OH Radical and Soot Concentration Structures in Diesel Sprays under Low Sooting and Non-Sooting Conditions

In an optically accessible high-pressure/high-temperature (HP/HT) chamber, OH radicals, soot concentration, and OH^* chemiluminescence images were captured simultaneously at a constant ambient temperature of 823 K and a gas density of 20 kg/m³, with injection pressures of 800-2000 bar using an injector with nozzle orifice having a diameter of 0.1 mm. Swedish market sold MK1 diesel fuel was used in this study. The optical diagnostic methods used were the two-dimensional laser extinction for the soot concentration measurement, planar laser induced fluorescence for the OH radical measurement, OH^* chemiluminescence imaging, and the natural flame luminosity imaging. The objective of this study is to explore the diesel spray structures under the low sooting and non-sooting conditions. In this study, it was found that the OH radical zone in the jet’s upstream region expanded to the jet center and the soot concentration decreased when the fuel injection pressure increased. The expansion of the OH radical zone correlated well with the reduction of the sooting zone in the radial and axial directions. Under the non-sooting conditions, the OH radicals occupied the entire reacting region of the jet. A longer lift-off length leading to a reduction in the equivalence ratio (i.e. fuel leaner mixture), which resulted in an expansion of the OH radical zone, a decrease in sooting zone width, and a decrease in the soot formation.