Transition hysteresis between open- and closed-wake gas flow for close-coupled gas atomization nozzles

Inert gas atomization is the state-of-the-art production method for manufacturing high-quality metal powder particles (1 µm to 150 µm) required in industries such as metal additive manufacturing, metall injection moulding, and thermal spraying. Previous research has highlighted the significance of pulsatile atomization mechanisms, which are linked to the efficiency and productivity of the gas atomization process. The melt pulsations are presumably connected to the transitions between open- to closed-wake flow. This work provides new insights into open-/closed-wake flow transition through experimental investigations at an industrial-scale atomization test bench, coupling high-frequency pressure measurements with high-speed Schlieren imaging. By triggering open-/closed-wake flow transition via time variations of the nozzle pressure d p 0 / d t , an aspiration pressure p A s p to nozzle pressure p 0 hysteresis is observed, related to the presence of a dual solution at such operating conditions. Based on Schlieren imaging the internal shock is identified as a boundary manifold for information transport between the ambient atmosphere and the jets core. In open-wake configurations, the single recirculation zone is influenced by the ambient pressure p a , whereas in closed-wake flows, two separated zones exist, with the upstream zone unaffected by p a and the aspiration pressure p A s p for the closed-wake flow depends on the edge shock post-shock pressure. • Wake transition show a hysteresis between nozzle pressure and aspiration pressure. • Schlieren imaging reveals differences between open-wake flow and closed-wake flows. • Gas oscillation within hysteresis indicate a dual solution domain. • Ambient conditions affect open- but not closed-wake recirculation zones.