Gas-assisted atomization of a liquid jet into a spray of small droplets is used in many industrial applications. The breakup of the liquid jet is the result of a series of instabilities. Shear-induced Kelvin-Helmholtz (KH) instabilities give rise to liquid undulations, and once their crests destabilize through Raleigh-type instabilities, they are stretched into ligaments by the fast gas flow and fragment into small droplets under the influence of capillary forces and gas Reynolds stresses. Spray production involves three main steps, (i) liquid jet deformation, (ii) primary breakup, and (iii) secondary breakup. Of these three steps, primary breakup is the least studied.
In this study, close-coupled gas atomization (CCGA) atomizers were investigated. In these atomizers, it is surmised that gas recirculation directly below the nozzle tip is instrumental in primary breakup. Experiments were conducted across 2 different Weber numbers, 8 different momentum flux ratios and 3 different apex angles. High-speed Digital Inline Holography (DIH) was employed as the imaging technique, utilizing nanosecond-duration laser pulses effectively freezing the breakup mechanism and providing high-quality images at high temporal resolution. Key findings from this study include the identification of the liquid jet’s flapping frequency, and the effect of the Weber number, momentum flux ratios, and apex angle on these frequencies. Furthermore, droplet velocities were successfully measured for the highest Weber number, providing new insights into droplet dynamics in CCGA.
