Computational Modelling of Close-Coupled Gas Atomization

The demand for metal powders have increased with the recent advancements in additive manufacturing technology and this increase in demand has led to a tight powder size distribution constraint. The production of powders through CCGA is a complicated process and the relationship between various process and geometrical parameters is not completely understood. Few features of CCGA process cannot be analysed in depth experimentally. The high temperature of melt prevents the interaction between the melt and the gas from being analysed. The pressure developed in front of the melt nozzle i.e., the aspiration pressure with the presence of melt in the chamber cannot be studied due to the high temperature of the melt. The flow features such as recirculation zone also cannot be analysed in detail. A three-dimensional model of the whole CCGA process is computationally intensive and requires appropriate approximations. The thesis can be divided into two parts. In the first part, the axisymmetric nature of the annular slit nozzle has been taken advantage and a two-dimensional model has been developed. This model has been validated against the Schlieren images of gas flow pattern produced by a conventional discrete-jet atomizer nozzle. This model is used to analyse the flow features in Eulerian approach and the melt-gas interaction in Lagrangian approach. The relationship between flow features (shocks, stagnation point, Mach disk) and melt droplet movement have been analysed in depth for the annular slit atomizer. The pulsation phenomenon observed in CCGA has also been analysed. Finally, the aspiration pressure has been analysed comprehensively in flow fields with and without melt. In the second part, flow features around two discrete-jet atomizer nozzles has been analysed in Eulerian approach. Periodic models have been implemented to overcome high computational requirements. The wake condition and aspiration pressure has been found for all the pressures considered for these both nozzles. Few of the main results obtained in this study are as follows.
• Presence of melt affects the aspiration pressure and hence gas-only flow field aspiration pressures are not good guide for commercial atomizers.
• Movement of particles depends on the relative position of shocks and the expansion waves. This in turn affects the residence times and the velocity possessed by the particles which influences the final particle size.
• A three-dimensional model is required to model the flow features in discrete-jet atomizers due to the presence of intermittent spacing between the discrete jets.