Particle self-diffusion has a significant effect on mixing and thus on performance of rotating cylinder systems such as rotary kilns and drum mixers. We study experimentally the radial mixing of monodisperse beads of different colors in a quasi-two-dimensional cylinder rotated in the continuous flow regime. In this regime a shallow surface layer of particles flows steadily while the rest of the material rotates as a solid body. The initial distribution of tracer particles is taken to be radially symmetric and cylinder is taken to be half full. Both facilitate estimation of the particle self-diffusivity since the evolving concentration distribution during mixing in this case is radial for most part and the mixing in these conditions is shown to be dominated by diffusion of particles. A qualitative study of the mixing is carried out using digital photography. Radial number fraction profiles of the tracer particles are obtained by bulk sampling. Since mixing occurs only in the flowing layer, mixing is considered in terms of "passes" defined as the number of times the material in the bed entirely flows through the layer. Experimental results indicate that the mixing per pass decreases with increasing rotational speed, increases with increasing particle size, and is nearly independent of cylinder size. The mixed state captured by digital photography and the measured radial concentration profiles are well described by a convective diffusion model, using diffusivity as a fitting parameter. The diffusivity obtained from the model follows the scaling proposed by Savage ["Disorder, diffusion, and structure formation in granular flow," Disorder and Granular Media, edited by A. Hansen and D. Bideau (Elsevier, Amsterdam, 1993), pp. 255-285] and a simple expression for the diffusivity is obtained in terms of the particle diameter and the static and dynamic angles of repose.
in Wiley InterScience (www.interscience.wiley.com).While an understanding of the dynamics of segregation has begun to emerge, controlling segregation continues to be a complicated problem. The use of time-modulation-via selective baffle placement-in order to suppress segregation in rotating tumblers is explored. Bidisperse (size or density), cohesionless granular materials in quasi-two-dimensional (2-D) rotating containers are studied by means of simulations and experiments. Results are presented in two main configurations for the placement of the baffles, (1) axial placement, and (2) attached to the periphery of the tumbler. Both experiments and simulations indicate that baffles attached at the periphery are ineffective in reducing segregation, while baffles axially located in the tumbler generate periodic flow inversions and dramatically reduce both density and size segregation. Qualitative and quantitative evidence is presented, in terms of the intensity of segregation. Theoretical and scale-up arguments are provided for the practical implementation of this approach.
Periodic flow inversions have been shown as an effective means to eliminate both density (D system) and size (S system) segregation. The frequency of these inversions, however, is the key to applying this technique and is directly related to the inverse of the characteristic time of segregation. In this work, we study size segregation (S system) and adapt a size segregation model to compliment existing work on density segregation, and ultimately aid in determining the critical forcing frequency for S systems. We determine the impact on mixing/segregation of both the binary size ratio and the length of each leg of a "zig-zag chute". Mixing is observed when L <Ū tS, where L,Ū , and tS denote the length of each leg of the zig-zag chute, the average streamwise flow velocity of the particle, and the characteristic time of segregation, respectively.
Size segregation is studied using a quasi-two-dimensional rotating cylinder system for mixtures of different size, near-spherical particles. Flow occurs only in a thin surface layer, whereas the remaining particles rotate as a fixed bed. In most of the systems studied, the measured radial weight fraction profiles in the bed show significant double segregation ͑a core of small particles as well as a thin layer of small particles at the periphery͒. The profiles are found to be sensitively dependent on the surface roughness of the particles in the mixture, and double segregation reduces with particle roughness. Double segregation is also sensitive to cylinder diameter and no double segregation is observed for the smaller diameter cylinders used. The system, however, shows two unexpected scalings: ͑i͒ the scaled profiles are nearly the same for different cylinder diameters, when the cylinder diameter to the cylinder length ratio is the same, and ͑ii͒ the profiles obtained are found to be insensitive to the size of the large particles in the mixture but depend strongly on the size of the small particles. DOI: 10.1103/PhysRevE.69.031304 PACS number͑s͒: 45.70.Mg, 64.75.ϩg, 83.80.Fg Flowing granular mixtures segregate, and this can have a significant impact in many industrial processes ranging from pharmaceutical tablet manufacture to cement production. The process, however, is not understood to the level that quantitative predictions can be made. Even predicting the direction of segregation is not trivial. For example, Hong et al. ͓1͔ showed using molecular dynamics simulations that fluidized binary mixtures of elastic spheres under gravity segregate with the larger particles on the top or the bottom of the layer, depending on the temperature and the diameter and mass ratios. They obtained a criterion to predict the direction of segregation based on competition between condensation and percolation. Jenkins and Yoon ͓2͔ showed that kinetic theory also yields a similar criterion for the direction of segregation. Reversals in the direction of segregation have been observed in flowing fluidized layers as well, but the situation in this case is complicated by dissipation and gradients across the layer ͓3,4͔.Segregation in fluidized layers has been studied in a number of systems including heap flows ͓5-7͔, chute flows ͓3,8 -10͔, and rotating cylinder flows ͓9,11-19͔. In most experimental systems involving size segregation, the larger particles are at the top of the flowing layer and small particles are at the bottom. There are a few exceptions. Nityanand et al. ͓11͔ reported reverse segregation in a rotating cylinder system ͑small particle concentrate at the top of the flowing layer at high rotational speeds͒. Dolgunin and Ukolov ͓8͔ found small particles concentrated at the top and bottom of the layer with larger particles concentrated near the middle of the layer in a chute flow. We refer to this as double segregation since it involves regions of regular and reverse segregation. Thomas ͓9͔ also found double segregation...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.