On Effective viscosity models for gas-stirred ladle systems

Metallurgical and Materials Transactions B -     

钢包底吹氩均混时间及临界流量水模型实验

针对钢包底吹氩工艺,通过改变透气砖数量、单透气砖吹气位置、双透气砖夹角、喷吹气体流量、渣厚等参数,对钢包的均混时间进行了水模型实验研究.提出临界流量的概念,发现吹气量超过临界流量后均混时间明显减小.结果表明:单透气砖喷吹时,相同吹气量下偏心喷吹时的均混时间比中心喷吹时短,临界流量小;双透气砖喷吹时,透气砖夹角越大,均混时间越短,临界流量越小.     

Critical fluid-flow phenomenon in a gas-stirred ladle

Measurement of the velocities of bubbles and liquid with a two-element electroresistivity probe and laser-Doppler velocimeter, respectively, during bottom injection of air into a water bath, has confirmed the existence of a critical gas-injection rate. Above the critical flow rate, the change of axial bubble velocity in the air jet, and of liquid velocity with increasing volume flow rate, diminishes markedly. The existence of the critical flow rate is explicable from high-speed motion pictures of the vertical gas jets, which reveal four zones of gas dispersion axially distributed above the orifice: primary bubble at the orifice, free bubble, plume consisting of disintegrated bubbles, and spout at the bath surface. With increasing gas-injection rate, the free-bubble zone expands such that the point of bubble disintegration rises closer to the bath surface. Above the critical flow rate, the free bubbles rise with minimal breakup and erupt from the bath surface with maximum energy discharge. The combined Kelvin-Helmholtz, Rayleigh-Taylor instability theory has been applied to analyze the bubble breakup in the bath and the critical gas-injection rate in a gas-stirred ladle. The criterion for the critical diameter of bubble breakup has been found to depend primarily on the surface tension and density of the liquid. In the analysis, the propagation time of a disturbance on a bubble surface at the “most unstable” wave number has been compared with the bubble rising time in the bath in order to determine the critical gas-flow rate. The predicted critical values are in close agreement with the measured results. M. ZHOU formerly was Post Doctoral Fellow with the Centre for Metallurgical Process Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 J.K. BRIMACOMBE holds the Alcan Chair in Materials Process Engineering     

Numerical modelling of the transport and removal of inclusions in an industrial gas-stirred ladle

A full-scale, three-dimensional, transient CFD modelling approach capable of predicting the three-phase fluid flow characteristics and the inclusion removal in a gas-stirred ladle was developed. The comparison with experimental data indicates that this model can accurately predict the multiphase fluid flow and slag eye behaviour. The transport and removal of the inclusions in the gas-stirred ladle were predicted by tracing the movement of individual inclusions through computing their particle trajectories and considering a fluctuant top slag layer. The effects of inclusion size, gas flow rates, and injected bubble diameters as well as various removal mechanisms including slag capture, bubble attachment, and ladle wall adhesion on the removal of inclusions were investigated. It is shown that the slag capture is the prevailing mechanism for inclusion removal and the gas flow rate is the most important parameter for enhancing the inclusion removal efficiency.     

Model experiments on mixing phenomena in gas-stirred melts

Mixing experiments were performed in a water model system of a gas-stirred ladle with both an optical decolourization method and conductivity measurements at different positions within the vessel. The experimental results show that the rate by which the concentration is homogenized depends on the location of the measuring probe as well as on the used stirring conditions. Using a centric nozzle at the bottom pronounced dead zones exist. The equalization is slowest between these dead zones and the remaining volume. The mixing process in the remaining volume is characterized by a circulating concentration cloud in which mixing takes place by random turbulent diffusion. With an eccentric nozzle arrangement dead zones can be avoided and thus, mixing times decrease and become more similar for different positions.     

Numerical computations of fluid flow and heat transfer in a gas-stirred liquid bath

The flow and temperature fields due to bottom air injection in a cylindrical vessel containing water were numerically analyzed. The Eulerian approach was used for the formulation of both the continuous and the dispersed phases. The computational domain was extended beyond the undisturbed height of the liquid in the bath to accommodate practical gas injection systems. Turbulence in the liquid phase was modeled using a two-equationk- ε model. Interphase friction and heat transfer coefficients were calculated by using correlations available in the literature. The general-purpose computer program PHOENICS was employed to predict the velocity, vol-ume fraction, and the temperature fields of each phase. Turbulent dispersion of the phases was modeled by introducing a “dispersion Prandtl number.” The predicted flow fields were com-pared with experimental measurements available in the literature. The results are of interest in the design and operation of a wide variety of material processing operations.     

LF精炼炉盖内惰性气氛的流动模拟

采用耦合数值求解的方法,计算并分析了LF精炼炉盖内气体流动的速度分布状态以及液面附近惰性气体氩(Ar)的流动行为和分布.结果表明:露弧加热期和埋弧加热期炉盖的合理抽气压力分别为-200~-250 Pa和-120 Pa;底吹氩气量过小不能在液面上部弥散,而增大到20~50 m³·h^-1,在液面附近弥散且流动分布状态相似,有利于液面附近惰性气氛的保持;液面距钢包上边缘的距离增大,氩气在液面上部回旋的区域扩大,可防止钢液增[N]或[O].     

