C. W. Wilson

Nonintrusive optical measurements of aircraft engine exhaust emissions and comparison with standard intrusive techniques

Nonintrusive systems for the measurement on test rigs of aeroengine exhaust emissions required for engine certification (CO, NO(x), total unburned hydrocarbon, and smoke), together with CO(2) and temperature have been developed. These results have been compared with current certified intrusive measurements on an engine test. A spectroscopic database and data-analysis software has been developed to enable Fourier-transform Infrared measurement of concentrations of molecular species. CO(2), CO, and NO data showed agreement with intrusive techniques of approximately ?30%. A narrow-band spectroscopic device was used to measure CO(2) (with deviations of less than ?10% from the intrusive measurement), whereas laser-induced incandescence was used to measure particles. Future improvements to allow for the commercial use of the nonintrusive systems have been identified and the methods are applicable to any measurement of combustion emissions.

Sustainability of supply or the planet: a review of potential drop-in alternative aviation fuels

The development of kerosene-like drop-in alternative aircraft fuels can be categorised into two groups, depending on whether the product increases supply security or provides a reduced environmental footprint. This paper uncovers this relationship through a review of commercially available process technologies (Fischer Tropsch and hydroprocessing) to produce alternative fuels, lifecycle results and recent flight test campaigns, before evaluating the prospects for future fuel development. Supply may be improved through the conversion of coal (with carbon sequestration) or natural gas using the Fischer Tropsch process. Refinement of these alternative fossil fuels, however, provides comparable total life cycle emissions to Jet A-1. The hydroprocessing of biomass feedstock provides for a reduced environmental footprint—approximately 30% reduction for sustainable cultivated feedstock, when blended 50/50 with conventional jet fuel. However, securing supply is a significant issue. Considering aviation is responsible for 2.6% of global CO2 emissions, converting 6% of arable land (representing 0.95% of the earth surface) to supply a 50/50 blend, thus offsetting 0.78% of global CO2 emissions, seems impractical based upon the current land use scenario. Furthermore, ground based sectors have significant environmental footprints compared to aviation, yet require little pre-processing of feedstock (i.e. power generation can burn raw feedstock), thus presenting a better biomass opportunity cost.

Combustion of Kerosene in Counterflow Diffusion Flames

Numerical modeling has become an essential tool in combustion research as a means of predicting combustion performance and pollutant formation, e.g., NO x and soot. In many combustion models the combustion of a commercial fuel such as kerosene has been represented by single-step empirical expressions. To predict kinetically controlled phenomena, a more detailed chemical kinetic reaction mechanism is required. This paper reports the development of such a mechanism for kerosene, where, for the purposes of modeling, kerosene is assumed to be 89%n-decane and 11% toluene. The mechanism is initially validated against experimental jet stirred reactor and rich premixed e ame studies to yield satisfactory results. The chemical structure of countere ow diffusion e ames is computed using the same mechanism. The effect on the e ame structure of increasing both the pressure and the strain rate is explored. The inclusion of a model for thermal radiation using the optically thin approximation demonstrates the large radiative heat losses encountered as the pressure is increased. The calculations form the foundation ofa e ameletlibraryforthemodelingofturbulentnonpremixed combustion ofkeroseneunderpractical conditions.

Impact of Alternative Fuels on Emissions Characteristics of a Gas Turbine Engine ? Part 1: Gaseous and Particulate Matter Emissions

Growing concern over emissions from increased airport operations has resulted in a need to assess the impact of aviation related activities on local air quality in and around airports, and to develop strategies to mitigate these effects. One such strategy being investigated is the use of alternative fuels in aircraft engines and auxiliary power units (APUs) as a means to diversify fuel supplies and reduce emissions. This paper summarizes the results of a study to characterize the emissions of an APU, a small gas turbine engine, burning conventional Jet A-1, a fully synthetic jet fuel, and other alternative fuels with varying compositions. Gas phase emissions were measured at the engine exit plane while PM emissions were recorded at the exit plane as well as 10 m downstream of the engine. Five percent reduction in NO(x) emissions and 5-10% reduction in CO emissions were observed for the alternative fuels. Significant reductions in PM emissions at the engine exit plane were achieved with the alternative fuels. However, as the exhaust plume expanded and cooled, organic species were found to condense on the PM. This increase in organic PM elevated the PM mass but had little impact on PM number.

