Evaporation and breakup behavior of high-pressure liquid ammonia sprays under various thermal conditions

As a zero‑carbon fuel, the atomization and evaporation characteristics of liquid ammonia are key to achieving efficient engine application. Diffuse back-illumination and Z-type schlieren imaging were used to investigate the effects of ambient conditions (300–500 K, 0.1–1.5 MPa) on liquid ammonia spray characteristics at 105 MPa injection pressure, revealing the dominant mechanisms governing spray development. The results show that ambient temperature and pressure play a significant regulatory role in spray morphology. Under low temperature and low pressure, flash boiling causes lateral expansion of the spray and a barb-like edge. When the ambient pressure increases to 1.0 MPa or above, flash boiling is suppressed and the spray transitions into a cold jet with greater penetration capability. Under high temperature, evaporation dominates, leading to rapid vaporization of the liquid phase and a narrowed spray contour. To characterize the spray regimes under different conditions, the characteristic parameter ratio ξ and the dimensionless temperature τ are introduced, dividing the development process of liquid ammonia sprays into three regimes: breakup-dominated, synergistic, and evaporation-dominated. The typical spray structure features of each regime are elucidated. Penetration models for liquid and gas phases incorporating the Weber and Ohnesorge numbers are established, achieving R ² ranges of 0.935–0.994 (liquid) and 0.873–0.982 (gas), indicating good predictive accuracy across ambient conditions. Analysis based on the proposed models shows that inertial forces drive spray development, while viscous forces substantially inhibit it, and the synergistic effects of the two govern the development of liquid ammonia sprays.