Large Eddy Simulations of the Effects of EMBr and SEN Submergence Depth on Turbulent Flow in the Mold Region of a Steel Caster

Transient turbulent flow in the mold region during continuous casting of steel is related to many quality problems, such as surface defects and slag entrainment. This work applies an efficient multi-GPU based code, CUFlow, to perform large eddy simulations (LES) of the turbulent flow in a domain that includes the slide gate, SEN, and mold region. The computations were first validated by comparing the predicted surface velocity with plant measurements. Then, seven LES simulations were conducted to study the effects of casting speed, electromagnetic braking (EMBr) field strength, and submerged entry nozzle (SEN) depth on the transient flow. The results show that EMBr has an important influence on flow inside the SEN, in addition to flow in the mold. With EMBr, an “M-shaped” flow profile is seen inside the SEN. The swirling flow behavior in the SEN and ports is more symmetrical at high casting speed and with higher EMBr strength. The position of the SEN ports relative to the peak magnetic field affects the EMBr performance. The results confirm and quantify how applying EMBr greatly lowers both the magnitude and turbulent variations of the surface velocity and level profile.     

Effects of Electromagnetic Brake,Mold Curvature and Slide Gate on Fluid Flow of Steel in a Slab Mold

The influence of mold curvature, slide gate and magnetic forces on the steel flow in a slab mold was studied with a 3‐D mathematical model. The slide gate application induces a biased flow toward the mold side where its opening is located in the submerged entry nozzle (SEN). Turbulence and asymmetry of flows are more intense in a curved mold than in a straight mold. The effect of an electromagnetic brake (EMBr), located in the discharging ports to control flow turbulence, is only appreciable when magnetic flux density is higher than 0.1 T. The magnetic flux density does not affect the velocity profile in the discharging ports in the SEN because its construction material is insulated. Increasing the magnetic flux density leads to a decrease of the discharging jets angle and to the elimination of the two upper roll flows. The use of the EMBr in a curved mold equipped with a slide gate eliminates the meniscus velocity spikes observed in the mold corners. These results help to demonstrate that EMBr eliminates the asymmetry in a curved mold even under the excessive turbulent conditions existing in curved continuous casting molds.     

Large Eddy Simulation of Turbulent Fluid Flow in Liquid Metal of Continuous Casting

The transient turbulent flow in continuous casting steel plays a key role in minimizing defects. Compared with the k-ε model, the large eddy simulation (LES) of turbulence provides much more accurate representation of turbulent flow by resolving large-scale dynamics. The turbulent flow in a liquid metal model of continuous casting has been simulated by LES and measured using ultrasonic Doppler velocimetry (UDV). The result of measurement and LES has been compared to validate the LES model and furthermore enhance the understanding of the transient turbulent feature in the flow field. The results show that the jet exiting from the nozzle port swings, which is not steady, and turbulent velocity variation frequencies decreased with distance from the nozzle port region and also the LES mode can capture the high frequency fluctuation, which the measurement cannot detect.     

Study on fluid flow in the mold with electromagnetic braking

Multiphase flow control w ith electromagnetic braking( EMBr) is w idely used in the continuous casting of steel slabs. The basic aim of the flow control system of the process is to deliver molten steel from the ladle through the tundish,upper tundish nozzle,slide gate,and submerged entry nozzle into the mold region w ith minimal defects. This requires the optimization of turbulence levels at a meniscus to avoid both an excessively fast flow( which creates high fluctuations of the meniscus level in addition to slag entrapment,surface nonuniformities,and surface defects) and insufficient slow flow( w hich leads to meniscus solidification,inadequate flux infiltration,and surface defects). In this study,a Eulerian-Lagrangian approach is used to investigate the effects of EM Br and Ar bubble injection on the surface flow velocity. The results show that high Ar injection rates can lead to an increase in surface velocity.     

