Numerical Investigation of the Fluid Flow in Continuous Casting Tundish Using Analysis of RTD Curves

 A detailed mathematical procedure of the optimization of the fluid flow in a tundish water model with and without flow control devices (weir and dam) was carried out using the commercial CFD code FLUENT 60. The (k ε) two equation model was used to model turbulence. The residence time distribution (RTD) curves were used to analyze the behavior of the flow in tundish. The location of flow control devices in the tundish was studied. The results show that the flow modifiers play an important role in promoting the floatation of nonmetallic inclusions in steel. Comparing the three geometric configurations that are considered (bare tundish, weir, weir+dam), the tundish equipped with the arrangement (weir+dam) is a best and optimal geometric configuration of tundish.     

非对称中间包稳态浇铸与换包操作的冶金效果

??To rationally assess and optimize the metallurgical effect of an industrially used three- strand asymmetric tundish?? the mixing characteristics of molten steel with different tundish configurations were investigated by residence time distribution??RTD??curves based on the analysis of fluid flow and temperature distribution at four flow control schemes?? and the behavior of the tundish level was tracked by employing volume- of- fluid??VOF??model. The fluid dynamics behavior of the tundish was studied in term of both steady and unsteady service situations with and without fluid flow control devices?? which made it possible to assess comprehensively the metallurgical effect of the given tundish. The results show that the flow control effect of bare tundish is weak. It is observed that there are distinct short- circuit flow and large proportion of dead zone?? together with the non- uniform temperature distribution and the flow state discrepancy among the three strands?? which will accordingly lead to the quality difference of the bloom castings. The improvements on the fluid flow characteristics and the temperature distribution have been observed in tundish with the combined application of baffles and turbulence inhibitor. The proportion of dead zone is decreased by 13. 28%?? the temperature difference is only 0. 5K among three outlets?? along with an improved fluid flow consistency. Additionally?? studies also show that this arrangement may have large steel level velocity and level fluctuation during ladle change period. That is likely to bring about exposure?? reoxidation and even slag entrainment of molten steel. Attentions should be paid to the control of the ladle change interval?? inlet flowrate and the moment height of liquid level.     

Experimental investigation of flow control devices effect on inclusion separation in a four strand tundish

In continuous casting, molten steel often contains non-metallic inclusions that have a very high melting temperature and thus remain suspended in the molten flow. Given the lower density of these substances compared to the melt, they can be filtered in the form of top slag by proper use of buoyancy forces. Thus, providing a desirable flow pattern, reduced flow turbulence, and adequate residence time for inclusions in the tundish will improve the inclusion separation performance and therefore the product quality. In this study, a reduced-scale water model of a tundish was used to evaluate the effect of melt level in the tundish on the flow pattern and inclusion separation performance. Then, the flow control devices are added to this simple tundish model to examine the effect of them on the flow behaviour and inclusion separation. Experimental investigation showed that raising the water level in the model tundish has desirable effects on inclusion separation and the use of the dam at the inlet with tall dam improves the flow pattern and inclusion separation almost 20% in the form of top slag.     

非对称大方坯连铸中间包内控流装置的对比研究

通过对5流T型非对称大方坯连铸中间包控流装置的水模研究,对比了不同控流装置对连铸中间包内流场的影响,发现结构合理的控流装置能够明显改善中间包内流体的流动特性。采用多流连铸中间包流动特性的评判方法,分析讨论各方案实验数据,得出了中间包内控流装置的优化设计方案,为中间包内控流装置的设计提供了参考。实验表明：直通孔过滤器能够更好地改善五流圆坯中间包内钢水的流动,并根据实验结果提出了最佳的过滤器结构。     

Transient steel quality under non-isothermal conditions in a multi-strand billet caster tundish: part II. Effect of a flow-control device

A mathematical model was built to simultaneously analyse the effects of non-isothermal conditions and flow-control device on steel quality in a real steelmaking tundish. Liquid steel was used as the operating liquid with a step-input of 23° in a full-scale delta-shaped multi-strand billet caster tundish fitted with a standard impact pad (SIP). The changes in flow pattern and temperature fields of liquid steel in the tundish under isothermal, step-up and step-down conditions were thoroughly studied. Similar to the case of a bare tundish, buoyancy effects were seen to dominate at regions away from the ladle shroud. The presence of SIP modified the flow patterns and resulted into markedly different values of RRI as compared to bare tundish. Finally, the calculated results were qualitatively compared to results obtained from a real steelmaking tundish.     

