Abstract: A numerical simulation method based on the VOF-DPM approach combined with adaptive mesh refinement technology was employed to investigate the water dropping process from the belly-cabin of the H-125 helicopter at three airspeeds (5, 10, 15 m/s). The water dropping process from the belly-cabin involves the atomization phenomenon of the fire extinguishing agent working fluid, which is manifested as the process of a jet liquid column generating a liquid film and breaking to form liquid bands and droplets. The study reveals that regarding the lateral expansion of the fire extinguishing agent, with slight increases in airspeed, the velocity of air relative to the liquid increases, causing the momentum ratio to decrease and resulting in increased maximum lateral expansion width. Additionally, as the longitudinal height decreases, the lateral expansion width of the continuous liquid phase exhibits a trend of first increasing and then decreasing. In the analysis of droplet characteristics, with slight increases in airspeed, the droplet Weber (We) number increases, leading to more intense droplet breakup. This is specifically manifested as the critical diameter for droplet breakup gradually decreasing, making large-diameter droplets more prone to breaking into small-diameter droplets, thereby causing the relative frequency of small-diameter droplets to increase. This study provides theoretical support for the water dropping operations of fire-fighting helicopters and facilitates a deeper understanding of the water dropping process and governing mechanisms, which is of great significance for improving forest fire extinguishing efficiency.

Key words: water-dropping from helicopter, VOF-DPM, liquid jet breakup, lateral expansion, droplet atomization