Modeling of liquid metal flow and heat transfer in delivery tube during gas atomization

A numerical model is developed to describe the flow and heat transfer behavior of liquid metals in a cold delivery tube during gas atomization. Numerical calculations for liquid In, Sn, Bi, Pb, Zn and Sb are performed to investigate the influence of processing parameters and material properties on the flow and cooling of the liquid metals, and to predict the minimum melt superheat that is necessary to prevent the liquid metals from premature solidification during delivery. Processing maps are developed to provide direct insight into the complex relationship between the minimum melt superheat, processing parameters and material properties. A quantitative correlation is derived from the numerical results by means of a regression analysis, which facilitates application of the numerical model. The calculated results reveal that the overpressure, tube length/diameter ratio, ambient temperature and thermal properties of the tube and liquid metals are the important factors influencing liquid metal flow and heat transfer. For the materials studied, the minimum melt superheat ranges from 0.001Tm to 0.17Tm, depending on the processing parameters and material properties. The dependence can be expressed usnig a correlation derived from the regression analysis such as ΔTTm=0.15μU2kcpρΔPTmTg0.1LD0.3 Increasing the overpressure can effectively decrease the minimum melt superheat, especially for a large tube length/diameter ratio and for materials possessing low densities. The minimum melt superheat asymptotically approaches its final steady value with increasing overpressure. The minimum melt superheat can also be decreased by reducing the tube length/diameter ratio, by selecting a smooth delivery tube with low thermal conductivity and a thick tube wall, and/or by enhancing the ambient temperature. Materials with high thermal conductivity, high thermal capacity and/or large density require a small melt superheat to prevent the liquid metals from premature solidification, while materials with high melting temperature and/or high viscosity require a large melt superheat.