Numerical analysis of droplet aerobreakup in supercritical flows

This study investigates the aerobreakup mechanisms of a liquid droplet initially at a temperature below its critical point impacted by a shockwave in a supercritical environment, i.e. transcritical conditions, occurring in high-pressure/speed liquid-fuelled propulsion systems. Aerobreakup droplet breakup mechanisms have been extensively studied at atmospheric conditions, not considering the significant changes in fluid properties past the critical point that occur within very short breakup time scales in shock-dominated flows. Furthermore, the effects of decreased surface tension forces due to the weakening of intermolecular forces at supercritical conditions on the droplet breakup behaviour have not been resolved to date. This study aims to address these major gaps by developing a direct numerical simulation method to investigate the governing mechanism of droplet aerobreakup at transcritical conditions considering the changes in surface tension. A diffuse interface method coupled with a real-gas equation of state is developed to capture the fluid behaviour beyond the critical point. The results show that simultaneous changes in surface tension and density ratio unique to transcritical flows dictate the droplet aerobreakup mechanisms and the resultant breakup modes. This study presents the first transcritical droplet breakup regime map as a function of Weber number and density ratio compared with the classical breakup criteria commonly accepted for subcritical conditions, proving that the breakup is facilitated at supercritical conditions. The findings are expected to significantly contribute to the development of transcritical droplet aerobreakup models to enable the simulation of spray-shock interaction needed for designing new high-speed/pressure liquid fuel injection systems.