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Gas injection reactors in general, and bubble column reactors in particular, are key elements of many pyrometallurgical flow charts. Nevertheless, the phenomena and interactions that govern these reactors are not yet fully understood. From a multiscale point of view, the main bottleneck is situated at a mesoscopic level on which individual bubbles are considered. While simulations and water models can be very helpful to widen this bottleneck, experimental observations of gas bubbles in real pyrometallurgical systems remain indispensable for validation and fine-tuning of mesoscopic system descriptions. Unfortunately, the opacity of these systems enforces the use of indirect imaging techniques with limited temporal or spatial resolution. In addition, accurate tracking of the gas-liquid interface requires tomography, further complicating the design of an experimental setup. In this doctoral study an alternative and innovative approach is suggested that circumvents these two main restrictions. By injecting gas in a thin sheet of liquid entrapped between two flat and closely spaced plates, bubbles in a Hele-Shaw flow regime are generated. The resulting quasi-two-dimensional multiphase flow phenomena can be fully captured from a single point of view. Moreover, when using a transparent plate material that is not wetted by the liquid, even bubbles in opaque liquids can be visualized directly. This approach is explored for inert gas injection in liquid metals. To demonstrate the feasibility of the suggested Hele-Shaw based approach, buoyancy driven nitrogen bubbles in liquid mercury are observed at room temperature in a Hele-Shaw cell of 1 mm thickness. By using a moving high speed camera to make continuous close up recordings of individual bubbles, the position and geometry of these bubbles are quantified with a high resolution along their entire path. After a thorough evaluation of the experimental accuracy, this information is used for a detailed analysis of bubble volume variations. It is clear that a hydrostatic pressure gradient accounts for the most of the observed bubble growth. Yet, a careful assessment of the variations for smaller bubbles suggests that an accurate bubble description should also account for significant dynamic pressure variations that are largely flow regime dependent. Contrary to aqueous systems, in mercury transitions between these regimes can readily be observed along the trajectory of individual, expanding bubbles. A mapping for bubbles with diameters between 4 mm and 20 mm shows the existence of two different regimes: smaller bubbles adopt a constant round-to-elliptical shape and travel along a linear path while larger bubbles exhibit a periodically distorted shape and follow a swirling trajectory. The transition between this linear and periodic regime can be attributed to a shift from capillary to inertia dominated flow with increasing bubble size. Furthermore, a quantitative mapping of the bubble velocity shows that the transition goes hand in hand with a steep acceleration, and that the linear regime is marked by a negative correlation between the Eötvös number and the Reynolds number while the opposite is true for the periodic regime. To demonstrate the applicability of the suggested Hele-Shaw based approach for industrially relevant systems, a high-temperature experimental setup is developed. With this setup, nitrogen bubbles are observed in liquid zinc at 700°C in a fused quartz cell with a thickness of 1.5 mm. At low oxygen levels, cell walls are not wetted by the liquid zinc and bubbles can be visualized directly through the transparent cell walls, using the same moving high speed camera as for mercury. In the range of diameters between 5.9 and 9.0 mm, this reveals a single periodic flow regime in which bubbles follow a sinusoidal path with a characteristic frequency of 3.31 Hz. In addition, systematic intermediate accelerations are observed of which the exact origin remains unexplained. A direct extrapolation of the observations in a Hele-Shaw cell to industrially relevant geometries is not straightforward. As a consequence, such experiments will never be able to completely replace three-dimensional observations. Nevertheless, the suggested approach seems very promising as a first step in the study of gas bubbles in liquid metals. After all, in a Hele-Shaw cell, the same effects and interactions that govern unconfined bubbles can be studied. In addition, the resolution of the observations is unseen for bubbles in liquid metals, especially at high temperatures. This makes them highly suitable for detailed validations of simulation results. Therefore it is expected that this approach can contribute to a better understanding of the mechanisms that govern gas injection in pyrometallurgy and support future developments in this field.

669 <043> --- Metallurgy--Dissertaties --- Academische collection --- Theses --- 669 <043> Metallurgy--Dissertaties