Measurement and Modeling of Heat Transfer Across Interfacial Mold Flux Layers

Abstract

Surface quality problems in continuous cast steel are greatly affected by heat transfer across the interfacial layers in the gap between the solidifying steel shell and the mold. An experimental apparatus has been constructed to measure temperatures in the steel, mold flux layers, and copper under conditions approximating those in continuous casting. The flux solidified in multiple layers similar to those observed from continuous casting molds and contained many gas bubbles. Flux conductivities average about 1.0 Wm K and appear to evolve with time. Contact resistances at both interfaces are significant and average about 0.0015 m². KW. Flux crystallization appears to be the only significant effect of flux composition. The one glassy flux tested had much greater thermal conductivities, presumably due to radiation transport. Temperature and gap thickness had a negligible effect on the properties. These properties depend on the model used to extract them. They are being implemented into a mathematical model to simulate heat transfer in the mold, interface, and solidifying shell of a continuous slab-casting machine. © 2000 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.

Résumé

Les problèmes de qualité de surface de l'acier de coulée continue sont grandement affectés parle transfert de chaleur à travers les couches interfaciales dans l'intervalle entre la gaine d'acier en train de se solidifier et le moule. On a construit un appareil expérimental afin de mesurer la température de l'acier, des couches d'écoulement du moule et du cuivre sous des conditions appro chant celles de la coulée continue. L'écoulement s'est solidifié en couches multiples similaires à celles observees dans les moules de coulée continue et celles-ci contenaient plusieurs bulles de gaz. La moyenne de conductivité de l'écoulement est d'environ 1.0 Wm K et semble évoluer avec le temps. La résistance de contact aux deux interfaces est importante et atteint en moyenne environ 0.0015 m².kw. La cristallisation de l'écoulement parait etre le seul effet important de la composition du flux. Le seul écoulement vitreux évalué avait une conductivité thermique beaucoup plus élevée, vraisemblablement dil au transport parradiation. La température et la largeur de l'intervalle avaient un effet négligeable sur les propriétés. Ces propriétés dépendent du modèle utilisé pour les extraire. On est en train de les développer en un modéle mathématique afin de simuler le transfert de chaleur dans le moule, à l'interface et dans la gaine de solidification d'un appareil de coulée continue de brame. © 2000 Canadian Institute of Mining and Metallurgy.Published by Elsevier Science Ltd. All rights reserved.     

Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Interfacial Phenomena

Experiments have been carried out to study the rates of mass transfer between liquid silicon and CaO-SiO₂ slag with impeller stirring at 1823 K (1550 °C). The occurrence of transient interfacial phenomena related to the mass transfer of calcium has been observed; the evidence suggests that the reduction of calcium oxide at the interface leads to a rapid, temporary drop in the apparent interfacial tension. At low apparent interfacial tension, mechanical agitation facilitates the dispersion of metal into the slag phase, which dramatically increases the interfacial area; here, it has been estimated to increase by at least one order of magnitude. As the reaction rate slows down, the apparent interfacial tension increases and the metal recoalesces. The incidental transfer of calcium very likely promotes the transfer of boron by increasing the interfacial area. Mechanical mixing appears to be an extremely effective means to increase the reaction rate of boron extraction and could feasibly be implemented in the industrial slag refining of silicon to improve reaction rates.     

Heat-Transfer Phenomena Across Mold Flux by Using the Inferred Emitter Technique

An investigation was carried out to study the heat-transfer phenomena across mold flux film by using infrared emitter technique (IET). With IET, it is possible to develop the mold fluxes with a liquid layer at the top and a solid layer in contact with copper mold with the degree of varying crystallization. The dynamic crystallization and melting process of the mold fluxes as well as their effects on the overall heat-transfer rate in the mold were successfully conducted. The single hot thermocouple technique (SHTT) was also employed in this investigation to study the melting and crystallization behaviors of mold fluxes for the interpretation of IET results. The results suggested that the interfacial thermal resistance between the solidified mold flux and copper mold would significantly influence the heat-transfer rate in continuous casting and the melting of the mold flux tends to enhance the overall heat-transfer rate. The technique established in this article by utilizing the IET can be well applied to the investigation of mold flux thermal properties, which in turn gives guidelines for the design of new mold flux for continuous casting.     

Interfacial Phenomena among Liquid Iron-Carbon Alloy, Liquid Slag, and Solid CaO

Interfacial phenomena between hot metal, liquid slag and solid CaO are important to the understanding of the desulfurization reaction in hot-metal treatment processes. In the current work, the surface tension of molten iron-carbon alloy and liquid slag as well as the interfacial tensions among molten iron-carbon alloy-solid CaO, liquid slag-solid CaO, as well as molten iron-carbon alloy-liquid slag were measured in the temperature range 1623?K to 1723?K (1350?°C to 1450?°C). The sessile drop method has been used for these measurements. To analyze the experimental results, two types of graphical analysis programs have been developed to determine the coordinates of the X-ray shadow or charge-coupled device (CCD) image of the droplet. Furthermore, a software package that uses the Gauss-Newton method to minimize an error function between the physically observed and a theoretical Laplacian curve has also been developed in this work.     

