Alan H. Epstein

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.

1997 Best Paper Award—Controls and Diagnostics Committee: Active Stabilization of Rotating Stall and Surge in a Transonic Single-Stage Axial Compressor

Rotating stall and surge have been stabilized in a transonic single-stage axial compressor using active feedback control. The control strategy is to sense upstream wall static pressure patterns and feed back the signal to an annular array of twelve separately modulated air injectors. At tip relative Mach numbers of 1.0 and 1.5 the control achieved 11 and 3.5 percent reductions in stalling mass flow, respectively, with injection adding 3.6 percent of the design compressor mass flow. The aerodynamic effects of the injection have also been examined. At a tip Mach number, M tip , of 1.0, the stall inception dynamics and effective active control strategies are similar to results for low-speed axial compressors. The range extension was achieved by individually damping the first and second spatial harmonics of the prestall perturbations using constant gain feedback. At a M tip of 1.5 (design rotor speed), the prestall dynamics are different than at the lower speed. Both one-dimensional (surge) and two-dimensional (rotating stall) perturbations needed to be stabilized to increase the compressor operating range. At design speed, the instability was initiated by approximately ten rotor revolutions of rotating stall followed by classic surge cycles. In accord with the results from a compressible stall inception analysis, the zeroth, first, and second spatial harmonics each include more than one lightly damped mode, which can grow into the large amplitude instability. Forced response testing identified several modes traveling up to 150 percent of rotor speed for the first three spatial harmonics; simple constant gain control cannot damp all of these modes and thus cannot stabilize the compressor a this speed. A dynamic, model-based robust controller was therefore us to stabilize the multiple modes that co prise the first three harmonic perturbations in this transonic region of operation.

Millimeter-Scale MEMS Gas Turbine Engines

The confluence of market demand for greatly improved compact power sources for portable electronics with the rapidly expanding capability of micromachining technology has made feasible the development of gas turbines in the millimeter-size range. With airfoil spans measured in 100’s of microns rather than meters, these “microengines” have about 1 millionth the air flow of large gas turbines and thus should produce about 1 millionth the power, 10-100 W. Based on semiconductor industry-derived processing of materials such as silicon and silicon carbide to submicron accuracy, such devices are known as micro-electro-mechanical systems (MEMS). Current millimeter-scale designs use centrifugal turbomachinery with pressure ratios in the range of 2:1 to 4:1 and turbine inlet temperatures of 1200-1600 K. The projected performance of these engines are on a par with gas turbines of the 1940’s. The thermodynamics of MEMS gas turbines are the same as those for large engines but the mechanics differ due to scaling considerations and manufacturing constraints. The principal challenge is to arrive at a design which meets the thermodynamic and component functional requirements while staying within the realm of realizable micromachining technology. This paper reviews the state-of-the-art of millimeter-size gas turbine engines, including system design and integration, manufacturing, materials, component design, accessories, applications, and economics. It discusses the underlying technical issues, reviews current design approaches, and discusses future development and applications.

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.

Microfabrication of a high pressure bipropellant rocket engine

A high pressure bipropellant rocket engine has been successfully micromanufactured by fusion bonding a stack of six individually etched single crystal silicon wafers. In order to test the device, an innovative packaging technique was developed to deliver liquid coolant and gaseous propellants to the rocket chip at pressures in excess of 200 atm at temperatures above 300°C. Testing continues on the 1.2 g devices, which have been run to date at a chamber pressure of 12 atm, generating 1 N of thrust, and delivering a thrust power of 750 W.

Active Stabilization of Rotating Stall in a Three-Stage Axial Compressor

A three-stage, low-speed axial research compressor has been actively stabilized by damping low-amplitude circumferentially traveling waves, which can grow into rotating stall. Using a circumferential array of hot-wire sensors, and an array of high-speed individually positioned control vanes as the actuator, the first and second spatial harmonics of the compressor were stabilized down to a characteristic slope of 0.9, yielding an 8 percent increase in operating flow range. Stabilization of the third spatial harmonic did not alter the stalling flow coefficient. The actuators were also used open loop to determine the forced response behavior of the compressor

Prestall Behavior of Several High-Speed Compressors

High-speed compressor data immediately prior to rotating stall inception are analyzed and compared to stability theory. New techniques for the detection of small-amplitude rotating waves in the presence of noise are detailed, and experimental and signal processing pitfalls discussed. In all nine compressors examined, rotating stall precedes surge. Prior to rotating stall inception, all the machines support small-amplitude (<1 percent offully deueloped stall) waues traueling about the circumference. Traveling wave strength and structure are shown to be a strong function of corrected speed. At low speeds, a ∼0.5 times shaft speed wave is present for hundreds of rotor revolutions prior to stall initiation. At 100 percent speed, a shaft speed rotating wave dominates, growing as stall initiation is approached fully developed rotating stall occurs at about 1/2 of shaft speed). A new, two-dimensional, compressible hydrodynamic stability analysis is applied to the geometry of two of the compressors and gives results in agreement with data. The calculations show that, at low corrected speeds, these compressors behave predominantly as incompressible machines. The wave that first goes unstable is the 1/2 shaft frequency mode predicted by the incompressible Moore-Greitzer analysis and previously observed in low-speed compressors. Compressibility becomes important at high corrected speeds and adds axial structure to the rotating waves. At 100 percent corrected speed, one of these hitherto unrecognized compressible modes goes unstable first. The rotating frequency of this mode is constant and predicted to be approximately coincident with shaft speed at design. Thus, it is susceptible to excitation by geometric nonuniformities in the compressor. This new understanding of compressor dynamics is used to introduce the concept of traveling wave energy as a real time measure of compressor stability. Such a wave energy-based scheme is shown consistently to give an indication of low stability for significant periods (100-200 rotor revolutions) before stall initiation, even at 100 percent corrected speed

High-Pressure Bipropellant Microrocket Engine

The development of high-aspect-ratio, high-precision micromachining in silicon or silicon carbide suggests the feasibility of microfabricated, high-chamber-pressure chemical rocket engines. With high-speed turbopumps and valvesincorporatedontotherocketchip,anumberofpropulsioncyclesarepossible.Suchanengine,approximately 20 ££ 15 £ 3 mm in size, operating at a 125-atm chamberpressure, would produceabout 15 N of thrust using 300-s Isp propellants at a thrust-to-weight ratio of 1000:1. The feasibility of these engines has been investigated, and a liquid-cooled, pressure-fed thrust chamber and nozzle have been successfully designed, fabricated, and operated at a 12.5-atm chamber pressure to further evaluate the concept.

Modeling for control of rotating stall

Abstract An analytical model for control of rotating stall has been obtained from the basic fluid equations describing the process at inception. The model describes rotating stall as a traveling wave packet, sensed—in spatial components—via the Fourier decomposition of measurements obtained from a circumferential array of evenly distributed sensors (hot wires) upstream of the compressor. A set of “wiggly” inlet guide vanes (IGVs) equally spaced around the compressor annulus constitute the “forced” part of the model. Control is effected by launching waves at appropriate magnitude and phase, synthesized by spatial Fourier synthesis from individual IGV deflections. The effect of the IGV motion on the unsteady fluid process was quantified via identification experiments carried out on a low speed, single-stage axial research compressor. These experiments served to validate the theoretical model and refine key parameters in it. Further validation of the model was provided by the successful implementation of a complex-valued proportional control law, using a combination of first and second harmonic feedback; this resulted in an 18% reduction of stalling mass flow, at essentially the same pressure rise.