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Bubbles in a viscoplastic liquid Pourzahedi, Ali


Gas rise in viscoplastic fluids is an interesting problem both from phenomenological and industrial points of view. Unlike Newtonian fluids, gas can remain entrapped inside a viscoplastic fluid if the yield stress is high enough. In studying the rise of gas bubbles in viscoplastic fluids, it is crucial to know this critical limit beyond which there is no flow. The main motivation of this thesis stems from the problem of methanogenesis in tailings ponds, which leads to the release of a significant amount of greenhouse gas emissions into the atmosphere. Tailings ponds are large density stratified settling ponds that store the waste materials generated during the mining process. They consist of multiple layers: a water layer on the top, a sludge-like layer around the middle, and a coarse sand layer mixed with sludge close to the bottom. In this thesis, we study the gas behavior in different layers of the tailings ponds through numerical, experimental and analytical methods. This thesis contains results that use three distinct methodologies in three complementary research areas. Firstly, it investigates the static behavior of buoyant bubbles in yield-stress liquids using numerical methods, determining the minimum yield stress needed to immobilize them, and studying contributing factors such as bubble shape and surface tension. The ratio of yield stress to surface tension (yield-capillary number) was found to play a key role. In the second area, the study delves into characterizing the steady motion of bubbles that do rise, often adopting an inverted teardrop shape with a pointed tail, distinct from predictions using basic yield-stress fluid models. We explore the origin of this shape using an experimental approach. Finally, the thesis explores gas propagation in porous media filled with yield-stress fluids, utilizing a pore-throat network model and a semi-analytical approach. It evaluates the impact of variables like throat radii, fluid yield stress, network size, exit time, gas volume fraction, and network stability in response to perturbations. Understanding the mechanics of gas propagation inside viscoplastic fluids offers valuable insights into a more sustainable resource management, minimizing the environmental impacts.

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