Influence of flash boiling on ammonia spray characteristics under different superheat degrees

Abstract

Ammonia as a carbon-free fuel is promising for the transportation sector. Its direct injection in the liquid phase results in flash-boiling, significantly affecting the spray characteristics. This article characterizes the influence of the degree of superheat (R_p) on spray characteristics by injecting liquid ammonia using a single-hole gasoline direct injection injector. R_p is varied by changing ambient pressure (1–6 bar) and fuel temperature (−10 to 50°C). These conditions lead to sprays that are in flash-boiling and nonflash-boiling, as well as in a transition region. Near-field and far-field spray fluctuations are studied systematically for the first time. In the nonflash boiling conditions, when R_p < 1, the spray parameters are mainly controlled by ambient conditions: spray cone angle, spray area, and spray width are reduced for lower ambient pressure. However, in transition and strong flash boiling conditions, spray parameters increased with increasing R_p. Increasing R_p > 20 contributes slightly to the radial expansion and increase in the spray area. For superheated ammonia spray, a significant cooling effect of vaporization is observed due to the higher latent heat of liquid ammonia. This resulted in the presence of more liquid mass near the nozzle field for strong flash boiling spray. These findings provide a database for liquid ammonia injections and can be used to validate models.