On the role of vortex-ring formation in influencing air-assisted spray characteristics of n-heptane

Air-assisted spray characteristic in a constant-volume vessel has been investigated experimentally. Influences of injection control parameters such as fuel injection duration ΔT_f, fuel-air injection interval ΔT_f-a and air injection duration ΔT_a on the spray characteristics were studied. Results show that two distinct spray shapes were observed under appropriate control parameters, namely, “spindle-shaped” spray and “anchor-shaped” spray. The “anchor-shaped” spray which promotes spray spread possesses relatively smaller spray tip penetration, but better spray spread which is beneficial to fuel/air mixing. The enhancement on spray spread for “anchor-shaped” spray is attributed to the formation of vortex-ring near spray tip under relatively low spray velocity. Non-monotonic spray dispersion area ratio R_d can be observed with increasing ΔT_f, ΔT_f-a and ΔT_a. Droplet velocity decreases with ΔT_f increasing, which suppresses droplet breakup therefore results in the increasing of droplet size. However, the formation of the vortex-ring weakens this increasing trend of droplet size. Increasing of ΔT_f-a tends to generate stable vortex ring hence stable droplet velocity and SMD. In terms of air injection duration ΔT_a, droplet velocity was observed to change non-monotonically with ΔT_a increasing, due to the “spindle-shaped — anchor-shaped — spindle-shaped” spray shape transition.