电磁制动对CSP漏斗形结晶器开浇过程影响的数值模拟

 建立了描述电磁制动(EMBr)下CSP漏斗形结晶器开浇过程暂态流动现象的三维数学模型,研究不同磁场强度对开浇过程中结晶器内流场影响。利用流体体积方法(VOF)捕捉钢液-空气界面。采用动网格模型实现引锭杆的移动。研究结果表明：电磁场可显著抑制钢液湍流流动,降低引锭杆启动瞬间的弯月面波动;开浇过程电磁制动的使用可改善拉坯过程中结晶器内钢液流态;电磁制动的使用可将自由稳定时间提前约10s;使用电磁制动可明显减轻漏斗形结晶器内钢液偏流现象。     

Numerical Analysis of the Influences of Operational Parameters on the Braking Effect of EMBr in a CSP Funnel-Type Mold

A three-dimensional mathematical model of the magnetic field, flow field, and temperature field in a 1500 mm × 90 mm CSP funnel-type mold is used to numerically study the effect of an electromagnetic brake (EMBr) on flow and heat transfer behavior of molten steel. A number of effects of EMBr on the flow pattern and temperature distribution of molten steel are simulated. The jet flow discharge from the submerged entry nozzle (SEN) is significantly suppressed. In addition, heat transfer in the upper part of the mold increases under the influence of EMBr, which can improve the mobility of liquid steel at the meniscus and achieve low superheat casting. The relations between casting speed and magnetic flux density, and between SEN submergence depth and the installation position of the EMBr device, are taken into account to study the effects of braking on molten steel. The results show that the braking effect is weakened with an increase in either the casting speed or the SEN submergence depth. In order to insure the efficient and stable operation of a continuous casting production, the magnetic flux density should be increased by approximately 0.1 T when the casting speed increases by 1 m/min. In addition, an optimal braking effect for molten steel can be obtained when the distance between the bottom of the nozzle and the upper surface of the EMBr device is 100 mm.     

Fluid Flow-Related Transport Phenomena in Steel Slab Continuous Casting Strands under Electromagnetic Brake

In the current study, a three-dimensinal (3D) numerical model is built to investigate the effect of a local-type electromagnetic brake (EMBr) on the fluid flow, heat transfer, and inclusion motion in slab continuous casting strands. The results indicate that the magnetic force affects the jet characteristics, including jet angle, turbulent kinetic energy, and its dissipation rate. To reduce the top surface velocity and stabilize the top surface, the magnetic flux intensity should be larger than a critical value. With a 0.39 T magnetic flux intensity, the top surface velocity and its fluctuation can be well controlled, and less slag is entrained. The motion of argon bubbles is also studied. More bubbles, especially >2.0-mm bubbles, escape from the top surface between the mold submerged entry nozzle (SEN) and \frac14 \frac{1}{4} width for the case with a 0.39 T EMBr. This may push the top slag away and create an open “eye” on the top slag. Small bubbles (≤1 mm) tend to escape from one side of wide face no matter with or without EMBr, which is induced by the swirl flow from the SEN outport. EMBr has a little effect on the overall removal fraction of inclusions; however, it affects the local distribution of inclusion in the slab. With EMBr, more inclusions accumulate the region just below the surface, thus a worse subsurface quality, whereas the inner quality of the slab is better than that without EMBr. For heat transfer in the mold, the heat flux on the narrow face and the area of possible break-out zones can be reduced by using EMBr. Prevention of bias flow and/or asymmetrical flow in mold by EMBr is also concluded.     

The Effects of Electromagnetic Brake on Liquid Steel Flow in Thin Slab Caster

Computational fluid dynamics (CFD) model with magneto hydro dynamics (MHD) is developed for a thin slab caster to investigate the effects of electromagnetic brake (EMBr) on liquid steel flow in continuous casting mold and to determine the EMBr practices which lead to optimal flow structure. Particle Image Velocimetry (PIV) tests in water model and meniscus flow measurements in real caster are performed to validate the predictions obtained with CFD models. The performance of different submerged entry nozzle designs, SEN 1 and SEN 2, are evaluated. The effects of nozzle submergence in relation to the applied magnetic field on mold flow structure are quantified. There are significant differences between flow structures obtained with SEN 1 and SEN 2, even though both designs have fundamental similarities and contain four ports. EMBr mainly reduces the meniscus velocities for SEN 2 as opposed to the foremost influence of EMBr for SEN 1 that is to significantly slow down the downward jet coming from the bottom ports. In addition, reducing the EMBr strength for shallow nozzle submergence and increasing the EMBr strength for deep nozzle submergence help to maintain similar meniscus activity for all conditions.     

