X-ray fluoroscopy observations of bubble formation and separation at a metal-slag interface
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V.F. CHEVRIER, Graduate Student, and A.W. CRAMB, Professor, are with the Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted December 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
the outer edge of the nozzles, rather than the inner diameter as in water. Direct observation of gases injected into molten metals with X-ray techniques[2–5] confirmed these results, but quantitative studies were not possible because the crucibles used had to be on the order of the bubble diameter to permit X-ray penetration through the metal. In the present work, the diameter was calculated using simultaneously a flowmeter and pressure gage and directly in the slag layer using the fluoroscope. Although great attention was paid to the nozzle assembly, the two methods did not compare well because of a lack of conservation of flow rate: the gas volume in the bubbles did not equal the injected gas volume flow rate. The observed frequency of bubble release was identical using both methods. In some cases, we observed a continuous film of gas along the crucible wall, as shown in Figure 1. The size of the bubbles formed, measured with the fluoroscope, was then compared to the predicted values from the equation derived in the case of a single bubble forming at the tip of a nozzle at low flow rate, where only the outer diameter do is considered:
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[1]
In some cases, when the nozzle tip is close to the bottom of the crucible, the resulting bubbles are larger than predicted by Eq. [1]. When the nozzle is pushed further inside the metal bath, the measured size is comparable. Irons and Guthrie[2] experimentally observed that nitrogen spread on a horizontal nozzle when injected into an indium-gallium alloy, but did not expect spreading to occur on an upward facing nozzle because of the opposition by buoyancy forces. This hypothesis was qualitatively investigated for carbon-saturated iron by taking radiographic pictures of the bubbles forming at the nozzle in a rectangular crucible (12-mm thick). Two separate experiments were performed and are shown in Figure 2: (a) with the nozzle about 3 mm from the bottom of the crucible and (b) with the nozzle about 11 mm from the bottom of the crucible. Figure 2 confirms that the bubbles are growing along the outside of the nozzle but also on the bottom of the crucible when the nozzle tip is close to the crucible. A double-bore nozzle was also used and the X-ray radiographs showed that only single bubbles are formed because of the spreading of the gas on the nozzle tip, confirming the results by Wang et al.[4] These photographs indicate that it is neither the inner nor the outer bore of the nozzle that controls bubble size and the mechanism of bubble detachment. The bubbles detach when the gas film growing from the injection point reaches an unstable configuration where the buoyant energy of the gas overcomes the interfacial energy of the film. This point is a function of the overall shape of the film, which is dependent upon
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