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1. Introduction Centrifugal flow tundish, in which the molten steel is horizontally rotated by electromagnetic force, has been de- veloped to produce high quality steel with high productiv- ity. The effect of the rotating electromagnetic force on in- clusions separation in a batch system1) and a pilot plant ex- periment using a 500 kg capacity tundish2) have been re- ported. Industrial plant test3) carried out at Chiba works showed that the amount of inclusions was reduced to half and defects of hot-rolled coils were reduced to 60%. The swirling flow in the rotation chamber is crucial for the ef- fectiveness of centrifugal flow tundish. Therefore, it is ben- eficial to understand the flow structure of centrifugal flow tundish. Models of turbulent flow can be classified into the Reynolds-averaged approach, large eddy simulation and di- rect numerical simulation. Because of its computational cost, the Reynolds-averaged approach, typically with the two-equation (k\u2013e) turbulence model,4,5) has been exten- sively adopted in previous studies and has produced valu- able insights about the flows in tundish. However, this ap- proach is not suited for studying the time evolution of un- steady flow structures. The direct numerical solution is to solve directly the Navier\u2013Stokes (NS) equations with high computational cost. A midway choice between two-equa- tion turbulence models and direct numerical simulation is the employment of a computational technique known as Large Eddy Simulation where the NS equations are filtered. The filtered equations are then simulated and the left small- est eddies are modeled.6\u201310) The objective of the present work is to understand the flow structure in centrifugal flow tundish by using the large eddy simulation (LES), and analyze the effect of electro- magnetic force and bending nozzle on the swirling flow. It is an innovative technique to enhance the swirling flow in rotation chamber using bending nozzle. 2. Mathematical Model Flow field in the 1/12 low melting point alloy model is solved using LES. The computational domain is shown in Fig. 1. The size of tundish model is 0.42 m\ufffd0.128 m\ufffd 0.12 m. The diameter of rotation chamber is 0.128 m, the size of the outlet gate of rotation chamber is 0.032 m\ufffd 0.04 m (length\ufffdheight) the angle and diameter of the bend- ing nozzle is 90 degree and 0.02 m. The depth of the bend- ing nozzle is 0.01 m below the level of fluid. The casing speed is 0.95 m/min. It also shows the coordinate directions of x, y and z. The horizontal velocity component U, V and the vertical component W are in x, y and z direction, respec- tively. 2.1. Filtering Operation In order to separate out the large eddies from the small- scale motions, a filtering operations11) is applied to the 568© 2007 ISIJ Large Eddy Simulation on Flow Structure in Centrifugal Flow Tundish Fang WANG,1) Baokuan LI1) and Fumitaka TSUKIHASHI2) 1) School of Materials and Metallurgy, Northeastern University, Wenhua road 3-11, Heping District, Shenyang, 110004, China. 2) Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561 Japan. (Received on August 4, 2006; accepted on January 5, 2007 ) Centrifugal flow tundish which the molten steel is horizontally rotated by electromagnetic force has been developed to produce high quality steel with high productivity. Because the swirling flow in the rotation chamber is crucial for the effectiveness of centrifugal flow tundish, it is beneficial to search the way to en- hance the swirling flow and understand the flow structure in centrifugal flow tundish. Large Eddy Simula- tion (LES) technique is developed to simulate the complicated flows. LES is computationally much more in- tensive than k\u2013e and cost less than direct numerical simulation (DNS), but offers a new level of insight into transient phenomena. In the present works, flow structures in three cases are simulated and analyzed, i.e. the swirling flow is produced by (a) electromagnetic force with the direct nozzle, (b) bending nozzle using height potential energy of molten steel, (c) combination of electromagnetic force and bending nozzle. The swirling flow and vortices are clearly displayed. The effectiveness of bending nozzle is validated by 17% in- crement of maximum swirling velocity in rotation chamber. KEY WORDS: large eddy simulation; centrifugal flow tundish; swirling flow; bending nozzle; electromag- netic force. ISIJ International, Vol. 47 (2007), No. 4, pp. 568\u2013573 Navier\u2013Stokes equation. A filterer variable, denoted by an overbar, is defined as ......................