The gas-dynamic and metal atomization performance of two different close-coupled nozzles

An overview is given of experiments conducted at Penn State University in which pure tin was atomized with compressed air using two different close-coupled nozzles at pressure ratios between 15 and 50 and gas-to-metal ratios between 0.9 and 2.5. The experiments were intended to examine the overall physics of metal atomization, including the primary and secondary breakup stages, and to investigate the influence of nozzle pressure ratio and nozzle design on the physics of atomization and on particle size. A converging (HPGA) and a converging-diverging (Unal) close-coupled nozzle, both based on nozzles described in the literature, were included to investigate whether supersonic improves atomization performance. The experiments revealed that most of the fine particles are produced during the secondary breakup stage, which takes place over an extended distance from the nozzle tip. As such, the characteristics of the supersonic gas jets produced by these close-coupled nozzles, including their supersonic length and dynamic pressure, strongly affect the particle size. Longer supersonic jets were produced at higher pressure ratios, resulting in fewer coarse particles in the atomized powder. The HPGA and Unal nozzles exhibited similar gas dynamic and atomization behavior over most of the range examined, revealing that the converging-diverging design, despite its supersonic expansion, was not generally superior. Considerations for achieving minimum average particle size at minimum gas-to-metal ratios, based on the importance of supersonic jet length and dynamic pressure to atomization performance, are discussed.