Experimental and numerical investigation on the superheated and subcooled ammonia spray characteristics under high injection pressure

As an efficient hydrogen carrier and carbon-free fuel, ammonia is a significant solution to reduce carbon emissions from internal combustion engines. However, the physical properties of ammonia are quite different from those of conventional fuels, which leads to significant difference in spray characteristics. In this paper, the development, atomization and evaporation characteristics of superheated and subcooled ammonia spray are studied using the diffused back-illumination imaging method and high-resolution numerical model. The novelty of the current work lies in revealing the special characteristics of superheated ammonia spray and evaluating the various phase-change models based on the experimental results. The experimental results show that flash boiling significantly inhibits the spray penetration at the initial stage, resulting in a lower initial penetration velocity. The penetration velocity of superheated ammonia spray increases first and then decreases, while that of sub-cooled ammonia spray decreases rapidly first and then remains stable. The evaluations of various phase-change models demonstrate that the Frossling model and flash boiling model have high accuracy in predicting the ammonia spray characteristics. Based on the high-resolution numerical model, the ammonia spray characteristics under different superheat degrees are studied. Superheat degree significantly affects the penetration characteristics, spray morphology, atomization and vaporization features of ammonia spray. With the increase of superheat degree, the spray temperature decreases, and the gas-phase ammonia concentration increases. Flash boiling can significantly promote the atomization and vaporization rate of ammonia spray, thus reducing the temperature of spray region. Due to the rapid vaporization of small droplets, the superheated ammonia spray presents significantly fewer but larger droplets than the subcooled ammonia spray.