Starting transient spray characteristics of an effervescent ramp injector: Slug flow

In this study, the starting transient characteristics of an effervescent ramp injector are experimentally investigated for the slug flow. The near-field high-speed shadowgraph images reveal the time evolution of spray characteristics. The developmental process during the starting transient of the spray consists of various distinct phases, classified as droplet ejection, liquid film breakup, liquid accumulation, slug expansion with ligaments, and slug expansion. Each phase exhibits a distinct breakup mechanism driven by the interaction between expanding internal gas and surrounding liquid, producing both coarse and fine droplets. Modal analyses on high-speed images for these phases exhibit progressive reduction in the fundamental oscillatory frequencies (124–10 Hz) associated with the spray. Conversely, corresponding to slug expansion and film breakup, high oscillatory frequencies (1930 and 2430 Hz) are observed. These high-frequency components are responsible for the rapid disintegration events such as ligament rupture, film collapse, and droplet shedding, indicating a clear demarcation between bulk spray dynamics and fine-scale atomization. These findings offer critical insights to control and optimize the starting transient of slug flow in effervescent injectors used for scramjet engine applications.