Effect of aspiration and gas–melt configuration in close coupled nozzle on powder productivity

Convergent–divergent close coupled nozzles are used for the production of spherical and clean powders. A close coupled nozzle consists of a convergent–divergent gas flow channel concentric to which a melt flow tube is inserted. These nozzles have been proved to be efficient for melt
atomisation. However, the aspiration created at the tip of the delivery tube and the gas–melt configuration of interaction play a key role in determining the atomisation efficiency, mean particle size and standard deviation. In this investigation, aspiration has been measured for different protrusion lengths of metal and ceramic tubes from the nozzle exit. Two designs of atomisation nozzles are investigated. In one of the nozzles, flow channel is directed away from the centreline, whereas, in the other, it is directed on the centreline of the flow tube. Atomisation experiments are carried out to study the effect of aspiration and the melt–gas configuration of interaction on median particle diameter, standard deviation and flake formation. Aspiration, at the tip of the delivery tube, shows strong dependence on the combination of protrusion lengths. A change in the melt–gas configuration of interaction results in variation in median particle diameter and standard deviation. The nozzle with flow channel directed on the centreline showed better atomisation efficiency. This is manifested in smaller amount of flakes and narrow powder size distribution. The results have been presented and discussed.