Fluid dynamics in bubble stirred ladles: Part I. experiments

Air is supplied through a porous plug placed in the center axis of a cylindrical perspex-water model of a ladle. A Laser-Doppler system is employed to measure radial and axial mean and fluctuating velocities. Velocities in the two-phase bubbly region can also be determined. Velocities are measured near the bottom, half-way up, and near the free surface. It is shown that the bubbles contribute to production of turbulence. The ladle has recirculation zones near the bottom, where the mean velocities are very low. Close to the free surface the radial mean and turbulent velocities are high, promoting mass transfer through the interface. The present measured velocity profiles cannot be reduced to a single profile by employing similarity scaling. Formerly a Visiting Scientist at SINTEF Formerly Engineer at SINTEF     

钢包底温度场和应力场数值模拟

钢包是连铸生产线上的重要设备，其内衬的热应力是影响钢包寿命的主要因素。本文运用有限单元法，对钢包底的温度场和应力场进行了数值模拟。计算结果表明，新包预热后仍处于吸热状态，经数次热循环后才达到“准稳态”；包底工作层应力值在预热完成后，钢水注入瞬时达到最大；包底工作层热面靠近包壁处的应力比靠近中心部位高，可在靠近包壁处增设保温装置降低该处应力。     

Mathematical simulation of fluid dynamics during steel draining operations from a ladle

Fluid flow dynamics during ladle drainage operations of steel under isothermal and nonisothermal conditions has been studied using the turbulence shear stress transport k-ε model (SST k-ω) and the multiphase volume of fluid (VOF) model. At high bath levels, the angular velocity of the melt, close to the ladle nozzle, is small rotating anticlockwise and intense vertical-recirculating flows are developed in most of the liquid volume due to descending steel streams along the ladle vertical wall. These streams ascend further downstream driven by buoyancy forces. At low bath levels, the melt, which is close to the nozzle, rotates clockwise with higher velocities whose magnitudes are higher for shorter ladle standstill times. These velocities are responsible for the formation and development of a vortex on the bath free surface, which entrains slag into the nozzle by shear-stress mechanisms at the metal-slag interface. The critical bath level or bath height for this phenomenon is 0.35 m (in this particular ladle design) for a ladle standstill time of 15 minutes and decreases with longer ladle standstill times. At these steps, the vertical-recirculating flows are substituted by complex horizontal-rotating flows in most of the liquid volume. Under isothermal conditions, the critical bath level for vortex formation on the melt free surface is 0.20 m, which agrees very well with that determined with a 1/3 scale water model of 0.073 m. It is concluded that buoyancy forces, originated by thermal gradients, as the ladle cools, are responsible for increasing the critical bath level for vortex formation. Understanding vortex mechanisms will be useful to design simple and efficient devices to break down the vortex flow during steel draining even at very low metal residues in the ladle.     

底吹氩钢包内夹杂物粒子上浮行为的水模实验研究

将修正伽利略准数作为对钢包内夹杂物上浮规律模拟的判据,以某厂150t底吹氩钢包为原型,建立了模型与原型尺寸比为1:4的物理模型。通过实验研究了吹气量、吹气孔与钢包中心距离、两吹气孔与钢包底中心连线的角度,对夹杂物粒子上浮去除的影响规律。实验结果表明:合理的吹气量为300 L/min;吹气孔靠近钢包中心、吹气孔与钢包中心连线角度小时,夹杂物上浮去除时间短,有利于夹杂物去除。     

Plume characteristics and liquid circulation in gas injection through a porous plug

