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Modeling Study of Turbulent Flow in a Continuous Casting Slab Mold Comparing Three Ports SEN Designs

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Modeling Study of Turbulent Flow in a Continuous Casting Slab Mold Comparing Three Ports SEN Designs ISIJ International, Vol. 59 (2019),ISIJ No. International, 1 Vol. 59 (2019), No. 1, pp. 76–85 Modeling Study of Turbulent Flow in a Continuous Casting Slab Mold Comparing Three Ports SEN Designs Ismael CALDERÓN-RAMOS,1)* R. D. MORALES,2,3) Rumualdo SERVÍN-CASTAÑEDA,1) Alejandro PÉREZ-ALVARADO,1) Saúl GARCÍA-HERNÁNDEZ,4) José de Jesús BARRETO4) and Sixtos Antonio ARREOLA-VILLA1) 1) Mechanical Engineering Department, UAdeC/F.I.M.E.-U.N., Barranquilla S/N, Monclova, Coahuila, C.P. 25280 México. 2) Department of Materials Engineering and Metallurgy, I.P.N.-E.S.I.Q.I.E., Ed. 7 UPALM, Col. Zacatenco, Cd.México, C.P. 07738 México. 3) K&E Technologies, Manizales 88, Residencial Zacatenco, Del. Gustavo A. Madero, CDMX, C.P. 07369 México. 4) Metallurgy Graduate Center, Instituto Tecnológico de Morelia, Av. Tecnológico No. 1500, Morelia Michoacán, C.P. 58120 México. (Received on July 20, 2018; accepted on September 10, 2018) Fluid flow of liquid steel in a slab mold influenced by three different submerged entry nozzles with the same bore sizes but different ports including rectangular, square, and round shape at immersion depth of 185 mm was studied. The analysis includes numerical simulations and physical modeling. The results show that the port shape has great effects over the fluid dynamics of the liquid steel inside the slab mold. The comparison among the three nozzle port designs indicates that the nozzle with square ports, (SEN-S), decrease the jets velocity, promote a symmetrical path inside the mold and decrease the bath level oscil- lations; representing the best choice to control the turbulence and decrease the quality problems. KEY WORDS: slab mold; continuous casting; fluid flow; SEN port design. standing the phenomena that dominate the behavior of the 1. Introduction process, which provides the basis for achieving optimization Fluid flow in continuous casting molds is important that has an impact on minimization of production costs. The because it governs production rate of the caster and qual- aim of this work was to assess the unsteady flow structures ity of the product. However, both aspects have opposite into the slab mold developed by the SEN-R (current noz- consequences. On one side, to get high production rate it is zle), SEN-S and SEN-C for the same operating conditions necessary, to increase the casting speed; on the other side, (see Table 1). The comparison among the three SEN port an increase of casting speed leads to flux entrapment to designs was carried out through physical experiments and form inclusions impairing the product quality. This oppo- numerical simulations. site relationship between production and quality has been the driving force of many research reports related to fluid 2. Presentation of the Case flow of liquid steel, particularly in slab molds.1–4) The fluid flow inside the casting mold is characterized by having a Recently, a company that produces peritectic steels, turbulent behavior, which is associated with risky possi- acquired a new SEN design with rectangular ports, with10- bilities such as shell-thinning breakout, formation of slivers degree upward ports angle, which is presented in Fig. 1(a). and inclusion entrapment.5–8) The turbulence intensity in This nozzle was designed to create greater stirring into the the mold depends on the submerged entry nozzle, (SEN), mold and send fresh steel toward the upper mold corners port design, the casting speed and the SEN immersion to cast crack sensitive steels. Nevertheless, recent reports depth. To reach both productivity and quality, it is neces- indicate that the current nozzle (SEN-R) promotes excessive sary to understand the effect that the SEN ports have on the turbulence at bath level when its maximum immersion depth unsteady flow structures in this process. Unfortunately, due is 185 mm (to use the complete zirconia band), entraining to the high temperature of steel, it is difficult to perform particles from mold flux, slivers, and even breakout prob- velocity measurements directly in molten steel.9) Physical lems.10) To solve these quality and operational problems, models and mathematical simulations are alternative ways the plant is considering two alternative SEN port designs to to study the behavior of liquid steel inside the mold. Previ- replace the actual nozzle; the first has square ports (SEN- ous approaches are of great help in diagnosing and under- S), and the second has round ports (SEN-C). The shape and dimensions of these nozzles are shown in Figs. 1(b) * Corresponding author: E-mail: [email protected] and 1(c), respectively. Both proposed designs have a port DOI: