Starting transient spray characteristics of a twin-fluid internal-mixing ramp injector: Annular flow

The atomization characteristics of a twin-fluid internal-mixing ramp injector during starting transient are experimentally investigated for the annular flow regime using high-speed shadowgraph images synchronized with pressure measurements. The present study demonstrates six phases: mist flow, mist flow with annular liquid ejection, annular liquid ejection, one-sided liquid ejection, slug expansion, and annular shearing. These phases unfold in a repeatable sequence, governed by the interplay of internal gas-liquid dynamics and external aerodynamic shear. A high-velocity droplet-laden core appears early in the starting transient, followed by progressive liquid sheet formation and circumferential shearing. Additionally, the intermittent slug expulsions result in rapid gas expansion and liquid fragmentation, which ultimately transform into a fully developed annular flow. The modal analyses on high-speed images reveal spatially coherent structures, oscillating at various dominant frequencies, which are in harmonics. It is observed that the annular flow produces fine droplets even during early transients, in contrast to bubbly and slug flows, where atomization is delayed or localized. These findings highlight the superior atomization performance of the annular flow and its potential suitability for ignition reliability in a scramjet engine.