Multi-strand Tundish Research Articles

New Insight into Combined Model and Revised Model for RTD Curves in a Multi-strand Tundish

The analysis for the residence time distribution (RTD) curve is one of the important experimental technologies to optimize the tundish design. But there are some issues about RTD analysis model. Firstly, the combined (or mixed) model and the revised model give different analysis results for the same RTD curve. Secondly, different upper limits of integral in the numerator for the mean residence time give different results for the same RTD curve. Thirdly, the negative dead volume fraction sometimes appears at the outer strand of the multi-strand tundish. In order to solve the above problems, it is necessary to have a deep insight into the RTD curve and to propose a reasonable method to analyze the RTD curve. The results show that (1) the revised model is not appropriate to treat with the RTD curve; (2) the conception of the visual single-strand tundish and the combined model with the dimensionless time at the cut-off point are applied to estimate the flow characteristics in the multi-strand tundish; and that (3) the mean residence time at each exit is the key parameter to estimate the similarity of fluid flow among strands.

Numerical Investigation of Inclusion Behaviour in a Multi-Strand Tundish During Strand Blockages

Inclusion removal in the continuous casting tundish is an essential technology for production of high-grade steels. In the present study, a three dimensional numerical investigation has been carried out on six strand delta shape tundish. The aim of the work is to investigate the inclusion entrapment and removal phenomenon in a tundish. A parametric study has been carried out to study the behavior of the inclusions in a steelmaking tundish and the results have been compared on the basis of densities and diameters of inclusions. Bare tundish as well as a tundish occupied with Advance Pouring Box (APB) have been studied to predict the transport behavior of inclusion particles. Further, effect of strand blockages has been considered. The turbulent Navier–Stokes equations have been solved to predict the flow velocity distributions and inclusion trajectories were calculated using a Lagrangian particle tracking method. The numerical results were validated using available experimental results and found to be in good agreement. The results indicate that the use of APB has changed the fluid flow pattern and consequently the inclusion particles movement. The density of inclusion particles plays an important role. It has been seen that low dense particles have higher chances to get trapped or removed from tundish. It was also found that larger diameter of inclusion particles have more chances of floatation.

Simulation of Flow and Heat Fields in a Seven-strand Tundish with Gas Curtain for Molten Steel Continuous-Casting

The present study involves physical and mathematical simulations to study the effect of gas bubbling curtains on the flow and temperature fields in a seven-strand tundish. In 1/3 scale tundish model experiments, RTD (Residence Time Distribution) curves were used to study the flow characteristics in the tundish with different configurations. The flow uniformity for all strands is evaluated through variance analysis of the RTD curves in different tundish cases. Besides, the tracer dispersion experiment was performed to visualize the fluid flow under the effect of the gas curtain. In order to investigate the effect of a gas curtain on the flow in the tundish, mathematical simulations were carried out mainly for two cases with or without gas bubbling curtains. The RTD curves were obtained from the species transport model. The results obtained show that gas bubbling curtains can help improve fluid flow characteristics in multistrand tundish, and that the location of the curtains plays a key role in optimizing multi-strand tundishes configuration. Gas bubbling curtains can decrease the differences in residence times and molten steel temperatures at different outlets in multi-strand tundishes effectively. Heat transfer between different areas is promoted and the temperature field is uniformed significantly in such tundishes.

Numerical Investigation of the Hydrodynamic Conditions in A Multi-Strand Cc Tundish with Closed Outlets

Abstract In industrial conditions there are situations when the CC machine works under emergency. It can be result of mechanical or electrical causes, breakout of billet or problem with supplying new parts of liquid steel to the CC machine. As a consequence one or two outlets of the tundish should be closed. However, closing one of the outlets influences the hydrodynamic and thermal conditions occurring in the tundish. Thus, the important information is which of the outlets should be closed to conduct further continuous casting process correctly.The following research was conducted to analyze the influence of liquid steel flow behaviour in the multi-strand tundish when all outlets do not work. Such problem was solved by means of numerical methods based on Navier-Stokes equations (k–ɛ standard turbulence model). Numerical simulations were done using the educational version of CFD program (Computational Fluid Dynamics) – ANSYSFluent. As a result forecasted velocity fields and RTD curves (Residence Time Distribution) were obtained. RTD characteristics were used to determine kinetics of liquid steel mixing and also to calculate parts of particular flow areas for studied cases.

