Influence of gas flow on performance of “Confined” atomization nozzles

Supersonic gas jets in “confined” nozzles were studied by Schlieren photography in blank atomizing tests (i.e., no liquid present). Tests in nitrogen at 1.56 MPa pressure showed that changes in the geometry of a nozzle altered the wave pattern and the height of the supersonic region in the jet. In particular, the protrusion height of the metal delivery tube (above the gas exit) had a profound influence. An expansion wave formed at the tip of the nozzle when the protrusion height was too high, and the jet became subsonic in a short distance. Longer supersonic wave patterns were observed at lower protrusion heights following the appearance of a shock wave at the tip of the nozzle. These results correlated well with the atomizing performance of the same nozzles determined previously. The nozzles which had long supersonic flow regions corresponded to those which produced fine powders, and short supersonic regions were associated with reduced efficiency in performance. This indicated that the preservation of high velocities in the gas was of primary importance for effective liquid breakup in atomization. A procedure (based on the characteristics solution of supersonic flow) was developed for assessing flow conditions in atomizing nozzles and for calculating the optimum height of the delivery tube for a given geometry to obtain the longest supersonic jet.     

Studies on metal build upon atomisers during free fall gas atomisation of liquid metals

Abstract

In free fall gas atomisation of liquid metals, the atomised droplets mainly move downward, while some of them fly in an upward direction. Under certain conditions the upward moving droplets will deposit on the surface of the gas nozzle and metal delivery tube of an atomiser, which may cause a hindrance to the flow of liquid metal and gas. The effect of atomisation parameters, such as gas pressure, focal length, apex angle and diameter of gas nozzle, on the metal buildup on the atomiser have been studied during the free fall gas atomisation of lead, zinc and aluminium. The plenum pressure of gas at which deposition of atomised droplets on the surface of the gas nozzle and metal delivery tube takes place has been termed as limiting plenum pressure, and the corresponding gas velocity at the impingement point as limiting gas velocity. It has been shown that the limiting plenum pressure is different for different metals in the same atomiser. The limiting plenum pressure has been found to increase with free fall distance or specific gravity of liquid metals, and with a decrease in apex angle of atomiser. A correlation is proposed to determine the limiting gas velocity for free fall gas atomisation of metals.     

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.     

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.     

导流管中液态金属热传输的数值分析

应用有限差分对喷射成形导流管中的液态金属的流场进行模拟,计算液态金属Fe,Al,Zn在不同过程参数条件下所需的最小过热度△T,得出了最小过热度和材料热物理性质及过程参数之间的半定量关系.计算结果表明,对于最小过热度,导流管的导热能力是最重要的影响因素;在常现使用的过压范围内,增大过压可显著减小过热度;导流管的构形设计也是一关键因素,缩小导流管长度直径比L/D可使△T减小,且对于高熔点金属或高导热能力的导流管影响更为显著.     

底吹气中间包内钢液流动及夹杂物去除模拟研究

通过40 t中间包的1:3水模型实验,对小方坯连铸6流中间包内钢液的流场进行了测定,研究了5种不同底吹气方案对中间包内钢液流动特性和夹杂物去除的影响。结果表明,在注流区吹气能延长钢水在中间包内的停留时间,减弱湍流强度,同时注流区内吹气可将气泡击碎成弥散小气泡促进了夹杂物上浮,夹杂物去除效果最佳。在近流与中流水口之间吹气,可改善各流的流动特性,中间包内的流动状况最佳,但近流处夹杂物去除效果略差。在中流和远流水口之间吹气,流场改善效果不理想。     

Effect of carbonaceous particles on slag foaming

Use of carbonaceous particles such as coke or coal char in controlling slag foaming is of great practical significance for bath-smelting and other steelmaking processes. The foamability of the liquid slag in terms of the foam index has been determined with the presence of different amounts of coke and coal char particles. Different sized and shaped particles were used in the experiments. It was found that the foam index decreased significantly as the ratio of the total cross-sectional area of the particles to the liquid slag surface area increased. When the foam was generated by argon gas injection through an alumina nozzle (i.d. = 1.5 mm), a liquid slag, CaO-SiO₂-CaF₂-(Al₂O₃), depending on the alumina content, could have an initial foam index of about 2 to 4 seconds at 1500 °C without any carbonaceous particles. When the slag surface was covered only 15 ~20 pct with either coke or coal char particles, the foam was totally suppressed regardless of the initial foam index. In order to understand the mechanism of the antifoam effect of the carbonaceous particles, interactions of a coke sphere, an iron ore pellet, an alumina tube, and a coal char particle with the liquid slag foam were examined by X-ray observation. It was concluded that the antifoam effect of coke or coal char particles is primarily contributed by the nonwetting nature of the carbonaceous materials with the liquid slag. Possible mechanisms of carbonaceous particles rupturing a slag film could be (1) the rapid thinning of the liquid slag film driven by a difference between the instantaneous contact angle and the equilibrium contact angle or (2) the “dewetting” of the liquid slag from the interface when the film is “bridged” by the particle. Formerly Graduate Student and Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University.     

