Stuart A. Jacobson

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.

An electrostatic, on/off microvalve designed for gas fuel delivery for the MIT microengine

The MIT micro-gas turbine engine requires an integrated fuel-metering device in order to implement on-board engine control. Graded fuel control can be achieved with an array of on/off valves. Each valve in the array must withstand an annealing temperature of 1100/spl deg/C during fabrication and open against 1 MPa of supply pressure at 400/spl deg/C operating temperature. This paper presents the design, fabrication and testing of an electrostatic, on/off silicon prototype valve. Tested with nitrogen at room temperature, the valve opened against a differential pressure of 0.9 MPa with 136 V and delivered a mass flow rate of 45 sccm (3.38 g/h). At 0.1y MPa upstream pressure, the helium leak-rate was measured to be 6/spl times/10/sup -3/ sccm. The valve showed no sign of failure after being continuously actuated for more than 10/sup 5/ cycles. The prototype valve will serve as the base-line design for the engine fuel valve array.

A self-acting gas thrust bearing for high-speed microrotors

Micromachines rotating at high speeds require low drag bearings with adequate load capacity and stability. Such bearings must be compatible with the capabilities of microfabrication technology. A self-acting (hydrodynamic) gas thrust bearing was designed, fabricated and tested on a silicon microturbine. Conventional thrust bearing design techniques were adapted from macroscale literature. Microbearing design charts are presented that relate bearing performance to geometry. Such bearings exhibit a design tradeoff between load bearing capability and maximum operating speed (as limited by instabilities). The specific geometry described herein was intended to replace externally pressurized, hydrostatic thrust bearings in an existing device (a 4-mm-diameter silicon microturbine), thus the hydrodynamic bearing design was constrained to be compatible in geometry and fabrication process. The final design consisted of 2.2-/spl mu/m deep by 40-/spl mu/ wide spiral grooves around the 700-/spl mu/m diameter bearing. The bearings were fabricated in silicon with standard RIE and DRIE techniques. Test devices demonstrated lift-off and operation up to 450,000 rpm with a load capacity of 0.03 N. Measurements of load capacity and stiffness were consistent with the analysis.

Fabrication and characterization of a micro turbine/bearing rig

This paper reports on a process to build, package, and instrument a 5-level wafer-bonded micromachined turbine/bearing rig. The process flow involves the use of 5 wafers, 16 masks, and 9 deep silicon etching steps, as well as utilizing aligned wafer bonding, double-sided deep reactive ion etching and laser-assisted-etching. This paper also shows experimental results on flow characteristics of the hydrostatic thrust bearings and the preliminary rotational performance of the device.

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.

Fabrication and Testing of a High-Speed Microscale Turbocharger

A microelectromechanical system (MEMS) turbocharger has been designed, fabricated, and tested as part of a Massachusetts Institute of Technology program aimed at producing a microfabricated gas turbine engine for portable power applications. A gas turbine engine requires high-speed high-efficiency turbomachinery operating at tip speeds of several hundred meters per second. This MEMS turbocharger serves to demonstrate these requirements. The turbochargers silicon rotor, which is supported on hydrostatic gas thrust and journal bearings in a silicon stator housing, was spun to 480 000 rpm, corresponding to a tip speed of 200 m/s. This paper discusses critical fabrication processes that enabled the capabilities of this device. Operational issues and test results are also presented. The turbochargers compressor demonstrated a pressure ratio of 1.21 at a mass flow rate of 0.13 g/s, with a combined compressor-turbine spool efficiency of 0.24. Under these conditions, the turbine produced about 5 W of power. Results from the simultaneous operation of a spinning rotor and burning combustor within the microscale turbocharger are also presented. Experimental results compare well with analytical models and computations.

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

Challenges for Lubrication in High Speed MEMS

Design considerations for lubrication in high speed rotating MEMS devices are discussed, using the MIT Microengine program as the primary example. This device relies on the rotation of a microfabricated silicon rotor at speeds well in excess of one million RPM, and uses gas bearings to support the rotor both in the axial and radial directions. MEMS fabrication places unique and novel constraints on the design of the gas bearing system and these lead to a relatively unusual design space for high-speed lubrication systems. The overall architecture for the microengine lubrication system is described using both experimental and computational data to illustrate the effects of geometric and manufacturing variations on the overall system performance.