Physical modelling investigation of influence of strand blockage on RTD characteristics in a multistrand tundish

In the continuous casting process, the tundish acts as a continuous molten steel distribution vessel. The importance of the tundish during the molten steel delivery becomes more significant when it supplies liquid steel to more than one mould. In the present work, a water model of a six strand billet caster tundish has been used to study the effect of strand blockage on the residence time distribution (RTD) characteristics at the strands. All the experiments were performed under steady state conditions. Potassium chloride was used as a tracer for the study. The effects of blockage on the strand dissimilarity among the open strands were also studied. Both single and dual strand blockage experiments were performed. Blockage of strands deteriorated the RTD characteristics at the open strands. The results reveal the most preferred options for strand blockage when the plant operation needs to do so.

Mathematical modelling of single and multi-strand tundish for inclusion analysis

In the present work a numerical study was carried out to investigate the inclusion behaviour in a four strand asymmetric billet caster tundish. A parameter called separation efficiency was used to compare inclusion behaviour quantitatively. An open source CFD code called OpenFOAM was used to model the inclusions through Lagrangian Particle Tracking approach. First, the Lagrangian particle class was defined in the existing simpleFoam solver of the OpenFOAM and the same customized solver was used to track particles in the already obtained velocity field. Inclusions were modelled as spheres and various forces acting were also considered. Present numerical results were validated using available experimental results and found to be in good agreement. Further, such inclusion behaviour was also extended for a real industrial case tundish. Inclusion flotation characteristics in thermally induced flow was analyzed in detail. This is, perhaps, for the first time such an exhaustive study on inclusion analysis being reported considering the forces acting on the inclusions particles and with OpenFOAM.

Effect of blockage of outlet nozzle on fluid flow and heat transfer in continuously cast multistrand billet caster tundish

Shop floor problems like strand breakout, mechanical or electrical failure in the strand or inadequate availability of molten steel from coming ladle sometimes lead to closing one/two strand in a continuous casting unit. Closing any of the outlets affects the flow distribution inside the tundish and so it is important to study which of the outlets should be closed to have proper results in terms of residence time distribution (RTD) and thermal behaviour of the tundish. The present study aims at finding the effect of closing one or two outlets on flow behaviour and heat transfer in a multistrand tundish. Navier–Stokes equation has been solved using standard K–ϵ turbulence model with the help of educational version of computational fluid dynamics software PHOENICS to find the steady state velocity field inside the tundish. Furthermore, tracer dispersion study was carried out using pulse input technique of the tracer to find the RTD curve of the tundish. Mixing parameters, namely ratio of actual to theoretical mean residence time and ratio of mixed volume to dead volume have been found out for different cases when either all the outlets are open or one/two outlets are closed. It was found that closing the near outlet increases the mixing capability of the tundish the most as compared with far or middle outlet. A larger recirculation zone created in the half of the tundish results in better mixing when near outlet is closed. Numerical code was validated against the experimental observation by performing the tracer dispersion study inside a multistrand tundish and reasonably good match between the experimental and numerical results in terms of RTD curves is observed.Des problèmes d’atelier comme la percée du cordon, la défaillance mécanique ou électrique du cordon ou la disponibilité inadéquate de l’acier fondu de la poche d’arrivée mènent quelquefois à la fermeture d’un ou deux cordons d’une unité de coulée en continu. La fermeture de n’importe laquelle des orifices de sortie affecte la distribution de l’écoulement dans le panier de coulée. Il est donc important de déterminer lequel des orifices de sortie devrait être fermé pour avoir des résultats appropriés par rapport au RTD et au comportement thermique du panier de coulée. La présente étude vise à trouver l’effet de la fermeture d‘un ou deux orifices de sortie sur le comportement de l’écoulement et sur le transfert de chaleur dans un panier de coulée à plusieurs lignes. On a résolu l’équation de Navier-Stokes en utilisant le modèle normal de turbulence K-ϵ, avec l’aide de la version éducationnelle du logiciel de CFD PHOENICS, afin de trouver le champ de vitesse à régime constant à l’intérieur du panier de coulée. En plus, on a effectué une étude de dispersion de traceur en utilisant une technique d’entrée de l’impulsion du traceur afin de trouver la courbe de Distribution du Temps de Séjour (RTD) du panier de coulée. On a trouvé le paramètre de mélange, soit le rapport du temps de séjour moyen actuel à théorique et le rapport de volume mélangé à volume mort, pour différents cas où soit que toutes les sorties étaient ouvertes, soit qu’une ou deux sorties étaient fermées. On a trouvé que la fermeture de la sortie la plus proche augmentait davantage la capacité de mélange du panier de coulée par rapport à la sortie la plus éloignée ou à celle du milieu. Une plus grande zone de re-circulation créée dans la moitié du panier de coulée avait pour résultat un meilleur mélange lorsque la sortie la plus proche était fermée. On a validé le code numérique contre l’observation expérimentale en effectuant l’étude de dispersion du traceur à l’intérieur d’un panier de coulée à plusieurs lignes. On a observé une correspondance raisonnable entre les résultats expérimentaux et numériques par rapport aux courbes de la RTD.

