MODELING SHEAR-INDUCED PARTICLE MIGRATION AND LUBRICATION LAYER PROPERTIES IN THE PIPE FLOW OF VISCOPLASTIC FLUIDS

Abstract

This thesis aims to explore shear-induced particle migration (SIPM) and lubrication layer properties of concentrated suspensions within viscoplastic fluid flows in piping systems. Recognizing the pivotal role these phenomena play in optimizing industrial processes, thesis investigates the complex behavior of concentrated suspensions within viscoplastic fluids in pipe flow, focusing on the phenomena of shear-induced particle migration (SIPM) and the properties of lubrication layers. Viscoplastic fluids, characterized by a yield stress below which they behave as solids, are prevalent in various industrial processes and natural phenomena. The study utilizes the Finite Element Method (FEM) within COMSOL Multiphysics 6.2 software to model the flow through three geometrical configurations: straight pipe, nozzle, and elbow bend. The Bingham-Papanastasiou model serves as the foundation for representing the fluid's viscoplastic behavior, providing a more accurate depiction of the transition from solid-like to fluid-like behavior under stress. The thesis outlines the methodology for simulating viscoplastic fluid flow, employing COMSOL modules like Laminar Flow for incompressible fluid flow, Phase Transport for particle migration analysis, and Multiphysics for coupling interactions between fluid and particle dynamics. By varying parameters such as yield stress and plastic viscosity, the study examines the impact of SIPM on flow characteristics and their relationships in each geometry. Findings reveal insights into the role of SIPM in altering flow profiles, particularly in bends and nozzles where shear rates vary substantially. Contrary to initial expectations, simulations did not identify dual regions within the lubrication layer, suggesting a homogeneous lubrication mechanism contrary to some theoretical predictions. The study's simulations across cylindrical pipes, nozzles, and elbow bends provide insights into SIPM and the evolution of volume fraction distribution in viscoplastic fluids. It was observed that particles tend to migrate towards areas of lower shear stress, forming a peaked profile indicative of SIPM. The research highlights how yield stress influences the fluid's velocity profile, with increased yield stress leading to a flattened profile, signaling a transition to a viscoplastic regime. Moreover, plastic viscosity was identified as a crucial parameter affecting the fluid's flow behavior, with lower viscosities enhancing velocity and higher viscosities increasing flow resistance. These findings have implications for the design and operation of piping systems transporting viscoplastic fluids. For future research, there is an opportunity to extend this study's findings by exploring the dynamics of lubrication layers more deeply, considering polydisperse suspensions, and investigating other non-Newtonian fluids exhibiting yield stress behavior. Moreover, applying this research to industrial processes like injection molding and extrusion could offer valuable insights for optimizing these operations.