Morphology of Two-Phase Layers with Large Bubbles

The understanding of formation and movement of bubbles nucleated during aluminum reduction is essential for a good control of the electrolysis process. In our experiments, we filmed and studied the formation of a bubble layer under the anode in a real-size air–water electrolysis cell model. The maximum height of the bubbles was found to be up to 2 cm because of the presence of the so-called Fortin bubbles. Also, the mean height of the bubble layer was found to be much higher than published previously. The Fortin bubbles were investigated more closely, and their shape was found to be induced by a gravity wave formed at the gas–liquid interface. In addition, large bubbles were always observed to break up into smaller parts right before escaping from under the anode. This breakup and escape led to a large momentum transfer in the bath.     

不同开槽阳极铝电解槽的节能性研究

基于有限体积方法,建立三维传统阳极、纵向开槽和横向开槽阳极铝电解槽非稳态数学模型,采用磁动力流体模型(MHD)中电势法计算电磁场,把电磁力作为动量方程的源项,通过流体体积函数(VOF)法追踪电解质-铝液界面的波动,用离散相模型(DPM)追踪气泡的运动路径.对比分析传统阳极、纵向开槽和横向开槽阳极铝电解槽中电解质-铝液界面波动和气泡分布情况.结果表明,纵向开槽阳极下电解质-铝液界面波动幅度小于横向开槽阳极下的电解质-铝液界面波动幅度,且都小于传统阳极下电解质-铝液界面波动幅度.纵向开槽阳极底部的气体体积分数最小.     

Experimental Study of the Morphology and Dynamics of Gas-Laden Layers Under the Anodes in an Air–Water Model of Aluminum Reduction Cells

The bubble layer formed under an anode and the bubble-induced flow play a significant role in the aluminum electrolysis process. The bubbles covering the anode bottom reduce the efficient surface that can carry current. In our experiments, we filmed and studied the bubble layer under the anode in a real-size air?Cwater electrolysis cell model. Three different flow regimes were found depending on the gas generation rate. The covering factor was found to be proportional to the gas generation rate and inversely proportional to the angle of inclination. A correlation between the average height of the entire bubble layer and the position under the anode was determined. From this correlation and the measured contact sizes, the volume of the accumulated gas was calculated. The sweeping effect of large bubbles was observed. Moreover, the small bubbles under the inner edge of the anode were observed to move backward as a result of the escape of huge gas pockets, which means large momentum transport occurs in the bath.     

非均一极距对稀土电解多相流影响的数值模拟及实验验证

为了研究极距非均一性对电解槽电、热、流场的影响, 引入一个函数D表示极距的非均一性, 建立了极距非均一性数学模型, 模拟了非均一性极距下电场、温度场的分布规律, 以及计算了非均一性极距下的流场、气泡和金属液滴的分布状况。通过研究4种不同极距非均一性D, 分析可得:当D在1~1.48范围内增大时, 阴极两侧极间电流分布变化逐渐不对称, 发热量减小, 气泡含量及紊流强度发生改变, 且阴极稀土金属液在滴落的过程中, 偏离电解槽中心线角度为0°~30°, 但此时电解槽槽况稳定, 可以落入槽底的坩埚内; 当D=1.73时, 偏离角度为43°, 槽况恶劣, 此时稀土金属便不能落入坩埚内, 需要更换新阳极, 以保证正常电解。本文通过等距压降法对阳极电流分布进行测量, 并对比分析了实际电流测量值与模拟电流值, 其计算误差为2.7%~5.6%, 因此通过数值模拟可以反映出极距非均一性对阳极电流分布不均的影响, 为优化实际生产中的流程操作, 提高电解效率, 降低能耗提供保障。      

Mathematical model of the movement of layers of metal during the rotary rolling of semifinished products made of hypereutectic silumins

A mathematical model has been constructed to describe the kinematics of the movement of metal on the surface of semifinished products made of hypereutectic silumin alloys as they undergo rotary rolling. The model is based on the dependence of the pitch of the helical lines of metal flow on the deformation ε of the product and the feed angle α. The pitch is minimal when ε = 50–55%, regardless of the feed angle α. The degree of deformation ε = 50–55% is optimum for ensuring that the unit number of cycles in which the semifinished product is loaded by the work rolls is such as to produce a fine-grained structure and maximize the ductility characteristics of hypereutectic silumins. __________ Translated from Metallurg, No. 6, pp. 57–59, June, 2006.     

