Transient flow and heat transfer in a steelmaking ladle during the holding period

Transient, turbulent flow and heat transfer in a ladle during the holding period are numerically investigated. The ladle refractories including the working lining, safety lining, insulation layer, and steel shell have been simultaneously taken into account. No assumptions are made for the heat transfer between the liquid steel and the inside ladle walls. Both the initial ladle heating and the heat loss from the slag surface are changed to examine their effect on thermal stratification in molten steel. A simplified model for the heat loss from the molten steel to the refractory is proposed. Correlations for the history of mean steel temperature, thermal stratification, and heat loss rate are obtained, which can be easily applied for industrial operations. Predictions are compared with experimental data in an industrial ladle and a pilot plant ladle, and those from previous studies.     

Mathematical Modelling of Molten Steel Flow Process in a Whole RH Degasser during the Vacuum Circulation Refining Process: Application of the Model and Results

A three‐dimensional mathematical model for the molten steel flow during the RH refining process has been applied to the circulatory flow processes in both a practical RH degasser and its water model unit. The model was presented earlier [1] and one of its characteristics is that ladle, snorkels and vacuum vessel are regarded as a whole. Using this model, the fluid flow field and the gas holdups of liquid phases and others have been computed respectively for a 90 t RH degasser and its water model unit with a 1/5 linear scale. The results show that the mathematical model can properly describe the flow pattern of molten steel during the refining process in an RH degasser. Except in the area close to the liquid's free surface and in the zone between the two snorkels in the ladle, a strong mixing of the molten steel occurs, especially in the vacuum vessel. However, there is a boundary layer between the descending liquid stream from the down‐snorkel and its surrounding liquid, which is a typical liquid‐liquid two‐phase flow, and the molten steel in the ladle is not in a perfect mixing state. The lifting gas blown is ascending mostly near the up‐snorkel wall, which is more obvious under the conditions of a practical RH degasser, and the flow pattern of the bubbles and molten steel in the up‐snorkel is closer to an annular flow. The calculated circulation rates for the water model unit at different lifting gas rates are in good agreement with experimentally determined values.     

Mathematical modelling of molten steel flow in a whole degasser during RH refining process

Abstract

A three-dimensional mathematical model for molten steel flow in a whole degasser during the RH (Ruhrstahl–Heraeus) refining process is proposed. The model has been developed considering the physical characteristics of the process, particularly the behaviour of gas–liquid two phase flow in the up snorkel and the momentum exchange between the two phases. The fluid flow fields and gas holdups of liquid phases, among other parameters, in a 90 t RH degasser and a water model unit of one-fifth linear scale have been computed using this mathematical model. The results show that the flow pattern of molten steel in a whole RH degasser can be well represented by the mathematical model. Apart from the area close to the free surface and the zone between the two snorkels in the ladle, the molten steel in an RH degasser, especially in the vacuum vessel, is reasonably fully mixed during the refining process. However, there is a boundary layer between the descending liquid stream from the down snorkel and the surrounding liquid, which is typical liquid–liquid two phase flow, and the molten steel in the ladle is not perfectly mixed. The blown lifting gas ascends mostly near the up snorkel wall, which is more obvious under the conditions of an actual RH degasser, and the flow pattern of bubbles and molten steel in the up snorkel is closer to annular flow. Calculated circulation rates for the water model unit at various lifting gas rates are in good agreement with values determined by means of water modelling experiments.     

Inclusion Growth and Removal in Gas‐Stirred Ladles

A static modelling approach was used to study the growth and removal of inclusions during gas stirring in a ladle. A mathematical model of a gas‐stirred ladle was used to predict the data necessary to calculate growth and removal of inclusions. Results indicated that inclusion growth resulting from laminar shear collisions is negligible in comparison with growth from turbulent and Stokes collisions. Furthermore, the need for a model describing inclusion flotation by spherical‐cap bubbles was identified. Since the existing models presented in the literature are only valid for spherical bubbles, a model for the removal of inclusions by spherical‐cap bubbles was developed. Inclusion removal to the slag, refractory and by bubble flotation was compared. The mechanism determined to be responsible for the removal of the majority of inclusions larger than 25 μm was Stokes flotation and for the majority of the smaller inclusions, bubble flotation by spherical‐cap bubbles (assuming plane contact between the inclusion and the bubble).     

