High-temperature flash-boiling spray characteristics of liquid ammonia under high-pressure direct injection

Ammonia (NH₃) is a promising carbon-free alternative fuel for internal combustion engines. Developing accurate ammonia spray and evaporation models requires precise experimental data on macroscopic spray characteristics and droplet size distributions. Based on the critical state theory of ammonia flash-boiling, the interactive influences of ambient temperature (T_amb = 300–600 K), ambient pressure (P_amb = 0.1–0.8 MPa) and injection pressure (P_inj = 40–120 MPa) on spray dynamics are systematically examined. Experimental results show that at T_amb = 300 K, flash-boiling is significantly suppressed when P_amb > 0.4 MPa. As T_amb increases from 400 K to 600 K, spray expansion decreases and the initial spray cone angle (SCA) reduces by approximately 30 %, indicating progressive weakening of flash-boiling effects. The high temperature sensitivity of Liquid ammonia enables excellent atomization, achieving sauter mean diameters (SMD) below 4 μm without auxiliary heating or ultra-high injection pressure. This characteristic demonstrates significant potential for engine applications, particularly in zero-carbon internal combustion engines. The study provides valuable theoretical and experimental foundations for improving spray models and advancing ammonia-fueled engine implementation.