Simulation of Magnetohydrodynamic Multiphase Flow Phenomena and Interface Fluctuation in Aluminum Electrolytic Cell with Innovative Cathode

A three-dimensional (3D) transient mathematical model has been developed to understand the effect of innovative cathode on molten cryolite (bath)/molten aluminum (metal) interface fluctuation as well as energy-saving mechanism in aluminum electrolytic cell with innovative cathode. Based on the finite element method, the steady charge conservation law, Ohm’s law, and steady-state Maxwell’s equations were solved in order to investigate electric current field, magnetic field, and electromagnetic force (EMF) field. Then, an inhomogeneous multiphase flow model of three phases including bath, metal, and gas bubbles, based on the finite volume method, was implemented using the Euler/Euler approach to investigate melt motion and bath/metal interface fluctuation. EMF was incorporated into the momentum equations of bath and metal as a source term. Additionally, the interphase drag force was employed to consider different phase interactions. Thus, present work owns three main features: (1) magnetohydrodynamic multiphase flow are demonstrated in detail both in aluminum electrolytic cell with traditional cathode and innovative cathode; (2) bath/metal interface fluctuation due to different driving forces of gas bubbles, EMF, and the combined effect of the two driving forces is investigated, which is critical to the energy saving; and (3) the effect of innovative cathode on melt flow and motion of gas bubbles. A good agreement between the predicated results and measurement is obtained. The velocity difference leading to the melt oscillation decreases due to more uniform flow field. The average velocity of metal in the cell with innovative cathode decreases by approximately 33.98 pct. The gas bubbles in the cell with innovative cathode releases more quickly under the effect of protrusion on the cathode. The average bubble release frequency increases from 1.1 to 1.98 Hz. Hence, the voltage drop caused by gas bubbles would decrease significantly. In addition, the two large vortices are broken into many small vortices due to the protrusion. The final disappearance of the small vortices as a result of viscous dissipation is conducive to the suppression of bath/metal interface fluctuation. The average interface amplitude in the cell with innovative cathode reduces to 75.95 pct of that in the cell with traditional cathode.     

稀土电解槽气液两相流动数值模拟

利用CFD软件,建立了稀土电解槽的阳极气泡及熔体整体流场数学模型。对稀土电解槽的阳极表面化学反应自动生成气体的流场进行了数值模拟,得出了电解槽的熔体整体流场分布图及在不同位置气体浓度分布曲线图,使之更贴近于电解槽的实际工况,为熔盐稀土电解槽的槽型优化提供依据。     

NiFe_2O_4基惰性阳极的润湿性及气泡析出行为

利用座滴法和双室透明电解槽对NiFe2O4基惰性阳极的润湿性和气泡析出行为进行研究。结果表明,电解质对NiFe2O4基惰性阳极的润湿性要优于碳素阳极。在低电流密度情况下电解,阳极气泡的析出是一个动态过程,它先在阳极表面形核,以球形方式长大,小气泡在长大过程逐渐汇聚偏移,然后逸出。惰性阳极上析出的气泡尺寸比碳素阳极小,在阳极上的逗留时间也更短。大电流密度情况下,气泡的生成速度加快,尺寸降低,很难准确测量气泡的直径。     

The Effect of Detaching Bubbles on Aluminum-Cryolite Interfaces: An Experimental and Numerical Investigation

