Experimental Study on Spray Characteristics of Nano-TiO₂/Propylene Glycol Nanofluids

Nanofluids have become more and more attractive to the modern industry due to their high performance in heat transfer and combustion. As one of the key influencing factors, the atomization mechanism of nanofluids induced by airflow is more complex than that of pure liquids, which has attracted great attention recently. In this study, the spray characteristics of TiO₂–propylene glycol nanofluids were investigated by focusing on the effects of nano-TiO₂ additives on atomization. On the basis of high-speed shadow imaging, the spray morphology of the base fluid injected from an air atomizing nozzle under different pressures was analyzed first. Then a laser particle size analyzer was used to obtain the droplet size distribution of the nanofluids. The air pressure at the inlet of the atomizer was found to have more contribution to the atomization of nanofluids. And when the air pressure exceeded 0.35 MPa, the Sauter mean diameter (D₃₂) of the base fluid droplets was insensitive to the change of air pressure. Subsequently, the contribution of nanoparticles to atomization was discussed. An empirical correlation was proposed between the Sauter mean diameter (D₃₂) and the dimensionless parameters such as the gas Weber number (We_g), liquid–gas momentum ratio (q), and the volume fraction of nano-TiO₂, etc. Results indicated that adding nanoparticles enhanced the jet stability by increasing the viscosity at a low jet velocity, while the jet instability was increased due to the nanoparticles promoting the cavitation at a high jet velocity.