Abstract
Gas bubbles are of major importance in most metallurgical processes. They promote chemical reactions, homogenize the melt, or float inclusions. Thus, their dynamics are of crucial interest for the optimization of metallurgical processes. In this work, the state of knowledge of bubble dynamics at the bubble scale in liquid metals is reviewed. Measurement methods, with emphasis on liquid metals, are presented, and difficulties and shortcomings are analyzed. The bubble formation mechanism at nozzles and purging plugs is discussed. The uncertainty regarding the prediction of the bubble size distribution in real processes is demonstrated using the example of the steel casting ladle. Finally, the state of knowledge on bubble deformation and interfacial forces is summarized and the scalability of existing correlations to liquid metals is critically discussed. It is shown that the dynamics of bubbles, especially in liquid metals, are far from understood. While the drag force can be predicted reasonably well, there are large uncertainties regarding the bubble size distribution, deformation, and lift force. In particular, the influence of contaminants, which cannot yet be quantified in real processes, complicates the discussion and the comparability of experimental measurements. Further open questions are discussed and possible solutions are proposed.
Highlights
Due to the small number of studies in liquid metals, conclusions drawn from aqueous systems are summarized first. Their transferability to liquid metals is discussed. This allows guidelines to be derived as to which models should currently be used for computational fluid dynamics (CFD), and the model uncertainty can be analyzed in more detail
For a combination of regime I and II, a high agreement with the empirical correlation of Mori et al [46] (Equation (7)) was found, assuming that the total flow rate is evenly distributed among the individual bubble formation sites and that they behave like single nozzles [71]
Both use the volume of fluid surface tracking method. They reported that bubbles are spherical after injection but become wobbly or ellipsoidal cap shaped, depending on their size and their Eötvös and Reynolds numbers. When evaluating these results, it should be noted that no turbulence model was used in both studies, but the mesh resolution used is too coarse for a real direct numerical simulation (DNS)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Metallurgical bubble column reactors are of major importance for the process industry. Their main advantages are a simple construction, low maintenance costs, good mass, and heat transfer [1] and applicability in case mechanic stirring is prohibited by high temperature or reactive flows. Bubbles react to pressure gradients with a deformation This leads to fascinating phenomena such as shape and path oscillation. This review covers the bubble formation mechanism at nozzles and purging plugs, bubble deformation, and the interfacial forces between bubbles and fluid. Their transferability to liquid metals is discussed This allows guidelines to be derived as to which models should currently be used for CFD, and the model uncertainty can be analyzed in more detail. Open questions are identified and possible solution strategies are proposed
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