MCE Ph.D. Thesis Seminar

This thesis presents experimental measurements of the rheological behavior of liquid-solid mixtures at moderate  Reynolds  (defined by the shear rate and  particle diameter) and Stokes  numbers, ranging from  3-1600 and 0.4 -195 respectively. The experiments use a specifically designed Couette cylindrical rheometer that allows probing the transition from transporting a pure liquid to transporting a dense suspension of particles. Measurements of the shear stress are presented for a wide range of particle concentration (10 to 60\% in volume) and for particle to fluid density ratio between 1 and 1.05. The effective relative viscosity exhibits a strong dependence on the solid fraction for all density  ratios tested. For density ratios equal to 1 the effective viscosity increases with Stokes number (St) for volume fractions  lower than 40\% and become constant for higher volume fractions. When the particles are denser than the liquid, the effective viscosity shows a stronger dependence on St. An analysis of the particle resuspension for the case with settling particles is presented and used to predict the local volume fraction where the shear stress measurements take place. When the local volume fraction is considered, the effective viscosity for settling and no settling particles is consistent, indicating that the effective viscosity is independent of differences in density between the solid and liquid phase. Shear stress measurements of pure fluids (no particles) were performed using the same rheometer and a deviation from laminar behavior is observed for gap Reynolds numbers above 4000 indicating the presence of hydrodynamic instabilities associated with the rotation of the outer cylinder. The increase on the  effective viscosity with Stokes numbers observed for mixtures with particle concentration below 40% appears to be affected by such hydrodynamic instabilities. The effective viscosity for the current experiments is considerably higher than the one reported in non inertial suspensions.