Dynamic details inside water-in-oil (W/O) emulsion droplet and its impact on droplet evaporation and micro-explosion

Abstract

Emulsion fuel technology has been widely accepted as a potential alternative fuel to improve engine fuel efficiency and meanwhile reduce pollutant emissions without additional modification of the engine. Micro-explosion happened in fuel spray has been considered as the most important reason to improve fuel combustion efficiency and reduce harmful emissions. However, a few studies focused on the detailed micro-dynamic processes inside the emulsion droplet before micro-explosion as the emulsion droplet is heated. Understanding these micro-dynamic processes is significantly vital for the explanation of micro-explosion. Therefore, in this study, an experiment was conducted to investigate these micro-dynamic processes inside water-in-oil (W/O) emulsion droplet under high temperature environment using droplet suspension method. During the experiment, a thermocouple was applied to suspend the droplet and meanwhile measure droplet temperature, two high-speed cameras were applied to acquire the micro-dynamic processes inside emulsion droplet and micro-explosion respectively. Based on the collected data including droplet image and droplet temperature, the effect of micro-dynamic processes inside W/O emulsion droplet on droplet evaporation and micro-explosion is analyzed. It is found that the dispersed water particles in emulsion droplet can gradually coalesce into large water particles during the heating process in some cases, and these processes have a great effect on droplet evaporation and micro-explosion: As water coalescence happens in emulsion droplet, the evaporation of water is restrained. As the droplet temperature reaches to the superheating limit, flash boiling of coalesced water can instantly release tremendous energy to induce catastrophic micro-explosion. Without coalescence of dispersed water particles in emulsion droplet, just weak puffing intermittently occurs during the heating process. This result indicates that the precondition of catastrophic micro-explosion for emulsion droplet is water coalescence before droplet temperature reaches the superheating limit. In addition, the mechanism of water coalescence is also investigated in this study from the view of oil–water interfacial tension. It is found that the water coalescence in emulsion droplet is induced by the fast rise of oil–water interfacial tension as droplet temperature exceeds critical temperature of emulsion stability, but increasing the surfactant concentration in emulsion fuel can retard the water coalescence in emulsion droplet.