Heat-Transfer Measurements in the Primary Cooling Phase of the Direct-Chill Casting Process
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direct-chill (DC) casting process, presented schematically in Figure 1, is a semicontinuous process used in the production of nonferrous metals and their alloys.[1–3] DC casting can produce rectangular ingots for rolling as well as cylindrical billets for extrusion and forgings. The process uses cooling water to remove heat from the molten metal or alloy. Heat removal takes place both indirectly through a water-cooled mold (primary cooling) and directly via water jets impinging on the emerging solid surface of the ingot (secondary cooling). Heat is also extracted from the metal through the bottom ‘‘dummy’’ block (tertiary cooling) during process startup. ETIENNE J.F.R. CARON, Postdoctoral Fellow, is with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada. Contact e-mail: ejfcaron@ uwaterloo.ca AMIR R. BASERINIA, formerly Postdoctoral Fellow, Department of Mechanical and Mechatronics Engineering, University of Waterloo, is now CFD and Aerodynamics Engineer, Harvistor Canada, Picton, ON K0K 2T0, Canada. HARRY NG, formerly M.A.Sc. Student, Department of Mechanical and Mechatronics Engineering, University of Waterloo, is now Mechanical Engineer, Thumbprint Solutions, Stouffville, ON L4A 7X5, Canada. MARY A. WELLS and DAVID C. WECKMAN, Professors, are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo. Manuscript submitted October 21, 2011. Article published online June 12, 2012. 1202—VOLUME 43B, OCTOBER 2012
In the primary cooling zone, liquid metal is initially in contact with the mold wall, but pulls away from the mold as it solidifies to form a solid metal shell, leaving an air gap which reduces heat transfer to the mold. The solid shell pulls away from the mold because of thermal contraction during cooling.[4–6] The cooling water exits the mold bottom through a series of holes or a slot. Secondary cooling, in which the cooling water is in direct contact with the ingot or billet surface, is thought to be responsible for 80 pct to 95 pct of the total heat removal during steady-state operation.[2,7,8] Compared with secondary cooling, primary cooling plays only a limited role in the overall heat extraction process. However, it has been shown to be an important factor in the formation of surface defects like cold shuts and in determining process parameters such as minimum and maximum casting speed.[9–12] Little precise experimental data for the heat-transfer coefficient (HTC) or heat flux in the primary cooling zone of the direct-chill casting process are available in the literature. Mathematical models of the direct-chill casting process have often used the HTC in the primary cooling zone as a fitting parameter. Typical values used in the region of direct contact between the cast metal and the mold have been 1000 to 5000 W/m2ÆK, whereas lower values ranging from 100 to 500 W/m2ÆK have been used when an air gap is thought to form between the ingot surface and the mold.[13–19] In comparison, the heat-transfer coefficients reported for
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