Discrete particle simulation of radial segregation in horizontally rotating drum: Effects of drum-length and non-rotating end-plates

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Abstract

The radial segregation phenomena of a mixture of two different size grains in a horizontal rotating drum are studied by DEM simulations. The grano-dynamics of radial segregation phenomena is examined as a function of the axial length and the friction between grains and not-rotating end-plates of the drum. The results indicate that, in the longer drums, the radial segregation ratio is higher and the friction on the end-plates shows little effect. Whereas in the shorter drums, the radial segregation is very slow or negligible; however, decreasing the friction on non-rotating end-plates increases the segregation ratio. If we increase the friction further (greater than the frictions between the grain-grain and the grains and the inner wall), the segregation ratio drops in the longer drums while in the shorter drums mixing is seen instead. The cause of these phenomena lies in the mechanism of diffusion in granular flows due to shearing strain by the end-plates. For more roughened end-plates, this shearing activity increases the granular temperature of the system and only the mixing can be observed instead of the segregation.

Highlights

► DEM simulations of radial segregation in horizontal granular. ► Effect of roughened end-plates and drum length on radial segregation. ► In longer drums the segregation ratio is higher than for the shorter drum. ► Lower friction on non-rotating side-plates shows higher segregation. ► These phenomena are due to the diffusive collisions of particles in the active layer.

Introduction

Granular media have long been of interest to engineering and scientific communities; in particular, their role in mineral, polymer, food and pharmaceutical processing is very significant [1], [2], [3], [4], [5]. However, their dynamical properties have not been understood properly. It has been established that granular mixtures undergo segregation by the type of grain when they flow in hoppers, conveyor belts, vibrating beds, or rotating drums [6]. For example, when a bed of grains of different sizes is shaken parallel to the top surface, the larger grains move to the top surface and the smaller ones to the bottom [7], [8], [9]. This phenomenon of granular segregation occurs both in nature and in numerous industrial processes. In many industries, size segregation is a cause of significant frustration. Incorrect mixing or segregation can lower the quality of a product. There are a number of mechanisms accounting for size segregation in granular flows. Here, we are interested in the issue of radial segregation of binary granular materials in a rotating drum; a subject which has received much attention recently [7], [8], [9], [10], [11]. Though many system factors, such as size, shape, friction, side-plates, etc., can affect this mechanism, earlier reports on the segregation problem were mainly concerned with the geometric effects, friction-driven segregation, etc., [12], [13], [14]. However, a wall-induced segregation mechanism has been recently reported [12], but for periodic end-plates in a rotating cylinder. It has been pointed out that the segregation is never complete in some systems (shorter drums) because of diffusion produced by particles due to end-plates [15]. The shorter drum eliminates the axial segregation and one can only study the radial segregation; but the radial segregation in this drum is difficult to achieve. This difficulty can be overcome by increasing the number of smaller particles over larger ones. Some researchers have postulated that due to diffusive collisions in the active layer, segregation induced by end-walls is possible in the longer drum, but in the shorter drum the segregation could never be completed [12]. The main reason is the reduction of the sieving effect due to gravity, as was explained by Savage and Lun in their classical random sieving model [16]. In shorter drums, and the presence of roughened end-plates, the segregation mechanism is less pronounced due to diffusive mixing. Luckily, such systems can be useful for blending particulate matter in industries.

This paper focuses on such geometrical systems. Our primary goal is to study the effect of the axial length of the drum and the friction between grains and the non-rotating side-plates on radial segregation phenomena. We are mainly interested in the effect of the end-plates, such as roughness, on the radial segregation. Therefore, we make the side-plates stationary and choose different frictional values of the side-plates and drums of different lengths. The length of the drum is chosen in order to vary the gap between the two end-plates and observe the consequent effect on radial segregation. The drums are filled with a bi-mixture of granular particles of smaller grains with radii RS and larger grains with radii RL. We vary the coefficient of friction between the end-plates and the grains, μpg, so that μpg is less than or equal to the friction coefficients between grains, μgg, and between the grains and the inner wall, μgw. Because the values of μgg and μgw are the same throughout this paper, we simply write μ to define both quantities. In some trials, a value of μpg greater than μ is also chosen.

Section snippets

Simulation method

We employ the Discrete Element Method (DEM) which was first developed by Cundal and Strack to describe the mechanical behavior of disks and spheres [17], [18], [19]. Our simulation schemes are based on the DEM code “LIGGGHTS” applied initially by Kloss et al. [20], [21]. Originally, the code is improved version of the granular package of the classical molecular dynamics simulator LAMMPS [22] and is implemented to a new Hertz–Mindlin granular contact model. In this contact model, the

Results and discussion

The radially segregated states of the mixture are shown in Fig. 1. The snapshots are taken in the initial mix state (just after the start of rotation) and in the final state. The radial core of the smallest grains is clearly visible. First, starting from shorter drum, we analyze the results of simulations with μpg of 0.3 and 0.5 in turn. For simplicity, throughout this paper, we write end-plates for non-rotating end-plates.

The segregation is measured by calculating the population of smaller

Conclusions

DEM simulations of granular matter have been used to demonstrate the influence of the end-plate properties on the radial segregation in a rotating cylinder containing two types of grain which differ only in size. The frictional values between the grain–grain and the inner wall and the grains are the same, whereas the frictions on the end-plates are varied. The end-plates are stationary with respect to the drum rotation. To find out more about the influence of roughened end-plates on the radial

Acknowledgment

This work is supported by the Chinese National Science Foundation, Project No. 11172039.

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