Optical and numerical study on high-pressure liquid ammonia spray atomization and ignition characteristics under different injector nozzle diameters

Abstract

Ammonia spray diffusion combustion exhibits high efficiency and low pollutant emissions, making it a promising approach for achieving efficient and clean combustion in ammonia engines. It is crucial to investigate the spray development and atomization characteristics of high-pressure liquid ammonia for further understanding of the ammonia spray diffusion combustion mode. Despite research reports on ammonia spray, the influence of nozzle diameter on ammonia spray atomization and ignition characteristics remains unclear. Most existing studies focus on the flash boiling phenomenon or macro-scale spray development process, with limited attention to the diffusion and mixing characteristics of gaseous ammonia. In this paper, the development process and diffusion mixing features of ammonia spray were visualized and studied using optical and numerical methods. The experimental and numerical results of ammonia spray with different nozzle diameters were compared and analyzed under varing ambient conditions, especially the spray diffusion and fuel-air mixing features under engine-like conditions. The research results indicate that nozzle diameter significantly influences the ammonia spray development process, the atomization rate of liquid droplets, and the gaseous ammonia diffusion characteristics. As the nozzle diameter increases, the spray penetration is faster, the spray angle and spray area enlarge, and the ammonia spray atomization is compromised. Furthermore, the ammonia-air mixing features under engine-like conditions were studied with different nozzle diameters and the formation characteristics of the flammable mixture were revealed. Ammonia spray tends to form a large amount of dilute mixture under engine-like conditions due to the high mixing rate of ammonia. Only the middle part of the fully developed ammonia spray has good ignition conditions. Compared to the nozzle diameter of 0.2 mm, adopting the injector with a larger nozzle diameter of 0.24 mm significantly increases the flammability proportion of ammonia spray, which is more conducive to ignition.