(1) Where G is the filter function, and the integral is extended to the entire domain D. The filter employed here is the box type in real space: ....................(2) This procedure splits the problem into two parts: the first one consists of solving the filterer NS equations without the cumbersome task of evaluating the Reynolds stresses, and the second one consists of using relatively simple models to evaluate turbulent viscosity and the subgrid residual stresses (small eddies). The instantaneous velocity is the sum of the filtered velocity and of the residual velocity as given by u\ufffdu¯\ufffdu\u2019....................................(3) 2.2. Governing Equations The application of the filtering operation to the continu- ity and the Navier\u2013Stokes equations gives the resolved Navier\u2013Stokes equations, which, in incompressible flow, are ...........................................(4) where m eff\ufffdm0\ufffdm t .................................(5) The symbols p and ui in Eq. (4) represent the pressure and filtered velocities. The subscripts i and j represent the three Cartesian coordinate directions and repeated subscripts imply summation. We choose the basic Smagorinsky SGS model12) to study the present flow problem. m t\ufffd(CsD)2|S¯| ................................(6) D\ufffd(DxDyDz)1/3 ..............................(7) Cs is Smagorinsky\u2019s constant, Cs\ufffd0.1. Dx, Dy and Dz are the size of each cell in the Cartesian computational grid. |S \u2013 | is the magnitude of the strain-rate tensor, given by ...............................(8) where ........................(9) 2.3. Electromagnetic Force Electromagnetic force can be calculated using the Maxwell\u2019s equation and Ohm\u2019s law. Faraday law: ............................(10) Ampere\u2019s law: \ufffd\ufffdB\ufffdm J ................................(11) \u2207 \u2202\u2202\ufffd \ufffd\ufffdE B t S u x u xij i j j i \ufffd \ufffd 1 2 \u2202 \u2202 \u2202 \u2202 \uf8eb \uf8ed\uf8ec \uf8f6 \uf8f8\uf8f7 | |S S Sij ij\ufffd 2 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \u2202 \uf8eb \uf8ed\uf8ec \uf8f6 \uf8f8\uf8f7 u x u t x u u p x x u x u x i i i j i j i j i j j i \ufffd \ufffd \ufffd\ufffd \ufffd \ufffd 0 1 ( ) \u3c1 µeff G X X X ( ) , | | , \ufffd \ufffd \ufffd 1 2 0 2 \u2206 \u2206 \u2206 if if | | \uf8f1 \uf8f2 \uf8f4\uf8f4 \uf8f3 \uf8f4\uf8f4 f f x G x x dx D \ufffd \ufffd \ufffd \ufffd \ufffd( ) ( )\u222b ISIJ International, Vol. 47 (2007), No. 4 569 © 2007 ISIJ Fig. 1. Schematic of the physical model (all dimensions are in mm). Continuity of the magnetic field is written as: \ufffd ·B\ufffd0...................................(12) Moreover, the induced current is expressed by ohm\u2019s law: J\ufffds(E\ufffdV\ufffdB) ............................(13) Here, E: electric field intensity, B: magnetic flux density, J: current density, m : ermeability, s : conductivity, V: velocity. Lorentz\u2019s law is used to calculate electromagnetic force. f\ufffdJ\ufffdB ..................................(14) where the 500 A electric current, 6 poles and 5 Hz fre- quency with 5 mm inner copper wall of stirrer are used in the present calculation. Figure 2 shows the calculated in- stantaneous magnetic field and averaged electromagnetic force in half cylindrical space using the commercial soft- ware of ANSYS 8.0, these are used to calculate flow field in centrifugal flow tundish. 2.4. Boundary Conditions for Flow Field (1) At the outlet of blending nozzle: the velocity Vin is specified by the casting speed. (2) At the free surface of tundish: If the swirling flow in the rotation chamber is enough strong, the surface defor- mation occurs. In order to simplify