Quantitative observation on breakup of superheated liquid jet using transparent slit nozzle

• Transparent slit nozzle for simultaneous observation of bubble and jet breakup.
• Superheat degree (SD) dominates bubble formation and superheated fuel jet breakup.
• Superheated fuel jet breakup is correlated quantitatively to bubble number density.
• SD of 30 °C is a critical point for bubble formation, jet breakup and evaporation.

Flash-boiling atomization is an effective way to enhance fuel jet breakup and evaporation and to improve the quality of fuel spray in an SIDI engine especially at the cold start condition, compared to the conventional high pressure injection. However, what happened inside the nozzle and how it affects the breakup and atomization characteristics of a superheated liquid jet (spray) is still unknown. In this study, a two-dimensional transparent nozzle was developed for quantitatively observing the bubble formation inside the nozzle and liquid jet breakup near the nozzle exit by high-speed microscopic imaging. It was found that the liquid jet breakup characteristics is strongly affected by the number density and size distribution of bubbles inside nozzle. The superheat degree has predominant effect on both characteristics of the jet breakup and the bubble formation. Superheat degree of 30 °C is a critical point at which the bubble formation rate and the atomization characteristics of the superheated jet changes significantly. These results provide insightful information for understanding the breakup mechanism of a superheated liquid jet and modeling a flash boiling spray.