Transient Turbulent Flow in a Liquid-Metal Model of Continuous Casting,Including Comparison of Six Different Methods

Computational modeling is an important tool to understand and stabilize transient turbulent fluid flow in the continuous casting of steel to minimize defects. The current work combines the predictions of two steady Reynolds-averaged Navier–Stokes (RANS) models, a “filtered” unsteady RANS model, and two large eddy simulation (LES) models with ultrasonic Doppler velocimetry (UDV) measurements in a small-scale liquid GaInSn model of the continuous casting mold region fed by a bifurcated well-bottom nozzle with horizontal ports. Both mean and transient features of the turbulent flow are investigated. LES outperformed all models while matching the measurements, except in locations where measurement problems are suspected. The LES model also captured high-frequency fluctuations, which the measurements could not detect. Steady RANS models were the least accurate methods. Turbulent velocity variation frequencies and energies decreased with distance from the nozzle port regions. Proper orthogonal decomposition analysis, instantaneous velocity patterns, and Reynolds stresses reveal that velocity fluctuations and flow structures associated with the alternating-direction swirl in the nozzle bottom lead to a wobbling jet exiting the ports into the mold. These turbulent flow structures are responsible for patterns observed in both the time average flow and the statistics of their fluctuations.     

Effects of a Magnetic Field on Turbulent Flow in the Mold Region of a Steel Caster

Electromagnetic braking (EMBr) greatly influences turbulent flow in the continuous casting mold and its transient stability, which affects level fluctuations and inclusion entrainment. Large eddy simulations are performed to investigate these transient flow phenomena using an accurate numerical scheme implemented on a graphics processing unit. The important effect of the current flow through the conducting solid steel shell on stabilizing the fluid flow pattern is investigated. The computational model is first validated with measurements made in a scaled physical model with a low melting point liquid metal and is then applied to a full-scale industrial caster. The overall flow field in the scale model was matched in the real caster by keeping only the Stuart number constant. The free surface-level behaviors can be matched by scaling the results using a similarity criterion based on the ratio of the Froude numbers. The transient behavior of the mold flow reveals the effects of EMBr on stability of the jet, top surface velocities, surface-level profiles, and surface-level fluctuations.     

混合LES-RANS模型在结晶器钢液流场模拟中的应用

采用了一种新的混合LES-RANS（大涡模拟-雷诺平均模型）湍流模型模拟结晶器中钢液的流场.模型通过修正湍流黏度系数对水口和结晶器内湍流进行过滤,对大尺度的湍流直接采用Navier-Stokes方程求解计算,对小尺度的脉动采用标准k-ε模型进行计算.该模型能避免RANS的过分耗散并且能捕捉到更多的瞬态湍流信息.模型通过对连铸结晶器内液态金属GaInSn模型速度进行测量验证,速度测量方法为超声波多普勒测速仪（UDV）测速法.新模型与实验测量值吻合程度明显好于RANS模拟的结果,能更准确地预测结晶器和水口内的湍流行为.结晶器内瞬态流动特征表明,水口两侧流体呈周期性的偏流,周期约为5s.      