超音速气雾化喷嘴中抽吸压力变化规律的研究

对超音速气雾化喷嘴中抽吸压力ΔP的变化进行了试验研究,重点考察了雾化气体压力P0、气流夹角α、导液管锥顶角β及突出长度h对抽吸压力的影响。试验结果表明,在(0.5～4.0MPa)雾化压力范围内,抽吸压力皆呈负压状态。但抽吸压力变化分三阶段:随着雾化压力的增加,雾化压力在0.5～1.5MPa范围内,抽吸压力减小;雾化压力在1.5～3.5MPa之间时,抽吸压力逐渐增大,并达到一个最大值;当雾化压力超过3.5MPa后,抽吸压力又逐渐降低。锥顶角越大,抽吸压力越大;而雾化压力升高到或大于3.23MPa时,随着突出长度的增加,抽吸压力逐渐减小。     

Behavior of liquid metal droplets in an aspirating nozzle

Measurements of particle size, velocity, and relative mass flux were made on a spray field produced by aspirating liquid tin into 350 °C argon flowing through a venturi nozzlevia a small orifice in the throat of the nozzle. Details of the aspiration and droplet formation process were observed through windows in the nozzle. The spatial distribution of droplet size, velocity, and relative number density was measured at a location 10 mm from the nozzle exit. Due to the presence of separated flow in the nozzle, changes in nozzle inlet pressure did not significantly effect resulting droplet size and velocity. This suggests that good aerodynamic nozzle design is required if spray characteristics are to be controlled by nozzle flow. This article is based on a presentation made in the symposium “Spray Processing Fundamentals: Coating and Deposition” presented as part of the 1990 TMS Fall Meeting, October 9, 1990, in Detroit, MI, under the auspices of the TMS Synthesis and Analysis in Materials Processing Committee.     

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.     

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.     

The interfacial tension between aluminum and cryolite melts saturated with alumina

A method was developed for measuring the capillary depression of metallic aluminum in an alumina tube in a cryolite melt. As the tube was progressively lowered through the melt into the metal the volume of gas expelled was measured by the movement of a meniscus of liquid in a horizontal glass tube. No movement occurred after the tube entered the aluminum until it was far enough down that the metal could enter it. A correction was applied for the finite radius of the crucible holding the melt, the necessary theory being derived. The contact angle between metal, alumina, and cryolite was determined from the shape of the metal frozen in the crucible. It was found that the contact angle of the metal on alumina is very close to 180 deg, and that the interfacial tension at 1000°C is 460 ± 27 mN ⋅ m⁻¹ (standard deviation). Interfacial tension decreases with increasing NaFJAlF₃ ratio, and increases with addition of MgF₂ or Li₃AlF₆. CaF₂ has no significant effect. The hypothesis is advanced that the effect of ratio is due to adsorption of Na atoms, generated by the reaction 3 NaF + Al ⇌ AlF₃+ 3Na, at the interface. Application of the Gibbs adsorption isotherm suggests that at molar ratios NaF/AlF₃ above 2.8 the interface is covered with a monolayer of sodium atoms.     

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.     

Hydrogen transport in stagnant molten iron

A new experimental method is described for the study of gas absorption in liquid metals. The technique involves the sudden admission of the gas into a previously evacuated tube containing a column of stagnant liquid metal. Following a short initial period of pressure instability (< 10 sec), absorption rates can be accurately followed by monitoring changes in gas pressure (or volume) using a differential pressure transducer. In the present work, the solution of hydrogen in pure liquid iron was studied and confirmed to be a diffusion controlled process. The mean diffusion coefficient measured for hydrogen absorption in pure iron at 1 atm pressure over the temperature range 1550° to 1720°C was 8.76 x 10⁻⁴ sq cm sec⁻¹. This figure is about 35 pct lower than previously reported values by Parlee and coworkers.⁹⁻¹⁰     