Rise of Gas Bubbles Across the Interface Between Two Liquids

Metallurgical and Materials Transactions B - Water and different oils were used to represent liquid metal and slag, respectively, in a cold model simulation of gas bubbles rising through molten...     

Diffusional Mass Transfer at Sediment-Water Interface

Utilizing a miniature, Clark-type, dissolved oxygen (DO) microprobe and a laser-Doppler velocimeter (LDV), laboratory experiments were conducted to elucidate the effect of the turbulent flow field on the diffusive sublayer thickness, mass transfer coefficient, and DO flux over a smooth bed. Both an artificial and a natural sediment were tested under flow conditions ranging in Reynolds number from 0 to 7,000, for a total of 17 experiments. The vertical resolution achieved with the microprobe enabled measurement of DO concentrations within the diffusive boundary layer and provided a direct measurement of the concentration sublayer thickness. Velocity profile measurements obtained with the LDV were used to estimate the depth-averaged velocity and the shear stress velocity. Analysis of the data included formulation of dimensionless groups and the obtaining of empirical relationships that can facilitate the prediction of the diffusive sublayer thickness, mass transfer coefficient, and mass flux at the sediment-water interface. Although the experimental work focuses on DO transport, the approach undertaken represents a generalized theory of waterside-controlled mass transfer at the sediment-water interface in the presence of a moving fluid.     

Predicting Interfacial Diffusion Coefficients for Fluxes across the Sediment-Water Interface

Calculations of interfacial diffusion coefficients are often based on tracer flux or penetration into permeable media or sediments. Based on previous investigations, a new empirical relationship for tracer-based interfacial diffusion coefficients is derived. This relationship is a powerful tool for estimating interfacial fluxes over a range of environmentally relevant conditions. Support for this relationship was found in experiments targeting the slip velocity at porous media–boundary layer interfaces. Slip measurements from flume experiments using flat permeable sediment beds and from previous studies using high permeability media were converted to interfacial diffusion coefficients based on the momentum flux needed to drive the observed interstitial flow. Slip-based estimates compared well with the tracer-based predictions over the entire range of flow-permeability conditions. This study presents a relationship for predicting fluxes across the sediment-water interface that is driven by permeability-scale processes. Predicted fluxes are comparable to those documented for other processes driving interfacial transport, such as bed topography and sediment transport.     

A New Insight to Interfacial Phenomena Occurring at Slag‐Metal Interfaces

Interfacial dilatational modulus was evaluated for slag‐metal systems using oxygen and sulfur as tracers at 1823 K. The high values of the dilatational modulus (5–10 times that obtained for surfactant adsorption) was directly related to the higher change in apparent interfacial tension prevailing at the slag‐metal interface. The variation in the dilatational modulus was attributed to the non‐uniform distribution of surface active elements at the interface and also due to the varying surface pressure. Further, experiments were designed to estimate the surface shear viscosity. A relationship was established to find the surface/interfacial shear viscosity from the Newton's law of viscosity. The order of magnitude of the interfacial shear viscosity at the slag‐metal interface was estimated from the values obtained earlier for the interfacial velocity. The order of magnitude obtained for slag‐metal systems was roughly 10–100 times that usually occurring in colloidal systems. The same could be attributed to the high bulk viscosities of the individual phases in slag‐metal systems. The order of magnitude of the interfacial velocity was verified from the equation generated earlier by dimension analysis to be similar to those obtained from experiments.     

Slip Transfer Across Hetero-Interfaces in Two-Phase Titanium Aluminum Intermetallics

The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned (PST) crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity in both the γ and the α ₂ phases. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study details of the hard mode of the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl.     

硫化锰与奥氏体之间半共格界面比界面能的理论计算

根据错配位错理论，提出了对钢中MnS与奥氏体之间半共格界面比理论计算方法，包括比界面能σ1，侧面上的比界面能σ2，MnS沉淀初期侧面上的比界面能σ2和平均比界面能的计算。当MnS尺寸很小时(≤100m)MnS长短轴比为5．3，当继续长大时，长短轴比值降低至1．5左右。     

Interfacial Heat Transfer during Die Casting of an Al-Si-Cu Alloy

The relationship between in-cavity pressure, heat flux, and heat-transfer coefficient during high-pressure die casting of an Al-9 pct Si-3 pct Cu alloy was investigated. Detailed measurements were performed using infrared probes and thermocouple arrays that accurately determine both casting and die surface temperatures during the pressure die casting of an aluminum A380 alloy. Concurrent in-cavity pressure measurements were also performed. These measurements enabled the correlation between in-cavity pressure and accurate heat-transfer coefficients in high-pressure die-casting operations.     

