Dynamics of the Ammonia Spray Using High-Speed Schlieren Imaging

Ammonia (NH3), as a carbon-free fuel, has a higher optimization potential to power internal combustion engines (ICEs) compared to hydrogen due to its relatively high energy density (7.1MJ/L), with an established transportation network and high flexibility. However, the NH3 is still far underdeveloped as fuel for ICE application because of its completely different chemical and physical properties compared with hydrocarbon fuels. Among all uncertainties, the dynamics of the NH3 spray at engine conditions is one of the most important factors that should be clarified for optimizing the fuel-air mixing. To characterize the evolution and evaporation process of NH3 spray, a high-speed Z-type schlieren imaging technique is employed to estimate the spray characteristics under different injection pressure and air densities in a constant volume chamber. Three renewable fuels, including NH3, methanol and ethanol, are investigated to compare the differences in their spray behavior at engine-like conditions. The basic parameters of the spray geometry such as spray penetration, spray cone angle and cross-section area are quantified based on the schlieren images postprocessing. The results show that the spray geometry of NH3 differs from that of the other fuels, which exhibits a longer penetration, larger spray cone angle and cross-section area. Moreover, the NH3 also shows a faster evaporation rate than methanol and ethanol. To extract more information from the spray images, an optical flow algorithm is derived to visualize the velocity field based on the schlieren images. The results indicate that NH3 spray is driven to the spray axis under the effect of the vortices. The vortices are induced by the entrainment of the surrounding gas and act as the driving forces that push the spray plumes towards the axis at the same time. The two vortices of NH3 grow much bigger and stronger and move closer to the spray axis compared to the ethanol and methanol.