https://doi.org/10.2355/isijinternational.ISIJINT-2018-504 size greater than the actual nozzle (SEN-R), looking for jet © 2019 ISIJ 76 ISIJ International, Vol. 59 (2019), No. 1 Table 1. Operating conditions used in the Physical experiments and computational simulations. Parameter Value PHYSICAL MODEL Casting Speed, (m/min), (m/s) 0.9, 0.015 Equivalent flow rate, (m3/s) × 103 6.4 Slab mold size, (m3) 1.88 × 0.23 × 0.7 Air zone, (m) 0.1 Nozzle immersion*, (m) 0.185 NUMERICAL MODEL Casting Speed, (m/s) 0.015 Pressure inlet, (Pa) 101 325 Nozzle immersion*, (m) 0.185 Viscosity of the liquid steel, (Pa s) 0.0064 Kinematic viscosity of the steel, (m2/s) 1×10 −6 Density of the liquid steel, (kg/m3) 7 100 Viscosity of the air, (Pa s) 1.7894 ×10 −5 Density of the air, (kg/m3) 1.225 Interfacial tension between air and steel, (N/m) 1.6 Turbulence Model LES Interfacial model VOF Pressure-velocity couple SIMPLEC Convergence criterion Less than 10 −4 *Distance from the free surface to the upper exit port position Fig. 1. Geometric characteristics (in mm) of the three submerged velocity reduction and consequently to decrease the turbu- entry nozzles (SEN’s) tested, a) SEN-R, port area: 2 2 lence inside the mold without compromising productivity 0.002795 mm , b) SEN-S, port area: 0.0044225 mm , and c) SEN-C, port area: 0.003848 mm2. and quality. In other words, looking for a good symmetric flows and suitable stirring conditions to melt the mold flux and maintaining small meniscus disturbances by oscillation 3.1. Particle Image Velocimetry (PIV), Dye Injection waves, and vortex flows.11) and Ultrasonic Sensors To measure flow velocities in the water model, the PIV technique was employed. The principle of PIV is to deter- 3. Water Model minate the flow velocities by measuring the displacement A full-scale water model of the slab mold was built with vector of illuminated particle images during a known time transparent plastic plates with total height of 1 700 mm. interval. In this work, particles with diameters of approxi- To recreate the fluid flow of liquid steel into the mold, mately 20 μm and density of 1 020 kg/m3 were seeded into the model was partially inserted (250 mm) into a pit full the fluid prior to the measurements.12) A 1 mm thick laser of water to represent the continuity of the strand. The pit sheet was displayed in the central plane, at half the mold contains a submergible water pump to transport water thickness. The displacements of particles were recorded through a vertical pipe, which has a flow meter and a pre- with a CCD camera (DANTEC-Double Image 700) and the cision valve embedded along the line to control the flow signals were converted to velocity magnitudes and validated rate of water fed into the model. This pipe line feeds the through a Fast Fourier Transforms algorithm. The studied tundish fixing the bath height at the same level (1 m) as in area involves the upper-half side of the mold with a size of the actual tundish in the plant. This configuration permits 880 × 660 mm2 as shown in Fig. 2(a). To reveal the flow the water to be recycled. The flow rate from the tundish to pattern as a function of the nozzle port shape, a red dye the mold was controlled through a stopper rod as the actual tracer was injected as a pulse through an orifice located in system at the plant. The physical model includes the three the upper side of the SEN, Fig. 2(b). The mixing kinematics full scale plastic prototypes of the real SEN designs used of the dye was recorded using a conventional video-camera, at the plant. A detail explanation of the experimental setup which was fixed in front of the water model. Finally, the can be found in reference 10. The experimental techniques bath level in the mold was monitored in real time using six employed to study the fluid flow in the mold are described ultrasonic sensors. Three were placed at each side of the in the next lines. SEN; one close to the narrow mold wall (1 and 6), another at the midpoint between the SEN and narrow mold wall (sen- 77 © 2019 ISIJ ISIJ International, Vol. 59 (2019), No. 1 vi 0 .................................... (1) xi Dvi 1 p C vi v j S veff D T ............. (2) Dt xxij E x j xi U vvefft0 v ................................ (3) Where, the subscripts i and j represent the three Cartesian directions and repeated subscripts imply summation. The symbols p and vi in Eqs. (1) and (2) represent the pressure and filtered velocities. The residual stresses, which arise from the unresolved small eddies, are modeled using an Fig. 2. Experimental techniques employed to study the fluid flow eddy viscosity (vt). The SGS kinetic energy (SGS k) model in the slab mold, a) Schematics showing the PIV measure- employed here requires solving the following additional ments region, b) Dye tracer injection and c) Ultrasonic transport equation, which includes advection, production, sensors.
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