Mechanism of micro-explosion of water-in-oil emulsified fuel droplet and its effect on soot generation

Abstract

In this study, the mechanism of micro-explosion of single droplets of water-in-oil (W/O) emulsified fuels and its effect on the soot generation are investigated experimentally using suspended droplets. Two distinct breakup modes are observed as the droplets are heated, namely catastrophic micro-explosion after water coalescence at a lower surfactant concentration and puffing at a higher surfactant concentration without water coalescence. To understand the underlying physics of such transition, the water/oil interfacial tension is investigated. It is found that the increase of water/oil interfacial tension after the deactivation of surfactant can significantly accelerate the coalescence of dispersed water, and effectively increase the temperature of bubble nucleation. The deactivation of surfactant is a prerequisite for the occurrence of catastrophic micro-explosion. The reason is found to be that the catastrophic micro-explosion can be induced by the explosive vaporization of water only if the dispersed water coalesces rapidly into large droplets to explode violently at a high superheat degree. The surfactant concentration in emulsified fuels influences the deactivation temperature of surfactant. Therefore, the catastrophic micro-explosion can be controlled to occur by adjusting the surfactant concentration in emulsified fuels. In addition, it is also found that the micro-explosion of W/O emulsified fuel during the combustion can effectively decrease the soot generation due to uniform fuel/air mixture formed after the micro-explosion.