The determination of the true equilibrium flow curve for fluid mud in a wide-gap Couette rheometer.

The rheological properties of fluid mud, a gelling suspension of predominantly colloidal clay particles in water, are usually obtained with a vane spindle for its presumed advantage of avoiding slip with the smooth surface of a standard cylindrical spindle. The vane configuration is presumed to be equivalent to a cylinder configuration by entrapping mud between its blades when it rotates. However, recent studies show that non-axisymmetric flow patterns are observed in vane configurations which make this assumption questionable. Therefore, the rheological study of fluid mud is needed to be re-assessed with the latest developments. In the present study, an in-house created serrated spindle, which approximates an infinite number of blades, is investigated to study possible slip and axisymmetric flow compared to vane and cylinder configurations. A wide-gap configuration is chosen since the estimated mean particle size of 10 μ m (of the finest clay aggregates) is quite coarse, while even in a standard narrow-gap configuration one cannot avoid that the yield surface is located within the gap (certainly at low rotation speeds). Therefore, the data are processed with the iterative method of Toorman (1994) valid for both configurations. Moreover, it allows a more practical handling of mud samples. The experimental results show a good reproducibility of the equilibrium flow curve and avoidance of the slip effect for the serrated configuration. In addition, numerical simulation results confirm that the flow of fluid mud with the vane configuration is not circular and deviates by 10%–20%. On the other hand, the flow of fluid mud with a serrated configuration is circular and deviates by less than 5%, matching the flow of fluid mud in the cylinder configuration. • Novel serrated spindle configuration for accurate rheological analysis of fluid mud. • Comparison of fluid mud flow properties in vane, cylinder and serrated configuration. • Investigation of circular flow with vane configuration using CFD-OpenFOAM. [ABSTRACT FROM AUTHOR]