Inclusion Growth and Removal in Gas‐Stirred Ladles

A static modelling approach was used to study the growth and removal of inclusions during gas stirring in a ladle. A mathematical model of a gas‐stirred ladle was used to predict the data necessary to calculate growth and removal of inclusions. Results indicated that inclusion growth resulting from laminar shear collisions is negligible in comparison with growth from turbulent and Stokes collisions. Furthermore, the need for a model describing inclusion flotation by spherical‐cap bubbles was identified. Since the existing models presented in the literature are only valid for spherical bubbles, a model for the removal of inclusions by spherical‐cap bubbles was developed. Inclusion removal to the slag, refractory and by bubble flotation was compared. The mechanism determined to be responsible for the removal of the majority of inclusions larger than 25 μm was Stokes flotation and for the majority of the smaller inclusions, bubble flotation by spherical‐cap bubbles (assuming plane contact between the inclusion and the bubble).     

Measurement of physical characteristics of bubbles in gas-liquid plumes: Part I. An improved electroresistivity probe technique

This paper presents an experimental study of the structure of turbulent air-water bubble plumes in upwardly injected jets. Part I of the paper describes a microcomputer-aided two-element electro-resistivity probe technique developed for simultaneously determining various important local parameters of the gas phase: gas fraction, bubble frequency, bubble velocity spectrum, and bubble-pierced length spectrum. The measurement of the last two parameters, under the turbulent conditions investigated, necessitated the development of special electronic instrumentation and software to analyze, in real time, the signals produced by the contact of the bubbles with the sensor. The signal analysis, based on pattern recognition logic and the statistics of outliers, eliminated the uncertainties associated with the stochastic nature of the interception of the bubbles with the probe contacts. This permitted the measurement of the velocity of bubbles traveling vertically and undisturbed between the two contacts of the probe. The measuring technique developed was found to be reliable based on the determination of the velocity of single spherical cap bubbles and the consistency between measured and known gas volume flow rates in turbulent gas-liquid plumes.     

Understanding the Mixing Pattern in an Anaerobic Expanded Granular Sludge Bed Reactor: Effect of Liquid Recirculation

An anaerobic expanded granular sludge bed (EGSB) reactor is considered to be an improvement over upflow anaerobic sludge blanket reactors owing to the former’s ability to recycle the effluent and its modified reactor geometry. However, the mixing pattern in EGSB reactors, which greatly influences the design and the performance of this reactor, has not yet been studied in detail. In this research, the mixing pattern in a lab-scale EGSB reactor treating a synthetic dye wastewater was studied using lithium chloride as a tracer. The tracer exit curve indicated a complete-mix behavior. A simulation study was conducted on identical reactors using conductivity probes, inserted through the sample ports along the height of the reactors and connected to a data acquisition system. The reactors were operated at three different hydraulic retention times (3.3, 5.5, and 9 h) and at four different upflow liquid velocities (1.10, 2.66, 5.33, and 8.68 m/h). The data showed the existence of a plug-flow regime in the basin at lower upflow liquid velocities although the tracer response curves resemble complete-mix behavior. With increasing upflow liquid velocity the flow pattern in the basin deviates from a plug-flow pattern and approaches a complete-mix condition. The EGSB reactor can be modeled as a plug-flow reactor with recycle and dead space, and with a large vessel dispersion number (D/uL>0.01).     

钢包内气泡的形成与运动过程的数值模拟

 以钢包精炼底吹氩气过程中氩气泡为研究对象,运用VOF模型追踪氩气与钢液的界面,在层流条件下三维数值模拟了氩气泡的生成和运动过程。采用Tecplot进行模拟结果的后处理,研究了不同气体流量条件对气泡脱离直径的影响以及气泡的运动特性。模拟结果显示,氩气泡的形成历经膨胀、脱离两个阶段,气泡脱离直径随着气体流量的增加而增加,针对2 mm直径的孔口,其鼓泡流量上限为1.325 L/min。气泡从孔口脱离后历经近似球形、扁平的椭球形、规则的椭球形和不规则的球帽形的形状变化;气体流量越大,气泡的变形程度越剧烈,气泡位于钢包高度中上部位置时,全部呈现近似规则的球形,不随流量的改变而改变。     

Measurement of physical characteristics of bubbles in gas-liquid plumes: Part II. Local properties of turbulent air-water plumes in vertically injected jets

