Spray characteristics of flash-boiling hollow-cone liquid ammonia sprays under direct injection conditions

Ammonia, a carbon-free compound, holds significant potential as a fuel in internal combustion engines. Recent research has revealed the limitations of gaseous ammonia use and highlighted the need to inject it in the liquid phase at higher pressures into the combustion environment to mitigate these shortcomings. This study focuses on investigating the flash-boiling effects of high-pressure liquid ammonia spray in a constant volume combustion chamber (CVCC) using a hollow-cone gasoline direct injection injector. Based on high-speed Schlieren and shadowgraph imaging data, the spray characteristics under varying ambient conditions are characterized. In particular, parameters such as the spray cone angle, penetration length, and plume ratios are studied to characterize the transformation of liquid ammonia spray into different spray regimes, characterized by flash-boiling phenomena. Furthermore, this work establishes a correlation between an increase in the ambient temperature of the CVCC and the transition into different regimes, as evidenced by the characteristics of the spray plume. Furthermore, the cooling effects of high fuel injection pressures at elevated CVCC ambient pressures are investigated to study the impact of pressure differentials on the drop in temperature caused by the injection of liquid ammonia. The findings of this study can serve as a foundation for the development of high-pressure liquid ammonia direct injection techniques, leveraging the rapid flash-boiling behavior of liquid ammonia.