Macroscopic and microscopic characteristics of flash boiling spray with binary fuel mixtures

Flash boiling has drawn much attention recently for its ability to enhance spray atomization and vaporization, while providing better fuel/air mixing for gasoline direct injection engines. However, the behaviors of flash boiling spray with multi-component fuels have not been fully discovered. In this study, isooctane, ethanol and the mixtures of the two with three blend ratios were chosen as the fuels. Measurements were performed with constant fuel temperature while ambient pressures were varied to adjust the superheated degree. Macroscopic and microscopic characteristics of flash boiling spray were investigated using Diffused Back-Illumination (DBI) imaging and Phase Doppler Anemometry (PDA). Comparisons between flash boiling sprays with single component and binary fuel mixtures were performed to study the effect of fuel properties on spray structure as well as atomization and vaporization processes. Adding ethanol into isooctane increased the overall vapor pressure which led to higher superheated degree under given ambient pressure. Stronger flash boiling behavior as well as advanced spray collapsing were observed. Smaller droplet size and shorter liquid penetration were achieved with low ethanol blend ratios under relatively high ambient pressures. Opposite behaviors were found for sprays with high ethanol content and pure ethanol. Such phenomenon was caused by ethanol's high latent heat of vaporization which inhibited vaporization of child droplets.