On the flash-assisted atomization of a two-component immiscible mixture spray

• Flashing hexane assists water atomization in a hexane–water spray without surfactant addition. • Spray puffing occurs at temperatures above hexane’s boiling point, causing instability in the mixture spray. • The flashing hexane significantly promotes the primary and secondary breakup of water. • The droplets produced by the mixture spray fluctuate in size and velocity with certain frequencies. This paper employs blends of water and hexane to study the flash-assisted atomization of immiscible two-fluid sprays without the use of surfactants. Equal volumes of each fluid are heated separately to the same temperature above the boiling point of hexane (69 °C), mixed quicky and ejected through a small nozzle to the atmospheric environment. The results are compared with those obtained for single fluids of hexane and water. High-speed imaging and phase Doppler particle analyser (PDPA) measurements at different downstream and radial locations were taken to map the spray morphology and droplet characteristics. A puffing phenomenon was observed in the near field of the spray when temperatures exceed the boiling point of the single fluid or one of the fluids in the blend. This phenomenon significantly widens the spray, with the spray width and liquid core length of the immiscible water-hexane falling between those for pure hexane and water. A novel finding from the PDPA results indicate that the puffing phenomenon induces instabilities in the immiscible blend as the explosions of hexane bubbles promote spray mixing, decreasing the droplet size and increasing the droplet velocity. Micro-explosion of bubbles from the evaporating component underlies the mechanism of flash-assisted atomization and this, in immiscible blends, results in additional instabilities through spray puffing induced pulsating breakup which eventually improves the breakup and fragmentation of both components in the mixture, producing clusters of fine droplets at a certain frequency. The results provide the potential of utilizing the immiscibility between fluids to further assist the atomization of a superheated spray.