The structural development of air-water bubble plumes during upward injection into a ladle-shaped vessel has been measured under different conditions of air flow rate, orifice diameter, and bath depth. The measured radial profiles of gas fraction at different axial positions in the plume were found to exhibit good similarity, and the distribution of the phases in the plume was correlated to the modified Froude number. Different regions of flow behavior in the plume were identified by changes in bubble frequency, bubble velocity, and bubble pierced length which occur as bubbles rise in the plume. Measurement of bubble velocity indicates that close to the nozzle the motion of the gas phase is strongly affected by the injection velocity; at injection velocities below 41 m/s, the velocity of the bubbles along the centerline exhibits an increase with height, while above, the tendency reverses. High-speed film observations suggest that this effect is related to the nature of gas discharge,i.e., whether the gas discharge produces single bubbles or short jets. In this region of developing flow, measurement of bubble frequency and pierced length indicates that break-up of the discharging bubbles occurs until a nearly constant bubble-size distribution is established in a region of fully developed flow. In this largest zone of the plume the bubbles influence the flow only through buoyancy, and the spectra of bubble pierced length and diameter can be fitted to a log-normal distribution. Close to the bath surface, a third zone of bubble motion behavior is characterized by a faster decrease in bubble velocity as liquid flows radially outward from the plume.     

Experimental Validation of a Computational Fluid Dynamics Model of Copper Electrowinning

The hydrodynamics that occur in the space between the electrode plates in copper electrowinning (EW) are simulated using a computational fluid dynamics model (CFD). The model solves for the phases of gas oxygen bubbles and electrolyte using the Navier–Stokes equations in a CFD framework. An oxygen source is added to the anode, which sets up a recirculation pattern. The gradients in copper near the cathode lead to buoyancy forces, which result in an uplift in the electrolyte close to the cathode. This study investigates the experimental validation of the CFD model using a small/medium-scale real EW system. The predicted fluid velocity profiles are compared with the experimental values, which have been measured along various cross sections of the gap between the anode and the cathode. The results show that the CFD model accurately predicts the velocity profile at several heights in the plate pair. The CFD model prediction of the gas hold-up and the recirculation pattern is compared with visualizations from the experiment. The CFD model prediction is shown to be good across several different operating conditions and geometries, showing that the fundamental underlying equations used in the CFD model transfer to these cases without adjusting the model parameters.     

Simulation of gas dispersion in a metallurgical bubble reactor

A better understanding of complex process phenomena and engineering of gas dispersion techniques in metallurgical processes can be obtained with the method of computer aided process simulation. Main target of this investigation is to analyze the influence of operational and design parameters such as gas flow rate, reactor diameter, liquid height, or injector size, number and location on the system quantities like mixing power, hold up or degassing rates. The bath aspect ratio should be between 0.8 and 1.2 if the desorption at the bath surface is playing an important role. An optimization of the injected gas quantitiy can be achieved with increased injector cross-section area (bigger bubbles ascending faster and increasing the mixing power) or higher aspect ratios (longer bubble residence times in the reactor). Dead zones can be avoided by distribution of gas through several injectors positioned symmetrically, increasing total mixing power and specific interface area in the system.     

表面曝气反应器中气泡尺寸分布测量

合理处置矿山开采过程中产生的废水是绿色矿山建设中必要且重要环节,表面曝气处理废水有助于节约能耗。本研究开展了表面曝气反应器内气泡局部性质分布的测量,将为反应器准确设计奠定基础。采用新研制的侵入式远心照相多相测量仪,对表面曝气反应器内气泡直径分布进行了系统测量,进一步获得了相界面积和气含率分布,重点考察了轴向位置和搅拌转速2个因素。研究结果表明:与相同转速下分布器供气的气液搅拌反应器相比,表面曝气反应器中气泡尺寸更小;沿轴向逐渐远离液面,气泡尺寸逐渐减小,同时气泡数量也逐渐变少;随着转速增大,吸入气泡增多,平均气泡尺寸变大。为了强化表面曝气反应器内气液分散过程,采用浅层床设计,同时在功率允许范围内可适当提高转速。本研究将为绿色矿山废水高效处理提供一种新的选择。     

