Cyclic Turbulent Instabilities in a Thin Slab Mold. Part I: Physical Model
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design of submerged entry nozzles (SENs) is critical for controlling steel flow turbulence in continuous casting molds and especially in confined spaces such as funnel-type thin slab molds, which operate at higher casting speeds than conventional thick molds. Controlling turbulence is very important to avoid flux particles entrainment, which transforms into slivers in the final product[1,2]; to avoid meniscus instability[3,4]; and distribute heat transfer evenly, aiming for uniform shell growth[5] and for the attainment of steady flow conditions. Some thin slab and conventional mold SEN designs induce strong meniscus instability and highamplitude oscillations of liquid in the mold.[6] For instance, Takatani et al.[7] found that the horizontal velocity under the meniscus fluctuates strongly through the width of a conventional slab mold. Using a physical model, Yoshida et al.[8] identified a meniscus-descending flow along the outer surface of the SEN as a result of differential pressure in this region. This descending flow carries out flux, which, once reaching the port position, E. TORRES-ALONSO, Graduate Student, R.D. MORALES, Professor, S. GARCI´A-HERNA´NDEZ, Graduate Student, and J. PALAFOX-RAMOS, Researcher, are with the Department of Metallurgy and Materials Engineering, Instituto Politecnico NacionalESQIE, Lindavista Zacatenco, Mexico, D.F. CP 07738. Contact e-mail: [email protected] Manuscript submitted February 19, 2009. Article published online April 2, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
is easily entrained by the discharging jet-distributing flux particles inside the steel bulk. According to those authors, driving force for the descending flow is the instantaneous meniscus velocity difference between metal and liquid flux provided by bath oscillations. Morales et al.[9] reported that in a four-port SEN, strong backflows from the upper roll flow are responsible for the existence of strong bath oscillations, indicating that full port utilization is a key factor for SEN design. A second key factor for SEN design is to avoid high free-shear-strain rates induced by long discharging jets, especially when the casting speed increases as Torres-Alonso et al.[10] indicated. In another work, the same authors also reported the existence of energetic vortexes formed very close to the SEN as a result of biased upper roll flows in a thin slab mold.[11] Other authors have claimed that bath oscillations, specifically in thin slab molds, originate in an intermittent-confined cross-flow that passes through the gap located between the outer SEN wall and the wide mold wall[12,13] during its journey from one of the upper corner narrow walls to the other. However, it is a fact that for a highly turbulent fluid flow in a continuous casting mold, the flow history plays the most important role on flow behavior. Indeed, that confined cross-flow is a consequence of wall shear and strain rates generated by walls of the SEN and discharging ports that shape the history of the flow downstream from the mold. In this work, the authors focused their effort o
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