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ABSTRACT
An experimental investigation on the ignition of a single lubricating oil droplet in combustible ambient gaseous mixture was performed on a rapid compression machine, under high-temperature (1060–1090 K) and high-pressure (13.0–36.5 bar) conditions. The lubricating oil droplet was suspended on a thin quartz fiber tip in the combustion chamber. Under the conditions presented in this study, the lubricating oil droplet almost has no distinct influence on the ignition delay of the ambient gaseous mixture with an initial radius of 600 μm due to the low evaporation rate. After the ignition of the ambient gaseous mixture, the droplet exploded severely, forming a flame ball-like soot cloud. With the increase of ambient pressure, the time for droplet explosion was advanced and the explosion get more intensive. Within the middle range of effective pressure (27–31 bar), the droplet explosion seemed to have a positive effect on the amplitude of the pressure oscillation after the autoignition of the end-gas of ambient mixture. Furthermore, the evaporation and its cooling effect of the lubricating oil droplet were investigated by a numerical method, so as to analyze the effect of the droplet size on the ignition order between the oil droplet and the ambient gaseous mixture. The results show that as for a bigger droplet, the cooling effect was more distinct and the evaporation mass ratio would be lower, which made the ignition around the droplet more difficult to occur.