Mixing and Segregation Processes in Turbula Blender
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Mixing and Segregation Processes in Turbula Blender Nathalie SOMMIER1, Patrice PORION2, and Pierre EVESQUE Laboratoire de Mécanique MSSM, UMR 8579 CNRS - Ecole Centrale Paris, F-92295 ChâtenayMalabry cedex, France 1 Laboratoire de Physique Pharmaceutique, UMR 8612 CNRS - Université Paris XI, 92296 Châtenay-Malabry cedex, France. 2 Centre de Recherche sur la Matière Divisée, UMR 6619 CNRS - Université d'Orléans, F45071 Orléans cedex 2, France. ABSTRACT Magnetic Resonance Imaging (MRI) technique was used to study the mixing and segregation processes of granular materials in a sophisticated tumbling blender (Turbula mixer) using binary mixtures of sugar beads of different diameters d. Its motion generates mixtures with complex patterns. Effects of some parameters (beads diameter ratio, rotation speed, mixing time) were checked on segregation and mixing processes. We report in this paper, a qualitative and quantitative analysis of these phenomena. A segregation index S was defined to study the homogeneity and the kinetics of the mixing/segregation processes. When the ratio of bead diameters dmax/dmin is approximately 1, mixing process is observed but segregation occurs as soon as dmax/dmin is greater than 1.1. INTRODUCTION The segregation of granular materials such as grains and powders is a significant problem when processing granular media in the pharmaceutical and food-processing industries. Indeed, heterogeneous mixtures tend to segregate by size, even in situations when one might expect good mixing such as shaking or rotation. Many experiments have been dealing with the segregation occurring in rotating cylinders or in drum mixers. For instance, the well-known axial segregation was already reported in 1939 by Oyama [1]; such axial bands of segregation appear after a few tens of rotation when a binary mixture of different size is rotated in a drum blender and it is now a standard problem in the field of granular-media studies [2-7]. More recently, Magnetic Resonance Imaging (MRI) studies were used to analyze the axial and radial segregation within the bulk of granular media [8] or to study granular flow [9-10] and convection [11-13]. From a more fundamental point of view, the understanding of the basic laws of segregation and mixing appears to be a fundamental challenge of the physics of granular matter, of which very little is known [14]. Furthermore, this topic is most likely related to other fundamental problems of fluid mechanics, such as stirring and turbulence [15], and is coupled to the problem of control and enhancement of the kinematics of chemical reactions [16-17]. In this paper, MRI experiments on mixing and segregation of granular matter performed with a sophisticated tumbling blender called Turbula mixer (Willy A. Bachofen AG Maschinenfabrik, Basel, Switzerland) are reported. This apparatus is commonly used in chemical and pharmaceutical researches as it is supposed to generate good homogenization of the granular samples due to the intricate motion of
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