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.     

Study on dimension and migration behavior of bubble under annular argon blowing at tundish upper nozzle

The water model experiments were carried out to study the bubble morphology in the tundish and mold with the process of annular argon blowing at tundish upper nozzle. The effects of the position of gas permeable brick, the casting speed and the argon flow rate on the bubble size distribution, the bubble migration behavior and the flow behavior of liquid steel near the liquid level in tundish were further investigated, coupled with the numerical simulation. The results show that with the process of annular argon blowing at tundish upper nozzle, a frustum cone shaped bubble plume can be formed around the stopper rod. The concentration of argon bubbles gradually decreases outward along the radial direction of the stopper rod. Owing to the wall attached effect, the bubble plumes float upward along the stopper rod, which can increase the collision probability between bubbles and the velocity of bubble plumes, causing a larger impact strength on the liquid level in tundish. In addition, a part of small bubbles are wrapped into the nozzle and the mold due to the drag force of liquid steel. With increasing argon flow rate, the number of bubbles in annular bubble plumes and the vertical velocity of liquid steel near the liquid level in tundish increase significantly. With increasing casting speed, the width and the bubble number of annular bubble plumes gradually decrease, leading to a decrease of the vertical velocity of liquid steel near the liquid level in tundish. Increasing the distance between the annular gas permeable brick and the center of tundish upper nozzle, the dispersion of bubbles and the width of bubble plumes increase, and the impact strength of bubbles acting on the liquid level in tundish becomes weaker. As the argon flow rate and the casting speed increase, and the distance between the gas permeable brick and the center of tundish upper nozzle decreases, the gas volume and bubble size in the mold increase. Under the experimental conditions, when the inner and outer diameters of the annular gas permeable brick are 110mm and 140mm, respectively, and the casting speed is 1.2m/min, the appropriate argon flow rate is 4L/min.     

Bubble formation at nozzles in pig iron

An experimental study was undertaken to determine how several variables affect the size of gas bubbles formed at nozzles in liquid pig iron. The frequency of bubble formation was measured by an acoustic device, which could detect the vibrations produced by the bubble release. Accurate knowledge of the gas flow rate then enabled the calculation of bubble volumes. The use of large baths (60 Kg), melted by induction heating, permitted a wide range of experimental parameters: gas flow rate (0.5 to 1000 cc/s), outside nozzle diameter (0.64 to 5.1 cm), inside diameter (0.16 to 0.64 cm), chamber volume (23 to 2200 cc), nozzle depth (7.6 to 20 cm), surface tension (700 to 1500 dynes/cm) and nozzle orientation (up, down and sideways). The resulting bubble volumes were between 0.5 and 100 cc. The bubbles were found to form at the outer diameter of the nozzles due to the nonwettability of the nozzles. Furthermore, the bubbles were of a uniform size at low flow rates, but increased in volume with the flow rate, so that a constant frequency was established. In addition, the bubble volume was strongly dependent on the chamber volume upstream from the nozzle. This is known as a “capacitance” effect and is due to compressibility of the gas. “Doublets” or “double bubbles” at small chamber volumes and bubble “pairs” at large chamber volumes were also observed. These phenomena result in smaller bubbles, which make precise predictions of bubble size difficult. The results are compared with those obtained by other investigators in aqueous and metallic systems.     

Bubble formation at nozzles in pig iron

An experimental study was undertaken to determine how several variables affect the size of gas bubbles formed at nozzles in liquid pig iron. The frequency of bubble formation was measured by an acoustic device, which could detect the vibrations produced by the bubble release. Accurate knowledge of the gas flow rate then enabled the calculation of bubble volumes. The use of large baths (60 Kg), melted by induction heating, permitted a wide range of experimental parameters: gas flow rate (0.5 to 1000 cc/s), outside nozzle diameter (0.64 to 5.1 cm), inside diameter (0.16 to 0.64 cm), chamber volume (23 to 2200 cc), nozzle depth (7.6 to 20 cm), surface tension (700 to 1500 dynes/cm) and nozzle orientation (up, down and sideways). The resulting bubble volumes were between 0.5 and 100 cc. The bubbles were found to form at the outer diameter of the nozzles due to the nonwettability of the nozzles. Furthermore, the bubbles were of a uniform size at low flow rates, but increased in volume with the flow rate, so that a constant frequency was established. In addition, the bubble volume was strongly dependent on the chamber vol-ume upstream from the nozzle. This is known as a “capacitance” effect and is due to compressibility of the gas. “Doublet” or “double bubbles” at small chamber volumes and bubble “pairs” at large chamber volumes were also observed. These phenomena re-sult in smaller bubbles, which make precise predictions of bubble size difficult. The re-sults are compared with those obtained by other investigators in aqueous and metallic systems.     

