Voulgaropoulos, Victor; (2018) Dynamics of spatially evolving dispersed flows. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This dissertation provides a unique insight into the flow dynamics of evolving dispersed pipe flows. Kinetically unstable liquid-liquid dispersions are actuated in two horizontal flow loop systems. Novel conductivity and optical laser-based experimental methods are developed and applied at several axial locations capturing the flow characteristics and separation properties of the dispersions downstream the pipe with combined measurements of drop sizes, phase fractions and velocities. Flow pattern transitions are recorded for low mixture velocities as the dispersions flow. Drops segregate and coalesce forming a second continuous layer. Drop size measurements exhibit growth of the drops along the streamwise direction independent of the flow pattern, with larger drops recorded closer to the direction of buoyancy. A phenomenological model based on batch vessel settlers is modified and is found to predict well the axial evolution of the dispersions. Holdup and velocity measurements acquired from laser diagnostics are compared with CFD predictions obtained using a mixture approach implementing an effective viscosity model. Good comparisons are obtained by considering sedimentation, shear-induced diffusion and lift. The dispersions behave as suspensions of solid rigid spheres for the conditions investigated. Asymmetry in the velocity profiles is found for both experiments and simulations as the dispersions separate, with the maxima of the velocity located in the drop-free layer. Due to the prominent role of coalescence in the system, its dynamics are studied both during pipe flow and in a Hele-Shaw cell. For the former, high resolution velocity field measurements illustrate the vortices generated from the rupture point of the film inside a coalescing drop and its expanding neck until it fully merges with the bulk, being in agreement with scaling laws for immobile systems. The latter cases are used to investigate the effect of surface active agents and complex fluids. Surfactants are found to deform the interface, increase locally their concentration at the neck and change the propagation direction of the vortices. Xanthan gum addition in the coalescing phase slows down the neck expansion velocity and causes a spatial variation of the viscosity affecting the velocity field inside the drop.

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