Micro-explosion and autoignition of composite fuel/water droplets

The results of experimental investigations of puffing, micro-explosions and autoignition of composite fuel/water droplets free falling in a high temperature air are presented. The measurements were performed for non-mixed and premixed Diesel fuel/water and rapeseed oil/water droplets. Air temperature was in the range 850–1100 °C, initial droplet radii were in the range 0.62–1.34 mm, and relative volume fractions of fuel (Diesel fuel or rapeseed oil) were in the range 3–97%. It is shown that the time to micro-explosion decreases with increasing temperature, weakly depends on the volume fraction of fuel, and increases with increasing droplet sizes. The latter trend is shown to be consistent with the prediction of the previously developed simple model for droplet puffing/micro-explosion based on the assumption that the water component forms a spherical sub-droplet located in the centre of a fuel droplet. It is shown that the autoignition delay time for composite droplets, close to the time to micro-explosion for both fuels under consideration, is almost an order of magnitude less than that for pure fuel droplets.