This thesis presents an in-depth investigation of Taylor bubbles in counter-current flows, employing a dual approach of high-fidelity numerical simulations and state-of-the-art experimental techniques. Taylor bubble is one of the manifestations of two-phase slug flow in a pipe. It consists of large gas bubbles separated from each other by slugs of the liquid phase. The dynamics of Taylor bubbles in air-water mixture were examined in two different regimes - the transitional flow regime with Reynolds number $Re = 1400$ and the fully turbulent flow with $Re = 5600$. The study focused on bubbles of various lengths in stagnant conditions, where buoyancy is counterbalanced by inertial drag in the downward turbulent flow.
The experimental study focuses on three main aspects: the analysis of bubble disintegration, the detailed analysis of the bubble interface dynamics and measurement of velocity fields using Particle Image Velocimetry (PIV) technique. By developing specialized algorithms for interface reconstruction from the obtained high-speed camera images, the research identified asymmetric, bullet-train-like shapes of Taylor bubbles in fully turbulent flow, but axi-symmetric bubbles in transitional liquid flow. This was a significant finding, demonstrating also the presence of disturbance waves along the bubble interface, which were tracked with high sensitivity. These waves exhibited correlated movements across the bubble surface, adding new understanding to the behavior of such flows.
The numerical study focused on the bubble behavior and breakup mechanisms, highlighting the limitations of the numerical methods. Using the OpenFOAM framework, the research implemented a high-order Runge-Kutta time-integration scheme combined with the Volume-Of-Fluid (VOF) method and the geometric reconstruction of bubble interfaces. The study concentrated on the transitional flow regime with a liquid $Re=1400$, comparing algebraic and geometric capturing techniques. The results highlighted the superiority of geometric reconstruction in capturing the nuances of Taylor bubble breakup. A novel discovery was also the emergence of a secondary vortex in the turbulent wake of the Taylor bubble, particularly noticeable at finer mesh resolutions.
This thesis contributes to the field of nuclear engineering and fluid dynamics by providing a more comprehensive understanding of Taylor bubble behavior in counter-current flows. The combination of experimental observations and numerical simulations offers a holistic view, paving the way for improved designs and operations in various engineering fields where such flow phenomena are encountered.
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