The impact of internal circulation on water coalescence within water-in-oil emulsion fuel before micro-explosion

Emulsion fuel has gained widespread acceptance as a potential alternative fuel in diesel engine due to its advantages of reducing pollutant emissions and simultaneously improving engine operating efficiency without necessitating extensive engine modifications. One of the key factors contributing to these advantages is micro-explosion. Prior research has predominantly focused on the macroscopic characteristics of micro-explosion, neglecting the crucial microscopic processes taking place within emulsion droplet before micro-explosion. In this study, we focus on the internal circulation within water-in-oil emulsion fuel during heating process and find its impact on the coalescence of dispersed water droplets before micro-explosion. To delve into this phenomenon, we conducted three distinct experiments: a single drop experiment based on Leidenfrost effect, a microscope experiment and an interfacial tension testing experiment. The outcomes of single droplet experiments reveal that there are two symmetric downward vortices formed in emulsion droplet during its heating process which induces violent movement of dispersed water droplets, consequently facilitating their coalescence. The outcomes of interfacial tension testing experiment indicate that the primary driving force behind the generation of internal circulation is Marangoni-driven convection, which arises due to an escalated tension gradient after the surfactant loses its efficacy at high temperature. The concentration of surfactant has a decisive effect on the interfacial tension in emulsion fuel: Increasing the concentration of surfactant contributes to the high-temperature stability of emulsion fuel, but it retards the processes of internal circulation and water coalescence which is disadvantageous for occurrence of micro-explosion.