the best way to operate the bearing is in a hybrid mode where the bearing relies on hydrostatics at low speeds and hydrodynamics at high speeds. However, in transitioning from hydrostatic to hydrodynamic operation, the model shows that the bearing is prone to instability problems and great care must be taken in scheduling the bearing pressurization system in the course of accelerating through low and intermediate rotational speeds.

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

Silicon based power MEMS applications require the high-speed micro-rotating machinery to operate stably over a large range of operating conditions. The technical barriers to achieve stable high-speed operation using 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. Over the past few years, a large body of research was conducted at MIT to address these technical challenges; many lessons were learned and new theories were developed related to the dynamic behavior of micro-gas journal and thrust bearings. Based on the above mentioned experience, a gas-bearing supported micro-air turbine was developed with the objectives of demonstrating repeatable, stable high-speed gas-bearing operation and verifying the new micro-gas-bearing analytical models. The key challenge in this endeavor involved the synthesis and integration of the newly-developed gas-bearing theories and insight gained from extensive experimental work. The focus of this paper is on the process and the outcomes of this synthesis, rather than the details and results of the underlying theoretical models which have been previously published. 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 non-uniformity, and a new rotor micro-fabrication methodology. 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 million rpm (equivalent to 370 m/s blade tip speed or a bearing DN of 7 million mm-rpm) was achieved demonstrating the feasibility of MEMS based micro-scale rotating machinery and validating key aspects of the micro-gas-bearing theory.Copyright

Unsteady Flow and Whirl-Inducing Forces in Axial-Flow Compressors: Part I — Experiment

An experimental and theoretical investigation has been conducted to evaluate the effects seen in axial-flow compressors when the centerline of the rotor is displaced from the centerline of the static structure of the engine. This creates circumferentially non-uniform rotor-tip clearances, unsteady flow, and potentially increased clearances if the rotating and stationary parts come in contact. The result not only adversely affects compressor stall margin, pressure rise capability and efficiency but also generates an unsteady, destabilizing, aerodynamic force, called the Thomas/Alford force, which contributes significantly to rotor whirl instabilities in turbomachinery. Determining both the direction and magnitude of this force in compressors, relative to those in turbines, is especially important for the design of mechanically-stable turbomachinery components. Part I of this two-part paper addresses these issues experimentally and Part II presents analyses from relevant computational models.Our results clearly show that the Thomas/Alford force can promote significant backward rotor whirl over much of the operating range of modern compressors, although some regions of zero and forward whirl were found near the design point. This is the first time that definitive measurements, coupled with compelling analyses, have been reported in the literature to resolve the long-standing disparity in findings concerning the direction and magnitude of whirl-inducing forces important in the design of modern axial-flow compressors.Copyright