气体流量及水口底部形状对板坯结晶器内流场影响的水模型试验

采用1:1水模型研究了气体流量（010 L/min）和水口底部形状（凹底和尖底）对结晶器内流场的影响。在结晶器断面为230mm×1 200 mm,浇铸速度为1.6 m/min的模拟工况条件下,凹底水口其流体形态优于尖底水口;在结晶器液面波动稳定性方面凹底水口亦优于尖底水口;气体流量在08 L/min,使用尖底水口的流体其表面流速明显高于使用凹底水口的流体;对凹底水口而言,气体流量超过8 L/min,其流体表面流速低于0.2 m/s;120炉IF钢生产结果表明,使用优化的凹底水口和吹氩流量7 L/min,浇铸过程结晶器液面波动在±3 mm以内,铸坯夹杂物比优化前降低24%。     

Investigation of Fluid Flow and Steel Cleanliness in the Continuous Casting Strand

Fluid flow in the mold region of the continuous slab caster at Panzhihua Steel is investigated with 0.6-scale water model experiments, industrial measurements, and numerical simulations. In the water model, multiphase fluid flow in the submerged entry nozzle (SEN) and the mold with gas injection is investigated. Top surface level fluctuations, pressure at the jet impingement point, and the flow pattern in the mold are measured with changing submergence depth, SEN geometry, mold width, water flow rate, and argon gas flow rate. In the industrial investigation, the top surface shape and slag thickness are measured, and steel cleanliness including inclusions and the total oxygen (TO) content are quantified and analyzed, comparing the old and new nozzle designs. Three kinds of fluid flow pattern are observed in the SEN: “bubbly flow,” “annular flow,” and an intermediate critical flow structure. The annular flow structure induces detrimental asymmetrical flow and worse level fluctuations in the mold. The SEN flow structure depends on the liquid flow rate, the gas flow rate, and the liquid height in the tundish. The gas flow rate should be decreased at low casting speed in order to maintain stable bubbly flow, which produces desirable symmetrical flow. Two main flow patterns are observed in the mold: single roll and double roll. The single-roll flow pattern is generated by large gas injection, small SEN submergence depth, and low casting speed. To maintain a stable double-roll flow pattern, which is often optimal, the argon should be kept safely below a critical level. The chosen optimal nozzle had 45-mm inner bore diameter, downward 15 deg port angle, 2.27 port-to-bore area ratio, and a recessed bottom. The pointed-bottom SEN generates smaller level fluctuations at the meniscus, larger impingement pressure, deeper impingement, and more inclusion entrapment in the strand than the recess-bottom SEN. Mass balances of inclusions in the steel slag from slag and slab measurements show that around 20 pct of the alumina inclusions are removed from the steel into the mold slag. However, entrainment of the mold slag itself is a critical problem. Inclusions in the steel slabs increase twofold during ladle changes and tenfold during the start and end of a sequence. All of the findings in the current study are important for controlling slag entrainment.     

Impingement of particles-gas jet on a uniformly flowing water

Solid particles of different diameters and wettabilities were injected into a uniformly flowing water of relatively low flowrate in order to visualize with a high-speed cinecamera the penetration of a particles-gas-liquid, three-phase flow jet. The velocity of particles injected from the nozzle was measured by a laser Doppler velocimeter, and the kinetic energy of the particlesgas jet was estimated. Two kinds of penetration depth were defined. One is the penetration depth of an “envelope” in which the particles-gas jet moves downward at a very high speed with virtually no liquid fragmentation. The particles remaining at the bottom of the cavity formed by the impinging gas jet are of primary importance for the initiation of this “envelope.” The wettability of particles has less influence on this phenomenon. Another penetration depth is concerned with the trajectory of the particles-gas-liquid, three-phase flow jet. The particles-gas jet entrains the surrounding water beyond the envelope, and a three-phase flow jet is formed. Its behavior is affected considerably by the wettability of particles. It also was observed that the sectional area of the particles-gas jet was contracted slightly after impinging on the liquid surface due to the induction of a downward water flow. The penetration depths were predicted based on the kinetic energy of the impinging particles-gas jet. The effect of particle diameter on these phenomena was influenced significantly by the movement of the ensemble of particles. T. Shimada, formerly Graduate Student at Kyoto University T. Nishihara, formerly Graduate Student at Kyoto University H. Tanemura, formerly Graduate Student at Kyoto University     

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。