Visualization study on flash boiling spray characteristics of high-pressure liquid ammonia with different nozzle diameters

•The macro and micro characteristics of high-pressure liquid ammonia with different nozzle diameters were researched.•Explored the influence mechanisms at different flash boiling stages.•The 0.32 mm nozzle diameter is better for ammonia injection in this study. Zero-carbon fuel liquid ammonia is prone to flash boiling under actual engine conditions. However, the influence rule of liquid ammonia flash boiling spray characteristics under different nozzle diameters remains unclear. Therefore, the macroscopic and microscopic flash boiling spray characteristics were investigated by utilizing long focal length high-speed shadowgraphy and long working distance microscopic high-speed shadowgraphy methods. Three nozzle diameters are 0.12 mm, 0.32 mm, and 0.52 mm. The results indicate that the nozzle diameters exhibit stage variation effects on the peak spray cone angle in different flash boiling stages. In the flare flash boiling stage, the peak cone angle is controlled by the degree of superheat and decreases with increasing nozzle diameter; In the transition/critical flash boiling stage, the peak cone angle is regulated by ambient pressure and increases with nozzle diameter. Furthermore, introducing coefficient fitting using Oh and Ja, representing dynamic and thermodynamic fragmentation, reveals that the peak cone angle is slightly more influenced by Oh than Ja. The time-averaged Sauter Mean Diameter increases with nozzle diameter, and the growth rate of the average Sauter Mean Diameter between adjacent diameters is 5 %–10 % in the flare flash boiling stage, 5 %–15 % in the transition flash boiling stage, and 15 %–35 % in the critical flash boiling stage. The most probable droplet diameters do not exhibit a monotonic increase with nozzle diameter but show a significant increase in the distribution of large droplets. Considering the comprehensive evaluation of macroscopic and microscopic tests, the 0.32 mm nozzle diameter exhibits the optimal liquid ammonia spray characteristics in this study.