Investigation of the urea-water solution atomization process in engine exhaust-like conditions

•Droplet tracking at very high imaging speed is performed in the urea-water injection.•Higher pressure produced more droplets with smaller diameter.•Near the nozzle-exit, the radial component of velocity increased with the pressure.•Gas temperature only has an effect in the distribution of the developed spray.•Blobs and non-spherical structures where identified near the nozzle exit. The injection process of the urea-water solution (UWS) determines the initial conditions for the mixing and evaporation of the fluid in the selective catalytic reduction system. In this study, the liquid atomization process of a UWS dosing system is investigated using optical diagnosis through back-light imaging. The droplet diameter distribution and the droplet velocity (in the axial and tangential components) of the liquid spray are quantified under different air flow and injection conditions. A new test facility was designed to study UWS spray under conditions that resemble those of the engine exhaust pipe, which is capable of reaching an air flow of 400 kg/h and air temperatures up to 400 °C. The test matrix consisted of variations in the air flow temperature, air mass flow and UWS injection pressure. A high speed camera was used for capturing the images of the liquid spray, comparing the atomized liquid behaviour in three different regions of the plume: the first one near the nozzle exit, and the other two in the developed region of the spray (one aligned with the injector axis and the other at the spray periphery). Increasing the injection pressure affected the atomization process producing smaller particles with higher velocities in the axial and tangential components, promoting wider global spray angles, that combined with high air flow temperatures could improve the evaporation and mixing process in the SCR system. The main contribution is the development of an alternative technique for the quantification of the droplet size and velocity.