Mathematical Modeling of Fluid Flow in Bath during Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Application of the Model and Results

The mathematical model developed for the molten steel flow in the combined side and top blowing AOD refining process of stainless steel has been used to compute and analyze the flow fields of the liquid phases in the baths of the 120 t AOD converter and its water model unit with a 1/4 linear scale. The influence of the side tuyere number and the angle between each tuyere on the flows has been examined. The results demonstrate that the mathematical model can quite reliably and well model and predict the fluid flow in an AOD bath with the combined blowing. The liquid flow in an AOD converter bath with the combined blowing is resulted from the gas side blowing streams under the influence of a gas top blowing jet. The streams play a governing role on it; and the liquid in the whole bath is in active agitation and circulatory motion during the gas blowing process. The gas jet from the top lance does not change the essential features of the gas stirring and liquid flow in the bath, but can make the local flow pattern of the bath liquid obviously vary and its turbulent kinetic energy enhance. The changes in the tuyere position and number have similarly not altered the basic characteristics and patterns of the gas agitation and liquid flow and turbulent kinetic energy distribution in the bath. At a given tuyere number and gas side blowing rate or a given angular separation between each tuyere and gas side blowing rate, however, the variation of the angle between each tuyere or the tuyere number can locally change them. Using 6 tuyeres with 27° can reach the more uniform flow field and turbulent energy distribution of the liquid in the bath than taking 7 tuyeres with 18° or 22.5° and 6 tuyeres with 22.5°.     

Study on Mass Transfer Characteristics in an AOD Converter Bath under Conditions of Combined Side and Top Blowing

The mass transfer characteristics in a steel bath during the AOD refining process with the conditions of combined side and top blowing were investigated. The experiments were conducted on a water model unit of 1/4 linear scale for a 120‐t combined side and top blowing AOD converter. Sodium chloride powder of analytical purity was employed as the flux for blowing, and the mass transfer coefficient of solute (NaCI) in the bath was determined under the conditions of the AOD process. The effects of the gas flow rates of side and top blowing processes, the position arrangement and number of side tuyeres, the powdered flux particle (bubble) size and others on the characteristics were examined. The results indicated that, under the conditions of the present work, the mass transfer coefficient of solute in the bath liquid is in the range of (7.31×10^?5‐3.84×10^?4) m/s. The coefficient increases non‐linearly with increasing angle between each tuyere, for the simple side blowing process at a given side tuyere number and gas side blowing rate. The gas flow rate of the main tuyere has a governing influence on the characteristics, and the gas jet from the top lance decreases the mass transfer rate, the relevant coefficient being smaller than that for a simple side blowing. Also, in the range of particle (bubble) size used in the present work and with all other factors being constant, raising particle (bubble) size increases the coefficient. Excessively fine powder particle (bubble) sizes are not advantageous to strengthening the mass transfer. With the oxygen top blowing rate practiced in the industrial technology, the side tuyere arrangements of 7 and 6 tuyeres with an angular separation of 22.5° and 27° between each tuyere, as well as 5 tuyeres with an angle of 22.5° between each tuyere can provide a larger mass transfer rate in the bath. Considering the relative velocity of the particles to the liquid, the energy dissipation caused by the fluctuation in the velocity of the liquid in turbulent flow and regarding the mass transfer as that between a rigid bubble and molten steel, the related dimensionless relationships for the coefficient were obtained.     

Water modelling study of fluid flow and mixing characteristics in bath during AOD process

Abstract

The fluid flow and mixing characteristics in the bath during the argon–oxygen decarburisation (AOD) process have been investigated on a water model of an 18 t AOD vessel blown through two annular tube type lances of constant cross-sectional area. The geometric similarity ratio between the model and its prototype (including the lances) was 1 : 3. Based on theoretical calculations of the parameters of the gas streams in the lances, the gas blowing rates used for the model were determined fairly precisely. Thus, sufficiently full kinematic similarity between the model and its prototype was ensured. The influence of the gas flowrate and the angle included between the two lances was examined. The results demonstrated that the liquid in the bath underwent vigorous circulatory motion during blowing, and there was no obvious dead zone in the bath, resulting in excellent mixing and a short mixing time. The gas flowrates, particularly that of the main lance, had a key influence on these characteristics. However, the gas jet of the sublance had a physical shielding effect on the gas jet of the main lance, and mixing efficiency could be improved by a suitable increase in the gas blowing rate of the sublance. The angular separation of the two lances also had a marked influence on the flow and mixing in the bath. An excessively large or small separation of the two lances would reduce the stability of blowing and would also be unfavourable to mixing. The optimum range of separation is 60–100° under the conditions of the present work. The relationships between the mixing time and the gas blowing rate, the stirring energy, the modified Froude numbers for the main lance and sublance, the lance arrangement, etc. have been obtained.     