Various forms of plumes have been identified following the injection of air at different rates through a porous plug into water contained in a ladle-shaped vessel. Discrete bubbles form at the plug and rise uniformly through the column of liquid at gas flow rates up to 14 cm³/s cm² of plug surface; at higher flow rates, groups of bubbles increasingly coalesce into larger gas pockets, and beyond about 40 cm³/s cm², the gas globes are large enough to cover the entire plug surface before detachment and gradual disintegration as they rise through the body of liquid. The gas fraction, as well as bubble frequency, bubble velocity, and bubble size, have been measured in the various dispersion regimes by means of an electroresistivity probe. The radial distributions of gas fraction and bubble frequency are approximately bell-shaped about the axis of flow, and the reduced values are close to Gaussian functions of the reduced radial distance from the axis. The gas fraction along the axis has been correlated to the reduced height of the plume; it increases with decreasing distance above the plug and with increasing gas flow rate. The axial bubble frequency shows a decrease in the vicinity of the plug with the onset of bubble coalescence, but the values of the frequencies at all gas injection rates converge to about 12 s⁻¹ toward the surface of the bath. The mean bubble velocity increases with increasing flow rate but drops once coalescence is fully established. Conversely, there is a sudden increase in the mean bubble diameter with the onset of coalescence. The axial and radial components of the velocity of the liquid surrounding the plume have been measured by means of a Laser-Doppler Velocimeter (LDV), and the results show that the circulation patterns are identical, irrespective of the dispersion regime. The axial flow which is upward in the vicinity of the plume decreases in magnitude with increasing radial distance, ultimately reversing to an in-creasing downward flow beyond a certain distance from the plug axis. Similarly, the radial flow which is outward from the plume near the liquid surface decreases steadily with depth and eventually reverses to an inward flow at a depth independent of the gas injection rate. The profiles of the axial velocities are almost sigmoidal, except in the coalescence regime, where the effect of turbulence is profound at the upper liquid layers. The radial liquid velocities are generally small relative to the axial components, only about one-fifth as large, considering the maximum average values.     

Effect of holding time and surface cover in ladles on liquid steel flow in continuous casting tundishes

Mathematical modeling of fluid flow and heat transfer of melt in a typical two-strand slab caster tundish has been done for a complete casting sequence. The complete casting sequence consists of 1 minute of tundish emptying period during the ladle transfer operation followed by 1 minute of tundish filling period by the new ladle and pouring at the normal operating level of the tundish for 46 minutes. The effect of varying ladle stream temperature conditions on the melt flow and heat transfer in the continuous casting tundish has been studied. When the ladle stream temperature decreases appreciably over the casting period, corresponding to heat loss of the melt in the ladle from the top free surface, the incoming melt temperature becomes lower than that of the melt in the bulk of the tundish after about 30 minutes from the start of teeming. This results in melt flow along the bottom of the tundish instead of the normal free surface directed flow. The ladle melt stream temperature shows little variability when the ladle has an insulated top. Corresponding to this situation, the temperature of the incoming melt remains higher than that of the melt in the bulk of the tundish and the normal free surface directed flow is maintained throughout the casting period. The product cast under such condition is expected to have a uniformly low inclusion content. The heat loss condition from the top of the ladle has been shown to be the dominant factor in determining fluid flow and heat-transfer characteristics of the melt in the tundish rather than the holding time of the melt in the ladle. Formerly Graduate Student, Department of Materials Science and Engineering, Ohio State University     

底吹钢包内流动及混合的两相流数值模拟

应用欧拉-欧拉模型建立了钢包内钢液流动及混合过程的数学模型,考察了吹气量对中心底吹及偏心底吹钢包内流场及均混时间的影响。计算结果表明,钢包底部四周为流动缓慢区域;吹气量越大,一方面可以降低均混时间,另一方面会导致钢包自由液面的钢液流速增大,从而容易造成卷渣;从缩短混合时间,提高生产效率考虑,偏心底吹更为有利。     

Effect of slag cover on heat loss and liquid steel flow in ladles before and during teeming to a continuous casting tundish

Mathematical modeling of transient fluid flow and heat transfer of melt in the ladle has been carried out, both before and during teeming of the melt to a tundish. The model involves solution of the transient, two-dimensional form of the turbulent Navier-Stokes' equation along with the equations of turbulence energy, energy dissipation rate of turbulence energy, and thermal energy conservation in the cylindrical coordinate system. Two different heat loss conditions have been assumed to occur from the top free surface of the melt in the ladle. When the ladle has an insulating layer of slag, temperature stratification occurs within the melt with the coolest melt in contact with the ladle bottom. The degree of temperature stratification increases with the increase in holding time. Pouring of the melt from such a ladle to the tundish, however, results in near uniform ladle stream temperature during the 47 minutes of pouring period considered in the present study. This is especially true if the melt in the ladle is held for a period of 20 minutes prior to teeming. When the melt in the ladle loses an appreciable amount of heat from the top due to a thin layer of slag, the average temperature of the melt drops considerably during the holding period although there is no temperature stratification. Pouring from such a ladle results in a continuous decline of the ladle stream temperature, even though the pouring starts after a holding period of 5 minutes. Formerly Graduate Student, Department of Materials Science and Engineering, Ohio State University     