中间包上水口环形吹氩下气泡大小和迁移行为研究

摘要：通过水模型实验研究了上水口环形吹氩工艺下中间包和结晶器内气泡形貌,并结合数值模拟分析了透气砖位置、拉坯速度和吹氩量对中间包和结晶器内气泡尺寸、气泡迁移和中间包近液面钢液流动的影响。结果表明：上水口环形吹氩形成以塞棒为中心的圆台状气泡羽流,气泡浓度沿径向向外逐渐减少;附壁效应使得气泡羽流偏向塞棒壁面流动,增大气泡的碰撞聚并概率和近塞棒壁面的羽流上升速度,对中间包液面产生较大冲击作用;同时,部分细小气泡会随钢液进入水口及结晶器内部;增大吹氩量,中间包内环形气泡羽流中气泡数目明显增多,中间包近液面钢液上升速度增大;增大拉坯速度,环形气泡羽流的宽度和气泡数量逐渐减小,近液面速度减小;增大透气环距水口中心距离,中间包内气泡弥散度增大,环形气泡羽流宽度也随之增大,气泡羽流对中间包液面冲击作用减弱;增大吹氩量和拉坯速度、减小透气环距水口中心距离,进入结晶器的气量和气泡尺寸逐渐增大。实验条件下,透气环内外径为110mm/140mm、拉坯速度为1.2m/min时,吹氩量为4L/min较为合适。     

Investigation of the kinetics of breakup of gas bubbles in liquid metals with digital simulation method

The method of digital system simulation can be effectively used to quantify the complex multiphase interactions within a gas injection process. Process simulation results yield a better understanding and a better aimed engineering of gas dispersion techniques in metallurgical processes. In this paper the breakup phenomenon of gas bubbles in stagnant liquids is simulated and the dependencies between breakup of bubbles and various parameters of a gas dispersion process such as operative parameters, system parameters and mass transfer rates are investigated. The bubble diameter after breakup is almost independent of the nozzle diameter and gas flow rate. The frequency of bubble breakup and critical bubble size depend on the rate of mass transfer into the bubble. An almost constant rising velocity is achieved only in those cases investigated where mass transfer and bubble breakup are considered. In all other cases no stationary rising velocity is obtained. The interplay between bubble size, rising velocity and the inertia of the surrounding liquid and the influence of mass transfer and breakup are investigated. Simulation results reveal that the behaviour of an ascending bubble is strongly influenced by the mass transfer rate, i. e. by the composition of the melt. Verification of the simulation results with empirical equations from literature shows a very good agreement in all dispersion systems investigated.     

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     

Model study of bubble and liquid-flow characteristics in a bottom blown bath under reduced pressure

Gas injection techniques are widely used in metals refining processes. Pressure on the bath surface of reactors is sometimes highly reduced to enhance the efficiency of refining. Many fundamental and practical investigations have been made to clarify the effects of reduced surface pressure on the mixing time and reaction rates of decarburization or desulfurization in the bath. However, details of these effects are not fully understood yet. Since the mixing time and chemical reaction rates are closely associated with fluid flow phenomena in the bath, information on, for example, the total surface area of bubbles rising in the bath and liquid flow induced by the buoyancy force of the bubbles should be accumulated as much as possible. In this study, the so-called water-model experiments were carried out to reveal the effects of reduced surface pressure on the bubble and liquidflow characteristics using a two-needle electroresistivity probe and a two-dimensional laser Doppler velocimeter. At an axial position near the nozzle, each bubble expanded to a volume corresponding to the hydrostatic pressure. The bubble and liquid-flow characteristics in the axial region located farther than this axial position were found to be approximately the same as those obtained under an atmospheric surface pressure.     

