Magnetorheological fluids for extreme environments : stronger, lighter, hotter

Abstract

The controllable properties of magnetorheological (MR) fluids offer reliable and efficient actuation means to a number of far-ranging engineering applications. In this thesis we are motivated by the applications of MR fluids in oil & gas exploration and production. These applications also bring about a number of operational requirements for the fluid such as generating large magnetically induced shift in rheological properties with tolerance to elevated temperatures and low fluid density in order to maintain manageable hydrostatic downhole pressures. In this thesis we investigate a number of these fluid design constraints. Firstly, the evolution of the rheological properties of MR fluids over a wide range of magnetic field and temperature was investigated. A magnetorheometry fixture with a unique combination of high-field and high-temperature capability was manufactured. With the experimental measurements and the results from a numerical model of interparticle magnetic interaction, a scaling law was identified between the applied magnetic field and the resulting MR yield stress. The aggregation phenomena and the evolution of fluid microstructure were also investigated in microfluidic geometries with strong particle-wall interactions. The results of this study highlighted design features and operational techniques that can improve the performance of MR fluid valves. Investigation of fluid flow in non-uniform magnetic fields showed that in these regions the motion of the particle phase is governed by a balance between hydrodynamic and magnetophoretic forces. Finally, the flow of MR fluids in spatially-inhomogeneous magnetic and deformation fields was studied. A slit-flow magnetorheometer was manufactured to measure the bulk MR response of the fluid under non-uniform fields. In order to understand the parameters governing these flows and to develop a predictive tool for further investigations, a two-fluid suspension-balance constitutive model was developed which captures the key features of multi-phase flow and fluid anisotropy. The model was numerically implemented using the finite element method and was used to study the transport of MR fluids in spatially-inhomogeneous flows such as those encountered in contraction and expansion channels. This model provides insight into the design and optimization of MR fluid devices that can enhance the magnetically-controlled gain in flow resistance under downhole conditions.