Impact of alternative jet fuels on gas turbine combustion systems

The replacement of conventional, petroleum-derived jet fuels with alternative jet fuels plays an important role in climate change mitigation, economic security, and energy independence. To ensure that alternative jet fuels can replace conventional jet fuels without engine modification, new fuels must be certified, a lengthy and expensive process. Understanding what fuel properties rate-limit the combustion process across a variety of engine-relevant operating conditions can dramatically shorten fuel certification times and decrease testing costs. The central objective of this work is to determine which fuel properties govern whether alternative jet fuels behave similarly to conventional jet fuels across different gas turbine-relevant regimes. The first part of this work focuses on the characterization of the spray and the flame dynamics and structure near the lean blowout (LBO) limit, when fuel flow rate is decreased to the point at which the flame is extinguished. To study spray droplet distributions at this condition, a 2D phase Doppler anemometer (PDA) is used to characterize alternative jet fuels’ sprays and compare them to the Jet-A fuel spray in a realistic single-cup swirl-stabilized ‘referee’ combustor, under the umbrella of the Aviation Sustainability Center’s National Jet Fuels Combustion Program. Average OH* chemiluminescence images are used to determine flame structure and dynamics with the goal of correlating spray characteristics with flame location. The alternative jet fuels studied exhibit unusual properties; a low cetane number fuel and a flat distillation curve fuel are compared to conventional Jet-A. Profiles of the SMDs reveal only minor differences between the fuel spray distributions, but OH* imaging revealed significant differences in the overall flame structure, dynamics, and stabilization mechanisms between the three fuels. The second part of this work is focused on studying the effects of variation in fuels and fuel properties under conditions that simulate a high-altitude relight scenario, where the engine needs to be restarted at high altitudes. Ignition probability measurements are made for a range of conventional and alternative jet fuels on a swirl-stabilized combustor, modeled on the NJFCP referee combustor, in a high-altitude chamber. The combustor, the Army Research Combustor-L1, was designed with input from engine OEMs to replicate key geometric features of the referee combustor while still maintaining simplicity for easier machining and offering better optical access than the referee combustor. Correlations between ignition probability and fuel properties show the strongest relationship for fuel properties related to atomization (viscosity, surface tension) and vaporization (temperature at 20% distillation, flash point). The final aspect of this research focuses on the use of X-ray phase-contrast imaging and X-ray radiography to study the breakup of sprays at conditions near the LBO limit in a model gas turbine combustor. First, these techniques are employed to examine the breakup of a water spray from a pressure-swirl atomizer in a single-sector, model gas turbine combustor. The combustor, the Army Research Combustor-M1, was designed with input from engine OEMs to replicate the key geometric features of a single combustor cup of a helicopter-sized engine. X-ray techniques are useful diagnostics particularly useful for studying sprays near the tip of the nozzle, where the spray is too dense for more powerful techniques, like PDA, to be effective. Phase-contrast imaging at 100 kHz is used to qualitatively examine the breakup of the water across three different liquid supply pressures. Measures of average droplet size, obtained from image processing, show that the droplet size decreases as the liquid supply pressure increases, and the droplet velocity magnitude also increases with increasing supply pressure. X-ray radiography is used to calculate the optical depth of the water spray across the three supply pressures, showing spray cone broadening with increasing liquid supply pressure. To examine spray breakup under more realistic conditions using real fuels, X-ray phase-contrast imaging is conducted at 90,517 Hz on a fuel spray to characterize the breakup of the bulk fuel into ligaments and then individual droplets under combusting conditions. Phase-contrast imaging is used to qualitatively assess spray breakup for two fuels: conventional Jet-A, which represents a normal viscosity fuel, and a blend of 64% by volume JP-5 and 36% by volume farnesane, which represents a high viscosity fuel. The two fuels are tested at three flow rates and two air inlet preheat temperatures. The high viscosity fuel is qualitatively observed to have markedly longer ligaments and larger diameters of the droplets after the primary breakup than the normal viscosity fuel. The same image processing and quantification strategy to assess spray breakup as for the water spray measurements is employed. The increase in preheat temperature from 50°C to 100°C results in an increase in the mean droplet velocity magnitude and a decrease in numerical average and SMD, meaning atomization improves. The high viscosity fuel was found to have larger average diameters at each test condition than the normal viscosity fuel. The effect on atomization from the increased preheat temperature was found to have a much greater effect on change in mean droplet sizes than the difference in the viscosities of the fuels at the same condition. The increased preheat is found to enhance the combustion efficiency, which is hypothesized to couple with the improved atomization due to the higher combustion temperatures. Examining spray atomization and breakup under realistic combusting conditions for relevant fuels is important as it directly couples with the atomization of the fuels. Overall, this work provides valuable insight into the effects of fuel properties on combustor performance, which provide valuable tools that can be used to expedite the certification of new alternative jet fuels