Study on the Formation of Open‐Eye and Slag Entrainment in Gas Stirred Ladle

Laboratory experiments were carried out to study the phenomena related to open‐eye formation in ladle treatment. Ga‐In‐Sn alloy with a melting temperature of 283 K was used to simulate the liquid steel, while MgCI₂‐Glycerol(87%) solution as well as HCl solution were used to simulate the ladle slag. No open‐eye was formed at lower gas flow rates, but, occurred when gas flow reached a critical rate. This critical gas flow rate was found to depend significantly on the height of the top liquid. No noticeable amount of top liquid was observed in any of the samples taken from the metal bulk during gas stirring. To confirm this aspect, samples of slag‐metal interface were taken around the open‐eye in an industrial gas stirred steel ladle. No entrapped slag droplet was found in the solidified steel within the region between the interface and 2 cm from the interface. The accordance of the laboratory and industrial results suggests that the entrainment of slag into the steel bulk around the open‐eye cannot be considered as the major contribution to inclusion formation.     

REDA与RH精炼过程钢液流动规律比较

以300 t REDA和RH精炼装置为研究对象,借助计算流体力学软件模拟REDA与RH两种精炼工艺下钢液流动行为,从精炼过程流场形态、循环流量、氩气行程及熔池表面湍动能等方面进行分析,研究结果表明:RH对钢包底部熔池的搅拌作用强于REDA,REDA的单浸渍管结构有利于延长浸渍管寿命及提高钢液循环流量,REDA只需RH提升气体流量的30%便能达到相同的循环流量。     

Mathematical Simulation of Flow Phenomena in CAS-OB Refining Ladle

SymbolList　　Ag———Projectareaofgasbubble ,m2 ;　　C1 ,C2 ,Cμ———Empiricalturbulentconstants;　　Cd———Dragcoefficient;　　Cf———Frictioncoefficient;　　db———Diameterofbubble ,m ;　　dmax———Maximumdiameterofbubble ,m ;　　dp———Diameterofporousplug ,mm ;　　ds———Innerdiameterofsnorkel,mm ;　　dt———Topdiameterofladle ,mm ;　　du———Bottomdiameterofladle ,mm ;　　F———Interphasefrictionforce ;　　Fi———Additionalforce ;　　g———Gravityaccelerationconstant,…     

底吹氩过程LF炉盖内气体流动的数值模拟

基于流体模型和湍流修正模型,借助流体工程模拟软件Fluent 6.3.26对吹氩过程中210 t LF精炼炉盖内气体的流动、混合和质量、动量传输进行了计算模拟,分析了其流动行为和分布状态。结果表明,随钢包净空高度增加,液面上部氩气回旋区扩大,"死区"减小;当氩气流量增至500 L/min时,吹氩孔位于0.68 R的盖内流动效果优于0.3 R的效果;正常工作状态下的合理抽气压力为-150 Pa;在合理的抽气压力和吹氩孔位置的情况下,300 L/min的氩气流量基本可以满足要求,强搅拌时可增至500 L/min。     

Influence of the Stirring Gas in a 170‐t Ladle on Mixing Phenomena – Formation and On‐line Control of Open‐Eye at an Industrial LD Steel Plant

3D calculations with Computational Fluid Dynamics were carried out to evaluate the flow pattern under industrial conditions with different gas flow rates at the steel plant of Saarstahl AG. The generated flow pattern consists of a circulating loop characterised by an upward flow driven by the argon gas and a downward flow close to the wall on the opposite side of the porous plug in the case of a gas flow rate of 27 STP m³/h. When this high gas flow rate is used, the gas bubbles are taking a straight way from the inlet, but further up the momentum from the circulating steel is affecting the path of the gas bubbles followed by a breakthrough zone at the top surface. Intensive experiments with the 170‐t ladle of Saarstahl AG revealed typical open‐eyes. Large open‐eyes coupled with turbulences in the surface were generated in the case of gas flow rates between 20 and 30 STP m³/h. Intensive turbulences and even smoke formation were identified when a gas flow rate of > 30 STP m³/h was applied. For the investigation of the influence of gas stirring processes on the mixing phenomena samples were taken from the melt immediately after alloying. It could be seen that the analyses of Al, C, Mn and Si increased to the target analyses due to alloying and introduction of Ar through the porous plug. The total time for complete alloying depended on the elements within these experiments. It seemed to be that the alloying time increased in the order of Al, C, Mn and Si. For on‐line control and analysis of open‐eyes in the melt surface during ladle stirring, a BFI image processing system was installed at the steel plant of Saarstahl. It consisted of a conventional digital camera equipped with an infrared filter and coupled to an image processing software. Primary tests showed a slight influence of the open‐eye diameter at the end of the ladle treatment on inclusion densities in the liquid steel and oxidic K0 values of the finished wire rod. Additional experiments were performed but only a small correlation existed between the stirring energy at the end of ladle treatment and the inclusion length index of the applied blue brittle tests. But as soon as an open‐eye came into existence, the inclusion length was higher compared to those heats produced under a closed top slag.     