Bubble formation during horizontal gas injection into downward-flowing liquid

Bubble formation during gas injection into turbulent downward-flowing water is studied using high-speed videos and mathematical models. The bubble size is determined during the initial stages of injection and is very important to turbulent multiphase flow in molten-metal processes. The effects of liquid velocity, gas-injection flow rate, injection hole diameter, and gas composition on the initial bubble-formation behavior have been investigated. Specifically, the bubble-shape evolution, contact angles, size, size range, and formation mode are measured. The bubble size is found to increase with increasing gas-injection flow rate and decreasing liquid velocity and is relatively independent of the gas injection hole size and gas composition. Bubble formation occurs in one of four different modes, depending on the liquid velocity and gas flow rate. Uniform-sized spherical bubbles form and detach from the gas injection hole in mode I for a low liquid speed and small gas flow rate. Modes III and IV occur for high-velocity liquid flows, where the injected gas elongates down along the wall and breaks up into uneven-sized bubbles. An analytical two-stage model is developed to predict the average bubble size, based on realistic force balances, and shows good agreement with measurements. Preliminary results of numerical simulations of bubble formation using a volume-of-fluid (VOF) model qualitatively match experimental observations, but more work is needed to reach a quantitative match. The analytical model is then used to estimate the size of the argon bubbles expected in liquid steel in tundish nozzles for conditions typical of continuous casting with a slide gate. The average argon bubble sizes generated in liquid steel are predicted to be larger than air bubbles in water for the same flow conditions. However, the differences lessen with increasing liquid velocity.     

Sludge Blanket Height and Flow Pattern in UASB Reactors: Temperature Effects

Two upflow anaerobic sludge blanket (UASB) reactors were operated for approximately 900?days to examine the feasibility of treating municipal wastewater under low temperature conditions. In this paper, a modified solid distribution model has been formulated by incorporating the variation of biogas production rate with a change in temperature. It was found that the model simulated the solid distribution well as confirmed by experimental observations of solid profile along the height of the reactor. Mathematical analysis of tracer curves indicated the presence of a mixed type of flow pattern in the sludge-bed zone of the reactor. It was found that the dead-zone and bypass flow fraction were impacted by the change in operating temperatures.     

RH真空室内气泡行为的研究

Ruhrstahl-Hereaeus (RH)上升管内的气液两相流是整个装置的重要动力源,并对钢液的流动、混匀及精炼过程有重要影响.上升管及真空室内的气液两相流决定了钢包内钢液的流动状态,为了研究真空室及上升管内气液两相流,通过1:6的300 t RH的物理模型模拟了RH上升管及真空室内气泡行为过程,并测量了RH循环流量的变化用于计算上升管内含气率以及气泡运动速度最终得到气泡在真空室内的停留时间,同时记录了气泡在真空室内的存在形式.气泡在真空室的存在形式的主要影响因素为提升气体流量,研究发现了气泡从规则独立的大气泡经历聚合长大,碰撞破碎成小气泡,最后变成小气泡和不规则大气泡共存的现象.液面高度达到80 mm之后,气泡在真空室内的停留时间达到一个平衡值,不再随真空室液面高度的增加而发生改变.当提升气体量达3000 L·min-1,气泡停留时间减小趋势弱,对应3000 L·min-1情况下,真空室内气泡开始聚合长大.研究认为对于300 t RH的真空室液面高度应为80 mm,提升气体量应在3500 L·min-1左右,优化后,脱碳时间由原工艺的21.4 min缩短至现工艺的17.5 min.      

Inactivation of Cryptosporidium Oocysts in a Pilot-Scale Ozone Bubble-Diffuser Contactor. I: Model Development

A mathematical model was developed to simulate the performance of a pilot-scale ozone bubble-diffuser column. The reactor hydrodynamics was represented with the axial dispersion reactor model. An analytical solution was developed for the liquid and gas phase ozone mass balances in which dissolved ozone decomposes by first-order kinetics. Numerical approximations were provided for the mass balances for viable microorganisms and the more general case of dissolved ozone decomposition through a second-order reaction with fast ozone demand in natural organic matter. Model components required to predict the liquid and gas phase ozone concentration and viable microorganism number density profiles throughout the bubble-diffuser column included input parameters (liquid and gas flow rates, influent gas and dissolved ozone concentrations, temperature, and countercurrent or cocurrent operation mode), empirical correlations (dispersion number, volumetric mass transfer coefficient, Henry’s law constant), and batch or semibatch kinetic information (ozone decomposition rate constants and fast-ozone demand, and microorganism inactivation lag phase and rate constant). A sample model run for the case of first-order ozone decomposition revealed that the analytical and numerical solutions were practically identical.     

