Revisit aerobreakup criteria for vaporizing micro-droplet

Aerodynamic deformation and breakup are common phenomena in droplet and spray applications. Past research has focused on millimeter-sized droplets, identifying various breakup topologies and characterizing breakup modes primarily through the Weber and Ohnesorge numbers. Less attention has been devoted to micro-droplets, with diameters of tens of microns, where the assumption is that they are too small to break. However, in applications such as fuel injection and droplet impact in high-speed flight, the relative velocity between the droplet and air can be sufficiently high to cause micro-droplet breakup. Moreover, these applications often involve high-temperature flows, where aerobreakup is accompanied by significant heat and mass transfer. The Stefan flow, induced by phase change at the droplet surface, significantly influences interfacial instability, droplet deformation dynamics, and resulting breakup morphology and outcomes. This study presents our recent simulation results on the aerobreakup of vaporizing micro-droplets, systematically characterizing the effects of Reynolds and Stefan numbers on droplet deformation and breakup dynamics. The sharp liquid-gas interface with phase change is tracked using the geometric Volume-of-Fluid (VOF) method. The Stefan flow at the interface is accounted for by solving the pressure Poisson equation with an additional source term for phase change, using a conservative and compact source distribution method. The two-phase flow model is implemented in the adaptive multiphase flow solver Basilisk, and comprehensive high-fidelity simulations were performed across various Weber, Reynolds, and Stefan numbers. The simulation results indicate that both a decrease in Re and an increase in St stabilize droplet breakup and increase the critical Weber number.