Effects of injection pressure and existence of a critical axial position on the evolution of microscopic spray characteristics

Air-assisted atomization is characterized by the presence of an internal high-velocity gas jet, which drives liquid fragmentation and atomization through intense gas–liquid interactions. In this study, a phase Doppler particle analyzer was employed to investigate the effects of injection pressure and axial position on the microscopic characteristics of air-assisted sprays. Based on droplet collision dynamics, the probabilities of different collision outcomes were calculated. Furthermore, the intrinsic relationships among spray variables were quantified using correlation analysis and principal component analysis (PCA). Results indicate the existence of a critical axial position, x crit. For x < x crit, gas–liquid interactions dominate, increasing the Sauter mean diameter (D 32) and droplet velocity (U d); for x > x crit, aerodynamic drag prevails, leading to fluctuating D 32 and reduced U d. Higher injection pressure enhances instability waves, broadens the droplet size distribution, and shifts x crit downstream. When the droplet Weber number remains below the empirical critical threshold, neither shear-nor turbulence-induced breakup occurs, leaving coalescence as the dominant collision outcome. Correlation analysis reveals a non-monotonic influence of pressure on spray microstructures, highlighting their spatial dependence. PCA further demonstrates a near-field trade-off between droplet velocity and size, while the far field is characterized by high turbulence intensity, large droplet size, and reduced velocity, reflecting a dynamic balance between coalescence and breakup.