Study of Open Eye Formation in an Argon Stirred Ladle

Two cold models were employed to simulate the formation of open‐eyes in a gas stirred ladle. In the first model, water and silicon oils were employed to simulate liquid steel and slag respectively. In the second one, liquid Ga‐In‐Sn alloy was used to simulate liquid steel, while 12% hydrochloric acid simulated the top slag. The experimental results indicated that the gas flow rate, height of the lower liquid and height of the top liquid had a strong impact on the open‐eye size. On the other hand, the viscosity of the top liquid and the interfacial tension between the two liquids had only little effect on the open‐eye size. A semi‐empirical model was developed to describe the size of open‐eye as a function of the heights of the two liquids and the gas flow rate. The two sets of parameters obtained for the water and Ga‐In‐Sn models were very different. Industrial trials were also conducted to examine the applicability of the models. The model developed based on the Ga‐In‐Sn model could well predict the formation of an open‐eye during ladle treatment. The model could be adopted by the industry to estimate the real gas flow rate by measuring the size of the open‐eye online. On the other hand, the mathematical model based on water model experiments was unsatisfactory when applied to the industrial ladle process.     

Numerical Simulations of Inclusion Behavior in Gas-Stirred Ladles

A computation fluid dynamics–population balance model (CFD–PBM) coupled model has been proposed to investigate the bubbly plume flow and inclusion behavior including growth, size distribution, and removal in gas-stirred ladles, and some new and important phenomena and mechanisms were presented. For the bubbly plume flow, a modified k-ε model with extra source terms to account for the bubble-induced turbulence was adopted to model the turbulence, and the bubble turbulent dispersion force was taken into account to predict gas volume fraction distribution in the turbulent gas-stirred system. For inclusion behavior, the phenomena of inclusions turbulent random motion, bubbles wake, and slag eye forming on the molten steel surface were considered. In addition, the multiple mechanisms both that promote inclusion growth due to inclusion–inclusion collision caused by turbulent random motion, shear rate in turbulent eddy, and difference inclusion Stokes velocities, and the mechanisms that promote inclusion removal due to bubble-inclusion turbulence random collision, bubble-inclusion turbulent shear collision, bubble-inclusion buoyancy collision, inclusion own floatation near slag–metal interface, bubble wake capture, and wall adhesion were investigated. The importance of different mechanisms and total inclusion removal ratio under different conditions, and the distribution of inclusion number densities in ladle, were discussed and clarified. The results show that at a low gas flow rate, the inclusion growth is mainly attributed to both turbulent shear collision and Stokes collision, which is notably affected by the Stokes collision efficiency, and the inclusion removal is mainly attributed to the bubble-inclusion buoyancy collision and inclusion own floatation near slag–metal interface. At a higher gas flow rate, the inclusions appear as turbulence random motion in bubbly plume zone, and both the inclusion–inclusion and inclusion-bubble turbulent random collisions become important for inclusion growth and removal. With the increase of the gas flow rate, the total removal ratio increases, but when the gas flow rate exceeds 200 NL/min in 150-ton ladle, the total removal ration almost does not change. For the larger size inclusions, the number density in bubbly plume zone is less than that in the sidewall recirculation zones, but for the small size inclusions, the distribution of number density shows the opposite trend.     

150 t钢包底吹氩卷渣行为的物理模拟

针对钢厂150 t双孔底吹氩钢包,根据相似原理建立几何比例为1:5的水力学模型,得出对应实际氩流量260~600 L/min时原型钢包及优化后钢包的液面裸露面积及渣钢卷混情况的变化规律和临界卷渣气量。研究结果表明,原型方案下两透气砖分别位于距钢包中心0.64 R和0.76 R处,两孔成90°(0.64 R+0.76 R,90°),临界卷渣气量为550 L/min;对于两个优化方案,双孔分别位于1/3 R和0.64 R,两孔成180°(1/3 R+0.64 R,180°)以及双孔位于0.5 R圆周上,两孔成135°(0.5 R+0.5 R,135°),临界卷渣气量分别为550 L/min与600 L/min。     

A Study on the mathematical modeling of turbulent recirculating flows in gas-stirred ladles

Computed results are presented describing the velocity field and the map of the turbulent kinetic energy in a water model of an argon-stirred ladle. The theoretical predictions agree well with the measurements, when an experimentally determined void fraction distribution is used in computing the body force driving the flow. The agreement is somewhat less satisfactory, particularly regarding the maps of the turbulent kinetic energy, when the no-slip or the drift flux models are used to predict the void fraction of the gas.     

