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Characterization of Granular Flow In a Bladed Mixer Using the Discrete Element Method

Brenda Remy1, Johannes G. Khinast2, and Benjamin J. Glasser1. (1) Dept. of Chemical and Biochemical Engineering, Rutgers University, 98 Brett Road, Piscataway, NJ 08854, (2) Institute for Process Engineering, Graz University of Technology, Inffeldgasse 21/A, Graz, A-8010, Austria

Bladed mixers are commonly used in a variety of areas, ranging from the bulk chemical to the food and pharmaceutical industries. Despite their extensive use, the flow of granular materials in agitated devices and the mechanisms by which particle motion is generated are still not fully understood. Experimental examination of granular flows in these mixers has been limited to off-line analysis of location-specific samples, and to on-line measurement of a limited set of process parameters. Numerical simulation techniques have the potential of bridging the knowledge gap since they allow for the study of experimentally difficult-to-measure parameters.

Utilizing the discrete element method the 3-dimensional flow of cohesionless particles in a cylindrical vessel agitated by a 4-blade impeller was studied. Instantaneous, averaged and fluctuating velocity fields show that the particle movement is governed by the angular motion of the blades. The granular bed deforms by forming heaps where the blades are present and valleys between blades passes. The presence of these heaps leads to the formation of a 3-dimensional recirculation zone in front of the blade, where particles flow upwards by the wall and downwards by the impeller shaft. This 3-D recirculation leads to enhanced mixing by promoting radial and vertical mixing. The particle's frictional characteristics are shown to strongly influence the flow structure and mixing kinetics within the mixer. At low friction coefficients the 3-D recirculation in front of the blade is not present reducing the convective mixing inside the mixer. Higher friction coefficients lead to an increase in granular temperature which is associated with an increase in diffusive mixing. Collisional stresses within the mixer showed periodic behavior indicating that the granular material compresses as the blade passes and dilates in between the blades. The long-time averages of the normal and shear stresses were found to vary with mixer height with maximum values near the bottom plate. Additionally, a strong dependence between the magnitude of the shear stresses and the friction coefficient of the particles was found. These stress tensor characteristics indicate that the granular flow in this bladed mixer occurs in the quasi-static regime.