Droplet Fragmentation in Gas-liquid Annular Flow with Disturbance Waves

A wall liquid film driven by turbulent airflow develops three-dimensional waves and eventually fragments into fine droplets at the trailing edge. At high gas–liquid flow rates, large disturbance waves emerge and intermittently eject droplets. While previous studies have provided detailed insights into individual stages of film dynamics and fragmentation, a comprehensive theoretical model that predicts the entire sequence of these phenomena remains lacking. In this study, we develop an experimental setup for annular flow where the turbulent gas is fully developed and the liquid film maintains a uniform thickness, allowing for general discussion from waviness to droplet formation. We quantitatively analyze the wave structures on the film surface, liquid sheets elongating at the trailing edge, and the resulting droplet statistics. We find that disturbance waves are formed by Kelvin–Helmholtz instability in the axial direction and transport the liquid film downstream. At the trailing edge, the wave crests are accelerated by aerodynamic forces and converge into a rim at the liquid sheet tip. Finally, we formulate a theoretical model that predicts the mean droplet diameter and size distribution with considering upstream wave dynamics, which is validated by experimental results.