Measurement and modeling of turbulence in the gas/liquid two-phase zone during gas injection

Averaged and turbulent fluctuating liquid velocities in the gas/liquid plume zone of a gas-stirred water model ladle were measured with a combined laser Doppler anemometer (LDA) and elec-trical probe technique. The measured turbulence fields, void fraction distribution, and gas and liquid velocities in the plume zone were used for evaluation of various turbulence models. It was found that, among all of the turbulence models tested, only a modified k-ε model, with extra source terms to take into account the generation and dissipation resulting from the inter-action of the bubbles with the liquid, yielded good agreement with both the mean liquid flow field and the turbulent kinetic energy distribution. However, the values of the coefficients orig-inally proposed by their authors were found inapplicable to the bubbly plume situation; more appropriate values of the coefficients were determined based on comparison with experimental measurement.     

The impact of bubble dynamics on the flow in plumes of ladle water models

Bubbly plumes are widely encountered in metallurgical processes when gas is injected into liquid metals for refining purposes. Based on the experimental findings from a water model ladle, this phenomenon was simulated with a mathematical model, paying special attention to the dynamics of the bubbles in the plume. In the model, the liquid flow field is first calculated in an Eulerian frame with an estimated distribution of the void fraction. The trajectories of bubbles are then computed in a Lagrangian manner using the estimated flow field, experimentally measured information on bubble drag coefficients, lateral migration due to lateral lift forces, and variation in bubble size due to breakup. Turbulence in the two-phase zone is modeled with a modifiedk-ε model with extra source terms to account for the second phase. The computed void fraction and turbulent liquid flow field distributions are in good agreement with experimental measurements.     

Flow pattern velocity and turbulence energy measurements and predictions in a water model of an argon-stirred ladle

Experiments were carried out using a simplified water model of an argon-stirred ladle system. The flow patterns were determined by a flow visualization technique and the velocity and turbulence energy fields were quantitatively measured using hot-film anemometry. The latter quantities were predicted by solving the turbulent Navier-Stokes equations using Spalding’sk-W model for the turbulence viscosity. There is semiquantitative agreement between predictions and measurements. Mixing lengths also were computed. This agreement between measurements and predictions provides further evidence that modeling is a promising approach for the study of recirculating turbulent flows in steel processing operations. J. SZEKELY, formerly of the State University of New York at Buffalo.     

Modelling of air ingress and pressure distribution in ladle shroud system for continuous casting of steel

The pressure distribution and fluid flow profiles whithin the slide gate and shroud nozzle for the continuous casting of steel have been investigated using a full scale water model and a CFD (computational fluid dynamics) model. The water modelling has shown that a large quantity of air can be drawn into the liquid stream if there is any breakdown of the seals in the vicinity of the slide gate. The 3-dimensional numerical solution for highly turbulent flow has predicted the pressure distribution and velocity profile within the slide gate and shroud. Based on the experimental and numerical modelling, it has been shown that cavitation can occur near the slide gate during ladle teeming. This can be a source for erosion of the refractories. Improvements to the design of the ladle shroud system are recommended.     

Numerical Simulation of Fluid Flow and Interfacial Behavior in Three‐phase Argon‐Stirred Ladles with One Plug and Dual Plugs

A numerical investigation is performed to describe the quasi‐steady fluid flow and interfacial behavior in a three‐phase argon gas‐stirred ladle with off‐centered bottom Ar injection through a plug and two plugs placed in 180° and 90°configurations, respectively. The flow of the fluid phase is solved in an Eulerian frame of reference together with the motion of every individually injected Ar bubble, tracked in its own Lagrangian frame. Volume of fluid (VOF) model is used to track any interface between two or more immiscible phases, which include slag/metal, slag/gas and metal/gas. The characteristics of fluid flow in a gas‐stirred ladle with one plug or two plugs configuration are described when the slag layer and the top gas are presented. The slag layer deformation and slag open‐eye formation at different Ar gas flow rates for three types of plug arrangements are given. The comparison of the mixing time, the deformation of slag layer and the behavior of slag/steel interface between one‐plug and two‐plug system is made. Several implications for ladle operational issues during a gas‐stirred ladle refining cycle are discussed. It is found that the proper selection of Ar gas flow rate and plug arrangements during a ladle refining cycle is required for different refining purposes considering the mixing and metallurgical reaction in a three‐phase ladle system.