渣罐底吹熔池表面突起特性的模拟研究

基于“选择性析出技术”,向渣罐内吹入空气氧化含钛高炉渣,以实现钛的选择性富集。渣罐内熔池的表面突起高度和宽度对于渣罐内自由空间的设计存在重要影响。通过摄像法测定了粘度、气体流量对底吹渣罐水模型表面突起高度和宽度的影响。结果表明：液面突起的形状服从高斯分布;液面突起高度随底吹气体流量的增大而变大,而基本不受熔池黏度变化的影响;液面突起宽度随气体流量和溶液黏度的增大而变大,但主要受气体流量的影响。通过对实验结果的分析讨论,得出了水模型溶液表面突起高度和突起宽度的经验关系方程式。     

Mixing Enhancement in Gas-Stirred Melts by Rotating Magnetic Fields

A model experiment of a submerged gas injection system in a cylindrical vessel under the influence of a rotating magnetic field and its effect on liquid metal mixing is presented. Argon gas is injected through a nozzle into a column of the eutectic alloy GaInSn, which is liquid at room temperature. Without a magnetic field, the bubble plume in the center region of the cylindrical vessel produces a recirculation zone with high fluid velocities near the free surface, while the fluid velocities in the bottom region are rather low. Our measurements revealed the potential of rotating magnetic fields to control both the amplitude of the meridional flow and the bubble distribution and to provide an effective mixing in the whole fluid volume. Various periodic flow patterns were observed in a certain parameter range with respect to variations of the magnetic field strength and the gas flow rate.     

钢包底吹氩卷渣临界条件的水模型研究

以钢厂70t钢包为原型,建立模型与原型尺寸比为1:2.75的水模型。通过水模型实验对钢包临界卷渣吹氩量进行测量,得到临界卷渣气量是450 L/min。实验测量不同吹气量下钢包液面水平流速,分析水平流速与钢包卷渣的关系,得到钢包临界卷渣的液面流速为0.652m/s、韦伯准数为6.967。讨论了采用临界卷渣韦伯准数计算临界卷渣液面流速,通过测量达到临界卷渣液面流速时的吹氩量,来确定临界卷渣吹氩量的方法。     

深插入浸罩CAS钢包流场混合特性的水模型研究

按照110t CAS钢包1/6的水模型,研究了浸罩深度(熔池液面深度0～20%)和直径(钢包底部直径的0.4～0.7)对熔池混匀时间的影响。结果表明,随着浸罩深度和直径的增加,罩内的循环流增强,在深插入浸罩(熔池液面深度的20%)条件下,钢包内流场发生显著变化,浸罩内形成了明显的循环流。通过无因次分析,得出底吹气量Q和浸罩深度H对混匀时间T影响程度的经验公式(T-T0)/T=3.13Q^-0.66(H/HL)^1.56。     

重钢炼钢厂80 t钢包热分析

通过建立钢包传热数学模型,计算了重钢炼钢厂80t钢包热损失与外表面温度,计算结果与实测值基本符合。利用此模型,分析了钢包壁、包底、覆盖剂材质及厚度对热损失的影响,提出了降低钢包热损失的主要途径,为重钢炼钢厂减少钢水温降提供了依据。     

Large-scale measurements of the physical characteristics of round vertical bubble plumes in liquids

The hydrodynamics of air/water plumes in a large-scale model of a metallurgical ladle were investigated. The dimensions of the cylindrical vessel were 1600-mm ID and 2250-mm total height. The air was injected through a centered nozzle. Axial and radial profiles of gas concentration, bubble frequency, and liquid and gas velocities were measured using electrical resistivity probes and a propeller flowmeter. It was found that the bubble plume is not at a fixed position but wanders away from the vertical vessel axis. This causes difficulties in the measurements, and special methods have to be designed to define and deduce reproducible values for the characteristic plume quantities. In the analysis of the data, the various physical characteristics were related toz ₀, the distance from the nozzle where the axial gas concentration is 50 Pct. The maximum values of the radial profiles are presented in nondimensional correlations. MARCO A.S.C. CASTELLO-BRANCO, formerly Scientist, Institut für Allgemeine Metallurgie, Technical University of Clausthal     