Study of the deoxidation of steel with aluminum wire injection in a gas-stirred ladle

In the present work, the deoxidation of liquid steel with aluminum wire injection in a gas-stirred ladle was studied by mathematical modeling using a computational fluid dynamics (CFD) approach. This was complemented by an industrial trial study conducted at Uddeholm Tooling AB (Hagfors, Sweden). The results of the industrial trials were found to be in accordance with the results of the model calculation. In order to study the aspect of nucleation of alumina, emphasis was given to the initial period of deoxidation, when aluminum wire was injected into the bath. The concentration distributions of aluminum and oxygen were calculated both by considering and not considering the chemical reaction. Both calculations revealed that the driving force for the nucleation fo Al₂O₃ was very high in the region near the upper surface of the bath and close to the wire injection. The estimated nucleation rate in the vicinity of the aluminum wire injection point was much higher than the recommended value for spontaneously homogeneous nucleation, 10³ nuclei/(cm³/s). The results of the model calculation also showed that the alumina nuclei generated at the vicinity of the wire injection point are transported to other regions by the flow.     

底吹钢包用圆孔透气砖冶金效果的水模型

采用水力学模型方法对钢包底吹氩工艺进行了实验研究,对比分析了圆孔型透气砖与狭缝式透气砖对冶炼效果的影响,并研究了圆孔透气砖的孔角和孔径对混匀时间、夹杂物去除率和渣眼面积的影响规律.结果表明:当吹气流量相同时,使用圆孔斜通透气砖时,钢包的混匀时间、夹杂物去除率和渣眼面积均优于狭缝式透气砖;相比于圆孔直通透气砖的钢包,使用圆孔斜通透气砖的钢包混匀时间更短,去除夹杂物效果佳,但渣眼面积略大;对于圆孔斜通透气砖,其孔径越小,钢包混匀时间越短,夹杂物去除率越高,渣眼面积越小.     

蓄热式钢包烘烤流动与传热过程数值模拟

采用ANSYS CFX10.0商业软件对蓄热式钢包烘烤燃烧、流动、传热耦合过程进行模拟计算,给出钢包内部的气流和温度分布情况,分析了不同空气预热温度对钢包烘烤温度、火焰辐射强度的影响,计算结果表明蓄热式钢包烘烤更有利于提高钢包温度及其均匀性。     

Numerical modelling of fluid flow and desulphurisation kinetics in an argon-stirred ladle furnace

A full-scale, three-dimensional, transient CFD modelling approach capable of predicting the three-phase fluid-flow characteristics and desulphurisation behaviour in an argon-stirred ladle was developed. The model can accurately predict the molten steel flow and slag eye behaviour. The predicted sulphur content in ladle as a function of time agrees well with the experimental data. The effects of the initial sulphur content, the gas flow rate and the slag layer thickness on the desulphurisation efficiency were also investigated. The predicted results show that the desulphurisation efficiency improves with the increase of the initial sulphur content, the gas flow rate and the slag layer thickness. Higher gas flow rate can improve the slag–steel interaction, which, in turn, helps improving the desulphurisation rate. The thinner the slag layer, the larger the slag eyes and the smaller the interfacial area between the slag and steel phases. The consequence is the decrease in the desulphurisation rate.     

Scaling estimations of thermal and flow field in gas-stirred ladles

To obtain a better understanding of the physical process involved in gas stirring of a steelmaking vessel, a scaling analysis approach is developed that accounts for the effects of natural convection and axisymmetric bottom gas injection in the vessel. The orders of magnitude of some important quantities such as the transient velocity scale, thermal boundary layer thickness, and the critical flow rate to homogenize the thermal stratification in the molten steel are predicted successfully.     

Fluid flow and bath temperature destratification in gas-stirred ladles

The transient fluid flow and temperature distributions in argon-stirred ladles have been investigated. The governing equations of unsteady fluid flow and energy were solved numeri-cally with a control-volume technique, while the turbulence was modeled by the two-equationk- ∃ model. The two-phase zone was described by novel experimental equations, which char-acterize the gas-fraction distribution in the bath for a wide range of variables in both aqueous and liquid metal systems. Fully transient computational results are presented and compared against transient temperature computations based on a steady-state velocity field. The resulting mixing times compare closely with industrial experience. A.H. Castillejos E., formerly Postdoctoral Fellow, The University of British Columbia,.     

钢包炉电磁搅拌磁场与流场的数值模拟

钢包炉电磁搅拌(electromagnetic stirring in ladle furnace,LF-EMS)是钢包精炼过程中的一种重要的搅拌方式.对某厂90 t钢包炉电磁搅拌器的磁场与流场的数值模拟结果表明,钢液在电磁力的作用下,形成一个大的单一循环流动,钢包内的速度和湍流动能分布均匀.     

100 t底吹氩钢包流场数值模拟及现场实践

为了研究国内某钢厂100 t双孔底吹氩钢包的流场现状,针对此钢包精炼过程流量200 L/min(方案1)和300 L/min(方案2)的钢水流动行为进行了数值模拟与现场实践。结果表明,每个吹氩流股均会在钢包形成两个循环流,分别位于流股与钢包壁之间和两个流股之间;方案2比方案1混匀时间缩短36 s,渣眼平均直径增大 0.055 m,耐火材料侵蚀加重。数值模拟通过现场实践验证了其正确性,为现场生产提供了有效指导。     

纯氧助燃预热铁包中废钢过程气流特性的数值模拟

低热值煤气铁包内燃烧预热废钢是提高转炉废钢入炉量的有效措施之一.基于标准k-ε湍流模型耦合涡-耗散燃烧模型,针对铁包内转炉煤气纯氧燃烧过程进行数值模拟研究,探讨气流分布特性及其对烘烤效果影响.结果表明,纯氧燃烧时高温气流速度大,从料堆中心进入、边缘排出,换热距离大,可有效提高低热值煤气燃烧的热利用效率;燃烧过程中铁包盖...     

钢包内气泡行为的数值模拟

气泡行为是钢水精炼过程中最重要的现象之一,用数值方法计算气泡在钢液中的运动,运用拉格朗日法跟踪气泡的运动轨迹,且应用四阶龙格库塔法求解气泡的速度变化,气泡的长大过程.