Investigation of detonation induced breakup of small droplets using digital off-axis holography

Liquid-fueled detonation engines are expected to play a pivotal role in the development of next-generation aerospace propulsion systems. However, knowledge gaps related to detonation-driven droplet breakup, vaporization, and reaction rates limit further development. In this work, secondary atomization of small liquid droplets from 5 to 25 μ m are studied for the first time after interacting with gaseous hydrogen-oxygen detonation waves. To minimize imaging distortions from windows and detonation waves, a novel focused beam digital off-axis holography technique is utilized. This method captures single-shot holograms with resolutions as high as 1.5 μ m , and also provides unique images showing phase gradients behind each droplet, highlighting the location of density gradients, droplet vaporization, and reaction zones. After the detonation wave passes, processed results show immediate droplet flattening along the vertical direction and edge roll-up at times near 0.3 μ s . After flattening, droplets grow in the horizontal direction, suggesting liquid at the edges are being stripped away. As the fragmentation begins, a unique protrusion appears on the windward side of the droplet at > 0.6 μ s . During the final stages, a thin streak of liquid remains surrounded by atomized droplets and phase gradients indicating zones of droplet vaporization and reaction with the surrounding gases. In addition to morphological changes, trends in the droplet width and height are extracted from the holography data to generate an ensemble time history for droplet breakup. These show an immediate increase in droplet height followed by a slower increase in droplet width before each droplet fully vaporizes. This unique work with small droplets can be used to improve atomization models and injector designs for future detonation engine development. Novelty and significance statement: This work is the first in the literature to focus on detonation-induced breakup of single liquid droplets as small as 5 to 25 μ m . Studies in the literature currently focus on > 100 μ m droplets, which have long breakup timescales that prevent them from contributing to detonation propagation. This work is the first to study detonation-driven breakup of droplets at these small size scales and the first to observe the formation of unique protrusion features for detonation-driven catastrophic breakup. This work also utilizes a novel focused beam digital off-axis holography technique, which not only achieves diffraction limited imaging but also enables quantitative phase visualization for identifying the presence of droplet vapor and reaction zones. This not only provides high-resolution data for detonation-driven droplet breakup models but can also be used to design new liquid-fueled detonation engines.