Insights into near nozzle spray evolution, ignition and air/flame entrainment in high pressure spray flames

Experimental investigations were performed to study the dynamic processes related to early development of high pressure sprays and their effects on ignition, local soot formation and its entrainment as well as spray to spray interactions in a six-hole common rail diesel injector. These spray-flame measurements were performed using ultra-high speed imaging technique in an optically accessible constant volume chamber maintained under reactive high pressure environment. The acquired back scattered light from fuel droplets and the broadband natural soot luminosity images and their analysis have revealed new insights on how the radial dispersions of fuel sprays during its early development at high injection pressures affect the ignition, flame stabilisation and soot formation in diesel spray flames. Analysis of the high speed data have shown that radial spreading also called as bulge in this paper occurs during the very early stages of fuel spray development closer to nozzle. The cloud of finely dispersed fuel droplets trapped within these bulges loses its moment and evaporates quickly relative to liquid core of the spray to form a locally ignitable mixture and this initiates ignition from these locations. Local flame kernels formed from these local ignition sites tend to develop into small pockets of sooting flame, which subsequently moves counter to the main spray towards the nozzle and gets entrained into the liquid core of spray. At times, these sooting pockets are also transported to into the next spray when the bulges are large. The sooting flames that are transported towards the nozzle and into the liquid spray core are quenched upon its interaction with the core of liquid spray. Quenched flames are rich in unburnt hydrocarbon and soot particles and they have the potential to significantly impact the pollutant formation through altering the equivalence ratio within the core of fuel spray. Wide variations in the sizes were observed on the formation of bulges, and it also varied between sprays from different orifices of a multi-hole injector. These anomalies are mainly related to the complexity of nozzle sac flow affecting the early spray development.