A new interpretation of viscosity and yield stress in dense slurries: Coal and other irregular particles

Abstract

Illinois coal was ground and wet-sieved to prepare three powder stocks whose particle-size distributions were characterized. Three suspending fluids were used (glycerin, bromonaphthalene, Aroclor), with viscositiesη _s that differed by a factor of 100 and with very different chemistries, but whose densities matched that of the coal. Suspensions were prepared under vacuum, with coal volume fractionsφ that ranged up to 0.46. Viscosities were measured in a cone-and-plate over a shear rate$(\dot \gamma )$ range 10⁻³−10² s⁻¹. Reduced viscosityη _r = η/η_s is correlated in the high-shear limit (η _∞) withφ/φ _M∞, whereφ _M∞ is the maximum packing fraction for the high-shear microstructure, to reveal the roles of size distribution and suspending fluid character. A new model that invokes the stress-dependence ofφ _M is found to correlateη _r well under non-Newtonian conditions with simultaneous prediction of yield stress at sufficiently highφ; a critical result is that stress and not$\dot \gamma$ governs the microstructure and rheology. Numerous experimental anomalies provide insight into suspension behavior.