Experimental investigation of flash-boiling ethanol-R-152a sprays

Abstract

Flash-boiling (flashing) sprays are characterised by explosive atomization generated by a superheated fluid exiting an orifice. This mechanism modifies spray structure and exhibits strong coherent structures. This study investigates the impact of flashing on spray atomization using high-speed imaging on near-nozzle sprays at 100,000 frames per second. The Jakob number (Ja) was varied by adjusting the degree of superheat while holding Weber (We) and Reynolds (Re) numbers near-constant to explore the effects on droplet atomization at the onset of flashing. Time-averaged analysis showed changes in spray morphology and spreading rate, while spectral analysis and a data-based modal decomposition technique provided insights into spray transient behaviour and structure. For Ja values greater than 0.2, spray profiles became more homogeneous, with reduced variability and increased stability in flashing flow. Flash-boiling flow is least stable when in an intermittent state. Flash evaporation suppressed high-frequency content (Strouhal number Str > 10⁻²), amplified low-frequency magnitudes, enhanced pre-existing flow instabilities and altered the instability structure, producing chevron-shaped waves. For this reason, decreasing Ja could affect injection systems or medical inhaler spray performance. Further investigation is needed to fully understand the complex dynamics of flashing sprays.