Numerical Simulation of Fluid Flow in Bath during Combined Top and Bottom Blowing VOD Refining Process of Stainless Steel

The fluid flow in a bath in combined top and bottom blowing vacuum‐oxygen decarburization (VOD) refining process of stainless steel has numerically been simulated. The three‐dimensional mathematical model used is essentially based on that proposed in our previous work for the flow in combined side and top blowing argon‐oxygen decarburization (AOD) process, but considering the influence of reduced ambient pressure. Applying it to the flow in the bath of a 120 t VOD vessel under the refining conditions, the results present that the model can fairly well simulate and estimate the flow phenomena. The flow pattern of molten steel in the bath with the combined blowing is a composite result under the common action of the jets from a three‐hole Laval top lance and gas bottom blowing streams. The jets have a leading role on it; the molten steel in the whole bath is in vigorous stirring and circulatory motion during the blowing process. The streams do not alter the basic features of the gas agitation and liquid flow, but can evidently change the local flow pattern of the liquid and increase its turbulent kinetic energy to a certain extent. The flow field and turbulent kinetic energy distribution in the combined blowing with three tuyeres are more uniform than those in the blowing with double tuyeres. Increasing properly the tuyere eccentricities is of advantage for improving the velocity and turbulent kinetic energy distributions, the stirring and mixing result in the practical VOD refining process.     

Bath mixing behaviour in top–bottom–side blown converter

Abstract

The influence of blowing process parameters on bath stirring was investigated in a model of a top–bottom–side blown converter using physical modelling experiments. It was shown that the side blowing gas flowrate has an important influence on bath mixing time which decreases as side tuyere gas flowrate increases up to a critical flowrate and then plateaus. Bottom gas injection is favourable for bath mixing for top–bottom–side blown converters; however, top lance height, top gas flowrate and bath level have little influence.     

Mathematical Modeling of Fluid Flow in Bath during Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Mathematical Model of the Fluid Flow

Considering that the liquid flow field under the conditions of the combined side and top blowing would be a combined result from the common action of the side blowing gas streams and a gas top blowing jet, as the first attempt, the three‐dimensional mathematical models for the flows of molten steel in an AOD converter bath during the simple side and top blowing processes have been proposed and developed, respectively. And the mathematical model of the flow in the bath during the combined blowing AOD refining process of stainless steel has been given by the composition and superposition of the two models. In the composed model, the gas‐liquid two‐phase flow is described and treated in terms of the two‐fluid (Eulerian‐Eulerian) model. The especially modified two‐equation k?ε model for the turbulence in the liquid phase is employed. And, the surface of the sunken pit formed by impact of the gas jet blown from a top lance at the central location of the bath liquid surface is regarded as a revolution paraboloid. The related details of the composed model are shown.     

电弧炉氧枪搅拌特性及熔池混匀效果的试验研究

采用1∶10水模型对100t电弧炉炉壁氧枪搅拌特性和熔池混匀效果进行了试验研究。研究了炉壁氧枪水平角、垂直角、气体流量及氧枪布置等对熔池各区域搅拌效果和混匀时间的影响。试验结果表明,相对于垂直角而言,水平角对熔池各区域的流动和混匀有更为显著的影响。熔池混匀时间随气体流量的增大而减小,但气体流量增大会导致熔池喷溅量增大,且熔池液体对炉底的冲刷增强。对比两种不同的氧枪布置方案,采用原型分布且氧枪水平角为10°时,整个熔池具有最好的混匀效果。     

