Regimes of Micro-bubble Formation Using Gas Injection into Ladle Shroud
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tundish operations, Gas bubbling[1–4] is an effective methodology to enhance the removal of inclusions, especially for those smaller than 50 lm. The inclusion removal by bubble flotation occurs by two phenomena: adherence of inclusion on the bubble surface[5] and inclusion capture by the bubbles’ wakes.[6] Furthermore, the upward flow caused by bubble flotation can also reduce the thermal stratification of liquid steel. Zhang and Taniguchi[7] reviewed the fundamentals of inclusions removal by gas bubbling. The probability of attachment and detachment between inclusions and bubbles was discussed on the basis of theories from particle flotation in mineral processing. They reported that small bubbles performed better in the removal of inclusions, owing to their high attachment probability for inclusions, long residence time, and large surface-to-volume ratio. Moreover, small bubbles are beneficial for maintaining a stable slag layer during gas blowing, which prevents heat loss and re-oxidization
SHENG CHANG is with the School of Metallurgy, Northeastern University, Shenyang, 110819, Liaoning, P.R. China and also with the McGill Metals Processing Centre, McGill University, Montreal, QC, H3A 2B2, Canada. Contact e-mail: [email protected] XIANGKUN CAO is with the McGill Metals Processing Centre, McGill University and also with the Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853. ZONGSHU ZOU is with the School of Metallurgy, Northeastern University. Manuscript submitted July 31, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
of liquid steel. Hence, minimizing the bubble size has been the focus of gas bubbling in tundish. At present, gas curtain technique is commonly used to produce bubbles in tundish operations, in which gas was blown through a porous plug, forming a bubble column. Many studies had already successfully obtained bubbles around 2 mm (or even smaller) in water modeling, through decreasing gas flow rate and reducing the size of gas ports. Irons and Guthrie[8] performed experimental work to investigate the bubble formation in liquid pig iron at 1523 K. Bubble sizes were calculated by the bubble formation frequency combined with the gas flow rate. According to their results, the smallest bubbles, formed by a very small nozzle (1.6 mm) with an extremely low gas flow rate (0.03 L/min), were around 16 mm in diameter, which were still much bigger than the bubbles obtained in water modeling. In the gas curtain technique, the porous plug was located at the bottom of tundish where the liquid flow was very slow and smooth, providing a quasi-static condition for bubble formation. The impact of liquid flow on bubble departure can be neglected. Since liquid steel has approximately 22 times higher surface tension and 7 times higher density than water, bubbles in liquid steel would bear a much higher capillary pressure (caused by the surface tension) and hydrostatic pressure (caused by gravity of the liquid) than those in water modeling. These pressures, acting on the gas li
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