| Studies on the nonpremixed ignition of hydrocarbon and oxygenated fuels were conducted to examine the individual and coupled effects of chemistry and transport on diffusive ignition. The counterflow configuration was selected for gaseous fuels, in which a heated oxidizer jet was directed downward against an upwardly flowing jet of cold (298K) fuel/inert mixture. For fuels that exist in the liquid state at room conditions, the conventional stagnation flow was adopted, in which the downwardly directed, heated oxidizer jet impinged upon a pool of liquid surface. Experimental, numerical and analytical studies were performed over a wide range of system pressure, strain rate and fuel type, with particular interest in the controlling mechanisms and responses of diffusive ignition. Experimentally, the ignition temperature was measured with a thermocouple and the local strain rate by Laser Doppler Velocimetry (LDV). Numerical simulation with detailed chemistry and transport was conducted, with the results compared with the measurements to assess the adequacy of the kinetic models. The investigation was facilitated by adopting the analytical tools of Chemical Explosive Mode Analysis (CEMA) and mechanism reduction.;The complexity of ignition in a chemically reacting flow was first demonstrated by an interesting observation in the methane/air system, namely the multiple criticality and staged ignition of methane in the counterflow. Similar to hydrogen ignition, up to three steady branches were computationally identified on the "S"-curve with a detailed kinetic mechanism, indicating that ignition could potentially take place in a staged manner. It was further shown that the first ignition is dominated by radical runaway, while the second requires thermal feedback. However, this multiple criticality response was not observed by using alternate mechanisms. The large uncertainty of reaction rates of the crucial pathways renders it difficult to draw a conclusion on the relative merits of the two results.;Studies on the counterflow ignition of pure fuels were then extended to fuel mixtures, specifically methane/ethylene mixtures, to understand the effects of fuel interaction in promoting/inhibiting ignition. The hierarchical structure of the kinetic model was also assessed by performing mechanism reduction for individual fuels and their mixtures.;To extend the study to liquid fuels, the counterflow apparatus was modified to a liquid-pool assembly. Measurements were taken on a series of fuels, including n-heptane, iso-octane, n-decane, n-dodecane, n-hexadecane, n-butanol, iso-butanol and methyl-butanoate. Results show that the ignition temperature in general decreases with pressure while increases with strain rate. The ignition events were simulated with a stagnation-flow code and the dominant reactions for ignition were identified and discussed by CEMA and sensitivity analysis. In addition to chemistry, the binary diffusivities between fuel and air were shown to be one of the crucial factors in governing the ignition response. It was found that the ignition temperature of large normal alkanes increases with the carbon number, essentially due to the smaller diffusivity as the molecular size increases. Simulation in diffusive systems was greatly facilitated by mechanism reduction, and a series of skeletal and reduced mechanisms were developed. |
