Ian A. Waitz

A six-wafer combustion system for a silicon micro gas turbine engine

As part of a program to develop a micro gas turbine engine capable of producing 10-50 W of electrical power in a package less than one cubic centimeter in volume, we present the design, fabrication, packaging, and experimental test results for the 6-wafer combustion system for a silicon microengine. Comprising the main nonrotating functional components of the engine, the device described measures 2.1 cm/spl times/2.1 cm/spl times/0.38 cm and is largely fabricated by deep reactive ion etching through a total thickness of 3800 /spl mu/m. Complete with a set of fuel plenums, pressure ports, fuel injectors, igniters, fluidic interconnects, and compressor and turbine static airfoils, this structure is the first demonstration of the complete hot flow path of a multilevel micro gas turbine engine. The 0.195 cm/sup 3/ combustion chamber is shown to sustain a stable hydrogen flame over a range of operating mass flows and fuel-air mixture ratios and to produce exit gas temperatures in excess of 1600 K. It also serves as the first experimental demonstration of stable hydrocarbon microcombustion within the structural constraints of silicon. Combined with longevity tests at elevated temperatures for tens of hours, these results demonstrate the viability of a silicon-based combustion system for micro heat engine applications.

Combustors for micro-gas turbine engines

The development ofa hydrogen-air microcombustor is described. The combustor is intended for use in a 1 mm 2 inlet area, micro-gas turbine engine. While the size of the device poses several difficulties, it also provides new and unique opportunities. The combustion concept investigated is based upon introducing hydrogen and premixing it with air upstream of the combustor. The wide flammability limits of hydrogen-air mixtures and the use of refractory ceramics enable combustion at lean conditions, obviating the need for both a combustor dilution zone and combustor wall cooling. The entire combustion process is carried out at temperatures below the limitations set by material properties, resulting in a significant reduction of complexity when compared to larger-scale gas turbine combustors. A feasibility study with initial design analyses is presented, followed by experimental results from 0.13 cm 3 silicon carbide and steel microcombustors. The combustors were operated for tens of hours, and produced the requisite heat release for a microengine application over a range of fuel-air ratios, inlet temperatures, and pressures up to four atmospheres. Issues of flame stability, heat transfer, ignition and mixing are addressed. A discussion of requirements for catalytic processes for hydrocarbon fuels is also presented.

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.

Investigation of a contoured wall injector for hypervelocity mixing augmentation

An experimental and computational investigation of a contoured wall fuel injector is presented. The injector was aimed at enabling shock-enhanced mixing for the supersonic combustion ramjet engines currently envisioned for applications on hypersonic vehicles. Three-dimensional flow field surveys, and temporally resolved planar Rayleigh scattering measurements are presented for Mach 1.7 helium injection into Mach 6 air. These experimental data are compared directly with a three-dimensional Navier-Stokes simulation of the flow about the injector array. Two dominant axial vorticity sources are identified and characterized. The axial vorticity produced strong convective mixing of the injectant with the freestream. Shock-impingement was particularly effective as it assured seeding of baroclinic vorticity directly on the helium/air interface. The vorticity coalesced into a counter-rotating vortex pair of a sense which produced migration of the helium away from the wall. The influences of spatial averaging on the representation of the flow field as well as the importance of the fluctuating component of the flow in producing molecularly-mixed fluid are addressed.

High Power Density Silicon Combustion Systems for Micro Gas Turbine Engines

As part of an effort to develop a microscale gas turbine engine for power generation and micropropulsion applications, this paper presents the design, fabrication, experimental testing, and modeling of the combustion system. Two radial inflow combustor designs were examined; a single-zone arrangement and a primary and dilution-zone configuration. Both combustors were micromachined from silicon using deep reactive ion etching (DRIE) and aligned fusion wafer handing. Hydrogen-air and hydrocarbon-air combustion were stabilized in both devices, each with chamber volumes of 191 mm 3 . Exit gas temperatures as high as 1800 K and power densities in excess of 1100 MW/m 3 were achieved. For the same equivalence ratio and overall efficiency, the dual-zone combustor reached power densities nearly double that of the single-zone design. Because diagnostics in microscale devices are often highly intrusive, numerical simulations were used to gain insight into the fluid and combustion physics. Unlike large-scale combustors, the performance of the microcombustors was found to be mole severely limited by heat transfer and chemical kinetics constraints. Important design trades are identified and recommendations for microcombustor design are presented.

