From vortices to droplets: Gas-phase instability effects on thin liquid film breakup

This study explores the role of gas flow dynamics in the fragmentation of thin liquid sheets using three-dimensional numerical simulations. A thin liquid sheet with a thickness of 25 μm is subjected to high-velocity co-flowing gas streams characterized by three distinct gas Reynolds numbers, namely, Reg=300,500, and 1000. Simulations show that the breakup behavior of the sheet is governed by the vortical structures formed in the gas flow, which range from oblique vortex shedding at lower Reg to rib-like vortical patterns at higher Reg. The gas-induced instabilities impose transverse and longitudinal oscillations on the liquid sheet, which govern the locations of hole formation, the development of ligaments, and ultimately the resulting droplet size distribution. For Reg=300, breakup occurs via rim instability and the end-pinching or Rayleigh–Plateau-type instability of liquid fingers, whereas for Reg=500 and 1000, sheet rupture through hole formation and collision dominates. A manifold death (MD) algorithm is used to enforce breakup at a grid-independent critical film thickness, improving robustness against numerical artifacts. Droplet size distributions obtained with and without the MD algorithm confirm that the dominant droplet size remains unchanged, while the formation of small, potentially spurious droplets is suppressed. The resulting distributions align more closely with a lognormal model, consistent with the presence of multiple breakup mechanisms. This work provides new insight into the coupling between gas-phase instabilities and liquid fragmentation, offering a pathway to more accurate predictive models for air-blast atomization processes.