Tese e Dissertação

Tese: Startup pipeline flow of thixotropic materials

Understanding the mechanical response of time-/strain history-dependent materials is essential for various industrial applications, notably in pipeline transportation. These materials exhibit complex rheological behavior, that hugely influence their flow characteristics. Despite decades of research, accurately modeling the mechanical behavior of these materials remains a challenge, with existing models often constrained by their inability to predict complex responses beyond simple shear flow configurations. Consequently, there is a pressing need for further investigation to refine our understanding and predictive capabilities in this field, ensuring the efficient and safe operation of industrial processes involving such materials. This work aims to investigate the flow restart of thixotropic fluids through both numerical and experimental methods. Specifically, this study involves modelling a thixotropic Laponite suspension 1.25% wt using a constitutive equation that faithfully accounts the real material’s behavior during processes that induce microstructural breakage or construction. Numerical simulations were used to analyze the restart processes in a pipe using the modelled suspension and compare it with a suspension present in the literature modelled with the same mathematical model to understand the role of thixotropy in the flow restart. Additionally, an attempt to validate the constitutive equation obtained was made. The numerical analysis indicated that both fluids exhibited similar qualitative behavior for the temporal fluidity evolution. While high thixotropy caused a gradual transition from rest to flow, low thixotropy resulted in sudden transitions. Higher imposed pressure gradients were associated with earlier resumption of flow and larger sheared regions across the spatial domain for both fluids. Additionally, for both fluids, the steady-state regime was solely defined by the imposed pressure gradient. Despite the limitations of the experimental apparatus and the notable differences between the numerical predictions and experimental results for the Laponite suspension formulated in this work, the mathematical model employed showed to be a useful tool to study the restart flow of thixotropic fluids.

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