Pré-visualização5 páginas

Melt Flow Control in a Multistrand Tundish Using a Turbulence Inhibitor R.D. MORALES, J. de J. BARRETO, S. LO´ PEZ-RAMIREZ, J. PALAFOX-RAMOS, and D. ZACHARIAS Water modeling and mathematical simulation techniques were used to study the melt flow under the influence of turbulence inhibitors in a multistrand bloom caster tundish. Three different cases were studied: a bare tundish (BT), a tundish with two pairs of baffles and a waved impact pad (BWIP), and a tundish equipped with turbulence inhibitor and a pair of dams (TI&D). Chemical mixing of tracer turbulence diffusion was also simulated and compared with actual experimental results. The TI&D arrangement showed an improvement of the fluid flow characteristics, yielding better tracer distribution among the outlets, lower values of back mixing flow, and higher values of plug flow. A mass transfer model coupled with k-« turbulence model predicted acceptably well the experimental chemical mixing of the tracer in the water model. The water modeling and the numerical simulation indicated that the TI&D arrangement retains the tracer inside the vessel for longer times, increasing the minimum residence time. These results encourage the use of turbulence-inhibiting devices in bloom and billet casters, which pursue excellence in product quality. I. INTRODUCTION TI on the fluid flow pattern, tracer diffusion, and tracer distribution in the different strands of a four-strand tundishMELT flow control in tundishes with one or two strands of a bloom caster. Other important aspects considered were using dams, weirs, and baffles has been widely studied using the melt flow performance comparisons, among the bare water models.[1–8] Melt flow in multiple-strand tundishes tundish, MFCD consisting of dams and a waved impact pad,for billet and bloom casters has been studied using water and the TI itself. In order to obtain these objectives, water modeling and mathematical simulation techniques.[9–14] and mathematical modeling techniques were applied simul-More recently, these techniques also have been employed taneously, reaching useful conclusions in a complemen-to develop turbulence inhibitors. These devices are essential tary fashion.to decrease fluid turbulence in the pouring region in one- and two-strand tundishes.[15–20] Turbulence inhibitors have been shown to be very useful II. WATER MODELING to avoid slag entrapment, pick up of oxygen and nitrogen A 1/3-scale model, using the Froude criterion, was con-from the surrounding air during ladle changes, and decrease structed using transparent Perspex (Bodega de Pla´sticos,downgraded steel during grade changes. It is a usual trend Mexico) plastic sheets with a thickness of 0.0127 m. Figurethat, similar to other melt flow control devices (MFCD), a 1 shows the geometric dimensions of this model. As can beturbulence inhibitor (TI) should be designed and manufac- seen, the positions of the outlets are nonsymmetrical withtured on a tailor-made basis. Every TI should be designed respect to the central axis of both sides of the tundish. Threeaccording to the tundish size, melt depth, flow rate, and types of tundishes were studied: the bare tundish (BT), atundish design. tundish with MFCD consisting of two pairs of baffles andIn the present work, melt flow control using a turbulence a waved impact pad (BWIP), and a tundish with MFCDinhibitor in a four-strand tundish of a bloom caster is thor- consisting of a TI and a pair of dams (TI&D).oughly studied. In this sense, this is the first report dealing Design of the high baffles for the BWIP arrangement iswith the employment of a TI in a multiple-strand tundish. shown in Figure 2(a). Figure 2(b) shows the design for theA multiple-strand tundish presents various challenges such low dams, while Figure 2(c) shows a scheme of the wavedas being able to maintain the same casting temperature, impact pad. Similarly, the TI&D arrangement is shown inhomogeneous chemistry, and similar steel cleanliness in Figures 3(a) and (b). The first one shows the dam designevery strand. and the second one the TI.The objective in this work was to study the effects of a The positions of the two pairs of dams inside the tundish for the BWIP arrangement and those corresponding to the TI&D arrangement are indicated in Figure 4. Stopper rods R.D. MORALES, Professor, and J. PALAFOX-RAMOS, Postdoctoral perform flow rate control of fluid, one for each outlet. TheStudent, are with the Department of Metallurgy and Materials Engineering, operating conditions of the tundish are reported in Table I.Institute Polytechnic National, ESIQIE, C.P. 07300, Mexico. J. de J. In order to determine the Residence Time DistributionBARRETO, Professor, is with the Materials Graduate Center, Institute Tecnologico de Morelia, C.P. 58120-Morelia, Mexico. S. LOPEZ-RAMIREZ, (RTD) curves, tracer experiments were carried out using a formerly Postdoctoral Student, Department of Metallurgy and Materials red dye that was injected in the inlet stream at a time zero. Engineering, Institute Polytechnic National, ESIQIE, is Researcher, The tracer concentration was measured in two of the outlets,FOSECO INC., 20200 Sheldon Road, Cleveland, OH 44142. one called the interior (the nearest one to the inlet) and theD. ZACHARIAS, Research Engineer, is with FOSECO INC. Manuscript submitted October 18, 1999. other one exterior (the nearest one to the end wall of the METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 31B, DECEMBER 2000—1505 (a) Fig. 1—Geometric dimensions of the water model (m). (b) tundish), using two UV spectrophotometers. The output sig- nals from these apparatuses were fed to a PC equipped with a data acquisition card provided with software to process the raw data into dimensionless variables according to well- known proce-dures.[21] Thus, assuming that the volumetric flow rates through the four strands are identical, the amount of injected tracer flowing out in a period dt through the i strand will be[22]. dmi 5 Ci (t)Qdt [1] where Ci is the tracer concentration in the outlet i, Q the volumetric flow rate, and mi the tracer mass at the strand i (List of Symbols). Allowing M to be the total mass of the pulse tracer (c) injected, we obtain, by definition, the following expression: Fig. 2—Geometric dimensions for the BWIP tundish arrangement (m): (a) tall baffles, (b) short baffles, (c) waved impact pad.dmi M 5 Ei (t)dt [2] model was designed to simulate the fluid flow of waterIntegrating this equation for all strands, inside the tundish model as well as the chemical mixing process of the tracer injected by a pulse in the incominge`0 E1 (t)dt 1 e ` 0 E2 (t)dt 1 . . . . 5 1 [3] stream. It involves the solution of the three-dimensional (3- or simply D) Navier–Stokes equations of turbulence, the mass transfer equation, continuity equation, and two equations for the k-«e`0 E (t)dt 5 1 [4] model chosen to represent turbulent viscosity. The equations were reduced to their finite-difference equivalents by integ- Once the RTD curve given by Eq. [4] was determined, the rating over the computational cells into which the 3-D flow parameters were calculated using the methods dis- domain was divided, as shown in Figure 5. Turbulent cussed in Reference 21. momentum equations were solved to yield steady-state con- ditions and the turbulent mass transfer equation was solved under unsteady-state conditions. This is a similar procedureIII. MATHEMATICAL MODELING to that employed in the physical model, i.e., the fluid is A. Fundamental Equations allowed to stabilize at a constant volumetric flow rate and at an arbitrary tune taken as zero, the tracer is injectedTwo cases were considered in this study, the bare tundish and the tundish with a TI&D arrangement. A mathematical starting its unsteady chemical mixing in the fluid. 1506—VOLUME 31B, DECEMBER 2000 METALLURGICAL AND MATERIALS TRANSACTIONS B Fig. 5—3-D computational mesh employed in the mathematical model. Continuity equation: (a) r t 1 xj (ruj) 5 0 [5]