Physical and mathematical models of gas‐liquid fluid dynamics in LD converters

Fluid dynamics of gas‐liquid interactions in a LD converter to refine steel was physically and mathematically simulated. Using a water model three cases of gas supply were considered, top blowing, bottom injection and combined process top blowing‐bottom injection. Mixing time in top blowing increases with bath height and the distance between the lance of the gaseous jet and the bath surface. The jet penetration was found to be dependent on the modified Froude number. The unstable and unsteady behaviour of the bath topography, as affected by the gaseous jet, was well simulated through a multiphase momentum transfer model. In top blowing, three zones of liquid splashing were found, penetration with low splash, heavy splash and dimpling with low splash intensity. These zones depend on the gas flow rate and the distance from the lance to the bath surface. During bottom injection mixing times decrease with the number of tuyères, increases of bath height and gas flow rate. In a combined process mixing time decreases considerably due to the recirculating flow formed by the action of the top jet and the submerged jets. When a submerged jet is located just below the top jet the mixing time does not decrease as compared with the separated processes either top blowing or bottom stirring.     

180 t转炉聚合射流流场的水力学模型的实验研究

采用几何相似比1∶10水模型对180 t顶底复吹转炉内射流与熔池相互作用进行模拟试验,研究了在最佳枪位(150 mm)时氧气流量(38~42 m³/h)对均混时间的影响以及最佳顶枪流量(39 m³/h)下聚合射流氧枪枪位(40~150 mm)对均混时间的影响。结果表明,聚合射流氧枪对熔池的搅拌效果完全能达到顶底复吹的搅拌效果,如能在转炉冶炼工艺中应用,可取消底吹系统,简化转炉设备,提高转炉炉龄。     

Model study of mixing and mass transfer rates of slag-metal in top and bottom blown converters

Water model experiments have been conducted to clarify mixing rates of molten steel and mass transfer rates between slag and metal in LD and Q-BOP furnaces using six different circular tuyere arrangements. Splashing and ‘spitting’ were also examined with a view to finding a quiet bath with minimum mixing time and maximum mass transfer rate. Froude’s similarity criterion was fulfilled to determine gas flow rate and bath depth. Complete mixing time of water determined by tracer technique had been 0.9 second to 1.8 seconds for Q-BOP as compared to 6 seconds to 13 seconds for LD. This shows that the stirring intensity in Q-BOP is remarkably larger than that of LD. A simple relationship τ = 5.9(Q/N) ^−0.49 was obtained with gas flow rateQ and number of tuyereN. This indicates that flow rate of gas per tuyere should be intensified to realize better mixing. Mass transfer coefficient K_Ba for bottom blowing was found to be almost double that for top blowing. Of all the tuyere configurations studied for Q-BOP’s, a half circular tuyere arrangement was found to be the best considering all aspects of mixing, mass transfer, and bath agitation.     

180 t转炉底吹和顶-底复吹射流与熔池作用的水模型研究

根据180 t转炉的实际生产情况,以修正的Froude准数为相似准数,建立几何相似比10 ： 1水模型,进 行了四孔对称单纯底吹试验,并在最佳的底吹工艺参数下(底吹最佳位置为喷嘴所在同心圆直径:转炉熔池直径= 0. 3处;最佳流量为0. 7 m³/h,均混时间18. 2 s),通过改变顶吹氧枪的气体流量和吹炼枪位进行了顶底复吹转炉射 流与熔池作用的试验。结果表明,在底吹条件下,增加顶吹工艺(最佳枪位150 mm,最佳流量39 m³/h),熔池平均 的均混时间减少了 5.6 s, 180 t转炉顶底复吹可显著提高经济效益。     

LBE转炉优化吹炼工艺参数的水模实验研究

根据相似原理 ,采用水模实验方法 ,以优化LBE转炉吹炼工艺为目的 ,研究了影响熔池搅拌的各工艺参数 ,主要包括顶枪枪位、底吹位置布置方案、底吹气体流量、顶吹气体流量与混匀时间的关系     

Design and calculation of gas property parameters for constant area lance under conditions of friction flow with heating

Abstract

Formulae for calculating the outlet property parameters of gas heating and friction streams in tubular and annular type lances with constant area (tuyeres) are given, and have been applied to the case of an annular type used for an AOD (argon–oxygen decarburisation) vessel of 18 t capacity. The distributions of both the inner wall temperatures of the tuyere and the gas stagnation temperatures along its length have been more reasonably fixed. The friction factors for the gas flows through the main and subtuyeres during blowing refining have been determined by comparison of the practically measured P–Q relationships with the results from trial calculations. The outlet parameters of the gas streams for the central tube (main tuyere) and annular slit pipe (subtuyere) of the tuyere have been calculated. The influences of the gas supply pressure, the length and diameter of the tuyere, and the type and composition of the gases, as well as the heating effect, on the gas outlet parameters have been considered. The results obtained may be expected to offer useful information and a reliable basis for tuyere design and determination, control, and optimisation of the gas blowing parameters and technology, as well as for the investigation of hydraulic modelling of the blowing processes.     

