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

In this study, the starting transient spray characteristics of an effervescent ramp injector are experimentally investigated for the bubbly flow. Toward this, near-field time evolution of the primary atomization process is captured through high-speed shadowgraph images. The results revealed that the starting transient consists of various phases that are cyclic in nature until the equilibrium condition of two-phase flow is reached. This includes droplet ejection and stratified flow, followed by intermittent liquid accumulation, liquid film formation, ligament expulsion, bag breakup, and progressive bubble-induced fragment events. We observed that the injector-specific asymmetry produces spatially segregated breakup zones, with bubble bursting and ligament formation preferentially occurred on opposite sides. Furthermore, it is noticed that once the flow is stabilized, the bursting frequency of the bubble is increased, forming a self-sustained bubbly spray. The data-driven modal decomposition techniques reveal the dominant frequencies associated with various starting transient phases, highlighting their quasi-periodic character to optimize the injector design for high-speed propulsion applications.