Numerical Simulation of the Interaction Between

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Numerical Simulation of the Interaction Between Supersonic Oxygen Jets and Molten Slag–Metal Bath in Steelmaking BOF Process QIANG LI, MINGMING LI, SHIBO KUANG, and ZONGSHU ZOU The impinging of multiple jets onto the molten bath in the BOF steelmaking process plays a crucial role in reactor performance but is not clearly understood. This paper presents a numerical study of the interaction between the multiple jets and slag–metal bath in a BOF by means of the three-phase volume of fluid model. The validity of the model is first examined by comparing the numerical results with experimental measurement of time-averaged cavity dimensions through a scaled-down water model. The calculated results are in reasonably good agreement with the experimental data. The mathematical model is then used to investigate the primary transport phenomena of the jets-bath interaction inside a 150-ton commercial BOF under steelmaking conditions. The numerical results show that the cavity profile and interface of slag/metal/gas remain unstable as a result of the propagation of surface waves, which, likely as a major factor, governs the generation of metal droplets and their initial spatiotemporal distribution. The total momentum transferred from the jets into the bath is consumed about a half to drive the movement of slag, rather than fully converted as the stirring power for the metal bath. Finally, the effects of operational conditions and fluid properties are quantified. It is shown that compared to viscosity and surface tension of the melts, operating pressure and lance height have a much more significant impact on the slag–metal interface behavior and cavity shape as well as the fluid dynamics in the molten bath. DOI: 10.1007/s11663-015-0292-3  The Minerals, Metals & Materials Society and ASM International 2015

I.

INTRODUCTION

THE impingement of the multiple supersonic oxygen jets, from a water-cooled lance installed with convergent-divergent nozzles, onto the free surface of molten slag–metal bath is inevitably encountered in modern BOF steelmaking. In this process, the jets deliver adequate oxygen into the molten bath for oxidation reactions. Consequently, the pig iron inside the bath is decarburized, the impurities like phosphorous, silicon, and other elements are transferred to slag, and then the target composition and temperature of steel is eventually achieved. Furthermore, the impingement of supersonic jets provides the essential stirring kinetic energy to the molten bath for gaining spontaneous emulsification that massively enhances the rates of heat transfer and chemical reactions.[1] Therefore, understanding the interaction characteristics between the supersonic oxygen jets and molten slag–metal bath inside a BOF is QIANG LI, Associate Processor, MINGMING LI, Ph.D. Candidate, and ZONGSHU ZOU, Professor, are with the School of Materials and Metallurgy, Northeastern University, Heping District, Shenyang 110819, P.R. China, and also with the Key Laboratory of Ecological Utilization of Multi-metallic Mineral of Education Ministry, Northeastern University, Heping District, Shenyang 110819, P.R. China. Contact e-mail: [email protected] SHIBO KUANG, Research Fellow, is with the School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. Manuscript submitted March 23, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B

crucial for accurate prediction and control of the flows, heat/mass transfers, and chemical reactions to achieve stable BOF operation and high operation efficiency. The cavity is one of the most important outcomes of the interaction between jets and a bath. Dogan et al.[2] reported that 55 pct of the total carbon is removed in the impact zone of the cavity during a BOF steelmaking blowing process. Thus, the determination of cavity profile and its spatiotemporal characteristics is of paramount importance to a better understanding of the complex phenomena in relation to spontaneous emulsification, mass transfer among oxygen/metal/slag, and so on.[1] More importantly, it provides the basic information for establishing comprehensive mathematical models that have been widely developed to improve the operation and control of BOF steelmaking.[3–12] However, the comprehensive models developed in the previous studies[9–12] were assumed to be static and/or one-dimensional, treated the multiple fluids inside the entire reactor as an ideal mixture of fluids. In other worlds, they ignored the spatiotemporal distributions of fluid hydrodynamics, heat and mass transfer, and chemical reactions. In principle, this deficiency can be overcome with the development of various state-of-theart numerical methods and emerging computer technology. But, to ultimately achieve this objective, fundamental phenomena inside a steelmaking BOF should be properly identified, and rational and reliable sub-models, which are capable of reasonably describing the timeand space-dependent characteristics of the phenomena

identified, need to be developed. An important aspect in this direction is the understanding and modeling of the interaction between the jets and molten bath. On the other hand, because of the high temperature and hazardous conditions, a direct measurement of transport phenomena is extremely difficult, if not impossible, for practical BOF steelmaking process. To overcome these problems, a vast majority of the past research efforts were contributed to the study of the primary phenomena associated with the impingement of jets onto a liquid bath through water model[13–23] and mathematical modeling.[5,7,8,13,16,24–33] These studies improve our understanding of BOF steelmaking but are largely limited to cold model conditions and a single kind of liquid bath. In practice, BOFs are often operated under high-temperature conditions, the bath consists of molten slag and metal, and the compressibility of supersonic oxygen flow is not negligible. As such, to date, insightful studies of the interaction between the jets and molten slag–metal bath inside a BOF under industrial conditions are very limited. The quantitative effects of pertinent operational parameters and liquid physical properties on the interaction process are few. This information is, however, useful not only for the operation and control of BOF but also for the establishment of a predictive BOF model. The objective of the present study is to reveal the fundamental sub-phenomena in the molten bath induced by supersonic oxygen jets inside a BOF under the steelmaking conditions by use of volume of fluid (VOF) approach. The validity of the VOF model is first examined. The model is then used to investigate the primary transport phenomena inside a 150-ton commercial converter under steelmaking conditions. Finally, the effects of operating pressure, lance height and physical properties of molten liquid such as viscosity and surface tension are studied.

II.

MATHEMATICAL MODEL

A. Outline of the BOF Modeling (1) The flows of molten slag and metal as well as oxygen gas are three-dimensional, unsteady and non-isothermal. (2) The gas phase is regarded as compressible Newtonian fluid, while the melts as incompressible Newtonian fluid. (3) Viscosity and surface tension of all the phases are assumed constant in each simulation. (4) Chemical reactions are not taken into consideration. B. Governing Equations To describe the sharp oxygen–slag–metal free interface, the VOF model applicable to complicated propagating interface problems is employed. The VOF method was first presented by Hirt and Nichols[34] and started a new trend in simulation of multiphase flows. It relies on the definition of an indicator function a, whose

value is unity at any point occupied by the fluid and zero otherwise. The average value of a in a cell represents the volume fraction of the cell occupied by the fluid. This information allows us to know whether single fluid or the mixture of fluids occupies the cell. If the value of a is in the range between zero and one in a cell, it suggests that the cell contains a free transient surface described by S(t). In a brief summary, three conditions can be defined according to the volume fraction of qth phase (aq): aq=0: The cell is empty (of phase q) aq=1: The cell is full (of phase q) 0