Spot turbulence, breakup, and coalescence of bubbles released from a porous plug injector into a gas-stirred ladle
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INTRODUCTION
REFINING metallurgical processes are often connected with gas injection from porous plugs and have been studied extensively in the last several years (Reference 1 provides a detailed review). The system studied is quite simple (Figure 1). It consists of a tank (ladle) containing the liquid metal; gas (usually argon) is injected from the bottom. Experiments are usually performed in the so-called water model in which the liquid metal is replaced with water and the gas used is air.[2,3,4] In our water model, the diameter and the height of the ladle are both 0.5 m, and the liquid level is 400 mm. Many experimental data and correlation describing the void fraction, liquid and gas velocity, and bubble frequency are available (for instance, Reference 5 through 7), but not enough attention has been paid to the bubble size distribution. On the other hand, numerical investigation often assumes a fixed bubble size.[8,9,10] This assumption is not valid at high flow rate, where coalescence and formation of large bubbles occur simultaneously with fragmentation. In this case, experiments show two transitions of phase among three different bubble dispersion patterns. The goal of this article is to describe the modeling of bubble behavior into a gas-stirred ladle including breakup and coalescence. It is important to discuss the presence of a porous plug instead of a simple nozzle gas injector. The use of the first [6,11] or the second[2,3] produces remarkable differences, especially in the region of primary bubbles (produced by the gas distributor). The main difference is that, from a porous plug, the gas is released in a form somehow comparable to bubbles. From a simple nozzle, at high gas flow rates, gas is released in a jet form. Since laws of coalescence and fragmentation used in this work are specific for bubbles, they apply mainly to the porous plug injector. ALESSIO ALEXIADIS, Researcher, formerly with ARMINES, École des Mines, 75272 Paris, Cedex 06, France, is with the School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney 2052, Australia. Contact e-mail: [email protected] PASCAL GARDIN and JEAN FRANÇOIS DOMGIN, Researchers, are with the Heat Transfer, Electromagnetism and Fluid Dynamics Department, IRSID, 57283 Maizières-Les-Metz, Cedex, France. Manuscript submitted October 29, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B
In the ladle, a volumetric gas flow QG of argon (air in the water model) is injected from the bottom. In the air-water model, air is injected: it is possible to observe different hydrodynamic regimes at different values of QG. In the first pattern (Figure 1(a)), the bubbles have, more or less, the same size in the range of 2 to 5 mm. In the intermediate gas-flow rate regime (Figure 1(b)), small bubbles are generated but some of them coalesce in larger bubbles. In the higher gas flow rate (Figure 1(c)), the bubbles forming at the nozzle exit are merged in one large pocket. First transition (discrete bubbling to incipient coalescence) and se
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