Effect of jet fuel composition on forced ignition in gas turbine combustors

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Wei, Sheng
Seitzman, Jerry M.
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The rapid growth in the aviation industry means increasing consumption of jet fuels, which is leading to greater interest in alternate and sustainable fuel sources. The overall properties of these alternative fuels can be designed to meet existing standards. Nevertheless, the compositional differences between alternative and conventional fuels can lead to important variations in chemical and physical properties that impact engine performance. For example, ignition is of paramount importance to ensure reliable operation, especially in extreme conditions like cold starts and high altitude relights. For aircraft engines, ignition is the process of creating self-sustaining flames starting with a high-temperature source located near a combustor liner. This thesis is devoted to studying the differences in ignition behavior due to the variations in fuel composition. Fuel variations in ignition are studied in a well-characterized test facility that is readily amenable to modeling and simulation. The experiments employ a sunken-fire ignitor, like those typically employed in aircraft engines, operating at 15 Hz with ~1.25J spark energy. Performance differences among fuels are characterized through their ignition probabilities. To understand both the chemical and physical fuel effects on ignition, both prevaporized fuels and liquid fuel sprays are examined. The purpose of prevaporizing the fuel is to remove the process of liquid to gas transition and to focus on combustion chemistry alone. In the forced ignition of liquid fuel sprays, which mimics the situation encountered in aviation gas turbine engines, both physical and chemical properties of the fuel are relevant. Statistically significant differences between fuel ignition probabilities are observed. The droplet heating time is shown to correlate well with ignition probability. A particle Doppler phase analyzer (PDPA) is used to study droplet size distribution near the ignitor. These droplet distribution measurements can be useful for future CFD modeling. In addition to differentiating fuel performances through ignition probability, advanced diagnostic techniques are employed to understand the evolution of a spark kernels as it interacts with combustible mixtures. These techniques include high speed OH planar laser induced fluorescence, OH* chemiluminescence, and schlieren imaging. The results reveal the entrainment of ambient fluid into the convecting spark kernel, the decomposition of vaporized jet fuel in the high temperature kernel, and the transition from local “hot spots” within the spark kernel to a self-sustaining flame. In addition to the experiments, reduced order modeling is used to better understand the physics and chemistry of ignition for both prevaporized and liquid fuels. Chemical differences are found to depend on the relative distribution between intermediate breakdown products (e.g., ethylene, propene and isobutene) from the parent fuels, as these intermediates have drastically different chemical rates as a function of temperature. The energy transfer mechanisms important in the ignition of liquid fuel sprays are also identified. The chemical heat release and the dilution cooling rates are orders of magnitudes larger than the heat required for the droplets’ heating and vaporization. However, the droplet heating time is shown to have the largest impact on ignition performance
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