Incorporation of physical bounds on rate parameters for reaction mechanism optimization using genetic algorithms

In this study a genetic algorithm (GA) approach for determining new reaction rate parameters ( A , g , and E a in the non-Arrhenius expressions) for the combustion of a hydrogen/air mixture in a perfectly stirred reactor (PSR) is assessed. A new floating-point coded GA and fitness function have been developed that dramatically increase both the rate of convergence and the predictive accuracy of the algorithm, thus promising the extension of the method to more detailed reaction schemes. Output profiles of species for 20 sets of PSR conditions, obtained from an original set of rate constants, are reproduced following a GA optimization inversion process. The new sets of rate constants following each iteration are constrained to lie between predefined boundaries that represent the uncertainty associated with the experimental findings listed in the National Institute of Standards and Technology (NIST) database. Comparisons with previous optimization work have demonstrated that those mechanisms generated using the NIST constraints can be applied to combustion scenarios outside those used in the mechanisms construction. In addition, the flexibility of the GA has been demonstrated by its success in generating reaction rate coefficients that reproduce a set of randomly perturbed species profiles.

Polycyclic Aromatic Hydrocarbon Emissions from the Combustion of Alternative Fuels in a Gas Turbine Engine

We report on the particulate-bound polycyclic aromatic hydrocarbons (PAH) in the exhaust of a test-bed gas turbine engine when powered by Jet A-1 aviation fuel and a number of alternative fuels: Sasol fully synthetic jet fuel (FSJF), Shell gas-to-liquid (GTL) kerosene, and Jet A-1/GTL 50:50 blended kerosene. The concentration of PAH compounds in the exhaust emissions vary greatly between fuels. Combustion of FSJF produces the greatest total concentration of PAH compounds while combustion of GTL produces the least. However, when PAHs in the exhaust sample are measured in terms of the regulatory marker compound benzo[a]pyrene, then all of the alternative fuels emit a lower concentration of PAH in comparison to Jet A-1. Emissions from the combustion of Jet A-1/GTL blended kerosene were found to have a disproportionately low concentration of PAHs and appear to inherit a greater proportion of the GTL emission characteristics than would be expected from volume fraction alone. The data imply the presence of a nonlinear relation between fuel blend composition and the emission of PAH compounds. For each of the fuels, the speciation of PAH compounds present in the exhaust emissions were found to be remarkably similar (R(2) = 0.94-0.62), and the results do provide evidence to support the premise that PAH speciation is to some extent indicative of the emission source. In contrast, no correlation was found between the PAH species present in the fuel with those subsequently emitted in the exhaust. The results strongly suggests that local air quality measured in terms of the particulate-bound PAH burden could be significantly improved by the use of GTL kerosene either blended with or in place of Jet A-1 kerosene.

MEASUREMENT AND ANALYSIS OF FLAME TRANSFER FUNCTION IN A SECTOR COMBUSTOR UNDER HIGH PRESSURE CONDITIONS

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. A flame transfer function describes the change in the rate of heat release in response to perturbations in the inlet flow as a function of frequency. It is a quantitative assessment of the susceptibility of combustion to disturbances. The resulting fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. Flame transfer functions for LPP combustion are poorly understood at present but are crucial for predicting combustion oscillations. This paper describes an experiment designed to measure the flame transfer function of a simple combustor incorporating realistic components. Tests were conducted initially on this combustor at atmospheric pressure (1.2 bar and 550 K) to make an early demonstration of the combustion system. The test rig consisted of a plenum chamber with an inline siren, followed by a single LPP premixer/duct and a combustion chamber with a silencer to prevent natural instabilities. The siren was used to induce variable frequency pressure/acoustic signals into the air approaching the combustor. Both unsteady pressure and heat release measurements were undertaken. There was good coherence between the pressure and heat release signals. At each test frequency, two unsteady pressure measurements in the plenum were used to calculate the acoustic waves in this chamber and hence estimate the mass-flow perturbation at the fuel injection point inside the LPP duct. The flame transfer function relating the heat release perturbation to this mass flow was found as a function of frequency. The same combustor hardware and associated instrumentation were then used for the high pressure (15 bar and 800 K) tests. Flame transfer function measurements were taken at three combustion conditions that simulated the staging point conditions (Idle, Approach and Take-off) of a large turbofan gas turbine. There was good coherence between pressure and heat release signals at Idle, indicating a close relationship between acoustic and heat release processes. Problems were encountered at high frequencies for the Approach and Take-off conditions, but the flame transfer function for the Idle case had very good qualitative agreement with the atmospheric-pressure tests. The flame transfer functions calculated here could be used directly for predicting combustion oscillations in gas turbine using the same LPP duct at the same operating conditions. More importantly they can guide work to produce a general analytical model.Copyright

