This thesis presents the results of a non-equilibrium kinetic study of oxidation and ignition in large volume hydrogen/air and ethylene/air mixtures by repetitively pulsed non-equilibrium plasmas. These measurements were conducted at low temperatures (~300K) and pressures (40 to 80 Torr), and a 40 kHz pulse repetition rate. Most measurements were also conducted at a 10 Hz discharge repetition rate. UV ICCD imaging and OH emission spectroscopy are used to ensure plasma uniformity and ignition delay time, respectively. Atomic oxygen Two Photon Absorption Laser Induced Fluorescence (TALIF) measurements, with Xenon calibration, give the absolute O atom concentration in a variety of mixtures. Finally, the sensitivity of a plasma chemistry kinetic model has been studied and gives insight into the chemical processes that are occurring.
UV ICCD camera imaging shows that the plasma remains stable and diffuse at the conditions explored in the atomic oxygen TALIF measurements (40 Torr pressure and 40 kHz discharge repetition rate). While plasma uniformity can be confirmed at these conditions, weak emission left ignition questionable. Images taken at higher pressures and equivalence ratios show emission between the pulses more clearly, indicating ignition. Also, a series of images taken at longer delay times after the burst, and with a larger camera gate, show the duration of the ignition event.
OH A to X(0,0) spontaneous emission spectroscopy is used to confirm ignition. The presence of a “footprint” in the emission trace (indicating emission, and thus ignition, between the pulses) is observed in pressures from 50 to 100 Torr and in equivalence ratios from f=0.3 to 1.0. Ignition delay time, as defined as the onset of continuous OH emission between pulses, is found to be a strong function of pressure, but only a weak function of equivalence ratio.
TALIF is used to measure the absolute atomic oxygen concentrations in air, hydrogen/air, and ethylene/air non-equilibrium plasmas as a function of burst size (15 to 1000 pulses, or 0.375 to 25 milliseconds). In air, there is a very rapid increase in O atom concentration (~2 milliseconds) that levels off into a near steady-state value. In the presence of very small amounts of hydrogen (f=0.05 or ~1.5%), there is significantly less atomic oxygen formed, by approximately an order of magnitude. The kinetic model predicts a very rapid rise in O atom concentration after 12 or 15 milliseconds in both the f=0.5 and 1.0 cases, however the experimental results only show this increase (more gradually) in the f=0.5 case.
Finally, sensitivity analysis of a plasma chemistry kinetic model shows that in a single pulse discharge, at low initial gas temperature (T=300K), the kinetics can be described by a reduced model that is independent of chain branching that rapidly increases the rate fuel oxidation. At higher temperatures (T=500 to 600K), this reduced model must include chain branching that becomes dominant and there is an increase in net energy release. In burst mode, a reduced model comprised of reactions from both the single pulse cases are found to be important in describing the kinetics of the system.