Modeling of the Steel Flow and Non-Metallic Inclusion Separation due to Flotation in a Six-Strand cc Tundish

Constantly increasing customers’ demands for the production of high and very high-quality steels, promote the intensive technological development of their production. Today, the dominating method of global steel production is continuous casting. A continuous casting plant includes the ladle turret, several steel ladles, a tundish, and a mold followed by a framework of rolls to support the strip. The tundish has become a very important metallurgical unit in the continuous casting process. Nowadays its role is not only to guaranty the link between the process of secondary metallurgy and the continuous casting process in the mould but it becomes an active metallurgical reactor. Therefore in recent years a lot of researches were done to establish a better understanding of the physical phenomena accompanying the steel flow through the tundish and non-metallic inclusion separation into the top slag cover. The article presents computational studies of the three-dimensional turbulent steel flow and non-metallic inclusions separation in a multi-strand tundish for steady-state casting conditions. Simulations of the steel flow in the tundish are performed with boundary conditions that are derived from the real casting process. The mathematical model used for simulations, was partly validated with experimental measurements. Numerical calculations are carried out by the finite-volume code Fluent using k-ε standard turbulence model. With the calculated flow field, micro-inclusions removal due to flotation to the coving slag is investigated numerically. For the particle separation at the interface a modified boundary condition is implemented.

Numerical investigation of the effect of transitory strand opening on mixing in a multistrand tundish

In a multistrand, the outlet near the inlet produces short circuiting flow. This leads to the formation of dead zones inside the tundish, and consequently, the mean residence time decreases. In the present study, numerical investigation of mixing inside a delta shaped tundish with sloping boundaries was carried out by solving the Navier-Stokes equation and employing the standard turbulence model. To decrease the dead zone volume inside the tundish, the effect of closing the outlet near the inlet for a small amount of time and further opening it on the mixing behavior of the tundish was studied. The outlets near the inlet were closed for varying amount of time, and the transient analysis of fluid flow and the tracer dispersion study were carried out to find the mixing parameters of the tundish, namely, mean residence time and the ratio of mixed to dead volume of the tundish. An optimum closure time of the near outlet has been found, which yields best mixing inside the tundish. The numerical code was validated against the experimental observation by performing the tracer dispersion study inside a multistrand tundish and the reasonably good match between the experimental and numerical results in terms of residence time distribution (RTD) curves. The results obtained from the present study confirm the strong role of choosing the right time for opening and closing the outlets to get improved characteristics for the fluid flow and mixing behavior of the tundish. The educational version of computational fluid dynamics (CFD) software PHOENICS was used to solve the governing equations and interpret the results in different forms.

Effect of vertical alignment of ladle shroud on transient steel quality output from multistrand tundish

Slight misalignments of the ladle shroud in a multistrand continuous casting tundish can lead to significant differences in the amounts of slag entrained into individual moulds during transient operations. In this full scale water modelling work, the effects of ladle shroud alignment on steel quality was measured in terms of ‘slag’ entrainment into the individual moulds. The ladle shroud was purposefully biased by about 4–5° off vertical, and the numbers of ‘slag particles’ entering individual strands of the four‐strand billet caster were measured during a ladle change and compared with the no bias condition. Given the great sensitivity of steel quality to this slight misalignment during a ladle change, possible remedial actions are discussed for equivalent steel plant operations.

