Linda Shafer

Pressure effects on flowing mildly-cracked n-decane

It is anticipated that traditional methods of cooling that employ the sensible heat transfer provided by fuels will not be sufficient to meet the cooling requirements of future high-performance aircraft. One potential solution is the use of endothermic fuels, which absorb heat through chemical reactions. However, few studies have analyzed the effects of pressure on a chemically reacting, flowing fuel. An experiment is described that studies the effects of pressure on flowing, mildly cracked, supercritical n-decane. The experimental results are studied with the aid of a unique two-dimensional computational fluid dynamics model that simulates the formation of cracked products from experimentally derived proportional distributions. This model is used to study the effect of pressure on the flow properties of the fuel. The experiments indicate that increasing pressure enhances bimolecular pyrolysis reactions, relative to unimolecular reactions. Increasing pressure also increases the overall conversion rate of supercritical n-decane flowing through a reactor. This is primarily because pressure increases the density, which increases the residence time of n-decane flowing through the reactor.

Combustion Products of Petroleum Jet Fuel, a Fischer–Tropsch Synthetic Fuel, and a Biomass Fatty Acid Methyl Ester Fuel for a Gas Turbine Engine

We report combustion emissions data for several alternatives to petroleum based Jet A jet fuel, including a natural gas–derived Fischer–Tropsch (FT) synthetic fuel; a 50/50 blend of the FT synthetic fuel with Jet A-1; a 20/80 blend of a fatty acid methyl ester (FAME) with jet fuel; and a 40/60 blend of FAME with jet fuel. The chief distinguishing features of the alternative fuels are reduced (for blends) or negligible (for pure fuels) aromatic content and increased oxygen content (for FAME blends). A CFM International CFM56-7 gas turbine engine was the test engine, and we measured NOX, CO, speciated volatile organic compounds (including oxygenates, olefins, and aromatic compounds), and nonvolatile particle size distribution, number, and mass emissions. We developed several new methods that account for fuel energy content and used the new methods to evaluate potential fuel effects on emissions performance. Our results are categorized as follows: (1) regulated pollutant emissions, CO, and NOX; (2) volatile organic compound emissions speciation; and (3) particle emissions. Replacing all or part of the petroleum jet fuel with either FAME or FT fuel reduces NOX emissions and may reduce CO emissions. Combustion of FT fuel and fuel blends increases selectivities and in some cases yields of oxygenates and some hydrocarbon volatile organic compound emissions relative to petroleum jet fuel. Combustion of FAME fuel increases propene and butene emissions, but despite its oxygen content does not strongly affect oxygenate emissions. Replacing petroleum jet fuel with zero aromatic alternatives decreases the emissions of aromatic hydrocarbons. The fuel effects become more pronounced as the size of the aromatic molecule increases (e.g., toluene is reduced more strongly than benzene). Particle emissions are decreased in particle size, number density, and total mass when petroleum jet fuel is replaced with the zero aromatic fuels. The effects of fuel composition on particle emissions are most pronounced at lower power conditions, i.e., when combustion temperature and pressure are lower, and less efficient mixing may lead to locally higher fuel/air ratios than are present at higher power.

Hydroprocessed Renewable Jet Fuel Evaluation, Performance, and Emissions in a T63 Turbine Engine

Due to potential beneficial environmental impacts and increased supply availability, alternative fuels derived from renewable resources are evolving on the forefront as unconventional substitutes for fossil fuel. Focus is being given to the evaluation and certification of Hydroprocessed Renewable Jet (HRJ), a fuel produced from animal fat and/or plant oils (triglycerides) by hydroprocessing, as the next potential synthetic aviation fuel. Extensive efforts have recently been performed at the Air Force Research Laboratory (AFRL) at Wright Patterson Air Force Base (WPAFB) to evaluate the potential of two HRJ fuels produced from camelina and tallow feedstocks. These have included characterization of the fuel chemical, physical fuel characteristics and Fit-for-Purpose properties (FFP). The present effort describes general combustion performance and the emission propensity of a T63-A-700 Allison turbine engine operated on the HRJs and 50/50 (by volume) HRJ/JP-8 fuel blends relative to a specification JP-8. In addition, engine and emission testing with a blend of the tallow-derived HRJ and 16% bio-derived aromatic components was completed. Fundamental engine performance characterization allows for determination of the suitability of potential synthetic fuels while quantitation of gaseous and particulate matter emissions provides an assessment of the potential environmental impact compared to current petroleum-derived fuels. In addition, an extended 150 h endurance test was performed using a 50/50 blend of tallow-derived HRJ with JP-8 to evaluate the long-term operation of the engine with the synthetic fuel blend. This paper discusses the laboratory testing performed to characterize HRJs and results from the basic engine operability and emissions studies of the alternative fuel blends.