Bubble and liquid flow characteristics in a wood’s metal bath stirred by bottom helium gas injection

A model study was carried out to elucidate bubble and liquid flow characteristics in the reactor of metals refining processes stirred by gas injection. Wood’s metal with a melting temperature of 70 °C was used as the model of molten metal. Helium gas was injected into the bath through a centered single-hole bottom nozzle to form a vertical bubbling jet along the centerline of the bath. The bubble characteristics specified by gas holdup, bubble frequency, and so on were measured using a two-needle electroresistivity probe, and the liquid flow characteristics, such as the axial and radial mean velocity components, were measured with a magnet probe. In the axial region far from the nozzle exit, where the disintegration of rising bubbles takes place and the radial distribution of gas holdup follows a Gaussian distribution, the axial mean velocity and turbulence components of liquid flow in the vertical direction are predicted approximately by empirical correlations derived originally for a water-air system, although the physical properties of the two systems are significantly different from each other. Under these same conditions, those turbulent parameters in high-temperature metals refining processes should thus be accurately predicted by the same empirical correlations.     

Influence of Hydrodynamics on the Performance of a Biofilm Airlift Suspension Reactor

Five biofilm airlift suspension (BAS) reactors filled with ceramic materials as biocarriers were used to investigate the hydrodynamics, liquid mixing, and biofilm detachment kinetics in the BAS reactor. A mathematical model was developed to describe the internal liquid circulation within the BAS reactor. The Froude number was introduced to correlate the relationship between the Froude number and superficial gas velocity at different biocarrier concentrations. The validity of the empirical model was verified over a wide range of experimental conditions and the result shows that the internal liquid circulation velocity was proportional to the square root of the reactor height and the superficial gas velocity. Because the internal liquid circulation flow rate was much larger than influent flow rate, the BAS reactor had a strong capacity to resist shock loading caused by the change in influent organic matter concentration. Shock loading resistance increased with the height of a BAS reactor. Although biofilm detachment was a very complicated process which involved many mechanisms, dimensional analysis was employed to successfully analyze the biofilm detachment kinetics. It was found that the biofilm detachment rate was proportional to the first power of the superficial gas velocity and biofilm thickness, and to the 2/3 power of the number of biocarriers in the reactor, respectively. Use of the Froude number and dimensional analysis provide an effective and accurate method to study the characteristics of the BAS reactor.     

Model study on mixing and mass transfer in ferroalloy refining processes

An experimental study has been performed to investigate the bath mixing intensity induced by a high-strength submerged gas injection in a bottom blown air-stirred one-seventh water model of Creusot-Loire Uddeholm (CLU) reactor using three different tuyere configurations. Experimental results have been discussed in terms of the mass transfer rate and mixing time. The air flow rates varied from 0.00599 to 0.01465 m³/s. The mixing time was determined at various gas flow rates, bath heights, and nozzle orientations, both in the presence and absence of a second phase. The mixing time was found to decrease with increasing gas flow rate and decreasing bath height. The influence of bath mixing intensity on mass transfer between metal (water) and slag (paraffin) was studied by measuring the transfer of benzoic acid from the gas-stirred water bath to paraffin as a function of the gas injection parameters. The bath mixing intensity was characterized by the value of the mass transfer rate constant. The rate constant of mass transfer between the metal and slag was found to increase with increasing gas injection rate and decreasing bath height.     

水口吹氩工艺板坯结晶器内气泡运动行为的物理模拟

以1300 mm × 230 mm板坯连铸结晶器的相似比0.4的物理模型,研究了拉速1.1 m/min、水口插入深度160 mm、水口吹气量0～15 L/min时连铸结晶器内气泡的运动行为,及其对钢液流股冲击深度、液面波动和液面裸露的影响。实验结果表明,随水口吹气量增加,结晶器内气泡的数量和尺寸都有所增加,气泡在钢液内水平方向扩散范围增大,且气泡最大穿透深度亦增加;当水口吹气量增大到5 L/min时,气泡逸出后在液面由全部向水口方向运动变为以集中逸出位置为中心的四散运动。     

