The Physical Mechanism for Melt Pulsation During Close-Coupled Atomization

High-speed filming (18,000 fps) has been used to study the phenomenon of melt pulsation in a discrete jet, high-pressure gas atomization system. Image processing routines have been developed to determine the velocity of material within the melt plume as it streams away from the melt delivery nozzle and to parameterize the shape of the melt plume. These data are then correlated with the volume of material in the melt plume, which is used as a diagnostic for melt pulsation. We find that during periods of low melt the constriction in the melt plume in which its diameter is a minimum is smaller and farther downstream. Both appear to be consistent with a transition from an open-wake structure during periods of high melt flow to a closed-wake during periods of low melt flow. Moreover, the average velocity of material in the plume appears to drop during periods of low melt flow, which we ascribe to its passage through the Mach disk. We conclude that there is sufficient evidence to assert that the closed-wake condition survives the introduction of a dense second fluid to the atomizer and that alternation between the open- and closed-wake conditions is the likely cause of the observed melt pulsation.