Transient fluid flow and superheat transport in continuous casting of steel slabs

The turbulent flow of molten steel and the superheat transport in the mold region of a continuous caster of thin steel slabs are investigated with transient large-eddy simulations and plant experiments. The predicted fluid velocities matched measurements taken from dye-injection experiments on full-scale water models of the process. The corresponding predicted temperatures matched measurements by thermocouples lowered into the molten steel during continuous casting. The classic double-roll flow pattern is confirmed for this 132×984 mm slab caster at a 1.52 m/min casting speed, with about 85 pct of the single-phase flow leaving the two side ports of the three-port nozzle. The temperature in the top portion of the molten pool dropped to about 30 pct of the superheat-temperature difference entering the mold of 58 °C. About 12 pct of the superheat is extracted at the narrow face, where the peak heat flux averages almost 750 kW/m² and the instantaneous peaks exceed 1500 kW/m². Two-thirds of the superheat is removed in the mold. The jets exiting the nozzle ports exhibit chaotic variations, producing temperature fluctuations in the upper liquid pool of ±4 °C and peak heat-flux variations of±350 kW/m². Employing a static-k subgrid-scale (SGS) model into the three-dimensional (3-D) finite-volume code had little effect on the solution.     

Modeling of transient flow phenomena in continuous casting of steel

This paper describes initial efforts to develop and apply 3D finite-difference models to simulate transient flow in the mold. These transient flow phenomena include flow pattern oscillations caused by sudden changes in nozzle inlet conditions and rapid fluctuations in the molten steel⧹flux interface level at the top surface of the mold. The flow model incorporates interactions with other transport phenomena, including turbulence, superheat removal and argon gas bubble injection. Predictions are shown for the oscillatory evolution of the flow pattern from biased steady flow to symmetrical steady flow after a sudden change in inlet conditions. In addition, the predicted turbulent kinetic energy levels at steady state are shown to correlate with measured surface level fluctuations. The effect of processing conditions are consistent with experimental findings. Without argon, the greatest level fluctuations are found near the narrow face, while increased argon moves the maximum towards the center. Fluctuations decrease with deeper submergence and lower casting speed. These transient phenomena are important because they may lead to defects in the final steel product from entrainment of slag, disruption of solidification at the meniscus and non-uniform heat transfer.     

Simulation of the submerged energy nozzle-mold water model system using laser-optical and computational fluid dynamics methods

The present work describes quantitative digital particle image velocimetry measurements of a full-scale water model of a thin slab mold. Different casting speeds and two submerged entry nozzles with one and two outlet ports have been investigated. The flow pattern of the single-port nozzle shows a counterclockwise-rotating double vortex that is nearly steady-state but leads to high stationary surface waves. The flow jets out of the two-port nozzle oscillate and produce a transient flow pattern with low wave amplitudes. The amplitudes for the one-port nozzle show a linear variation with the volumetric flow rate. The experimental results lead to a good interpretation of the flow phenomena and are used to validate steady-state numerical simulations with the commercial program, CFX, on the basis of the Reynolds equations. To describe anisotropic turbulence effects, the Reynolds stress model (RSM) is used for the flat single-port nozzle and the standard k-ɛ model for the mold flow. The calculated mean velocities and wave amplitudes, predicted from pressure distribution at the water surface, are generally in the consensus of the experimental data. An erratum to this article is available at .     

Cyclic Turbulent Instabilities in a Thin Slab Mold. Part I: Physical Model

A water-physical model of a funnel-type thin slab mold fed by a two-port submerged entry nozzle (SEN) was employed to characterize the flow of liquid steel using dye tracer, particle image velocimetry, and video recording experiments. The overflow fluid flow pattern was the typical double-roll flow. A cyclic, low-frequency (≈0.01 s⁻¹), and energetic flow distortion of a short-lived (8 to 12 seconds) inducing high meniscus oscillation was identified. Its intensity grew with high casting speeds (5 and 7 m/min) and with a shallow SEN immersion position (200 mm from the meniscus level to the SEN tip). This distortion originated from the apparent existence of vortex flows located below the two discharging jets, which are formed by shear stresses in their ends that act on the surrounding fluid. These vortexes exert momentum transfer upward through a cascade mechanism from the lowest part of the discharging jets until reaching the region close to the SEN tip. This cascade momentum transfer widens the separation of both jets, enhancing the fluid velocities of the upper rolls, which promotes a high-amplitude standing wave. It is inferred, based on the experimental results, that this flow distortion originated from an instantaneous unbalance of the turbulent kinetic energy in the discharging jets. The negative production of kinetic energy is ascribed as the source of this unbalance, which is compensated by a higher contribution of the turbulent kinetic energy through mean convection and turbulent transport mechanisms manifested through higher velocities. After the restoration of the energy balance, the system yields a stable meniscus to repeat the low-frequency cycle.     

