Water droplet dynamics and evaporation in airtanker firefighting

Abstract

This study presents the first systematic investigation of the dynamics of individual water droplets in the context of airtanker firefighting. While previous work has focused on ground-deposition patterns measured in standardized field tests, the droplet-scale mechanisms governing evaporation and transport have remained largely unexplored. A tailored model of the coupled momentum, heat, and mass transfer of an isolated water droplet in ambient air is proposed and applied to examine the evolution of droplets under a wide range of atmospheric conditions. The results demonstrate that droplet size governs the effectiveness of water delivery, the release height emerges as the dominant operational parameter, and relative humidity is the key atmospheric property. Increasing the release height lengthens the flight time and increases evaporative losses, while low relative humidity accelerates evaporation, particularly for droplets smaller than one millimeter. Only droplets within a narrow range of initial radii, $150\,\mu\mathrm{m} \lesssim r_{\mathrm{d},0} \lesssim 3\,\mathrm{mm}$, are able to reach the ground following an airtanker release, with smaller droplets fully evaporating during their fall and larger droplets being subject to secondary atomization. Although airtanker releases involve very large liquid volumes and complex spray dynamics, the present analysis deliberately isolates droplet-scale behavior and does not resolve collective spray effects, wake interactions, or turbulence. The findings therefore serve as a physically consistent baseline for droplet evaporation and transport, forming a foundation for spray-resolved modeling efforts aimed at improving airtanker delivery strategies.