Visualization of liquid ammonia spray using multi-hole injector

To reduce carbon dioxide emissions, carbon-free ammonia is being investigated as an alternative fuel for internal combustion engines. When injected into a low-pressure environment, liquid ammonia undergoes rapid vaporization (flash boiling) due to its low boiling point. In this study, the effects of ambient pressure and fuel temperature on spray morphology and tip dynamics of ammonia from a multi-hole injector were investigated under an injection pressure representative of port fuel injection. Visualization was performed in a constant volume chamber. Mie scattering was used to visualize the liquid phase distribution, while schlieren imaging was used to visualize the gas-liquid distribution. The results showed that, as the ambient pressure decreased and the superheat degree increased, the spray plumes from individual holes collapsed and merged into a single spray structure. The spatial distributions of liquid and gas phases remained similar, and the spray remained in the liquid phase up to the tip, regardless of ambient pressure and fuel temperature conditions. With increasing superheat degree, the spray region expanded near the nozzle and narrowed downstream. In addition, the penetration length increased. This increase was quantitatively correlated with the volume expansion caused by flash boiling.