Atomization and Sprays

A STUDY OF LIQUID METAL ATOMIZATION USING CLOSE-COUPLED NOZZLES, PART 2: ATOMIZATION BEHAVIOR

RESUMO

In this article we examine the gas dynamic and atomization behavior of a convergent and a converging-diverging (c-d) close-coupled nozzle used to fabricate fine metal powders by gas atomization. Part 1 of this article characterized and compared the gas dynamic behavior of the two nozzles over a range of nozzle stagnation pressures. Part 2 focuses on the liquid metal atomization behavior of these nozzles. A high-speed Schlieren technique is used to visualize liquid metal breakup behavior simultaneously with the supersonic gas flow pattern in which it occurs. At all conditions examined, atomization is characterized by a coarse but "prompt" primary breakup stage followed by a more energetic secondary breakup stage that takes place over an extended distance from the nozzle tip. The reduction in particle size observed with increasing stagnation pressure is believed to be due in large part to the increase in the supersonic length of the gas jets that extend away from the nozzle tip because of enhanced secondary breakup. As expected, the c-d nozzle did not produce finer powder than the convergent nozzle at stagnation pressures where they produced similarly long gas-only supersonic jets. Overall the present experiments suggest that particle size refinement is influenced by the ratio of the velocity decay length scale and the secondary breakup length scale, a quantity that is physically related to the gas-to-liquid metal mass flux ratio. Because such length scales have not been previously considered, new opportunities may exist to significantly improve particle size control and energy efficiency in this costly powder manufacturing process.