Stephen P. Lukachko

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.

Production of sulfate aerosol precursors in the turbine and exhaust nozzle of an aircraft engine

Recent in-flight measurements of aircraft engine exhaust have suggested much higher conversions of fuel sulfur to sulfuric acid aerosols than can be explained by gas-phase oxidation within the exhaust plume. This paper describes the effects of turbine and exhaust nozzle aerodynamics and chemical kinetics on the production of sulfate aerosol precursors in engine exhaust. Results from both one-dimensional (1-D) and two-dimensional (2-D) numerical simulations are presented for a range of flow and chemistry conditions. One-dimensional calculations for the entire postcombustor flow path of an advanced subsonic engine resulted in up to 6.3% sulfur oxidation through the turbine and exhaust nozzle. Results were most sensitive to species concentrations at the combustor exit, combustor exit temperature, and cooling flow mass addition in the turbine. Two-dimensional calculations for a representative turbine demonstrated that intraengine fluid mechanics can increase sulfur oxidation by a factor of 3 across a single blade row because of cooling-induced temperature nonuniformities. Comparisons of averaged 2-D results with 1-D simulations of the same blade row further showed that while 1-D simulations produce the correct trends, the magnitude of change in sulfur oxidation may be underpredicted by as much as 47% over a single blade row if spatial nonuniformities in flow field temperature are not included.

Assessing the impact of aviation on climate

We present an assessment of the marginal climate impacts of new aviation activities. We use impulse response functions derived from carbon-cycle and atmospheric models to estimate changes in surface temperature for various aviation impacts (CO 2 , NO X on methane, NO x on ozone, sulfates, soot, and contrails/induced cirrus). We use different damage functions and discount rates to explore health, welfare and ecological costs for a range of assumptions and scenarios. Since uncertainty is high regarding many aviation effects, we explicitly capture some uncertainty by representing several model parameters as probabilistic distributions. The uncertainties are then propagated using Monte Carlo analysis to derive estimates for the impact of these uncertainties on the marginal future climate impacts. Our goal is to provide a framework that will communicate the potential impacts of aviation on climate change under different scenarios and assumptions, and that will allow decision-makers to compare these potential impacts to other aviation environmental impacts. We present results to describe the influence of parametric uncertainties, scenarios, and assumptions for valuation on the expected marginal future costs of aviation impacts. Estimates of the change in global average surface temperature due to aviation are most sensitive to changes in climate sensitivity, the radiative forcing attributed to short-lived effects (in particular those related to contrails and aviation-induced cirrus), and the choice of emissions scenario. Estimates of marginal future costs of aviation are most sensitive to assumptions regarding the discount rate, followed by assumptions regarding climate sensitivity, and the choice of emissions scenario.

Aircraft and Energy Use

bypass ratio The ratio of air passed through the fan system to that passed through the engine core. contrail The condensation trail that forms when moist, high-temperature air in a jet exhaust, as it mixes with ambient cold air, condenses into particles in the atmosphere and saturation occurs. drag The aerodynamic force on an aircraft body; acts against the direction of aircraft motion. energy intensity (EI) A measure of aircraft fuel economy on a passenger-kilometer basis; denoted by energy used per unit of mobility provided (e.g., fuel consumption per passenger-kilometer) energy use (EU) A measure of aircraft fuel economy on a seat-kilometer basis (e.g., fuel consumption per seatkilometer). great circle distance The minimum distance between two points on the surface of a sphere. hub-and-spoke system Feeding smaller capacity flights into a central hub where passengers connect with flights on larger aircraft that then fly to the final destination. lift-to-drag ratio (L/D) A measure of aerodynamic efficiency; the ratio of lift force generated to drag experienced by the aircraft. load factor The fraction of passengers per available seats. radiative forcing A measure of the change in Earth’s radiative balance associated with atmospheric changes; positive forcing indicates a net warming tendency relative to preindustrial times. structural efficiency(OEW/MTOW) The ratio of aircraft operating empty weight (OEW) to maximum takeoff weight (MTOW); a measure of the weight of the aircraft structure relative to the weight it can carry (combined weights of structure plus payload plus fuel). thrust A force that is produced by engines and propels the aircraft. thrust specific fuel consumption (SFC) A measure of engine efficiency as denoted by the rate of fuel consumption per unit thrust (e.g., kilograms/second/Newton). turbofan engine The dominant mode of propulsion for commercial aircraft today; a turbofan engine derives its thrust primarily by passing air through a large fan system driven by the engine core.

Gas turbine engine durability impacts of high fuel-air ratio combustors: Part II: Near-wall reaction effects on film-cooled heat transfer

As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, the potential for significant chemical reactions on a film-cooled surface is enhanced. Currently, there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. A shock tube experiment was employed to generate short duration, high temperature (1000-2800 K) and pressure (6 atm) flows over a film-cooled flat plate. The test plate contained two sets of 35 deg film cooling holes that could be supplied with different gases, one side using air and the other nitrogen. A mixture of ethylene and argon provided a fuel rich freestream that reacted with the air film resulting in near wall reactions. The relative increase in surface heat flux due to near wall reactions was investigated over a range of fuel levels, momentum blowing ratios (0.5-2.0), and Damkohler numbers (ratio of flow to chemical time scales) from near zero to 30. For high Damkohler numbers, reactions had sufficient time to occur and increased the surface heat flux by 30 percent over the inert cooling side. When these results are appropriately scaled, it is shown that in some situations of interest for gas turbine engine environments significant increases in surface heat flux can be produced due to chemical reactions in the film-cooling layer. It is also shown that the non-dimensional parameters Damkohler number (Da), blowing ratio (B), heat release potential (H*), and scaled heat flux (Q s ) are the appropriate quantities to predict the augmentation in surface heat flux that arises due to secondary reactions.

