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Natural convection in liquid metals Stewart, Murray John

Abstract

Natural convection in liquid metals has been studied by direct observation of the fluid flow, using radioactive tracer techniques. The study is of importance in understanding the solidification of metals since fluid flow strongly influences the heat and mass transfer in the system which in turn strongly influences the structure, homogeneity, and mechanical properties of the solid metal produced. The system examined in this investigation was a rectangular liquid cell of variable thickness, positioned on edge. A small driving force for natural convection was imposed across the liquid cell and when steady state conditions were reached, a small amount of the same material containing a radioactive isotope was added to the top of the cell. The tracer material was picked up by the flow and after a given time interval the liquid was quenched to fix the tracer position. The resultant solid block was autoradiographed to determine the distribution of the added radioactive material. Thermal convection was observed in liquid tin and liquid lead using radioactive Sn¹¹³ and radioactive TI²º⁴ respectively. The results show that the flow rates increase with increasing temperature difference across the liquid cell, increasing average temperature, and increasing liquid cell thickness. Flow rates with Grashof numbers from 10⁶ to 10⁸ were experimentally observed. A finite difference numerical solution for the problem of thermal convection is presented for Prandtl numbers of 10.0, 1.0, 0.1, and 0.0127 with Grashof numbers from 2 x 10³ to 2 x 10⁷. The experimental results for liquid tin (Pr = 0.0127) are found to approach the theoretical analysis for large cell thicknesses and large temperature differences. The flow behavior of various types of fluids is compared with liquid metals to show that non-metallic analogies to .metallic flow problems have very limited value. Solute convection is experimentally considered from three different viewpoints; a) independent solute convection, b) the influence of solute convection on thermal convection, and c) the thermal and solute conditions for complete liquid mixing. It was found that there must be a horizontal density inversion across the whole liquid cell for complete mixing to occur throughout the liquid zone. Interdendritic liquid flow resulting from the natural convection in the residual liquid pool was observed in lead-tin alloys. The flow penetrated into the solid-liquid zone to a point of approximately 12 - 22 % solid for primary dendrite spacings of from 700 to 1000 microns. Several experimental models are presented for interdendritic flow. A three-dimensional wire mesh model predicts that the finer the dendrite structure, the greater the flow penetration into the solid-liquid zone. The experimental results for the lead-tin alloys compared favorably with the model. As an extension of the fluid flow considerations, an investigation was carried out to determine macrosegregation in castings which have imposed fluid flow patterns. The macrosegregation present in stationary, rotated, and oscillated castings of Al - 3 wt. % Ag was determined by measuring the distribution of radioactive silver added to the melt. It was found that, no significant macrosegregation was present in the stationary and rotated castings. Extensive macro-segregation was detected in the oscillated casting. For the oscillated case the macrosegregation can be accounted for on the basis of the long range movement of dendrite fragments which break and/or melt off in the solid-liquid interface region. This movement is a direct result of turbulent waves associated with the oscillation. The maximum silver concentration is shown to be related to the columnar-to-equiaxed transition.

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