Experimental analysis of temperature effects in supercritical-assisted atomization

Supercritical CO2 is proved as an excellent choice in supercritical-assisted atomization of nanoparticle suspensions for fabrication of micro/nano-powders. As the rheological properties of the supercritical fluids are strongly dependent on the temperature, the breakup mechanism of the CO2-liquid mixture upon injection is significantly affected by crossing the critical temperature of the binary mixture. In this study, we investigate the breakup of CO2-water mixture (CO2-A) at subcritical, critical, and supercritical states and compare it with the cases where N2 is utilized as the assisting fluid (N2-A) at the same injection conditions. High-speed imaging and laser diffraction systems are utilized to analyze the primary and secondary atomization of the injected CO2-water mixture (over 20 to 40 °C injection temperature range). In general, CO2-A showed smaller and more homogenous droplets compared to N2-A. Therefore, the use of CO2 as the atomization gas is superior to N2. The underlying mechanism in primary breakup of CO2-A involves the emergence, expansion, and burst of CO2 bubbles and formation of ligaments that break up into droplets. The core of the jet in CO2-A system expands up to 50% due to emergence of gas bubbles, while the expansion ratio remains unchanged in the N2-A jet. The finest and most homogenous droplet sizes are achieved by operating near the critical point at 31.5 °C and 7.5 MPa. High solubility of CO2 in water and low interfacial tension of the CO2-water mixture are the main contributors.