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Flash boiling is an effective way to enhance both atomization and evaporation characteristics of liquid fuel spray by introducing explosion of vapor bubbles, compared with the conventional high pressure injection. However, the evaporation characteristics and mechanism of flash boiling spray, especially, that associated with the explosion of vapor bubbles at high superheat degree, is still unknown. In this study, the vapor and liquid mass distributions in flash boiling sprays were quantified by use of ultraviolet/visible laser absorption/scattering (UV-LAS) imaging technique over a broad range of superheat degree. Based on the results of macro-structure characteristics of liquid/vapor phase spray and total evaporated vapor mass, influence of bubble explosion on detailed evaporation characteristics of flash boiling spray are investigated. And, by associating with our previous work on breakup mechanism of superheated droplet and liquid jet, the effect of two evaporation regimes, namely surface evaporation and flashing evaporation, on evaporation of flash boiling spray has been successfully identified for the first time. These results provide insightful information for understanding the evaporation mechanism of a superheated liquid jet and modeling a flash boiling spray.

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.

This investigation is to evaluate and quantify the influence of bubble formation inside the nozzle on breakup characteristics of a superheated liquid jet outside the nozzle. The effect of fuel properties was examined using methanol, ethanol and butanol. A unique optically-transparent slit nozzle with a high-speed micro-imaging system were utilized for quantifying the bubble formation inside and the liquid jet breakup outside the nozzle. Correlation between bubble number density and breakup of superheated liquid jet was obtained for all the fuel. The bubble is demonstrated as the main driving forces enhancing the breakup of superheated liquid jet. By introducing a parameter K , it is easier to correlate the superheated liquid jet breakup to the bubble number density as Δ f = K ⋅ n . K is related with the superheat degree, Reynolds number, Weber number, and the ratio of liquid to ambient gas viscosities, which has different form at the transition stage (0.2 P a / P s ⩽ 0.3) and the flare flashing stage ( P a / P s ⩽ 0.2), implying different breakup regimes.

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.

The macroscopic spray characteristics were quantified using dimensionless analysis by examining the role of the dominating forces associated with liquid-jet breakup. The Weber number, Reynolds number, and air-to-liquid density ratio dimensionless numbers were used to capture the primary forces including the inertia, viscous, surface tension, and aerodynamic drag forces. Planar Mie-scattering technique was applied to generate spray images over a broad range of conditions found in today’s spark-ignition-direct-injection (SIDI) engines, providing a relatively large range of dimensionless numbers. The effect of fuel properties were examined using gasoline, methanol and ethanol fluids. Six regions described on a Weber number versus Reynolds number domain were selected to identify the relative importance of the inertia force, surface tension force, and viscous force on macroscopic spray structure. The effect of aerodynamic drag was individually determined by characterizing the spray over a range of ambient air-to-liquid density ratios. As a result, for the non-flash-boiling multi-hole sprays in this study, the Weber number and air-to-liquid density ratio have much more profound effect on the spray penetration and spray–plume angle compared to the Reynolds number contribution. The inertia force and air drag force are more important factors compared to the viscous force and surface tension force. This analysis yielded dimensionless correlations for spray penetration and spray–plume angle that provided important insight into the spray breakup and atomization processes.