Flash boiling hollow cone spray from a GDI injector under different conditions

Good spray atomization facilitates fuel evaporation in a gasoline engine, thus contributing to higher fuel efficiency and lower emissions. During certain operations of a gasoline direct injection (GDI) engine, the combination of increased fuel temperature and sub-atmospheric cylinder pressure during injection can lead to flash boiling condition, which promotes droplet breakup and evaporation. In this study, experiments were carried out to study the flash boiling and non-flash boiling spray of a hollow cone GDI piezoelectric injector. By the combination of different temperature and ambient pressure, different superheat degrees (Tf-Tb) and different ambient-to-saturation pressure ratios (Pa/Ps) can be achieved. For a hollow cone injector, the flash boiling spray can cause the cone shape spray to expand, both inwards and outwards. The axisymmetric inward expansion would converge together and form a fast developing plume shape, and the transition point for plume front to appear is around 0.5 for Pa/Ps ratio. When Pa/Ps is larger than 0.5, the spray development is dominated by the injection momentum and the effect of boiling is minor. When Pa/Ps is reduced to below 0.5, the flash boiling effect takes place and changed the spray dynamics. The peak penetration velocity starts increasing rapidly with the superheated degree only, and a good linear relationship exists between plume ratio and the log(Pa/Ps). The spray axial penetration result at a certain time frame shows three regimes: Pa/Ps > 0.5, 0.1 < Pa/Ps < 0.5 and Pa/Ps < 0.1. When Pa/Ps is less than 0.1, flare flash boiling happens and the original spray shape is hardly maintained due to the micro-explosion, meantime the spray axial penetration further increases at a reduced rate. While cases with similar Pa/Ps value can exhibit similar penetration character, cases with similar Tf-Tb value can show some difference.