Development of a multineedle electroresistivity probe for measuring bubble characteristics in molten metal baths
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I.
INTRODUCTION
IT is widely recognized that the behavior of bubbles rising in the reactors of material-refining processes agitated by gas injection plays an essential role for the efficiency of the processes.[1–5] Theoretical and experimental investigations on the bubble behavior, therefore, have been carried out by many researchers on the basis of various kinds of cold and hot models. The bubble characteristics specified by gas holdup a, bubble frequency fB, mean bubble rising velocity uB, and mean bubble chord length LB can be measured with a two-needle electroresistivity probe which has been used extensively in this research field.[6–13] It should be noted that the first two values can be measured with a one-needle electroresistivity probe as well. Although metallurgical reactions between molten metal and bubbles in a bath are significantly influenced by the total gas-liquid interfacial area, the conventional two-needle electroresistivity probe cannot give us any information on the bubble shape, which is closely associated with the total interfacial area. Even an X-ray fluoroscope cannot give us detailed information on the shape of bubbles. It can detect only the contour of a bubble. Considering these circumstances, we decided to develop a multineedle electroresistivity probe capable of detecting the shape of bubbles in addition to the aforementioned gas holdup, bubble frequency, mean bubble rising velocity, and mean bubble chord length. It is said that a prototype of this kind of multineedle probe has been developed by Davenport et al.[14] to detect a single cap-shape bubble rising in mercury. The accuracy of a newly developed multineedle electroresistivity probe was first examined using an aqueous system. Experimental results of the gas holdup, bubble frequency, mean bubble rising velocity, and mean bubble chord length were compared satisfactorily with their reMANABU 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, and HIROTOSHI KAWABATA, Technician, Faculty of Engineering, are with Osaka University, Osaka, 565 Japan. Manuscript submitted March 12, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
spective values obtained using a two-needle electroresistivity probe. The shape of each bubble detected with the multineedle electroresistivity probe was very similar to that observed with a high-speed video camera. The multineedle electroresistivity probe was subsequently applied to a Wood’s metal-He system to clarify the shape of inert gas bubbles in molten metal baths. II.
EXPERIMENTAL APPARATUS AND MEASUREMENT METHOD
A. Water-Air System Figure 1 shows a schematic diagram of the experimental apparatus for an aqueous system. The cylindrical vessel made of transparent acrylic resin had an inner diameter D of 200 mm and a height H of 390 mm. Water was filled at a depth HL of 250 mm. In many cases, air was injected
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