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Abstract
The combustion characteristics of freely falling droplets, individually generated by the merging of colliding water and hexadecane droplets, were investigated and compared with those for pure hexadecane and emulsified water/hexadecane droplets. The merging of the nominally nonmiscible hexadecane and water was manifested either in an apparently adhesive manner, or with the water droplet inserting into the hexadecane droplet. The latter, however, is the prevalent mode in hot environments and/or for droplet burning, provided the water volume fraction is less than 0.54 so it can be completely covered by hexadecane. Results on the combustion characteristics show that the ignition delay increases with increasing water content that for the same droplet size it varies with t ig (hexadecane) < t ig (collision-generated) < t ig (emulsion) that the flame characteristics, including its color, were not affected by the water content that droplet burning was terminated with either complete combustion, extinction, or microexplosion and that the droplet volume at microexplosion is almost the same as the initial volume of water in the merged droplet. These results, together with considerations of the limit of superheat of water and the attainable droplet temperature, suggest that hexadecane was the primary and possibly only component undergoing gasification and subsequently combustion prior to the occurrence of microexplosion, and that microexplosion was predominantly induced by flash vaporization of the superheated water when it was exposed to the gaseous environment upon the near-complete gasification of the enveloping hexadecane component, instead of through homogeneous nucleation of the water component when it was still enveloped by hexadecane. With the aid of microexplosion, the effective burning rate K eff increased with increasing water content and for a given droplet size exhibited the ranking K eff (collision-generated) > K eff (emulsion) > K eff (hexadecane). The role of the air bubbles entrapped upon coalescence of the colliding droplets in facilitating microexplosion is also discussed.
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The work at National Taiwan University was supported by the National Science Foundation of Taiwan, Republic of China, and that at Princeton University was supported by the U.S. Air Force Office of Scientific Research.