Because of gas-induced flow in commercial aluminum reduction cells, deformations of the cryolite–aluminum interface will appear close to the side channel of the cell. In the present work, the dynamic nature of this phenomena is studied both in experiments and with help of a numerical model in the commercial computational fluid dynamics (CFD) code, FLUENT. Experiments are conducted in a full scale, two dimensional oil–water half-anode model, quantifying interfacial deformations over various operating conditions. Besides data on interfacial deformations, the experiments confirm previously published data on bubble motion under inclined anodes. The oil–water model is studied further with a three-fluid CFD model by means of a transient volume of fluid method. The model presented is verified and validated against experimentally measured data, yielding promising results. The CFD model is generalized further to realistic parameters for the Hall–Héroult cell. Averaged results from the transient model are compared with steady state calculations, showing good agreement. Average deformations up to 20 mm are observed under normal operating conditions. The transient approach shows that the dynamic behavior of the aluminum–cryolite interface is significant and is the same magnitude of the average deformations.     

On the mechanism of the anode effect in aluminum electrolysis

The dependency of the critical current density in aluminum electrolysis on the bulk concentration of alumina has been reported in various forms. Some workers found different relationships in restricted ranges of the alumina content and concluded on possible changes of the reaction mechanisms and various types of anode effect. A previously developed mathematical model could show that the anode effect is initiated as the actual current density equals the limiting one. The model is now applied to check some of the available theories. Comparison with experimental data shows that the varying effect of the alumina concentration can be described by a single relationship taking account of the combined action of mass transfer, fluid dynamics of gas release, and wettability for all values of the alumina content. The results suggest that there is every reason for the view that only one process occurs. A distinction of various types of mechanisms provoking the anode effect is unnecessary.     

Turbulence structure of bottom-blowing bubbling jet in a molten Wood’s metal bath

The coherent structure of turbulence in a vertical He-Wood’s metal bubbling jet formed in a cylindrical vessel was investigated using the four-quadrant classification method. Turbulent motions of molten Wood’s metal flow were classified into four distinct categories: ejection (higher-momentum fluid motion, directed outward), sweep (lower-momentum fluid motion, directed inward), outward interaction (lower-momentum fluid motion, directed outward), and inward interaction (higher-momentum fluid motion, directed inward). The relative occurrence in frequency of each turbulent motion and the contributions of each turbulent motion to the axial and radial turbulence kinetic energies and Reynolds shear stress were determined. These quantities were different from their respective values in an air-water vertical bubbling jet. Such differences were found to be closely associated with the fact that the shape and size of bubbles differs significantly between the two bubbling jets. Consequently, the coherent structure of turbulence in a bubbling jet is strongly dependent on the behavior of the wake behind the bubbles.     

Modeling of molten metal flow in a continuous casting process considering the effects of argon gas injection and static magnetic-field application

A mathematical model has been developed to analyze molten metal flow, considering the effects of argon gas injection and static magnetic-field application in the continuous casting process. The k-ɛ turbulence model is used to calculate the turbulent variables. A homogeneous fluid model with variable density is employed to tackle the molten metal-argon gas flow. The electromagnetic force is incorporated into the Navier-Stokes equation, and the effects of boundary conditions of the magnetic field on the velocity distribution near the mold wall are included. A good agreement between the numerically obtained flow-field results and measurements is obtained. The argon gas injection changes the molten metal flow pattern, mainly in the upper portion of the mold. By applying the magnetic field, values of the averaged velocity field in the bulk decrease significantly, and, especially at the top free surface, they become very small, which can cause meniscus freezing. When magnetic-field application and argon gas injection are used together, the external flow field out of the gas plume is significantly suppressed; nevertheless, flotation of gas bubbles is still active and is not affected directly by the magnetic field. Although the penetrating length of the gas plume is shortened, the argon gas bubbles in molten steel still cause fluctuation at the top free surface, which prevents the occurrence of freezing.     

