Burning behaviors of collision-merged water/diesel, methanol/diesel, and water+methanol/diesel droplets

► The colliding and burning of water/methanol/mixture and diesel droplets were studied. ► Water/diesel droplet might end with flash vaporization after diesel was consumed. ► Catastrophic explosion occurred for the adhesively merged methanol/diesel droplets. ► Adding water into methanol altered the burning mode and had tunable burning rates. ► Insoluble liquid fuels could burn effectively through direct impinging approach. The merging and combustion characteristics of droplets composed of water and diesel, methanol and diesel, and water+methanol (W+M) and diesel were investigated. For the merged droplets of water and diesel, the water droplets inserted into diesel in all test conditions. While only diesel vaporized during the burning period, the remaining water droplet terminated by either flash vaporization into water mist or extinguished as residue. For methanol and diesel, instead, the methanol droplet tended to enclose the diesel droplet when its size was relatively larger than that of diesel and adhered with the diesel droplet when its size was sufficiently small. As methanol content increased beyond ∼50%, the flame became almost invisible after ignition with a delayed, regular explosion occurring after the appearance of a yellowish flame. For those with lower methanol content, an explosion occurred shortly after the ignition with branched flame or slightly delayed with a flame ball. As for W+M and diesel, most of the merged droplets were in adhesive-like mode at the room temperature. However, the initial burning exhibited an insertive-like burning mode, with the yellowish flame as that of pure diesel or merged water and diesel droplet. The burning ended with droplet extinction, flash vaporization, or explosion at different mixing ratios of pre-mixture and diesel content. Practically, water and diesel emulsions could reduce smoke emissions from burning. Our results have shown that by adding methanol into water or alone and mixed with diesel can have superior burning behavior than water/diesel emulsions. Therefore, the potential of separate injections of water, methanol or W+M mixture, and oil in opposed jet arrangements, in direct-injection engines, is suggested.