Wave Velocities in Granular Materials under Microgravity

Velocities of primary (P) and shear (S) waves in granular materials are highly dependent on confining stress. These wave velocities are related to mechanical properties of the materials such as stiffness, density, and stress history. Measurements of the wave velocities using piezoelectric sensors provide scientists and engineers a technique for nonintrusive characterization of those mechanical properties. For aerospace engineering, measuring the wave velocities under microgravity, which simulates low loading and stress conditions, has a number of potential applications. It can help the understanding of the soil mechanics and the development of appropriate materials handling technologies in extraterrestrial environments, which will be crucial to meeting NASA’s future space exploration goals. This paper presents the technique and results of experiments conducted at NASA Glenn Research Center using the 2.2?s drop tower. Velocities of P and S waves in three sizes of glass beads and one size of alumina beads were measured under initially dense or loose compaction states. It was found that under microgravity, the wave signals were significantly weaker and the velocities were much slower. The material that makes up the beads has a strong influence on the wave velocities as well. The initial compaction state also has some influence on the wave velocities.     

Research on molten flow simulation of 30t VOD ladle smelting stainless steel

30tVOD ladle smelting stainless steel was used as the process background, and the numerical and physical simulations were applied to investigate the flow and mixing characteristics of molten steel during the 30tVOD refining process. The cold experiment was employed to investigate the mixing behaviors of molten steel under the bottom and combined blowing systems. The flow features of molten steel were analyzed by numerical simulation with different blowing conditions. The results show that when the plug position of the ladle is located on 1/4R(R is the radius of ladle bottom), whether top- bottom blowing or bottom blowing, the molten steel is mixed well, and mixing time compared to the central position is reduced by 46% and 14% respectively. In addition, the comparison of flow field and turbulent kinetic energy of the molten steel show that when the bottom blowing position is 1/4R, the flow direction of the molten steel caused by the top blowing and the bottom blowing is better near the bath surface, and active volume ratio of molten steel is higher than that of other blowing positions. Current results were validated in industrial VOD furnace, which indicates that 1/4R is regarded as the optimum injection position for combined stirring.     

Mathematical Modeling on Erosion Characteristics of Refining Ladle Lining with Application of Purging Plug

According to parameters of the refining ladle with argon bottom blowing, the mathematical model describing the erosion behavior of ladle lining materials was established, the flow process of molten steel and thermal transmission of ladle lining were coupled, and the erosion of ladle lining in the condition of blowing argon at the bottom was researched. It has been found that either single or double blowing is applied, the larger erosion rates are mainly distributed in the slag line and the area of ladle lining near purging plugs, and the erosion is accelerated with the gas flow rate increasing, so the areas with higher erosion rates of the lining should be enhanced to avoid early partial damage. The erosion rate of ladle lining with double blowing is larger and the distribution of erosion is obviously different as the gas flow rate is increasing; serious erosion areas are in the slag line region and the higher erosion areas are concentrated on the slag lining and extended toroidally. And, as the distance between the purging plug and the lining of ladle is shortened, the partial erosion is easy to deteriorate and the refractories in the area with higher erosion rate need specialized selection and design. Meanwhile, the purging plug should be located away from the lining wall under the condition of good blowing effect in order to avoid increasing of the partial erosion and shortening of the lining service life.     

吹气孔直径对钢包模型内流动的影响

采用相似比为1∶10的水模型研究了钢包底吹氩系统中吹气孔直径对钢液流动的影响,通过测量钢包中心面的速度场,得到流体流动随吹气孔直径的变化规律。研究结果表明,吹气孔直径在1~3 mm范围内,随吹气孔直径增加,气柱、液面和包壁附近的流体速度减小,整个钢包内速度场分布更均匀。随吹气孔直径增加,涡心坐标从(0.12,0.12)向(0.12,0.10)和(0.12,0.09)变化,涡心向上移动,横向移动不明显。随着吹气孔直径的增加,底部产生的气泡直径变大,混匀时间有所减小。     

Spout eyes formed by an emerging gas plume at the surface of a slag-covered metal melt

This article deals with the spout eyes developing, at the surface of a metal melt, in the ladle during argon stirring. Cold model experiments involving a mercury bath with an oil layer as slag and industrial experiments on a 350 t steel ladle have been carried out. The eye geometry as measured with a video technique is highly dynamic. The time average of the free surface area and the time fraction of complete coverage have been determined and are represented with dimensionless correlations.