Droplet cooling in atomization sprays

Transport between droplets/particles and a gas phase plays an important role in numerous material processing operations. These include rapid solidification operations such as gas atomization and spray forming, as well as chemical systems such as flash furnaces. Chemical reaction rates and solidification are dependent on the rate of gas-particle or gas-droplet transport mechanisms. These gas-based processes are difficult to analyze due to their complexity which include particle and droplet distribution and the flow in a gas field having variations in temperature and velocity both in the jet cross-section and in the axial distance away from the jet source. Thus to study and properly identify the important variables in transport, these gas and droplet variations must be eliminated or controlled. This is done in this work using models based on a single fluid atomization system. Using a heat transport model (referred to as thermal model) validated using single fluid atomization of molten droplets and a microsegregation model, the effect of process variables on heat losses from droplets was examined. In this work, the effect of type of gas, droplet size, gas temperature, gas-droplet relative velocity on the heat transport from AA6061 droplets was examined. It is shown that for a given gas type, the most critical process variable is the gas temperature particularly as affected by two-way thermal coupling and the droplet size. The results are generalized and applied to explain the difference in droplet cooling rate from different atomization processes. © Springer Science+Business Media, LLC 200

Mechanisms and control of macrosegregation in DC casting

Macrosegregation is a severe, unrecoverable defect often occurring in large-scale castings. This paper offers a critical review of mechanisms involved in the formation of macrosegregation during DC casting of aluminum alloys. These mechanisms include thermo-solutal and forced convection, shrinkage-driven flow and transport of solid crystals. It is demonstrated that the impact of melt flow on macrosegregation depends on the flow direction and pattern, and on the extent of the slurry zone in the sump. The shrinkage-induced flow contributes to the negative centerline segregation but this contribution depends on the shape of the macroscopic solidification front and on the permeability of the mushy zone. Accumulation of floating crystals will result in negative segregation but the occurrence of these grains and their composition depends on grain refinement and melt How pattern. It is shown that macrosegregation can be controlled by practical means such as process parameters, structure modification and melt flow management