Mixing time and fluid flow phenomena in liquids of varying kinematic viscosities agitated by bottom gas injection
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INTRODUCTION
IN steelmaking processes, gas injection techniques have been extensively utilized to enhance chemical reaction rates, homogenize temperature and chemical compositions, and remove impurities and nonmetallic inclusions. As one of the indices for the degree of mixing, the concept of mixing time has been widely employed. Numerous investigations on the mixing time for various types of reactors with gas stirring have been carried out using a deionized water bath and an electrical conductivity sensor.[1,2,3] Deionized water has commonly been used as a model for molten steel, because its kinematic viscosity is approximately the same as that of the molten steel. Dilute aqueous KCl solution is chosen as a tracer. The mixing time can be determined from the history of the electrical conductivity of liquid in the bath. In in-bath smelting reduction processes[4] and some kinds of refining processes, such as the desulfurization process,[3] a thick, top slag layer exists on the molten metal layer. Mixing in the top slag layer plays an important role in slag/metal reactions, i.e., in the efficiency of the processes, as the reactions are controlled by mass transfer in the top slag layer.[3] Transparent oily liquids such as silicone oil are usually used as models for top slag. It is, however, difficult to measure the mixing time in oily liquid baths by making use of the conventional electrical conductivity sensor,[1,2,3] because aqueous KCl solution is insoluble in the oily liquid baths. Accordingly, in a previous study,[5] we developed a new MANABU IGUCHI, Professor, is with the Division of Materials Science and Engineering, Graduate School of Engineering, Hakkaido University, Hokkaido, 060 Japan. KEI-ICHI NAKAMURA, formerly Graduate Student, Department of Materials Science and Processing, Osaka University, Osaka, 565 Japan, is Researcher, Steelmaking Section, Wakayama Works, Sumitomo Metal Industries Ltd., Wakayama, 690 Japan. RYOJI TSUJINO, Professor, is with the Department of Mechanical Engineering, Osaka Institute of Technology, Junior College, Osaka, 535 Japan. Manuscript submitted May 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
method for determining the mixing time in an oily liquid bath. Fine CaCO3 powder was used as a tracer. This method is based on the measurement of a temporal change in the transmittance of a laser beam crossing through a transparent vessel containing oily liquid. The output voltage of the sensor, which is uniquely related to the change in the transmittance, approaches a constant value as mixing proceeds, and, accordingly, the mixing time can be determined in the same manner as for the conventional electrical conductivity method. In this study, we measured the mixing time in a silicone oil bath agitated by central bottom air injection with this laser optical sensor. Four kinds of silicone oils with different kinematic viscosities were employed, and an empirical correlation for the mixing time was derived. Bubble-rising velocity and silicone oil flow velocity, being closely rela
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