复吹转炉钢-渣间容量传质系数的水-油模型

在熔池直径880mm、深258mm的冷态模型中，利用水模拟钢水、机油模拟渣、苯甲酸模拟钢-渣间传输物质来研究熔池直径5285mm、深1545mm的复吹转炉吹炼工艺参数对钢-渣间容量传质系数的影响。结果表明，枪位350mm，顶吹流量117m^3/h，底吹流量1．68m^3/h至2．36m^3/h时，传输物质苯甲酸的容量传质系数变化最显著，在采用枪位350mm，顶吹流量140m^3/h，底吹流量1．68m^3/h，底枪布置方式为8孔对称布置在0．66直径的圆上时，传输物质的容量传质系数最大。     

Splashing mechanism in combined blowing

Abstract

In combined BOF blowing, lance parameters and the combination of bottom or side wall tuyeres have an influence on splashing behaviour. The aim of this study was to clarify the interaction of the lance jet cavity with the bottom blowing plume and the side wall blowing jet and to determine its effects on splashing. According to the water model tests, three basic axioms existed in the combined blowing. First, when the bottom tuyere (or side wall tuyere) was located exactly beneath the lance jet, the lowered cavity turned the direction of splashes to lower trajectories. Second, the total amount of splashing was constant and the splashing peak was generated on the wall above the bottom plume. Third, both the plume and the side wall jet formed a so called protected zone beyond it. The model experiments showed clearly that the combination of bottom tuyeres and interaction of cavities and plumes play a very important role in splash generation in real converters.     

Effects of process conditions on mixing between molten iron and slag in smelting reduction vessel via water model study

Abstract

The present study has been conducted to investigate the effects of operating conditions, which include gas flowrate, tuyere size, tuyere number, and height of iron phase, on the extent of mixing between molten iron and molten slag in the direct iron ore smelting reduction process. A transparent acrylic water model, 30% of the size of the actual smelter, was constructed to study the mass transfer phenomena. In the water model, water and spindle oil were used to simulate molten iron and molten slag, respectively, while air was used to replace the bottom blown nitrogen gas. In addition, thymol (C₁₀H₁₄O₆) was used as a tracer material in the water model, added to the water at the beginning of the experiment. As mixing between water and spindle oil proceeded owing to stirring by the bottom blown gas, the concentration of thymol in the water decreased and that in the spindle oil increased. Water samples were taken from the bottom and 12 cm above the bottom of the water model at various operating times. Concentrations of thymol were then measured using a diode array ultraviolet visible spectrophotometer. By analysing the concentration data, the mass transfer rate k_wA, which is a direct index for evaluating the mixing efficiency, could be derived. The process conditions under investigation included 40-500 L min^-1 gas flowrate, 0·3^-1 cm tuyere size, four or five tuyeres, and 20-30 cm height of the water phase. The test results indicate that when the gas flowrate increases, the value of k_wA increases, which indicates better mixing between oil and water phases. However, as the gas flowrate approaches 40 L min^-1, the improvement becomes less obvious. The smaller tuyere gives better mixing, and the design of five tuyeres results in better mixing compared with four tuyeres when they are blown with the same total gas flowrate. However, mixing efficiency decreases with increased height of the water phase. Also, as the gas flowrate of bottom blowing approaches 40 L min^-1, gas blowing from the top has little effect on the mixing behaviour in the liquid bath. For a four tuyere system, the process conditions of height of oil phase 5 cm, height of water phase 25 cm, diameter of tuyere 0·75 cm, and gas flowrate for each tuyere 40 L min^-1, appear to be the optimal design.     

