An experimental study on the macroscopic behaviours of ammonia sprays in a constant volume chamber

Abstract

Green ammonia is a promising alternative fuel for future thermal propulsion systems. In its liquid form, ammonia provides an energy density by mass comparable to conventional hydrocarbon fuels. Already high levels of mass-production and widely available existing infrastructure enable a smooth transition from fossil fuels to ammonia on the supply side. However, on the application side, the evaporation and mixing processes of liquid ammonia in air, which are known to have significant impacts on the energy release and resulting emissions, differ notably from those of fossil fuels. Hence, existing spray models may not be applicable to liquid ammonia injection. This calls for a comprehensive experimental study of ammonia sprays under a variety of relevant test conditions in order to understand the nature of such processes and for model validation. In this work, liquid ammonia was injected into a constant volume chamber from a direct injector at three injection pressures (100 bar, 150 bar and 200 bar) at ambient pressures varying from 1 bar to 10 bar, with an increment as low as 0.5 bar. This selection of ambient pressures covers a range of flash boiling to non-flash boiling conditions. It was found that the flash boiling regime of ammonia sprays are much lower than its saturation pressure, indicating there exists a strong cooling effect presumably due to its high latent heat of evaporation. Also, the spray collapse phenomenon was observed in ammonia sprays, and the spray collapse point was determined by a morphological study. These tests provided a comprehensive validation dataset for spray models.