| The dissertation presents non-equilibrium chemical kinetic studies of large volume lean gaseous hydrocarbon/air mixture combustion at temperatures (∼300K) much below self ignition temperatures and low pressures (40-80torr), in ∼25 nanosecond duration repetitive high voltage (∼18kV) electric discharges running at 10 Hz.;Xenon calibrated Two Photon Absorption Laser Induced Fluorescence (TALIF) is used to measure absolute atomic oxygen concentrations in air, methane-air, and ethylene-air non-equilibrium plasmas, as a function of time after initiation of a single 25 nsec discharge pulse at 10Hz. Oxygen atom densities are also measured after a burst of nanosecond discharges at a variety of delay times, the burst being run at 10Hz. Each burst contains sequences of 2--100 nanosecond discharge pulses at 100 kHz.;Peak O atom mole fraction (at ∼50mus) in air increasing by ∼1.5 times from initial levels, after a single pulse, is measured to be ∼0.5x10 -4 at 60 torr, with decay occurring on a time scale of ∼2 msec. Peak O atom mole fraction (at ∼50mus) in a stoichiometric methane-air mixture is found to be approximately equal to that in pure air, but decays in less than 0.5 mus. In phi=0.5 ethylene-air, peak atomic oxygen concentration is reduced by a factor of approximately four, relative to air, and decays in ∼10mus due to much faster rate of reaction of atomic oxygen with ethylene, compared to methane, at room temperature. This is found to be due to transfer of energy from excited nitrogen molecules to oxygen molecules, resulting in dissociation into oxygen atoms.;Burst mode measurements show very significant (up to ∼0.2%) build-up of atomic oxygen density in air, and some build-up (by a factor of approximately three) in methane-air at phi=0.5. Burst measurements in ethylene-air at phi=0.5 show essentially no build-up, due to rapid O atom reactions with ethylene in the time interval between the pulses. Discharge modeling calculations, incorporating full air species kinetics complemented with GRI Mech 3.0 and Wang hydrocarbon oxidation mechanism, are shown to provide good overall agreement with the oxygen atom density measurements presented here. The reduced mechanism shows that low temperature reactive radicals such as CH3 and H from CH4, and radicals C2H3, C2H 2, H2, and H from C2H5, are formed by electron impact. In addition, electronically excited nitrogen molecules collide with fuels methane and ethylene to form radicals CH3, H and C2H3, H respectively.;Nitric oxide density is also measured using single photon Laser Induced Fluorescence (LIF), in a manner similar to oxygen atoms, and compared with kinetic modeling. Fluorescence from a NO (4.18ppm) +N2 calibration gas is used to calibrate the NO densities. Peak density in air is found to be ∼ 3.5ppm at ∼ 225mus, increasing from almost initial levels of ∼ 0 ppm directly after the pulse. The NO densities in CH4 (ϕ=0.5)/air are found to be approximately the same as in air and follow similar rise and fall. In C2H4 (f=0.5)/air, peak NO density (at ∼ 225mus) is ∼ 2.4 times less than that in air. Kinetic modeling using only the Zeldovich mechanism predicts a slow increase in NO formation, in ∼ 2 ms, which points towards the active participation of excited N2 and O2 molecules and N atoms in forming NO molecules.;Ignition delay at a variety of fuel/air conditions is studied using OH emission measurements at ∼ 308nm as ignition "foot prints". The ignition delay is found to be in the range of 6-20ms for ethylene/air mixtures. No ignition was observed in the case of methane/air mixtures. These measurements agree well with kinetic modeling in which heat loss by conduction is included. |
