Experimental insights into the supersonic close-coupled atomization process employed for metal powder production

Abstract

The growing demand for high-quality metal powders as the raw product for metal additive manufacturing requires a better understanding of the physics involved in their production by means of supersonic close-coupled atomization. However, so far, the dominant breakup mechanisms have neither been identified nor theoretically described, hampering the development of suitable atomization models.In this experimental study, the spray produced by a generic supersonic close-coupled atomizer operated with water and air is visualized for a wide range of set points of operation. High-speed imaging is used to observe the primary atomization in a time-resolved manner. The secondary atomization is captured with a high spatial resolution employing double-frame imaging and an ultra-short illumination time, which additionally allows for the evaluation of the liquid motion and velocity.The primary atomization is shown to be governed by the interaction between the liquid jet and the recirculating gas flow in the wake downstream of the liquid nozzle, resulting in the liquid being driven into the surrounding high-velocity gas jet. The initial secondary atomization is found to be due to the violent interaction within the forming shear layer of the gas jet. Notably, a novel breakup mechanism based on the upstream formation of detached bow shocks is observed.