Upward Molten Flow for Optimization of Fluid Flow in Continuous Casting Tundish

Fluid flow pattern and buoyancy force support the motion of nonmetallic inclusions toward the tundish slag. Upward molten flow was investigated. To understand the fundamentals of the process, physical modelling was carried out with the utilization of a 1∶4 scale model. Numerical modelling was carried out in line with the physical modelling to examine details of the flow pattern and rotational effect caused by the upward flow with the commercial CFD (Computational Fluid Dynamics) package environment, FLUENT. The two-equation k-ε model was used to simulate the turbulence. Multiphase fluid flow was numerically simulated by using the Volume of Fluid (VoF) method. The simulation can predict free surface waves and other phenomena, which can be used to optimize these important metallurgical operations.     

Effect of Thermal Buoyancy on Fluid Flow and Inclusion Motion in Tundish without Flow Control Devices--Part Ⅰ: Fluid Flow

The κ-ε two-equation model is used to simulate the fluid flow in the continuous casting tundish coupling with the effect of thermal buoyancy. The natural convection induced by the thermal buoyancy generates an upward flow pattern especially at the outlet zone, and has little effect on the fluid flow in the inlet zone. The maximum viscosity is 700 times larger than the laminar viscosity, which indicates the strong turbulent flow in the tundish. The maximum temperature difference in the whole tundish is 8.2 K. The temperature near the stopper rod and the short wall is obviously lower than that in the inlet zone. The existence of the stopper rod has a big effect on the fluid flow entering the SEN and the mold. All the characteristics of the tundish geometry should be considered to accurately simulate the fluid flow in the tundish.     

Inertial and buoyancy driven water flows under gas bubbling and thermal stratification conditions in a tundish model

Steel flow dominated by inertial and buoyancy flows under gas bubbling and thermal stratification conditions, in a one-strand tundish, was studied using a 2/5 scale water model. The use of a turbulence inhibitor yields plug flow volume fractions well above 40 pct for a casting rate of 3.12 tons/min under isothermal conditions. Small flow rates of gas injection (246 cm³/min), through a gas curtain, improved the fluid flow by enhancing the plug flow volume fraction. Higher flow rates originated an increase of back-mixing flow, thus forming recirculating flows in both sides of this curtain. Step inputs of hot water drove streams of this fluid toward the bath surface due to buoyancy forces. A rise in gas flow rate led to a thermal homogenization in two separated cells of flow located at each side of the gas curtain. Step inputs of cold water drove streams of fluid along the tundish bottom. Use of the gas curtain homogenized the lower part of the tundish as well as the upper part of the bath at the left side of the curtain. However, temperature at the top corner of the tundish, in the outlet box, remained very different than the rest of the temperatures inside this tundish. High gas flow rates (912 cm³/min) were required to homogenize the bath after times as long as twice the mean residence time of the fluid. Particle image velocimetry (PIV) measurements corroborated the formation of recirculating flows at both sides of the gas curtain.     

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.     

吹氩连铸中间包内钢液流动特性的水模型实验研究

建立了模拟连铸中间包底吹氩流动过程的水模型.通过测定分析水模型的RTD（停留时间分布）曲线研究了吹气流量、吹气位置及控流元件对中间包内流动特性的影响规律,并对此进行优化.结果表明：条形透气梁安放位置对中间包内的流动特性影响最为显著,均匀开孔的多孔挡墙和条形透气梁喷吹氩气二者的配合使用能获得理想的流动特性.     

六流中间包场协同分析及流场优化

采用场协同理论分析了连铸中间包内钢液流场与温度分布之间的关系,并对钢厂六流中间包进行了数值模拟计算。结果表明,当中间包不采用控流装置时,大部分区域钢液流场与温度梯度场协同性好,钢液温度梯度大,最大温差达27 K,出口最大温差达3 K,(火积)耗散值大,各出口RTD(停留时间分布)曲线相差较大;当中间包内设置斜孔挡墙后,钢液流场与温度梯度场间的协同程度降低,钢液温度分布较均匀,最大温差为12 K,各出口最大温差仅为0.3 K,(火积)耗散值降低了57.6%,各出口RTD曲线近似。     

五流非对称结构中间包流动及传热特征数值模拟

为优化某钢厂非对称结构中间包内流体流动、均匀各流钢液温度、延长近水口平均停留时间,以达到提高铸坯质量的目的,对工厂原方案及两个优化方案进行流场、温度场及停留时间的数值模拟计算.结果表明无控流装置的原中间包流动及温度场分布不合理,优化方案有了明显的改善,并确定了最佳方案.     

