Size Segregation in Granular Beds Subject to Discrete and Continuous Vertical Oscillations
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Size Segregation in Granular Beds Subject to Discrete and Continuous Vertical Oscillations Dimuth N. Fernando and Carl R. Wassgren School of Mechanical Engineering Purdue University West Lafayette, IN 47907-1288, U.S.A. ABSTRACT Size segregation of particulates is of concern in a number of industries that handle materials such as chemicals, pharmaceuticals, fertilizers, and food products. Of particular interest in this paper is segregation resulting from externally applied vibration. In industrial applications this vibration may either be applied intentionally in devices such as vibrating conveyors or “live wall” hoppers, or unintentionally during material handling and transport. This paper investigates size segregation in granular beds subject to discrete “taps” and continuous, sinusoidal vertical vibration. The results from discrete element computer simulations indicate that the rise rate of a single impurity increases monotonically with amplitude for discrete vibrations but for continuous vibrations the rise rate increases, reaches a maximum value, then decreases as the oscillation amplitude increases. INTRODUCTION Size segregation of particulates is of concern in a number of industries that handle materials such as chemicals, pharmaceuticals, fertilizers, and food products. As a result, researchers have investigated size segregation for a number of geometries including rotating drums [1], rotating blenders [2], along inclined chutes [3], and in hoppers [4]. Of particular interest in this paper is size segregation occurring in granular beds subject to external vibration. In industrial applications this vibration may either be applied intentionally in devices such as vibrating conveyors or “live wall” hoppers, or unintentionally during material handling and transport. Rosato et al. [5] utilized 2D Monte Carlo simulations to investigate the rise rate of a single, large impurity in an otherwise monodisperse bed of particles subject to discrete, vertical “taps.” Their studies indicate that the position of the impurity increases linearly with the number of vibration taps until reaching the free surface of the bed. The rise rate, defined as the average increase in the impurity’s vertical position over a single oscillation cycle, increases monotonically with the amplitude of the taps. Consequently, the rise rate is independent of the position within the bed. The impurity rises to the free surface because the small particles are more likely to fill the voids formed underneath the large particle when the particle bed is in flight and thus the large particle is forced toward the free surface. The results of Rosato et al. stand in contrast to the recent experimental investigations by Hsiau et al. [6]. Their experiments investigated size segregation of binary mixtures subject to continuous, sinusoidal vertical vibration. They found that the degree of segregation of the material first increases as the amplitude of the vibrations increases, reaches a maximum value, and then decreases as the amplitude continues to increase. Thu
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