Motion dynamics of anodic gas in the cryolite melt–alumina high-temperature slurry

The results of physical simulation of the behavior of bubbles formed due to the electrochemical evolution of oxygen on an inert anode during the high-temperature electrolysis of alumina slurry in the fluoride melt are presented. Similarity criteria are calculated, the experiments for a water model with vertically oriented electrodes are performed, and the data on the behavior of bubbles in the slurry are found with the help of video recording. The 20% aqueous solution of sulfuric acid with an alumina content of 30 vol % was used as the model electrolyte. The experiments were performed in a range of current densities from 0.05 to 0.25 A/cm². Video was recorded using a Nikon D3100 camera with a recording frequency of 30 frames/s. The data on the motion dynamics of the bubbles, the quantitative data that characterize coalescence, and the bubble lifting velocity are found. To determine the average lifting velocity, 125 bubbles were analyzed. They were 0.8–2.3 mm thick. The bubble motion is performed in the slug regime with lifting velocity of 1.0–2.3 cm/s. The bubble layer thickness was about 5 mm. Further investigations will be directed to finding new data on the behavior of bubbles for various solid phase contents, current density, electrode slope angle, and granulometric composition.     

超混沌电流对金属锰电解阳极电位振荡的调控

金属锰湿法电冶过程是一个典型的远离平衡态的非线性体系,直流作用下会出现电化学振荡、金属分形等非线性行为而引发体系额外的能耗。本文提出一种超混沌电流电解的新模式,通过引入超混沌电路代替原有直流电源来实现。超混沌电流作用下,采用恒电流极化曲线、阳极极化曲线、塔菲尔测试等分析方法和X射线衍射分析、扫描电子显微镜的表征方法,研究铅合金阳极电化学振荡行为与阳极沉积的锰氧化物之间的关联。研究结果表明,在电流密度为350 A·m?2恒电流极化30 min后,超混沌电流极化作用下电位振荡的平均振荡周期较直流极化提高5.6 s,平均振幅降低 38 mV;超混沌电流作用下阳极生成的MnO₂,其表面较为致密平整,在一定程度上可以提高铅合金阳极析氧反应活性和耐腐蚀性。综合分析可知,将超混沌电流运用于金属锰电解过程,可以实现对阳极电化学振荡的有效调控,为进一步降低电解过程能耗和污染排放提供新思路。      

柳钢板坯连铸结晶器流场优化

结合柳钢炼钢厂3号板坯连铸机的工艺参数,采用水力学模型物理模拟,通过测量结晶器液面波高和注流冲击深度,研究了浸入式水口侧孔形状、侧孔倾角、插入深度和拉速对结晶器流场的影响规律;采用数值模拟,分析了优化后结晶器的流场特征及钢液面的运动速度.结果表明:水口侧孔形状为椭圆形,侧孔倾角为18°,插入深度为160 mm时,在中高拉速下都能获得合理的结晶器流场.应用生产后,铸机年平均漏钢次数下降了43.1 %,铸坯表面清理率下降了44.9 %,冶金效果明显.     

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.     

Behaviour of Argon Gas Bubbles in an Electromagnetic Driven Swirling Flow

An innovative steelmaking process is suggested using an electromagnetic driven swirling flow in the up‐leg of an RH vacuum degassing vessel. The effectiveness of this new process depends on the two‐phase flow behaviour of molten steel and argon gas. A physical and a mathematical model are developed to understand the effect of electromagnetic driven swirling flows on the behaviour of gas bubbles in the up‐leg of an RH vessel. Both water model experiments and numerical simulation show the distribution and trajectories of the gas bubbles. The gas bubbles’ trajectories are spiral and move towards the centre of the up‐leg in the swirling flow field. The accumulation of gas bubbles depends on the swirling number. At the same time, the swirling flow can prolong the residence time and trajectories of non‐metal inclusions in the vessel. The viscous drag force becomes important for small bubbles in the RH degassing vessel, and small bubbles have the trend to rotate with the swirling flow.