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The dissertation presents an experimental study of ignition and flameholding of high speed, room temperature fuel-air flows using a diffuse, large volume, low temperature plasma produced by a repetitive nanosecond pulse discharge sustained in a cavity. Experiments are performed in premixed, partially premixed, and non-premixed ethylene-air and hydrogen-air flows in a pressure range of P = 0.2 – 0.3 atm. The dissertation also incorporates kinetic modeling of plasma assisted ignition of ethylene-air and hydrogen-air mixtures, to study the effect of radical generation in the plasma on ignition delay. The experimental results demonstrate that repetitive nanosecond pulse plasma assisted ignition occurs via formation of multiple arc filaments in the fuel-air plasma, although air plasma remains diffuse and low-temperature until the fuel is added. Comparison of ignition and flameholding achieved in premixed ethylene-air flows using a repetitive nanosecond pulse discharge and a DC arc discharge of approximately the same power (100 W) demonstrated that DC discharge resulted in sporadic ignition and flame blow-off, much lower burned fuel fraction, and significantly lower velocity (35 m/sec) at which ignition is achieved.For premixed and partially premixed near-stoichiometric ethylene-air flows, ignition and stable flameholding have been achieved up to a flow velocity of 100 m/sec at P=0.2 atm. During these experiments, nearly complete combustion is achieved. For partially premixed hydrogen-air flows, stable ignition and flameholding at P=0.2 atm has been achieved at flow velocities of up to 100 m/sec and equivalence ratios of φ=0.44-0.96. Time averaged plasma temperature measurements using nitrogen emission spectroscopy showed that the air plasma temperature is within 70° C to 200° C, while plasma temperature in presence of a stable flame is 700-1000° C. During non-premixed combustion experiments in ethylene-air at P=0.2 atm, ignition and stable flameholding is observed up to a flow velocity of 90 m/s at global equivalence ratio of φ=0.1. The highest flow velocity at which stable flameholding is observed in non-premixed hydrogen-air flows at P=0.26 atm is 190 m/sec, at φ≈0.04. Flow choking in the combustor is observed for average flow velocities above 200 m/sec. High frame rate NO Planar Laser Induced Fluorescence (PLIF) imaging and schlieren imaging have been performed to observe the dynamics of fuel-air mixing in the cavity during fuel injection. Kinetic modeling is used to study the mechanism of low-temperature nanosecond pulse plasma assisted ignition. The reduced kinetic mechanism of plasma assisted ignition of hydrogen has been identified and compared with the full mechanism in a wide range of temperatures and pressures, showing good agreement. Kinetic modeling calculations performed to study the effect of non-thermal radical generation in nanosecond pulse discharge plasma on oxidation/ignition of hydrogen-air mixtures demonstrated that removal of plasma chemical radical generation processes inhibits low-temperature exothermic chemical reactions, thus blocking ignition. It is also observed that presence of radicals produced by the plasma accelerates ignition process significantly and reduces ignition temperature. Finally, the kinetic model has been used to interpret the results of flameholding experiments in premixed ethylene-air and hydrogen-air flows. |
