Low-Pressure Injection of Water and Urea-Water Solution in Flash-Boiling Conditions

The flash-boiling effect in sprays has been shown to have the capability to decrease droplet diameters and spray penetration, as well as improve the vaporization process in gasoline direct injection systems. It has also been shown that in certain conditions, the effect on spray penetration can be the opposite. Under a high degree of overheating the spray collapse effect may appear and lead to increased spray penetration - flare flash boiling. Moreover, when the boiling is already strongly present in the nozzle it may lead to flow choking. Thus, the process is difficult to be controlled. While most studies so far have focused on high-pressure direct injection systems, we investigate the process in a low-pressure system to verify if the effects observed in high-pressure systems will be similar. The study was performed on two liquids, water and a urea-water solution, to limit any individual substance’s effects on the results and to improve the practical importance of the study. The results showed no increased penetration in any of the flash-boiling cases, although at the highest considered temperatures the two plumes tend to form a single spray cloud. Although the characteristics were similar for both studied liquids, in certain measurement conditions the differences in almost all analysed spray parameters were considerable. Moreover, the study has shown that the flash-boiling effect can be considered as an effective way to improve jet and droplet break-up while avoiding a pressure increase in low-pressure injection systems.