以出口暂定税率调控未锻造锰出口

为进一步控制高能耗、高污染和资源性产品出口，我国自2006年11月1日起，调整部分进出口商品暂定税率，对未锻造锰出口加征15％关税。文章分析了此次的调整对未锻造锰生产及出口的影响，并针对这些影响提出了几点建议。     

提高锰硅合金品级率途径的探讨

就如何采取有效措施提高锰硅合金品级率进行了论述,提出了提高混合锰矿二氧化硅含量,以贵州焦作为主要还原剂,采用升锰料批、升硅料批对产品质量波动进行及时调整的途径。     

Study on Transfer and Interfacial Kinetics of Neodymium and Samarium in Membrane Based Extraction

Ｔｈｅｅｘｔｒａｃｔｉｏｎｔｅｃｈｎｏｌｏｇｙｏｆｈｏｌｌｏｗｆｉｂｂｅｒｍｅｍｂｒａｎｅ (ＨＦＭ ) ,ｗｈｉｃｈｗａｓｄｅｖｅｌｏｐｅｄｉｎｅａｒｌｙ 1 980ｓ ,ｉｓａｎｅｗｋｉｎｄｏｆｓｅｐａｒａｔｉｏｎｍｅｔｈｏｄｃｏｍｂｉｎｅｄｐｒｏｃｅｓｓｅｓｏｆｌｉｑｕｉｄ -ｌｉｑｕｉｄａｎｄｍｅｍｂｒａｎｅｅｘｔｒａｃｔｉｏｎ .Ｔｈｉｓｍｅｔｈｏｄｃａｎｏｆ ｆｅｒｓｅｖｅｒａｌａｄｖａｎｔａｇｅｓｃｏｍｐａｒｅｄｗｉｔｈｃｏｎｖｅｎ ｔｉｏｎａｌｅｘｔｒａｃｔｏｒｓ[1～ 2 ] .Ｉｔｃａｎａｌｓｏｐｒｏｖ…     

一种薄带连铸工艺中瞬态界面传热模拟测量方法

研究和开发了一种可以测量钢水在铜辊表面凝固过程中瞬态界面传热的模拟装置及相应的界面热流计算方法.结果显示,钢水和铜基体接触的瞬间,热流达到最大值9 MW/m2左右,随后很快降低至4 MW/m2,然后维持在2～5 MW/m2之间.对比前人的研究结果,该模拟装置可以有效地用于研究薄带连铸中的界面传热现象.     

Simulation of Magnetohydrodynamic Multiphase Flow Phenomena and Interface Fluctuation in Aluminum Electrolytic Cell with Innovative Cathode

A three-dimensional (3D) transient mathematical model has been developed to understand the effect of innovative cathode on molten cryolite (bath)/molten aluminum (metal) interface fluctuation as well as energy-saving mechanism in aluminum electrolytic cell with innovative cathode. Based on the finite element method, the steady charge conservation law, Ohm’s law, and steady-state Maxwell’s equations were solved in order to investigate electric current field, magnetic field, and electromagnetic force (EMF) field. Then, an inhomogeneous multiphase flow model of three phases including bath, metal, and gas bubbles, based on the finite volume method, was implemented using the Euler/Euler approach to investigate melt motion and bath/metal interface fluctuation. EMF was incorporated into the momentum equations of bath and metal as a source term. Additionally, the interphase drag force was employed to consider different phase interactions. Thus, present work owns three main features: (1) magnetohydrodynamic multiphase flow are demonstrated in detail both in aluminum electrolytic cell with traditional cathode and innovative cathode; (2) bath/metal interface fluctuation due to different driving forces of gas bubbles, EMF, and the combined effect of the two driving forces is investigated, which is critical to the energy saving; and (3) the effect of innovative cathode on melt flow and motion of gas bubbles. A good agreement between the predicated results and measurement is obtained. The velocity difference leading to the melt oscillation decreases due to more uniform flow field. The average velocity of metal in the cell with innovative cathode decreases by approximately 33.98 pct. The gas bubbles in the cell with innovative cathode releases more quickly under the effect of protrusion on the cathode. The average bubble release frequency increases from 1.1 to 1.98 Hz. Hence, the voltage drop caused by gas bubbles would decrease significantly. In addition, the two large vortices are broken into many small vortices due to the protrusion. The final disappearance of the small vortices as a result of viscous dissipation is conducive to the suppression of bath/metal interface fluctuation. The average interface amplitude in the cell with innovative cathode reduces to 75.95 pct of that in the cell with traditional cathode.