The spray combustion of diesel fuel under conditions of very low ambient O2 concentrations was examined in this study. The detailed combustion characteristics were evaluated using two different sets of experimental apparatuses. A high-temperature, high-pressure combustion vessel with dual observation windows was employed to visualize the spray flame. The sequential images were obtained by using a high-speed color video camera and were analyzed using the two color method to quantify the temporal variation of the two dimensional distribution of soot temperature and KL factor. Both the ambient O2 concentration and the CO2 mixing ratio were varied as experimental parameters. A second constant volume vessel with a smaller internal volume was also employed as an experimental apparatus to conduct analyses of heat release rates based on temporal variations of pressure.
Based on a series of systematic experiments, we confirmed that O2 concentration is the primary factor affecting both the ignition delay and combustion period, while the level of CO2 mixing has little effect. Decreasing O2 concentrations were associated with delays in the appearance of the luminous flame following the onset of light emission from OH radicals. The heat release rate study showed the possibility of the existence of endothermic reactions during this period. The flame temperature was observed to decrease as the O2 concentration decreased and as the CO2 ratio increased, resulting in reduced NOx emissions. The amount of soot inside the flame initially increased with decreasing O2 concentrations, but then decreased starting at an O2 concentration of approximately 11%, such that minimal amounts of soot were generated at very low O2 levels. Both visual observations and emissions measurements confirmed that the simultaneous reduction of NOx, soot and CO can be successfully achieved under very low O2 concentrations.