Experimental analysis of supercritical-assisted atomization

Supercritical CO2 is used in supercritical-assisted atomization (SAA) systems to promote the atomization of nanoparticle suspensions in powder generation in pharmaceutical, electronics, and coating applications. Due to the sensitivity of the mixture properties to the operational conditions, the SAA process is not fully resolved to date. This study experimentally investigates the underlying mechanisms behind SAA utilizing CO2 or N2 as the assisted-atomization fluid (CO2-A or N2-A) using high-speed imaging and laser diffraction techniques. The effects of injection temperature, pressure, and gas-to-liquid ratio (GLR) are explored, and empirical droplet size models are developed. It is found that the primary breakup of CO2-A is governed by the emergence of the near-nozzle gas bubbles originated from the dissolved CO2, which expand radially and squeeze the liquid due to the inertial forces. As a result, the edges of the liquid core become thinner and deform into relatively long ligaments that further break up into droplets. CO2-A exhibited a shorter liquid length, wider spray angle, and smaller droplet size compared to N2-A. The discrepancies observed in the breakup process are mainly attributed to the higher solubility of CO2 in water and lower surface tension of the CO2–water system. The smallest droplet size distribution and the narrowest droplet size distribution are detected for CO2-A injected at the critical pressure of the CO2–water binary system where the solubility of CO2 in water significantly rises. Linear instability analysis indicates that both shear and acceleration that indirectly incorporate the experimentally observed bubble expansion are the main factors driving the instabilities.