Unstable rim vs impact wave dominated atomization: Spatiotemporal characteristics and droplet statistics across Weber number regimes

This study experimentally investigates the spatiotemporal dynamics and droplet statistics of impinging jet atomization under varying Weber numbers (We) and impact angles (2α), focusing on unstable rim regime and impact wave regime. High-speed imaging, combined with Proper Orthogonal Decomposition (POD), is employed to characterize the dynamic evolution and breakup behavior of the liquid sheet. Two distinct atomization mechanisms are identified: an unstable rim regime at low (81.53–226.47), and an impact wave regime at higher (326.12–579.77). POD spatial modes and their associated power spectral densities reveal that the rim breakup corresponds to low-frequency large-scale structures, whereas impact-wave-driven fragmentation exhibits high-frequency fluctuations. Droplet statistics show that diameters follow a log-normal distribution under the impact wave regime, while velocities exhibit a normal distribution across all regimes. The 2α significantly influences droplet velocity dispersion but has a limited effect on droplet size for impact wave. The droplet Reynolds number demonstrates a consistent scaling relationship with normalized diameter. An empirical model is developed to predict droplet sizes in the impact-wave-dominated regime, incorporating POD-derived disturbance wavelengths, breakup length, and ligament-to-droplet correlation. The model enables reliable estimation of mean droplet diameters based on injector geometry and flow parameters. These findings offer critical insights for the design and optimization of impinging jet atomizers in engineering applications such as aerospace propulsion, micro-reactors, and pharmaceutical sprays.