Extent and Impacts of Hydrocarbon Fuel Compositional Variability for Aerospace Propulsion Systems

Abstract : Kerosene fuels possess physical and chemical properties which make them attractive for aerospace propulsion applications from operational and performance standpoints. However, variation in fuel properties and performance owing to differences in chemical makeup can be significant as operating environments and fuel composition fall outside the realm of current experience. Both circumstances are increasingly frequent, given the incorporation of new fuels in existing systems and a desire to increase vehicle performance. The Air Force Research Laboratory (AFRL) is actively engaged in deriving relationships between fuel composition, properties, and performance in realistic operating conditions. Ideally, these models will be implemented in the optimization of fuel composition to meet requirements for future systems. Moreover, current engine development activities prompt an assessment of as-supplied rocket kerosene, the set of requirements used for its specification, and the potential impacts of compositional variations on engine operability and performance. To address these needs, several lab scale RP-1 formulations were obtained which met specification requirements but were blended from chemically unique feedstocks, thereby representing the expected compositional variation for currently produced fuel. Chemical composition was characterized in terms of hydrocarbon types and was compared between the various formulations.

Comparisons of Emissions Characteristics of Several Turbine Engines Burning Fischer-Tropsch and Hydroprocessed Esters and Fatty Acids Alternative Jet Fuels

A summary of the impacts of alternative fuel blends on the gaseous and particulate matter (PM) (mostly soot) emissions of aircraft turbine engines is presented. Six engines were studied under several US Air Force and NASA sponsored programs to assess the impacts of the alternative (non-petroleum) fuels on emissions and/or to support the certification of military aircraft for the use of 50/50 (by volume) alternative fuel/JP-8 blends. One turboshaft (T63) and five turbofan (CFM56-7, CFM56-2, F117, TF33 and PW308) engines were studied. Fuels derived from coal and natural gas produced via Fischer-Tropsch (FT) synthesis, and fuels from animal fats and plant oils produced via hydroprocessing [Hydroprocessed Esters and Fatty Acids (HEFA)] were evaluated. Trends of alternative fuel impacts on emissions compared to conventional fuel for the different engine types are discussed. Results consistently show significant reductions in PM emissions with the alternative fuel blends compared to operation with conventional fuels. These relative reductions were observed to be lower as engine power increased. Engines operated with different alternative fuel blends were found to produce similar slopes of normalized particle number to engine power with only the magnitude of the reductions being a function of the fuel type. These results suggest that it may be plausible to predict particle number emissions from turbine engines operated on alternative fuels based on engine, engine setting, limited PM data and fuel composition. Gaseous emissions measurements show modest reductions of carbon monoxide, unburned hydrocarbons and hazardous air pollutants (HAPs) with the alternative fuels for several engines; however, no clear dependency of fuel impacts based on engine characteristics were observed.Copyright

Experimental and Computational Studies of Jet Fuel Flow near the Freeze Point

To study the flow behavior of jet fuel at low temperatures, a wing-tank thermal simulator, which represents the fuel tank of a commercial aircraft, was fabricated. The simulator was subjected to cooling in an environmental chamber. Experimental results show that fuel flowability and pumpability decrease substantially as temperature is reduced. Time-dependent temperature and velocity distributions were numerically simulated for static cooling. Viscosities were obtained from viscometer measurements using different jet fuel samples. It was observed that near the freeze-point temperature, low freeze-point fuels tend to have higher viscosities than high freeze-point fuels. Measured viscosities were used in computational-fluid-dynamics simulations of jet fuel that was cooled. The calculations show that stringers, ribs, and other structures strongly promote fuel cooling. Also, the cooler, denser fuel resides near the bottom surface of the fuel tank simulator. The presence of an ullage space within the tank was found to strongly influence the fuel temperature profile by sometimes reducing cooling from the upper surface. In other instances, the ullage space enhanced cooling.

Hydroprocessed Renewable Jet Fuel Evaluation, Performance, and Emissions in a T-63 Turbine Engine

Due to potential beneficial environmental impacts and increased supply availability, alternative fuels derived from renewable resources are evolving on the forefront as unconventional substitutes for fossil fuel. Focus is being given to the evaluation and certification of Hydroprocessed Renewable Jet (HRJ), a fuel produced from animal fat and/or plant oils (triglycerides) by hydroprocessing, as the next potential synthetic aviation fuel. Extensive efforts have recently been performed at the Air Force Research Laboratory (AFRL) at Wright Patterson Air Force Base (WPAFB) to evaluate the potential of two HRJ fuels produced from camelina and tallow feedstocks. These have included characterization of the fuel chemical and physical fuel characteristics, and Fit-for-Purpose properties (FFP). The present effort describes general combustion performance and emission propensity of a T63-A-700 Allison turbine engine operated on the HRJs and 50/50 (by volume) HRJ/JP-8 fuel blends relative to a specification JP-8. In addition, engine and emission testing with a blend of the tallow-derived HRJ and 16% bio-derived aromatic components was completed. Fundamental engine performance characterization allows for determination of the suitability of potential synthetic fuels while quantitation of gaseous and particulate matter emissions provides an assessment of the potential environmental impact compared to current petroleum-derived fuels. In addition, an extended 150 hour endurance test was performed using a 50/50 blend of tallow-derived HRJ with JP-8 to evaluate the long-term operation of the engine with the synthetic fuel blend. This paper discusses the laboratory testing performed to characterize HRJs and results from the basic engine operability and emissions studies of the alternative fuel blends.Copyright