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UBC Theses and Dissertations

Visco-plastically lubricated multi-layer flows with application to transport in pipelines Sarmadi, Parisa


The thesis presents a novel triple-layer core-annular flow method in which we purposefully position an unyielded skin of a visco-plastic fluid between the core and the lubricating fluid to eliminate the possibility of interfacial instabilities. Specifically, the skin layer is shaped which allows for lubrication force to develop as the core rises under the action of buoyancy forces. The motivation originally stems from lubricated transport of heavy viscous oils. The objective is to reduce the frictional pressure gradient while avoiding interfacial instabilities. For this aim, first, we study this methodology for a steady periodic length of established flow, to establish the feasibility for the pipelining application. Second, we address the equally important issue of how in practice to develop a triple-layer flow with a sculpted visco-plastic skin, all within a concentric manifold by control of the flow rates of the individual fluids. The axisymmetric simulation establishes that these flows may be stably established in a controlled way. We develop a long-wavelength analysis of the extensional flow to predict the minimal yield stress required to maintain the skin rigid. Third, we extend the feasibility of the method to large pipes and higher flow rates by considering the effects of inertia and turbulence in the lubricating layer. We show that the method can generate enough lubrication force for wide range of parameters if the proper wave shape is imposed on the unyielded skin. Then, three-dimensional computations are performed to capture the buoyant motion of the core to reach its equilibrium position. The study shows that development lengths (times) for the core to attain equilibrium are relatively long, meaning extensive computation. We also present a simplified analytical model using the lubrication approximation and equations of motion for the lubricant and skin layers, to quickly estimate motion to the balanced configuration for a given shape and initial conditions. Finally, we show an explicit advantage of the proposed method in producing stable core-annular flows in regimes where conventional core-annular flows are unsuitable. In summary, we establish the potential of this new method for the stable and efficient transport of highly viscous fluids along pipelines.

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