The historical fuel efficiency characteristics of regional aircraft from technological, operational, and cost perspectives

To develop approaches that effectively reduce aircraft emissions, it is necessary to understand the mechanisms that have enabled historical improvements in aircraft efficiency. This paper focuses on the impact of regional aircraft on the US aviation system and examines the technological, operational and cost characteristics of turboprop (TP) and regional jet (RJ) aircraft. Regional aircraft are 40–60% less fuel efficient than their larger narrow- and wide-body counterparts, while RJs are 10–60% less fuel efficient than TPs. Fuel efficiency differences can be explained largely by differences in aircraft operations, not technology. Direct operating costs per revenue passenger kilometer are 2.5–6 times higher for regional aircraft because they operate at lower load factors and perform fewer miles over which to spread fixed costs. Further, despite incurring higher fuel costs, RJs are shown to have operating costs similar to TPs when flown over comparable stage lengths.

Global Mortality Attributable to Aircraft Cruise Emissions

Aircraft emissions impact human health though degradation of air quality. The majority of previous analyses of air quality impacts from aviation have considered only landing and takeoff emissions. We show that aircraft cruise emissions impact human health over a hemispheric scale and provide the first estimate of premature mortalities attributable to aircraft emissions globally. We estimate ∼8000 premature mortalities per year are attributable to aircraft cruise emissions. This represents ∼80% of the total impact of aviation (where the total includes the effects of landing and takeoff emissions), and ∼1% of air quality-related premature mortalities from all sources. However, we note that the impact of landing and takeoff emissions is likely to be under-resolved. Secondary H(2)SO(4)-HNO(3)-NH(3) aerosols are found to dominate mortality impacts. Due to the altitude and region of the atmosphere at which aircraft emissions are deposited, the extent of transboundary air pollution is particularly strong. For example, we describe how strong zonal westerly winds aloft, the mean meridional circulation around 30-60°N, interaction of aircraft-attributable aerosol precursors with background ammonia, and high population densities in combination give rise to an estimated ∼3500 premature mortalities per year in China and India combined, despite their relatively small current share of aircraft emissions. Subsidence of aviation-attributable aerosol and aerosol precursors occurs predominantly around the dry subtropical ridge, which results in reduced wet removal of aviation-attributable aerosol. It is also found that aircraft NO(x) emissions serve to increase oxidation of nonaviation SO(2), thereby further increasing the air quality impacts of aviation. We recommend that cruise emissions be explicitly considered in the development of policies, technologies and operational procedures designed to mitigate the air quality impacts of air transportation.

Endwall Blockage in Axial Compressors

This paper presents a new methodology for quantifying compressor endwall blockage and an approach, using this quantification, for defining the links between design parameters, flow conditions, and the growth of blockage due to tip clearance flow. Numerical simulations, measurements in a low-speed compressor, and measurements in a wind tunnel designed to simulate a compressor clearance flow are used to assess the approach. The analysis thus developed allows predictions of endwall blockage associated with variations in tip clearance, blade stagger angle, inlet boundary layer thickness, loading level, loading profile, solidity, and clearance jet total pressure. The estimates provided by this simplified method capture the trends in blockage with changes in design parameters to within 10 percent. More importantly, however, the method provides physical insight into, and thus guidance for control of, the flow features and phenomena responsible for compressor endwall blockage generation.

Adoption of active learning in a lecture-based engineering class

Three years ago, the Department of Aeronautics and Astronautics at MIT expanded its repertoire of active learning strategies and assessment tools with the introduction of muddiest-point-in-the-lecture cards, electronic response systems, concept tests, peer coaching, course web pages, and web-based course evaluations. This paper focuses on the change process of integrating these active learning strategies into a traditional lecture-based multidisciplinary course, called unified engineering. The description of the evolution of active learning in unified engineering is intended to underscore the motivation and incentives required for bringing about the change, and the support needed for sustaining and disseminating active learning approaches among the instructors.

Enhanced mixing with streamwise vorticity

Abstract A quantitative description is presented of mixing augmentation mechanisms associated with embedded streamwise vortices. The specific context of interest is the flowfield downstream of convoluted (lobed) mixers, but the concepts developed apply to a range of devices that generate such vortices for enhanced mixing. Arguments are presented to illustrate the dependence of mixing augmentation on the strain field associated with the vortices; this strain field increases both the area available for mixing between two streams and the local gradients in fluid properties which provide the driving potential for mixing. Computations and experiments have been carried out to assess the influence of the streamwise vortices on both momentum interchange and mixing on a molecular level. Based on these investigations, scaling laws have been developed for the overall parametric trends of flow field structure and mixing rate as functions of lobe geometry, Reynolds number, stream-to-stream velocity ratio and Mach number.