WHOLE ANALYSIS APPROACH FOR RESIDUE TIME DISTRIBUTION CURVE IN MULTI--STRAND CONTINUOUS CASTING TUNDISH

The study of residue time distribution(RTD) curve is usually one of the effective methods to estimate the flow characteristics in the continuous casting tundish.In the past,the analysis model for RTD curve of one-strand is usually applied to estimate the individual strand data in multi-strand tundish,but the result obtained by such a model can not agree with the physical fact sometimes. In this paper,an analysis model is proposed to analyze flow characteristics in multi-strand tundish. First,the whole RTD curve of multi-strand tundish is obtained by the individual strand data.Second, classic analysis model is applied to analyze the whole RTD curve of multi-strand tundish.Third, average residence time is chosen as a key parameter to estimate the similarity of flow characteristics of each strand in the multi-strand tundish.Such an analysis approach can avoid the occurrence of the negative dead zone.And the volume ratio of dead zone satisfies the physical facts.

Water modeling of molten steel flow in a multi-strand tundish with gas blowing

Fluid flow characteristics in a four-strand tundish with gas blowing were studied by water modeling experiments. It is found that gas blowing can greatly improve the flow characteristics in the tundish with a turbulence inhibitor. It dramatically increases the peak concentration time, and greatly decreases the dead volume, and reduces the minimum residence time. The gas blowing location, gas flow rate, and porous plug area greatly influence the flow characteristics in the tundish; the gas blowing location near the baffle, smaller gas flow rate, and smaller porous plug area are better for improving the fluid flow characteristics. Using gas blowing can reduce the difference of flows at the middle outlets and side outlets for the multi-strand tundish. Bubbles produced by gas blowing can absorb small inclusions and provide the condition for inclusion collision and aggregation. Therefore, introducing gas blowing into a tundish and combining the turbulence inhibitor can improve inclusion floating and removal, and the cleanness of molten steel can be advanced.

Mathematical model of flow characteristic in multi-strand continuous casting tundishes

In this paper, the subject of mathematical model is a series of math expressions, which is used to calculate different regions volume fraction and analyze flow characterization in multi-strand tundish. But research about mathematical model for multi-strand tundish is few, and so far, there has been no acknowledged math model for multi-strand tundish to describe its flow characteristic. If Sahai s model, which is originally proposed for the case of single-strand tundish (proposed in reference [10] , and this model is widely used in the world), is applied to describe flow feature in multi-strand tundish, the calculation results would be unreasonable. Based on the data of water-model experiment results, the sum of each strand s dead region s volume fraction is bigger than 100%, and this obviously doesn t agree with reality; and the value of dead region s volume fraction is calculated to be minus according to mathematical simulation results data in another case. What s more, Sahai s model does not propose the standard of plotting the RTD-curve, and this makes scholars around the world can t achieve consensus of views about plotting RTD-curve. And the model doesn t consider the bypass flow and can t calculate its volume fraction, but bypass flow is critical to tundish metallurgy. And through Sahai s model, the calculation result of plug flow region s volume fraction is also not reasonable, because the model doesn t well describe the essence of plug flow. So these suggest that it is not reliable to apply Sahai s single-strand tundish model to multi-strand tundish case. Then a new model is attempted to propose in this paper for your discussion. In the new model, the standard of plotting RTD curve is definitely proposed, and relative calculation method is also proposed; and the feature of dead region is carefully studied and the model proposes a new method to calculate its volume fraction, and the calculation formula about its volume fraction can be adjustable according the actual demand; what s more, the new model considers the bypass flow and proposes a method to calculate its volume fraction for the first time, and then volume fraction of plug flow region, backmix flow region, dead region and bypass flow can be calculated and obtained at the same time; and this new model can better capture the deviation of reality flow pattern from ideal plug flow pattern, and reflects the feature of plug flow.

Dissymmetric flow phenomenon in a multistrand tundish

The dissymmetric flow phenomenon exists in a symmetric multistrand tundish. It was studied by the physical simulation experiment. The fundamental flow characteristic of dissymmetry was analyzed. The asymmetry of the flow field, the temperature field, and the inclusions distribution without flow-control devices (FCDs) were compared with those with FCDs. It is proved that the asymmetry of the flow and temperature field along the outlets at the long range is more obvious. The symmetric FCDs installation has a slight effect on the dissymmetric temperature field, simultaneously, the symmetry of the average residence time and the fluid flow pattern has improved, and the fluid flow in the tundish has been more reasonable. In case of a symmetric multistrand tundish having a large volume, the influence of the dissymmetric phenomenon should be considered and the flow behaviors in the whole tundish should be studied completely.

