(554c) Development of a CFD-DEM Model in Non-Inertial Frame for Solid-Liquid Mixing Applications

Solid-liquid mixing is a significant process that is critical to pharmaceutical, biomechanical, food processing and other industries. Despite the relevance of solid-liquid mixing applications, the physics governing the suspension of particles in agitated vessels is still poorly understood. This can lead to sub-optimal designs and important energy waste or even an incapacity to achieve the desired mixing state. Important criteria that characterize solid-liquid mixing, like the just-suspended speed (N_js) and the homogeneity of the suspension, are difficult to predict. Consequently, engineers often rely on correlations to design solid-liquid mixing vessels. There is a need for more studies on solid-liquid mixing and, more particularly, for new and robust simulation models to predict suspension dynamics.

Complex geometries can be used to meet specific process requirements. For example, the double helical ribbon has shown to achieve good mixing in the laminar regime. However, the simulation of solid-liquid flow in such rotating systems is challenging. To deal with this issue, common techniques such as the sliding mesh and multiple reference frame can be used, with known limitations related to computational cost and accuracy .The present work concerns the development of an Euler-Lagrange model in a non-inertial frame of reference that is suitable to a wide range of geometries regardless of their complexity. It is based on an unresolved CFD-DEM approach that combines the Discrete Element Method (DEM) for the solid particle dynamics and CFD techniques for the fluid phase [1].

First, the full model is introduced. Using simple numerical experiments, the DEM sub-model in a rotating frame of reference is then assessed, including its stability, which is not simple because of the addition of a velocity-dependent force related to the Coriolis effect. Next, our coupling strategy for the fluid and the solid phases is described and the full CFD-DEM model is validated against experimental data and numerical studies previously carried in our lab in the case of simple geometries like the pitch blade turbine [2]. Finally, the model is applied to the double helical ribbon in order to evaluate the impact on the quality of mixing of factors such as the rotation speed and the fluid viscosity.

[1] B. Blais, M. Lassaigne, C. Goniva, L. Fradette, and F. Bertrand, "Development of an unresolved CFD–DEM model for the flow of viscous suspensions and its application to solid–liquid mixing," Journal of Computational Physics, vol. 318, pp. 201-221, 2016/08/01/ 2016.

[2] M. Lassaigne, B. Blais, L. Fradette, and F. Bertrand, "Experimental investigation of the mixing of viscous liquids and non-dilute concentrations of particles in a stirred tank," Chemical Engineering Research and Design, vol. 108, pp. 55-68, 2016.