The shape of bubbles rising near the nozzle exit in molten metal baths
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I.
INTRODUCTION
THE shape of bubbles is a key parameter for evaluating the interfacial area between bubbles and molten metal. A two-needle electroresistivity probe has been widely used to study bubble characteristics in refining processes agitated by bottom gas injection.[1–17] This probe permits us to determine gas holdup, bubble frequency, mean bubble rising velocity, and chord length. However, it cannot detect the shape of the rising bubbles, and, consequently, most of the interfacial area calculations assume spherical bubbles, which is not the case.[18] This assumption leads to considerable errors for the interfacial area under certain blowing conditions. The shape of a single bubble rising in a still bath has been extensively investigated by many researchers in a variety of engineering fields. The results are summarized by Clift et al.[19] as functions of the physical properties of gas and liquid, blowing conditions, etc. On the other hand, information on the shape of bubbles generated successively in molten metal baths has not been obtained except for a very low gas flow rate regime.[20,21] This is mainly because molten metals are not transparent and each bubble is highly influenced by turbulent liquid motion induced by preceding bubbles. Even an X-ray fluoroscope cannot detect precisely the shape of bubbles in molten metals.[20,21] Therefore, this work is focused on the detection and classification of the shape of bubbles generated successively in molten metal baths agitated by bottom gas injection. The shape of bubbles in molten Wood’s metal and mercury baths was observed using a previously developed multineedle electroresistivity probe. With this probe, the relationship between the shape of bubbles and bubble characteristics was determined. It was possible to create a bubble shape diagram by correlating the shape of bubbles as a MANABU IGUCHI, formerly Associate Professor, Faculty of Engineering, Osaka University, is Professor, Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Hokkaido, 060 Japan. TADATOSHI NAKATANI, Graduate Student, is with Osaka University, Osaka, 565 Japan. HIROHIKO TOKUNAGA, Engineer, is with Kakogawa Works, Kobe Steel Ltd., Hyogo, 675 Japan. Manuscript submitted May 6, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
function of a modified Reynolds number and a modified Weber number. The radial distributions of bubble characteristics from previous investigations in different gas-liquid systems have also been plotted on this diagram. II.
EXPERIMENTAL APPARATUS AND MEASUREMENT METHOD
A. Experiments Using Molten Wood’s Metal Bath A schematic of the experimental apparatus for molten Wood’s metal is shown in Figure 1. The origin of the cylindrical coordinates system (r, u, z ) was placed on the exit of a single-hole central bottom nozzle. Wood’s metal with a melting point of 72 7C was melted in a stainless steel cylindrical vessel with an inner diameter D of 20 cm and a height of 30 cm. The bath depth was 15 cm. Electroresistivity probe
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