Simulation of fluid flow inside a continuous slab-casting machine

A finite element model has been developed and applied to compute the fluid flow distribution inside the shell in the mold region of a continuous, steel slab-casting machine. The model was produced with the commercial program FIDAP, which allows this nonlinear, highly turbulent problem to be simulated using the K- ε turbulence model. It consists of separate two-dimensional (2-D) models of the nozzle and a section through the mold, facing the broad face. The predicted flow patterns and velocity fields show reasonable agreement with experimental observations and measurements conducted using a transparent plastic water model. The effects of nozzle angle, casting speed, mold width, and turbulence simulation parameters on the flow pattern have been investigated. The overall flow field is relatively insensitive to process parameters. Formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Illinois-Urbana     

Study of transient flow and particle transport in continuous steel caster molds: Part II. Particle transport

Particle motion and capture in continuous steel casters were simulated using a Lagrangian trajectory-tracking approach, based on time-dependent flow fields obtained from large-eddy simulations (Part I of this article). A computation was first conducted on a water model of a full-scale standard slab caster, where measurements were available. It simulated the transport of 15,000 plastic particles and their removal by a screen positioned near the mold top surface. The computation shows the screenremoval fractions to be 27±5 pct for 0 to 10 seconds and 26±2 pct for 10 to 100 seconds, which agrees with previous measurements. The flow exiting the nozzle was relatively uniform, and turbulent motion in the domain was very chaotic, so particle removal did not depend on the initial location of particles introduced in the nozzle port. A computation of motion and capture of 40,000 small inclusions (10 and 40 μm) was then performed in an actual thin-slab steel caster. The particles moved through the mold region with an asymmetrical distribution, which was caused by transients in fluid turbulence in the lower recirculation region, rather than by inlet variations at the nozzle port. Only about 8 pct of these small particles were removed to the top surface. This removal fraction was independent of both particle size and density, likely because all the simulated particles were too small to deviate significantly from the surrounding fluid flow. Finally, the computational results were further processed to predict the ultimate distribution of impurity particles in the solid thin slab after a short burst of inclusions entered the mold. They were reprocessed to reveal the distribution of total oxygen content for a steady inclusion supply from the nozzle. The results of this work confirm the important role of flow transients in the transport and capture of particles during continuous casting and can serve as a benchmark for future simplified models.     

Large Eddy Simulations of Double-Ruler Electromagnetic Field Effect on Transient Flow During Continuous Casting

Transient flow during nominally steady conditions is responsible for many intermittent defects during the continuous casting of steel. The double-ruler electromagnetic field configuration, or “FC-Mold EMBr,” is popular in commercial slab casting as it provides independent control of the applied static field near the jet and free surface regions of the mold. In the current study, transient flow in a typical commercial caster is simulated in the absence and in the presence of a double-ruler magnetic field, with rulers of equal strengths. Large eddy simulations with the in-house code CU-FLOW resolve the important transient behavior, using grids of over five million cells with a fast parallel solver. In the absence of a magnetic field, a double-roll pattern is observed, with transient unbalanced behavior, high surface velocities (~0.5 m/s), surface vortex formation, and very large surface-level fluctuations (~±12 mm). Applying the magnetic field suppresses the unbalanced behavior, producing a more complex mold flow pattern, but with much lower surface velocities (~0.1 m/s), and a flat surface level with small level fluctuations (<±1 mm). Nail board measurements taken at this commercial caster, in the absence of the field, matched reasonably well with the calculated results, both quantitatively and qualitatively.