Abstract
The rheological characterization of concentrated suspensions is complicated by the heterogeneous nature of their flow. In this contribution, the shear viscosity and wall slip velocity are quantified for highly concentrated suspensions (solid volume fractions of 0.55–0.60, D4,3 ~ 5 µm). The shear viscosity was determined using a high-pressure capillary rheometer equipped with a 3D-printed die that has a grooved surface of the internal flow channel. The wall slip velocity was then calculated from the difference between the apparent shear rates through a rough and smooth die, at identical wall shear stress. The influence of liquid phase rheology on the wall slip velocity was investigated by using different thickeners, resulting in different degrees of shear rate dependency, i.e. the flow indices varied between 0.20 and 1.00. The wall slip velocity scaled with the flow index of the liquid phase at a solid volume fraction of 0.60 and showed increasingly large deviations with decreasing solid volume fraction. It is hypothesized that these deviations are related to shear-induced migration of solids and macromolecules due to the large shear stress and shear rate gradients.
Highlights
The accurate determination of the rheological behaviour of highly concentrated suspensions in pressure driven flows is relevant for many processing operations and industries
The rheological characterization of materials generally focusses on the shear viscosity, which is ratio between the imposed shear stress and the resulting shear rate
The characterization of the shear viscosity of highly concentrated suspensions is complicated by the heterogeneous nature of their flow profile
Summary
The accurate determination of the rheological behaviour of highly concentrated suspensions in pressure driven flows is relevant for many processing operations and industries. For pure liquids and dilute suspensions, the material and resulting flow profile are homogeneous and the shear viscosity can be directly determined using classical rheological measurements (Barnes 2000). The characterization of the shear viscosity of highly concentrated suspensions is complicated by the heterogeneous nature of their flow profile. As the relative contribution of these regions varies depending on the flow conditions, it is important to accurately discriminate between them and increase our understanding of their individual dynamics. This understanding will improve the generality of resulting flow models and increase their accuracy when applied to practical situations
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