The variation of droplet parameters during the spray breakup process affects the droplet deposition behavior and accurate application. The aim of this study was to experimentally investigate droplet behaviors along the penetration direction with respect to spray propagation. Particle image analysis (PIA) was applied to obtain the characteristics of droplets at three representative stages (namely, initial, quasi-steady, and end stages) after the start of injection (ASOI). The effects of timing and location on the spray characteristics were thoroughly investigated. First, different morphological changes of spray (droplets, ligaments, and bags) during spray breakup were observed. The experimental results show that droplet size and velocity distinctly increase from upstream to downstream at the initial stage. However, at the quasi-steady and end stages, droplet velocities are similar, and the effects of location are not evident. This indicates that location has a significant effect on droplet behaviors at the initial stage. The mean minimum distance (MD) of droplets first increases considerably and then decreases from upstream to downstream, suggesting that the droplets disperse better at midstream. Moreover, the mean MD at the initial stage exceeds that at the quasi-steady and end stages, denoting that the droplets disperse better with time. Finally, the geometric parameter of droplets and the key stage selection are important for predicting the interaction between the droplets and surfaces.
