Close-coupled gas atomization: High-frame-rate analysis of spray-cone geometry

The geometry of the spray cone during atomization of Ni31.5Al68.5 in a close-coupled gas atomizer operating with a generic die and nozzle design has been studied utilizing high speed digital video techniques. Details of the region extending 5 cm from the spray nozzle at frame rates of up to 18,000 frames/s were recorded. The material was sprayed at a temperature ∼1830 K (corresponding to a superheat ~200 K), wherein sufficient thermal radiation was emitted for images to be recorded without any additional lighting. In order to quantitatively analyze the large number of still frames that result (up to 65,536), image processing routines capable of automating this process have been developed and used to measure the optical brightness and the position of the optical intensity maximum of the material passing though a narrow window at a fixed distance from the nozzle tip. The results of this analysis show that the spray cone consists of a jet that precesses around the center axis of the atomizer in a regular manner at a frequency ~360 Hz. In order to understand the origin of this motion further experiments were conducted with a laboratory-scale analogue atomizer which atomizes a water jet. It was found that the frequency of precession is essentially independent of the atomizing gas pressure, but does depend upon the geometry of both the die and nozzle.