Effects of Engine Aging on Aircraft NOx Emissions

The impact of representative paths of engine degradation on aircraft cruise NOx emissions was investigated. Engine cycles corresponding to older, current, and future subsonic and supersonic technologies were developed and used to determine the sensitivities of combustor conditions to various changes in component efficiencies and flow capacities due to aging. Estimates of relative changes in NOx emissions levels were made using established empirical correlations that relate emissions to combustor flow parameters. The analysis methodology was validated through comparisons to test data where available.It was found that the sensitivity of specific fuel consumption (SFC) and combustor flow parameters to component aging is enhanced by increases in cycle temperatures and pressures. This ultimately results in a higher sensitivity of NOx emissions to engine degradation for cycles representative of more advanced technology. At constant thrust, turbine degradation typically acts to decrease NOx emissions while compressor aging results in an increase in NOx emissions; both occurring at the expense of SFC. Sample degradation scenarios were used to highlight the balance between turbine and compressor aging effects. Changes in NOx emissions in the −1% to +4% range were predicted for typical aging scenarios. The applicability of these results to the formulation of aircraft emissions inventories is considered.Copyright

MILITARY AVIATION AND THE ENVIRONMENT: HISTORICAL TRENDS AND COMPARISON TO CIVIL AVIATION

Trends in the environmental impact of military aviation between 1960 and 2000 are articulated. The focus is on community noise, local air quality, and global climate impacts and the discussion is restricted to fixed-wing aircraft. Comparisons are made to trends within the commercial air transport industry. The unique features of military aircraft technology and operations responsible for the differences in environmental impacts are described. The discussion also considers the effects of environmental restrictions on the deployment and combat readiness of military aviation services. Regulations designed to mitigate environmental impacts from military and civil aviation are also reviewed

Engine Design and Operational Impacts on Particulate Matter Precursor Emissions

Aircraft emissions of trace sulfur and nitrogen oxides contribute to the generation of fine volatile particulate matter (PM). Resultant changes to ambient PM concentrations and radiative properties of the atmosphere may be important sources of aviation-related environmental impacts. This paper addresses engine design and operational impacts on aerosol precursor emissions of SO x and NO y species. Volatile PM formed from these species in the environment surrounding an aircraft is dependent on intraengine oxidation processes occurring both within and downstream of the combustor. This study examines the complex response of trace chemistry to the temporal and spatial evolution of temperature and pressure along this entire intraengine path after combustion through the aft combustor, turbine, and exhaust nozzle. Low-order and higher-fidelity tools are applied to model the interaction of chemical and fluid mechanical processes, identify important parameters, and assess uncertainties. The analysis suggests that intraengine processing is inefficient. For in-service engine types in the large commercial aviation fleet, mean conversion efficiency (e) is estimated to be 2.8-6.5% for sulfate precursors and 0.3-5.7% for nitrate precursors at the engine exit plane. These ranges reflect technological differences within the fleet, a variation in oxidative activity with operating mode, and modeling uncertainty stemming from variance in rate parameters and initial conditions. Assuming that sulfur-derived volatile PM is most likely, these results suggest emission indices of 0.06-0.13 g/kg fuel, assuming particles nucleated as 2H 2 SO 4 ·H 2 O for a fuel sulfur content of 500 ppm.

Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors—Part I: Potential for Secondary Combustion of Partially Reacted Fuel

Demand for greater engine efficiency and thrust-to-weight ratio has driven the production of aircraft engines with higher core temperatures and pressures. Such engines operate at higher fuel–air ratios, resulting in the potential for significant heat release through the turbine if energetic species emitted from the combustor are further oxidized. This paper outlines the magnitude and potential for turbine heat release for current and future engines. The analysis indicates that in the future, high fuel-air ratio designs may have to consider changes to cooling strategies to accommodate heat release resulting from secondary combustion. A characteristic time methodology is developed to evaluate the chemical and fluid mechanical conditions that lead to combustion within the turbine. The local concentration of energetic emissions partly determines the potential for energy release. An energy release parameter, here defined as a maximum increase in total temperature (∆Tt), is used to specify an upper limit on the magnitude of impact. The likelihood of such impacts relies on the convective, mixing, and chemical processes that determine the fate and transport of energetic species through the turbine. Appropriately defined Damkohler numbers (Da)—the comparative ratio of a characteristic flow time (τflow) to a characteristic chemical time (τchem)—are employed to capture the macroscopic physical features controlling the flow-chemistry interactions that lead to heat release in the turbine.