Quantitative observation on characteristics and breakup of single superheated droplet

Flash-boiling atomization is an effective way to enhance fuel jet breakup by introducing explosion of vapor bubbles and thus improve the evaporation of fuel spray, compared with the conventional high pressure injection. However, the break-up mechanism of a superheated jet, especially, that associated with the explosion of vapor bubbles inside the droplets, is still unknown. In this study, a superheated droplet generator was developed for observing the droplet morphology variation and the breakup process resulting from the vapor bubbles inside a superheated droplet by microscopic imaging. It was found that the droplet morphology is mainly influenced by droplet temperature, but micro bubbles formation and the breakup of the superheated droplet are dominated by superheat degree, and the superheat degree of 25 °C is an important critical point at which the droplet breakup occurs resulted from the ever-increasing void fraction exceeding a value of approximately 50% and the breakup mode shifts from aerodynamic mode to thermodynamic mode. The surface tension of superheated droplet was also evaluated by the droplet morphology, and the results show that the maximum reduction in surface tension reaches 70% as superheat degree increases to approximately 25 °C, and this explains the sharp decrease in SMD for a flash boiling spray when the superheat degree approaches this level. These results provide insightful information for understanding the breakup mechanism of superheated droplets and liquid jet and its modeling.