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。     

The shape of bubbles rising near the nozzle exit in molten metal baths

A previously developed multineedle electroresistivity probe was used to investigate the shape of bubbles generated at the exit of a central single-hole bottom nozzle in molten Wood’s metal and mercury baths. This probe is capable of detecting the vertical cross section of rising bubbles. The shape of bubbles just after the detachment from the nozzle exit was correlated as a function of a modified Reynolds number and a modified Weber number. Furthermore, the relations between the shape of bubbles and the radial distributions of bubble characteristics specified by gas holdup, bubble frequency, etc. were derived. As a result, it is possible to predict the shape of the bubbles by measuring the bubble characteristics with a conventional two-needle electroresistivity probe.     

Characterization of gas bubbles injected into molten metals under laminar flow conditions

Velocity and volume measurements of gas bubbles injected into liquid metals under laminar flow conditions (at the orifice) have been achieved. A novel experimental approach utilizing noises generated by bubbles was used to collect the necessary data. Argon gas was bubbled through tin, lead, and copper melts, and gas bubble formation frequencies (and hence bubble sizes) were determined. It was found that the bubble size generated for a particular orifice diameter was dependent upon the magnitudes of the orifice Froude and Weber numbers. Maximum formation frequencies increased slightly with decreasing orifice diameter, and the transition point from varying to constant frequency occurred at an orifice Weber number of approximately 0.44. Velocities of gas bubbles rising through the metals were greater than those previously reported for studies in which only one bubble was in the melt at any time. Effective drag coefficients of the rising bubbles were found to agree with data previously generated in aqueous systems. Formerly Graduate Student, Michigan Technological University     

Water model study of the frequency of bubble formation under reduced and elevated pressures

Air was injected vertically upward into a water bath through a bottom nozzle or a bottom orifice. The surface pressure was reduced or elevated from an atmospheric pressure in order to change the hydrostatic pressure around the nozzle and orifice. The gas delivery system was designed so that bubbles were generated in the middle and high gas flow rate regimes under a constant flow condition. The frequency of bubble formation, f _B , decreased as the surface pressure, P _s , decreased when the volumetric gas flow rate, Q _g , was kept constant. The measured f _B values were predicted satisfactorily by an empirical equation proposed previously by the present authors. This equation was derived originally to correlate the frequency of bubble formation both in aqueous and molten metal systems under an atmospheric surface pressure. The effect of surface pressure on the frequency of bubble formation was considered in terms of the density of gas, ρ _g , and the volumetric gas flow rate Q _g in the aforementioned empirical equation. These two quantities, ρ _g and Q _g , were evaluated at the nozzle exit by using the hydrostatic pressure there.     

Characterization of gas bubbles injected into molten metals under laminar flow conditions

Velocity and volume measurements of gas bubbles injected into liquid metals under laminar flow conditions (at the orifice) have been achieved. A novel experimental approach utilizing noises generated by bubbles was used to collect the necessary data. Argon gas was bubbled through tin, lead, and copper melts, and gas bubble formation frequencies (and hence bubble sizes) were determined. It was found that the bubble size generated for a particular orifice diameter was dependent upon the magnitudes of the orifice Froude and Weber numbers. Maximum formation frequencies increased slightly with decreasing orifice diameter, and the transition point from varying to constant frequency occurred at an orifice Weber number of approximately 0.44. Velocities of gas bubbles rising through the metals were greater than those previously reported for studies in which only one bubble was in the melt at any time. Effective drag coefficients of the rising bubbles were found to agree with data previously generated in aqueous systems. R. J. ANDREINI, Formerly Graduate Student, Michigan Technological University,     

Experimental studies on the bath oscillation during gas blowing into liquids,part 1: measurements using a single nozzle

Experimental studies on the wave motion during gas blowing into a liquid were carried out on the systems water/air and Wood's metal/nitrogen under various blowing conditions. Variables of the measurement were the gas flow rate, the bath depth, the vessel diameter and the nozzle diameter. The measured results showed that a wave motion appears only if the gas flow rate exceeds a critical value which depends on the bath depth. The blowing conditions for wave formation can be characterized by the relation of the bath depth to the vessel diameter and by the specific gas flow rate per unit of volume of liquid. The frequency or the oscillation period of the wave mainly depends on the vessel diameter. The amplitude of the wave follows a logarithmic function of gas flow rate. The density of liquid and the nozzle diameter have hardly any influence on the wave formation and the wave motion.     

