A study of liquid metal atomization using close-coupled nozzles, Part 1: Gas dynamic behavior

Liquid metal atomization using close-coupled nozzles is an established technique for fabricating fine (< 100-mum) metal powders for a variety of industrial uses. Despite its widespread use, however, the interrelationships among gas dynamics, nozzle geometry, processing parameters, and particle size remain ill-defined. As a result, efforts to reduce powder costs by improving particle size control and energy efficiency remain hindered. This study examines and compares examples of a convergent and a converging-diverging (c-d) close-coupled nozzle on the basis of their gas dynamic behavior (Part 1) and their liquid metal atomization performance (Part 2). In Part 1, Schlieren photography and Mach number and Pitot pressure measurements are used to characterize the gas dynamic behavior of the nozzles (without liquid metal present) operating at stagnation pressures between 2 and 5 MPa. In Part 2, their liquid metal atomization behavior is examined by high-speed Schlieren photography, and particle size distributions are measured to compare their atomization performance. Results showed that the two nozzles performed similarly in gas flow and atomization tests over most of the range of P. examined, despite their significantly different geometries. The active atomization zone appeared to extend far downstream, indicating that gas velocity decay by turbulent diffusion may play a limiting role in atomization. This also suggests that the importance of the gas-to-liquid mass flux ratio has a physical basis associated with a ratio of velocity decay length to breakup length scales. These observations have potentially important implications for designing efficient liquid metal atomization processes for producing low-cost metal powders.