Orthogonal analysis of water model study on the optimization of flow control devices in a six-strand tundish

Improper flow control devices in a multi-strand tundish can cause some problems, for example, liquid steel cannot reach every nozzle at the same time and the liquid steel in nozzles far away from the entry zone has a lower temperature. The water model experiment of a six-strand tundish of Tianjin Iron & Steel Co. Ltd. was performed, a new "U" type baffle was obtained, and its parameters were defined by perpendicular analysis. The "U" baffle can not only improve those imperfections, but also prolong the residence time of nonmetallic inclusions, which is good for their flotation and separation.

Basis for Systematic Hydrodynamic Analysis of a Multi-strand Tundish

Despite significant work done till date on single/dual strand tundish (those applied to slab casting) not much information on an equivalent multi-strand tundish (which are of significance to billet casting) is available regarding residence time distribution (RTD) measurements and its analysis. The proportions of various flow volumes estimation (e.g., dead, plug, well mixed, etc.) on the basis of a single/dual strand tundish cannot be directly extrapolated to a multi-strand tundish from its individual strand data only. In the present study, therefore, the principle and the methodology underlying the derivation for concentration vs. time curves (C curves) of a multi-strand tundish is addressed to provide a basis for the evaluation of RTD parameters. To this end, several experiments in a four-strand water model tundish were carried out for RTD measurements and estimation of associated flow volumes. The tracer dispersion experiments were performed by injecting potassium chloride (20 mL solution) as a pulse into the submerged inlet stream and the variation of conductivity of water caused by the differential mixing of the injected tracer into the bath were recorded continuously at the exits of the water model tundish with a 75 L volume. These conductivity values of individual strands were converted into corresponding concentrations, and the dimensionless concentrations were plotted against dimensionless time to derive the characteristic C curves. The individual C curves effect the strand to strand variations whereas the overall C curve (average of individual C curves) is used to determine the flow volumes in a multi-strand tundish system.

Effect of outlet positions and various turbulence models on mixing in a single and multi strand tundish

The Navier‐Stokes equation and the species continuity equation have been solved numerically in a boundary fitted coordinate system comprising the geometry of a large scale industrial size tundish. The solution of the species continuity equation predicts the time evolution of the concentration of a tracer at the outlet of a single strand bare tundish. The numerical prediction of the tracer concentration has been made with three different turbulence models; (a standard k‐ε, a k‐ε RNG and a Low Re number Lam‐Bremhorst model) which favorably compares with that of the experimental observation for a single strand bare tundish. It has been found that the overall comparison of k‐ε model with that of the experiment is better than the other two turbulence models as far as gross quantities like mean residence time and ratio of mixed to dead volume are concerned. However, it has been found that the initial transient development of the tracer concentration is best predicted by the Lam‐Bremhorst model and then by the RNG model. The k‐ε model predicts the tracer concentration much better than the other two models after the initial transience (t>40 per cent of mean residence time) and the RNG model lies in between the k‐ε and the Lam‐Bremhorst one. The numerical study has been extended to a multi strand tundish (having 6 outlets) where the effect of outlet positions on the ratio of mix to dead volume has been studied with the help of the above three turbulence models. It has been found that all the three turbulence models show a peak value for the ratio of mix to dead volume (a mixing parameter) when the outlets are placed 200 mm away from the wall (position‐2) thus signifying an optimum location for the outlets to get highest mixing in a given multi strand tundish.

Melt flow control in a multistrand tundish using a turbulence inhibitor

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-ɛ 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.

Control del flujo de acero líquido en una artesa de colada continua de salidas múltiples

Physical and mathematical modelling techniques were used to study the molten steel now behaviour in a multi-strand bloom tundish. Three different cases were studied: a bare tundish (BT), a tundish equipped with two pairs of baffles and a waved impact pad (BWIP) and a tundish equipped with a turbulence inhibitor and a pair of dams (TI&D). Chemical mixing of tracer turbulence diffusion was also simulated and compared with experimental results. The TI&D arrangement showed a better tracer distribution among the outlets and an improvement of the fluid flow characteristics yielding lower values of back mixing flow and a good relation qf mixed and plug volume flows. The mass transfer model coupled with the k -e turbulence model predicted acceptably well the experimental chemical mixing of the tracer in the water model.