Study on the macroscopic and microscopic characteristics of flash boiling spray of liquid ammonia

As an efficient hydrogen carrier and carbon-free fuel, ammonia is considered a key solution for reducing carbon emissions in internal combustion engines. However, its physical properties differ substantially from those of conventional hydrocarbon fuels, resulting in marked differences in spray behavior. In this study, the macroscopic spray development and microscopic droplet distribution of liquid ammonia under both flash-boiling and non-flash-boiling conditions were investigated using a constant-volume optical visualization platform. To more accurately characterize the phase-change process of liquid-ammonia sprays, the traditional chemical-potential-difference formulation was revised to incorporate the effect of ammonia’s exceptionally high latent heat of vaporization. The results show that the revised chemical-potential-difference model effectively reveals the radial expansion characteristics of flash-boiling sprays. The spray radial expansion width at 20d₀ downstream of the nozzle exhibits a clear linear relationship with ambient pressure and the modified flash-boiling chemical potential difference, with an R² value of 95%. Unlike conventional sprays, the axial evolution of flash-boiling ammonia sprays displays a distinct three-stage behavior, where the combined effects of phase-change driving force and reduced ambient pressure lead to superlinear penetration growth in the second stage under high superheat conditions. At the microscopic scale, the Sauter mean diameter (SMD) of flash-boiling sprays decreases by approximately 65% relative to non-flash-boiling sprays. Moreover, flash-boiling sprays exhibit a more homogeneous radial droplet distribution, whereas droplet velocity shows pronounced attenuation in the radial direction. This study provides both experimental evidence and theoretical insight into the phase-change-driven atomization mechanisms of liquid-ammonia sprays.