Experimental Study of Three-Dimensional Flow Field around a Complex Bridge Pier

In this paper, three-dimensional turbulent flow field around a complex bridge pier placed on a rough fixed bed is experimentally investigated. The complex pier foundation consists of a column, a pile cap, and a 2×4 pile group. All of the elements are exposed to the approaching flow. An acoustic-Doppler velocimeter was used to measure instantaneously the three components of the velocities at different horizontal and vertical planes. Profiles and contours of time-averaged velocity components, turbulent intensity components, turbulent kinetic energy, and Reynolds stresses, as well as velocity vectors are presented and discussed at different vertical and horizontal planes. The approaching boundary layer at the upstream of the pile cap separated in two vertical directions and induced an upward flow toward the column and a contracted downward flow below the pile cap and toward the piles. The contracted upward flow on the pile cap interacts with downflow in the front of the column and deflects toward the side of the pier, which in return produces a strong downflow along the side of the pile cap. The flow at the rear of the pile cap is very complex. The strong downward flow at the downstream and near the top of the pile cap in interaction with the reverse flow behind the column and upward flow near the bed produce two vortices close to the upper and lower corners of the pile cap with opposite direction of rotation. On the other hand, the back-flow from the wake of the pile cap is forced into the top region resulting in a secondary recirculation at the wake of the column. The contracted flow below the pile cap and toward the piles, a strong downflow along the sides of the pile cap at the upstream region, and a vortex flow behind the pile cap and an amplification of turbulence intensity along the sides of the pile cap at the downstream region are the main features of the flow responsible for the entrainment of bed sediments.     

Multiphase Flow and Thermo-Mechanical Behaviors of Solidifying Shell in Continuous Casting Mold

 The metallurgical phenomena occurring in the continuous casting mold have a significant influence on the performance and the quality of steel product. The multiphase flow phenomena of molten steel, steel/slag interface and gas bubbles in the slab continuous casting mold were described by numerical simulation, and the effect of electromagnetic brake (EMBR) and argon gas blowing on the process were investigated. The relationship between wavy fluctuation height near meniscus and the level fluctuation index F, which reflects the situation of mold flux entrapment, was clarified. Moreover, based on a microsegregation model of solute elements in mushy zone with δ/γ transformation and a thermo-mechanical coupling finite element model of shell solidification, the thermal and mechanical behaviors of solidifying shell including the dynamic distribution laws of air gap and mold flux, temperature and stress of shell in slab continuous casting mold were described.     

折流式移动流化床改质焦炉煤气预还原铁矿粉的数值模拟

 建立了折流式移动流化床内利用改质焦炉煤气进行气基粉铁矿预还原的数学模型。模型求解采用FLUENT和PHOENICS的联合求解。冷态工况的数值模拟结果和试验结果进行了比较。通过比较床层平均压降和分析气固相的流动行为,对提出的数学模型的可靠性进行了验证。利用所建立的数学模型对利用该反应器和采用改质COG(焦炉煤气)对铁矿粉预还原的工艺过程进行了热态模拟。在模拟的工况条件下,指出了反应器内分布板布置上的缺陷;反应器必须采用气体分布板振动才可以保持气固正常流动,同时保持较小的流化气速。还原气温度的整体降幅达到770K,气相还原势的利用率达到35%,矿粉的还原分数达到0.7,反映出该反应器内良好的气固换热和对还原势的利用率。该反应器在一个紧凑的结构下实现了对还原气热能和还原势的梯级利用。     

Quantification and Taxonomy of Pores in Thermal Spray Coatings by Image Analysis and Stereology Approach

Porosity is one of the most important microstructural features in thermal spray coatings and has been actively studied and measured by many methods. Image analysis techniques have become popular techniques in determining porosity in coatings because of simplicity, accessibility, and an ability to measure both open and closed porosities as well as pore characteristics such as size, shape, orientation, and spatial distribution. In the current study, an image analysis technique has been complemented by several stereology procedures to determine the porosity level and characteristics of pores within coatings. Stereology protocols such as Delesse, DeHoff, and Cruz-Orive analyses were used to derive the porosity level, pore size, and shape distributions, and the effectiveness of each stereology protocol was compared. Standoff distance (SOD) and annealing process did not alter the distribution trend of number of pores but influenced the distribution of pore volume fractions significantly. The bivariate size–shape distribution of the pores was used to predict the dominant pore type and fractions of pores that arose from different formation mechanisms. It was found that nearly spherical pores that originated from gas bubbles and entrapped gas pockets dominate at shorter SOD, while the different types of pores become more evenly distributed when the SOD was increased.