High-speed imaging and computational modelling of close-coupled gas atomization

Gas atomization process, especially Closed Coupled Gas Atomization (CCGA) is a very efficient processing method to produce ultra fine, spherodised metal powders. In this process, the high-pressure gas jet is used to disintegrate the molten metal stream in to the spherical powders. Due to hydrodynamic and thermal interaction between high-pressure gas jet and molten metal stream especially near melt delivery nozzle, this technique is very complex and challenging for atomization industries. Melt delivery nozzle design is one of the key factors to control powders properties. The optical Schlieren technique and analogue water atomizer along with Computational Fluid Dynamics (CFD) numerical methods are practical methods for observing the single or two-phase flow of gas-metal interaction during CCGA. This research is focused on the optical Schlieren and numerical CFD techniques to observe single-phase gas flow behaviour with different melt nozzles tip design and gas dies profile. The CFD numerical results are validated by the experimental Schlieren test results. The effect of melt tip design on open to closed-wake condition near melt nozzle was investigated. Comparing the CFD velocity field and velocity streamlines of different nozzle design at different atomization gas pressure could help to propose a new hypothesis of how open to closed-wake condition occurs at different nozzle tip design. In addition, the flow separation problem around melt nozzle by two different gas die systems was investigated. The results showed there are two major mechanisms for this phenomenon, which depends on gas die system set-up, melt nozzle tip protrusion length and mis-match angle of external nozzle wall to the gas jet direction.