Development of an unresolved CFD-DEM model for the flow of viscous suspensions and its application to solid-liquid mixing.

A new image analysis technique is proposed to determine the macromixing time in a transparent stirred tank. It consists of capturing on video a decolorization process by using a fast acid−base indicator reaction and employing image analysis to quantify the color evolution. The color change is quantified by means of individual thresholds on the RGB color model and provides a direct measurement of the macromixing evolution as it can be seen by an operator in front of the vessel. It is shown that this technique removes the subjectivity of the estimation of macromixing time by the naked eye, has a high degree of reliability and repeatability, and can yield accurate macromixing information by considering the possible presence of segregated regions and dead zones. Moreover, applications show that the macromixing curves bring new insights to study and compare mixing efficiency of different impellers or multiple impeller mixing systems.

The performance of a coaxial mixer combining an anchor and a Rushton turbine was investigated for different diameter ratios and rotating modes in the laminar and transition flow regimes. Based on mixing efficiency criteria, this study confirmed that the co-rotating mode is consistently yielding the best results in the laminar and early transition regimes. The best performance was obtained for a turbine to tank diameter ratio of 1/3. In the upper transition and turbulent regimes, the best performance was obtained with the anchor at rest acting like two opposed baffles. With such a configuration, the optimum diameter ratio was found to be 1/2. New definitions of Reynolds number and power number were also introduced based on new characteristic diameter and speed. These correlations shown to be applicable for radial and axial impellers and are significantly better than the ones proposed in the literature.

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Solid-liquid mixing as a unit operation still faces considerable challenges, notably regarding the prediction of the impeller speed required to suspend the particles (N js ), the fraction of suspended solids and the homogeneity of the suspension at a given speed.In this work, we extend to the turbulent regime, by means of large eddy simulation (LES), a CFD-DEM model developed recently in our group for solid-liquid mixing. The resulting model is used to study the mixing of glass particles in a baffled stirred tank