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 atomizati on. Numerical calculations for liquid In, Sn, Bi, Pb, Zn and Sb are pe rformed to investigate the influence of processing parameters and mate rial properties on the flow and cooling of the liquid metals, and to p redict the minimum melt superheat that is necessary to prevent the liq uid metals from premature solidification during delivery. Processing m aps are developed to provide direct insight into the complex relations hip between the minimum melt superheat, processing parameters and mate rial properties. A quantitative correlation is derived from the numeri cal results by means of a regression analysis, which facilitates appli cation of the numerical model. The calculated results reveal that the overpressure, tube length/diameter ratio, ambient temperature and ther mal properties of the tube and liquid metals are the important factors influencing liquid metal flow and heat transfer. For the materials st udied, the minimum melt superheat ranges from 0.001T(m) to 0.17T(m), d epending on the processing parameters and material properties. The dep endence can be expressed using a correlation derived from the regressi on analysis such as Delta T/T-m = 0.15(mu U-2/kc(p) rho Delta P T-m/T- g)(0.1)(L/D)(0.3) Increasing the overpressure can effectively decrease the minimum melt superheat, especially for a large tube length/diamet er ratio and for materials possessing low densities. The minimum melt superheat asymptotically approaches its final steady value with increa sing overpressure. The minimum melt superheat can also be decreased by reducing the tube length/diameter ratio, by selecting a smooth delive ry tube with low thermal conductivity and a thick tube wall, and/or by enhancing the ambient temperature. Materials with high thermal conduc tivity, high thermal capacity and/or large density require a small mel t superheat to prevent the liquid metals from premature solidification , while materials with high melting temperature and/or high viscosity require a large melt superheat.