Water model study of the frequency of bubble formation under reduced and elevated pressures
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I.
INTRODUCTION
MANY kinds of steelmaking processes subjected to gas injection have been used extensively. The surface pressure of the molten metal bath of the processes is often highly reduced to enhance the efficiency of degassing.[1–5] The efficiency is closely associated with the total interfacial area between molten metal and rising bubbles. Liquid flow motion induced by bubbles is also responsible for the efficiency. Precise information on the behavior of rising bubbles under reduced pressure, therefore, is required for further enhancement of the efficiency of current degassing processes as well as development of new degassing processes. The behavior of rising bubbles can be characterized by gas holdup, a, bubble frequency, fB, mean bubble rising velocity, u B, mean chord length, L B, and the shape of bubbles. These quantities are governed primarily by the shape and size of bubbles formed from a nozzle or an orifice, and the bubble formation is dependent strongly on the physical properties of liquid and gas, blowing conditions, and the configuration of baths.[6,7] Previous model studies of bubble formation in molten metal baths have been carried out, mainly under an atmospheric surface pressure.[6–10] Only a few articles have reported on the volumetric mass transfer coefficient[11] and bubble and liquid flow characteristics under reduced surface pressures.[12,13] Information on the bubble formation under reduced pressures, however, is very limited, even for water models.[11,12] The reduced surface pressure, Ps, is uniquely associated
MANABU IGUCHI, Professor, is with the Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628 Japan. TOMOYUKI CHIHARA, formerly Graduate Student, Graduate School of Engineering, Osaka University, is Engineer, Daido Special Steel Co. Ltd., Aichi, 477 Japan. Manuscript submitted December 31, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
with the hydrostatic pressure, Ph, at the nozzle and orifice exits: Ph 5 Ps 1 rL g HL
[1]
where rL is the density of liquid, g is the acceleration due to gravity, and HL is the bath depth. Knowledge about the effect of the hydrostatic pressure around bubble generating devices on the bubble formation is also desired in other fields of engineering such as waste water processing.[14] Unfortunately, useful information is not available at the present stage. The main purpose of this water model study is to elucidate the effect of reduced and elevated bath surface pressures on the frequency of bubble formation from a single-hole nozzle or a single-hole orifice placed at the bottom center of a cylindrical water bath. The frequency of bubble formation was determined using a high-speed video camera. II.
EXPERIMENTAL APPARATUS AND PROCEDURE
Figure 1 shows a schematic diagram of the experimental apparatus capable of changing the surface pressure of a water bath. The cylindrical test vessel is made of transparent acrylic resin, having an inner diameter, D, of 20 cm and a height, H, of 40 cm
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