Flash Freeze-Thaw Phenomena in Sprayed Evaporating Micrometer Droplets

Two-fluid nozzles are widely used in combustion, chemical processing, pharmaceutical coating, environmental control, and spray drying to atomize liquids with high-velocity pressurized gas. Yet, the adiabatic gas expansion-induced cooling of atomized droplets at the nozzle exit remains underexplored. Ice nucleation occurs homogeneously in pure water droplets or heterogeneously in the presence of ice nucleating particles (proteins, viruses, nanoparticles). Using computational fluid dynamics, we show that atomizing gas temperatures at the nozzle exit can fall below -90 °C, initiating rapid ice nucleation and growth within sprayed droplets. Temperature reduction during isentropic gas expansion depends on the initial pressure and expansion ratio. We examine how droplet size and nozzle design affect gas cooling and droplet freezing. Results reveal that smaller droplets have higher ice fractions due to enhanced cooling. Swirling flow increases ice formation by intensifying gas cooling, while non-swirling flow promotes ice formation by extending residence time in cold zones. These findings have implications for natural and industrial processes sensitive to freeze-thaw thermomechanical stresses.