Fluid flow in solidifying monotectic alloys

Use of a two-wavelength holographic technique results in a simultaneous determination of temperature and composition profiles during directional solidification in a system with a miscibility gap. A shadowgraph technique is employed for flow visualization. By these methods, flow regimes are identified and related to particular melt compositions. We discuss the relationships among fluid flow, phase separation, and mass transport during the solidification of the monotectic alloy. The primary sources of fluid motion in this system are buoyancy and thermocapillary forces. These forces act together when phase separation results in the formation of droplets (this occurs at the solid-liquid interface and in the bulk melt). While buoyancy forces arise due to density differences between the droplet and the host phase, thermocapillary forces (associated with temperature gradients in the droplet surface) may predominate. In the absence of phase separation, buoyancy results from density gradients related to temperature and compositional gradients in the single-phase bulk melt. The effects of buoyancy are especially evident in association with water- or ethanol-rich volumes created at the solid-liquid growth interface.     

Investigation of transient fluid flow and heat transfer in a continuous casting tundish by numerical analysis verified with nonisothermal water model experiments

The tundish process is complicated due to the periodic nature of its operation during the changing of ladles, which makes it both a transient and nonisothermal process. A nonisothermal water model, with temperature variations in the inlet stream, is described in this article. A three-dimensional (3-D) transient mathematical model to simulate the fluid flow and heat transfer in this water model is also introduced. From the experimental and numerical results, it is shown that there is a significant thermal buoyancy force contributing to the flow in this nonisothermal water model.     

六流方坯中间包内流场的数值模拟及优化

以唐钢150mm×150mm铸坯6流T型中间包为研究对象,用商业软件ANSYS对中间包内钢液流场进行了三维数值模拟与优化,研究了中间包内钢液的流动特征和设置导流挡墙后中间包内钢液的流动方式。结果表明,设置合理的挡墙和导流孔可以有效改善中间包冶金效果;3个导流孔直径(mm)分别为93、72、36,位置(mm)为(220,140)、(350,280)、(700,300),倾斜角度为(21°,15°)、(8°,23°)、(10°,28°)时中间包流场最为合理。     

中间包钢液非等温现象的水模型研究

钢液的非等温现象对中间包内流体流动和传热有着重要的影响。笔者用热态水模型研究了中间包内钢液的流动规律。结果表明：温度变化导致钢液的密度不均匀,使自然对流影响钢液流动;还给出了不同温差、不同控流装置下中间包内钢液温度的分布,并讨论了在不同条件下自然对流对钢液流动的影响。     

Fluid Flow and Interfacial Phenomenon of Slag and Metal in Continuous Casting Tundish With Argon Blowing

The fluid flow and the interfacial phenomenon of slag and metal in tundish with gas blowing were studied with mathematical and physical modeling, and the effects of gas flowrate, the placement of porous beam for the generation of bubbles, and the combination of flow control devices on the flow and slag-metal interface were investigated. The results show that the position of gas bubbling has a significant effect on the flow in tundish, and the placement of porous beam and gas flowrate are the two main factors affecting the entrapment of slag in tundish. The closer the porous beam to the weir, the more reasonable is the flow, which is in favor of the control of slag entrapment in tundish.     

连铸中间包不同控流装置使用效果比较的模拟研究

采用水力学数学模拟的方法,研究了3号多孔挡墙和U挡墙对昆钢小方坯连铸中间包流动特性的影响。水力学模拟依据相似原理,用"刺激—响应"实验方法,测定不同工况下停留时间分布(RTD)曲线,得到两种挡墙对中间包流体流动的影响。然后用数学模拟的方法验证水力学模拟的合理性。研究结果表明:3号多孔挡墙可以延长1号、2号水口响应时间及平均停留时间,缩短了3号水口响应时间及平均停留时间,控制各流流动趋于一致。中间包内流体流动特性得到明显改善。使用U挡墙后,钢液的平均停留时间Ta有了大幅增加,但各水口的初次响应时间差距较大,各流的流动特征一致性较差。因而,3号多孔挡墙控流效果优于U挡墙。     

薄板坯连铸中间包控流装置的数理模拟

采用水力学数理模拟的方法,研究了不同尺寸和安装位置的控流装置对马钢薄板坯连铸中间 包流动特性的影响。结果表明：中间包在无流动控制时,存在明显的短路流及较大死区;西马克公司提供的控 流装置其平均停留时间小,死区较大,实验效果并非最佳;改进后的优化方案使中间包内示踪剂开始响应时间 为无控流装置下的2倍多,平均停留时间由264.0s 增加到301.4 s,死区由无控流装置的25.54%降低到 15.39%,中间包的冶金性能有了明显改进。     

昆钢直弧形板坯连铸中间包水力学模拟试验研究

通过板坯铸机中间包水模试验，研究了多种控流装置下钢水流场变化情况，优化了中间包挡墙和堰的安装位置，改善了中间包冶金效果。