Effect of near-field characteristics on the two-phase distribution of superheated ammonia spray

Abstract

The development of low-carbon or zero-carbon fuel-driven engines has become a key technology to help reduce emissions under the background of carbon neutrality. Ammonia fuel has the advantages of low boiling point and high octane rating, but it also has the disadvantages of high self-ignition temperature and slow combustion speed. Direct injection of liquid ammonia has the potential to reduce greenhouse-gas emissions while maintaining engine power output, especially under high replacement ratio conditions. Meanwhile, liquid ammonia has a lower boiling point than traditional fossil fuels and is more likely to enter superheat state with plenty of bubbles nucleated inside the nozzle. After injection, bubble puffing/micro-explosion occurs inside the jet and thus accelerates the atomization and evaporation process of the spray. In this study, a micro-imaging technique was adopted to investigate the near-field characteristics and the gasification process inside the nozzle. Besides, both Schlieren and diffused backlit imaging techniques were applied to characterize the two-phase distribution evolution. By scanning a wide range of flash-boiling conditions, it is found that 3.5% of gasification ratio and 200.0% of near-field volume increment were achieved respectively inside and outside the nozzle at high fuel temperature conditions. Moreover, the linear increment of the gasification ratio accelerates the two-phase distribution area of ammonia spray exponentially. These results would provide insightful information for two-phase evolution and developing simulation models of ammonia sprays.