Detailed Modeling of Gas Flow in Liquid Steel: Bubble Size Distribution and Voidage Calculation

Although the role of gas purging in liquid steel systems is well recognized, it has yet to be adequately analyzed. One key aspect of this process is the prediction of gas voidage in the bath, which has been studied in great detail beginning with water modeling in the early days and using advanced multiphase models more recently. Still, there are significant unresolved issues with gas purging systems. When gas is introduced through a nozzle at high flow rate, a jet may form which is undesirable. The break‐up of this jet into bubbles is a separate topic of research. The more common practice in the steel industry is to use porous plugs for gas injection. Gas entry through a porous plug can be characterized by the stretched bubble regime, and the laws of coalescence and fragmentation used to analyze bubble column reactors are generally applicable. Calculation of the bubble size distribution is important for two reasons. First, the voidage distribution in the bath is significantly modified by the injection system and flow rates used, primarily due to changes in flow regime and bubble dynamics (collision, break‐up, coalescence). Second, the voidage distribution directly determines the buoyancy, that influences the physical mixing process, and the specific‐area‐density, that influences surface reactions (for example, decarburization, desulfurization and nitrogen pick‐up). In this paper, a numerical study is presented that combines a bubble dynamics model with an Eulerian multiphase model. The results of the simulation are compared with the experimental data from Anagbo and Brimacombe (1990). Relevant discussion and reviews will be presented to distinguish the differences of this detailed bubble dynamics model with the uniform bubble diameter approximations reported in various recent studies.     

Molten wood’s-metal flow in a cylindrical bath agitated by cold bottom-gas injection

Investigation was made of the heat-transfer effect on the motions of cold bubbles and molten metal in a bottom-blown bath. The heat transfer between the bubbles and the molten metal finished at an axial position near the nozzle exit. The bubble and liquid-flow characteristics measured above this position were in good agreement with those in a bath agitated by isothermal gas injection of the same mass flow rate. A simplified mathematical model was proposed to describe the two characteristics. The experimental results of gas holdup and mean liquid-flow velocity were satisfactorily predicted by it. The accuracy of the prediction became higher as the distance from the nozzle exit increased, due to disintegration of bubbles.     

Nonuniformity of NaOH concentration and effective bubble diameter in CO2 injection into aqueous NaOH solution

Mixed CO₂-N₂ gas was blown into an aqueous NaOH solution through a submerged nozzle of 3 mm ID, and the net absorption rate of CO₂ from the gas bubbles during their ascent was determined. The size distribution and the rising velocity of bubbles were also measured. The enhancement factor was estimated from the reported reaction rate constant as 1.16 to 8.20 at the NaOH concentration from 0.01 to 0.3 mol · dm^-3. It was deduced that NaOH concentration in the plume zone in which gas bubbles ascended was markedly lower than that of the bulk solution when NaOH concentration of the bulk solution was lower than 0.1 mol · dm^-3. The measured size distribution of bubbles had two peaks at approximately 0.15 and 2.3 cm. However, the effective bubble diameter defined as mean diameter based on the amount of absorbed CO₂ was 2.3 cm and it was close to the mean of larger bubbles.     

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.     

Bubble and liquid flow characteristics during horizontal cold gas injection into a water bath

In refining processes such as the AOD process cold gas is blown horizontally into the molten metal bath of the processes. The spatial distribution of bubbles in the bath is one of the important factors influencing the efficiency of the processes. In this study, a water model study was carried out to understand the characteristics of bubbles and liquid flow generated by horizontal gas injection. The bubble and liquid flow characteristics were measured using an electro‐resistivity probe and a laser Doppler velocimeter, respectively. In the flow field near the nozzle the bubble characteristics for the horizontal cold gas injection can be predicted by empirical equations derived for isothermal gas injection systems. The liquid flow characteristics could not be measured in this region. On the other hand, in the region far from the nozzle the two characteristics for the cold gas injection became different from those for the isothermal gas injection because of enhanced buoyancy force acting on expanding cold bubbles due to heat transfer.     

Fluid dynamics of vertical submerged gas jets in liquid metal processing systems

High speed cinematography and a pressure trace technique have been used to investigate the fluid dynamics of inert gas jets injected vertically upward into water, molten tin, lead-tin alloy, and iron. Two flow regimes of jet behavior were observed: one in which unstable bubbles were produced at the jet nozzle, and one in which a steady cone of gas emerged from the nozzle and broke up continuously into small bubbles. The transition between bubbling and continuous jet flow was controlled by the mass flow of gas per unit area of the jet and occurred at a flow rate of approximately 40 g/cm² s in all of the systems studied.