Unsteady Flow Predictions of Initial Surface Formation in Aluminum Strip Casting
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well over a decade now, the steel and aluminum industries have been pursuing new approaches to eliminate processing steps between an initial cast and the final sheet products. An over-arching paradigm for this effort has been termed near-net-shape casting technology and, over the years, has met with considerable success. There are challenges, however, in bringing such an approach to increasingly smaller sections (as measured by the cast thickness) in strip casting where, because of the decreased cross-sectional material, there is less opportunity to correct for defects. This is particularly true for surface defects, and for some aluminum alloys, this is a key issue to resolve to enable its introduction into new markets. Therefore, in order to take full advantage of the nearnet-shape casting of aluminum sheet, it has required increasingly detailed knowledge of the thermal-fluid dynamic history of the melt in its earliest stages of surface formation. To obtain this knowledge experimentally under real process conditions is difficult with high-temperature liquid metals. From a macro perspective, this type of process information can be obtained using computational fluid dynamics (CFD), which has been applied at increasing levels of detail to all types of materials processing phenomena. For aluminum and steel continuous casting processes, there is now a large body of CFD literature applying the Reynolds Averaged A.G. GERBER, Professor, and A. HEALY, Graduate Student, are with the Department of Mechanical Engineering, University of New Brunswick, P.O. Box 4400 Fredericton, New Brunswick, Canada E3B-5A3. Contact e-mail: [email protected] Manuscript submitted August 23, 2009. Article published online March 19, 2010. 660—VOLUME 41B, JUNE 2010
Navier–Stokes (RANS) equations with solidification[1–5] and, increasingly, attempts to combine these predictions with micro-scale phenomena in a coupled manner.[6,7] Such coupled approaches often are compromised by the complicated boundary conditions present in solidifying flows. For example, considerable research, both experimental and computational, has gone into the examination of contact heat-transfer conditions between a solidifying melt and its mold.[8–10] Macroscale CFD simulations often include the boundary condition complexity (to some approximation) in obtaining improved predictions at process scale (e.g., including the mold wall, air-gaps/films, and external water sprays as part of the solution).[3,11] Subsequent detailed microscale simulations can be considered based on these results, thus, avoiding direct coupling with the macroscale solution (which involves excessive computing times resolving scales from process level to microstructure) while compromising somewhat on accuracy. In the case of predicting thermo-fluid states at the initiation of a cast surface, the boundary conditions are also of considerable importance. In a real strip casting situation, there are normally a number of physical regions active at the same time that are critical to the thermal-fluid dynamic conditions when a
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