双流体模型在AOD熔池流场数学模拟中的应用

基于气液双流体模型和湍流的修正k-ε模型,考虑了多股气流侧吹操作对熔池流场的影响,以及AOD熔池内气液两相流的行为和两相间的动量传输,建立了AOD多股气流侧吹精炼过程中熔池内流体流动的数学模型,并对宝钢股份不锈钢事业部120 tAOD原型和线尺寸为其1/4的水模型熔池内流体的流动作了模拟,结果表明,确实可以采用双流体模型来模拟AOD精炼过程中熔池内流体的流动;用该模型计算的结果表明,整个熔池流体处于活泼的搅拌和循环运动状态。     

RH喷吹参数对循环流量和均混时间影响的物理模拟

以某厂300tRH真空精炼装置为研究原型,建立1∶6.5的水力模型对RH喷吹精炼工艺进行物理模拟。研究了喷吹位置、喷吹气量及驱动气体流量对循环流量和均混时间的影响。结果表明:不同喷吹气量、驱动气体流量条件下,获得大循环流量和短均混时间的最优喷吹位置不同。较小的喷吹气量(2.98~3.53m3/h)或者较小的驱动气体流量(0.93~1.02m3/h)时,宜采用低顶枪枪位(153.8mm)喷吹;喷吹气量大于3.91m3/h或者驱动气体流量大于1.12m3/h时,宜采用真空槽底部喷吹角度120°的侧喷嘴喷吹。顶枪与侧喷嘴复合喷吹有利于提高RH喷吹工艺的适应性及循环效率。     

Preliminary Investigation of Mathematical Modeling of Stainless Steelmaking in an AOD Converter: Mathematical Model of the Process

Mathematical modeling of stainless steelmaking in an AOD (argon‐oxygen decarburisation) converter with side and top combined blowing has been preliminarily investigated. The actual situations of the side and top combined blowing AOD process were analysed. A mathematical model for the whole refining process of stainless steel has been proposed and developed. The model is based on the assumption that one part of the oxygen blown through a top lance reacts with CO escaping from the bath, another part of the oxygen oxidizes the elements in the molten steel droplets splashed by the oxygen jet, and the remaining oxygen penetrates and dissolves into the molten steel through the pit stroked by the jet. All the oxygen entering into the bath oxidizes C, Cr, Si, and Mn dissolved in the steel and also the Fe of the steel melt, but the FeO generated is also an oxidant of C, Cr, Si, and Mn in the steel. During the process, all possible oxidation‐reduction reactions occur simultaneously and reach their equilibria, respectively their combined equilibrium, in competition at the liquid/bubble and liquid/slag interfaces. In the simple side blowing after the top blowing operation is finished, the possible reactions take place simultaneously and reach a combined equilibrium in competition at the liquid/bubble interfaces. The overall decarburization rate in the refining process is the sum of the contributions of both the top and side blowing processes. It is also assumed that at high carbon concentrations, the oxidation rates of elements are mainly dependent upon the supplied oxygen rate, and at low carbon contents, the rate of decarburisation is primarily related to the mass transfer of carbon from the molten steel bulk to the interface. It is further assumed that the non‐reacting oxygen blown into the bath does not accumulate in the steel and will escape from the bath and react with CO in the atmosphere above the bath. The study presents calculations of the refining rate and the mass and heat balances of the system for the whole process. Additionally, the influences of the operating factors, including addition of slag materials, scrap, and alloy agents, the non‐isothermal conditions, the changes in the amounts of metal and slag during the whole refining process, and others have all been considered.     

复吹转炉顶吹枪位与非均衡底吹搅拌的水模拟

为优化120 t复吹转炉冶炼效果,通过水模型试验,模拟研究了复吹转炉底吹强度、顶枪枪位、底吹排布方式和流量分配比对渣 钢间传质效果和熔池混匀时间的影响。研究表明,在相同底吹条件下,顶枪低枪位操作时渣 钢间传质系数比高枪位操作时渣 钢间传质系数大,且随着底吹流量分配比增大,渣 钢间传质系数先增大后减小。在相同顶枪枪位操作条件下,底吹元件间隔排布时,渣 钢界面传质效果总体优于连续排布。随着底吹流量分配比增大,高枪位和低枪位混匀效果变化趋势近似相反。在高枪位下,底吹元件连续排布混匀时间总体少于底吹元件间隔排布时对应的混匀时间;在顶枪低枪位操作条件下,底吹元件连续排布混匀时间总体大于间隔排布混匀时间。通过顶枪枪位、底吹排布和流量分配比的合理匹配,较传统均衡流量底吹模式,传质系数可提高30%～125%,混匀时间可减少8.6%～51.5%。