Critical fluid-flow phenomenon in a gas-stirred ladle
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I.
INTRODUCTION
THE bottom
injection of gas into molten baths has been practiced widely throughout the metallurgical industry. Consequently, numerous studies have been conducted to understand various aspects of submerged gas jet behavior. By way of example, in this laboratory, Castillejos and Brimacombet~] measured the distributions of gas fraction, bubble velocity, and bubble diameter in air-water s y s t e m s I21 and in gas-mercury systems.[3] Anagbo and Brimacombe[4] obtained the bubble characteristics in a submerged gas jet injected through a porous plug, while Sahajwalla et al. ts] determined the gas fraction distribution in the spout at the bath surface as a function of gas-flow rate. The gas fraction correlations emanating from these injection studies have been incorporated into a mathematical model of turbulent recirculating fluid flow in a gas-stirred liquid.tr] One of the most interesting findings from the study of Sahajwalla et al. t5] was the existence of a critical gasinjection rate above which the discharge of energy at the bath surface increases markedly. In their investigation (the model bath had an internal diameter of 500 mm and a height of 400 ram), the critical gas-flow rate was 1350 NcmS/s./6] Based on an analysis of the specific input power densities from four different systemst7-1~ exhibiting a critical gas-injection rate, a critical specific input power density of 0.065 to 0.13 W/kg was determined (dimensions of vessels are listed in Table I). Knowledge of the critical gas-injection rate is important for the control of fluid flow in gas-stirred processes. Above M. ZHOU formerly was Post Doctoral Fellow with the Centre for Metallurgical Process Engineering, University of British Columbia, Vancouver, BC, Canada V6T IZ4. J.K. BRIMACOMBE holds the Alcan Chair in Materials Process Engineering and is Director of the Centre for Metallurgical Process Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1ZA. Manuscript submitted February 19, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS B
the critical injection rate, the elevated discharge of energy at the bath surface by the disengaging gas promotes slag-metal reactions in this location. Below the critical injection rate, a greater fraction of the gas buoyancy energy is transferred to the liquid, which enhances bulk mixing. Thus, the present work has been undertaken to study the critical injection rate further and to develop a theoretical understanding of this phenomenon. Measurements have been made of velocities in the two-phase jet (electroresistivity probe) and in the bath (laser-Doppler velocimeter) as a function of gas-injection rate, while a combined Kelvin-Helmholtz and Rayleigh-Taylor instability analysis was conducted to examine the breakup of freely rising bubbles. II.
PREVIOUS WORK
A critical fluid-flow phenomenon in bottom gas-stirred processes, manifested in the mixing time, tH-14] the fluid flow, |11'13]the spatial average shear, ts] and the mass-lransfer coefficient,[gJ has been reported by other investigator
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