Impact of Alternative Fuels on Emissions Characteristics of a Gas Turbine Engine − Part 2: Volatile and Semivolatile Particulate Matter Emissions

The work characterizes the changes in volatile and semivolatile PM emissions from a gas turbine engine resulting from burning alternative fuels, specifically gas-to-liquid (GTL), coal-to-liquid (CTL), a blend of Jet A-1 and GTL, biodiesel, and diesel, to the standard Jet A-1. The data presented here, compares the mass spectral fingerprints of the different fuels as measured by the Aerodyne high resolution time-of-flight aerosol mass spectrometer. There were three sample points, two at the exhaust exit plane with dilution added at different locations and another probe located 10 m downstream. For emissions measured at the downstream probe when the engine was operating at high power, all fuels produced chemically similar organic PM, dominated by C(x)H(y) fragments, suggesting the presence of long chain alkanes. The second largest contribution came from C(x)H(y)O(z) fragments, possibly from carbonyls or alcohols. For the nondiesel fuels, the highest loadings of organic PM were from the downstream probe at high power. Conversely, the diesel based fuels produced more organic material at low power from one of the exit plane probes. Differences in the composition of the PM for certain fuels were observed as the engine power decreased to idle and the measurements were made closer to the exit plane.

A Novel Approach to Mechanism Reduction Optimization for an Aviation Fuel/Air Reaction Mechanism Using a Genetic Algorithm

This study presents a novel multiobjective genetic-algorithm approach to produce a new reduced chemical kinetic reaction mechanism to simulate aviation fuel combustion under various operating conditions. The mechanism is used to predict the flame structure of an aviation fuel/O 2 /N 2 flame in both spatially homogeneous and one-dimensional premixed combustion. Complex hydrocarbon fuels, such as aviation fuel, involve large numbers of reaction steps with many species. As all the reaction rate data are not well known, there is a high degree of uncertainty in the results obtained using these large detailed reaction mechanisms. In this study a genetic algorithm approach is employed for determining new reaction rate parameters for a reduced reaction mechanism for the combustion of aviation fuel-air mixtures. The genetic algorithm employed incorporates both perfectly stirred reactor and laminar premixed flame data in the inversion process, thus producing an efficient reaction mechanism. This study provides an optimized reduced aviation fuel-air reaction scheme whose performance in predicting experimental major species profiles and ignition delay times is not only an improvement on the starting reduced mechanism but also on the full mechanism.

Evolution of carbonaceous aerosol and aerosol precursor emissions through a jet engine

This study conducted during the summers of 2000 and 2001 represents the first measurement and model intercomparison that tracks detailed gaseous and aerosol emissions through a gas turbine engine. Its primary objective was to determine the impacts of engine operational state on the evolution of carbonaceous aerosol and aerosol precursors. Emissions measurements were performed at the exit of a combustor and at the exit of a full engine for a gas turbine engine typical of the in-service, commercial aircraft fleet. Measurements were compared to model simulations of changes in gaseous chemistry. As predicted by the model simulations, results show no significant modifications to the aerosol distribution along the postcombustor flowpath. The oxidation of NO to HONO was measured. Trends with engine power setting and sulfur loading were at the level of estimated uncertainty limits. Simulations of the fluid and chemical processes through the turbine and exhaust nozzle correctly captured HONO trends and matched experimental data within measurement uncertainty. This suggests that the employed modeling approach is valid for HONO chemistry, and more generally, because HONO results from NO oxidation via the hydroxyl radical, indicates the importance of OH-driven oxidation through the engine. These results indicate that the chemical and physical processes occurring in the turbine are important in determining aircraft engine emissions.