Research on size segregation dynamics and processes of a binary mixture dense granular flow (2022)

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Minerals Engineering

Volume 186,

August 2022

, 107722

Abstract

This study employed an experimental approach to investigate the effects of various filling degrees of the granular bed and the rotation speed on size-induced particle segregation behavior in an almost fully filled double-walled rotating drum. The distributions of the concentrations of bicolor particles in the system were obtained via image processing to determine the segregation state. The dynamics of the mixture flow including the statistical distribution, the azimuthal angle and radial direction granular temperature profiles were measured and calculated via particle-tracking velocimetry (PTV). The results were similar for the Brazil-nut effect and its reverse in the radial direction at high or low rotation speeds. When the rotation speed or filling degree was changed, the effect on two different particle sizes in the binary mixture system was inconsistent, and different flow mechanisms led to different segregation patterns. Two phase diagrams with the rotation speed and filling degree and the magnitude and position of the maximum granular temperature as the space enabled the prediction of segregation states.

Graphical abstract

Center position evolution of particles over time. The red circles and blue squares indicate the big and small particles, respectively.

Phase diagram of the binary-size mixture segregation pattern of the ωf space. BNE, MS, and RBNE were indicated by the circle, diamond, and square, respectively.

Research on size segregation dynamics and processes of a binary mixture dense granular flow (5)
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(Video) Experimental studies of granular flow

Introduction

Rotating drums or rotary drum mills have been widely used over the past few years to investigate granular flow mechanics (Ma and Zhao, 2018, Zhang et al., 2019, Chou et al., 2020, Venier et al., 2021), partly because they exhibit a simple closed geometry. Rotating drums usually comprise a cylinder rotating on its central axis to drive the motion of the particles. Devices based on this configuration are commonly used to process granular materials in the mineral, ceramic, cement, metallurgical, chemical, pharmaceutical, calcination, and waste industries. They are suited to drying, heating (Nafsun and Herz, 2016, Nafsun et al., 2017), chemical reactions (Desogus et al., 2016, Brück et al., 2019), mixing, segregation (Khakhar et al., 2003, Jain et al., 2005), and milling (Morrison et al., 2009, Cleary and Morrison, 2016). As chemical reactors, they are designed using empirical procedures. For milling, heating, or drying, the most common treatment of granular products typically require high-frequency rotations with shafts or inserts of the equipment to mediate mixing or heat transfer requirements. Therefore, there is significant economic incentive to develop a more fundamental understanding of rotating drums for processing granular solids.

Although the rotating drum was simple and could be operated relatively easily, the granular dynamic behavior was more complicated. The granular flow behavior in rotating drums can be separated into several flow regimes based on the particle motion. Moreover, the particle flow behavior and mixing/segregation mechanisms may differ in each flow regime. Six identifiable flow regimes in a dry granular system may be used to describe the particle motion in a rotating drum depending on different operational conditions, including the rotation speed, wall friction coefficient, and filling degree. The flow regimes are slipping, slumping, rolling, cascading, cataracting, and centrifuging (Henein et al., 1983, Rajchenbach, 1990). In the slurry system, relatively different flow regimes (Chou and Hsiau, 2012) appear, owing to the different properties of the interstitial fluid relative to the dry system.

The mixing of granular materials is economically important in numerous industries, such as foodstuffs, pharmaceutical products, detergents, chemicals, plastics, and construction industries (Cooke et al., 1976, Fan et al., 1990, Bridgwater, 1995). In many cases, a better mixing process can increase the quality and product value; thus, mixing is a key process and requires further study. However, the general understanding of the fundamentals of granular mixing is incomplete, and granular mixing has received less attention than fluids (Ottino, 1989, Ottino and Khakhar, 2000). Regardless of the particle size, when more than one kind of particle coexists in a system, segregation occurs between the particles as they move. Moreover, particle segregation is influenced by external driving conditions (Golick and Daniels, 2009, Liao et al., 2010, Xiang et al., 2010), interstitial fluid (Jain et al., 2004, Chou et al., 2011), and even container geometry (Jain et al., 2005, Gonzalez et al., 2015). Recently, rotating drums have been widely used to investigate mixing/segregation processes. In partially filled rotating drums, radial segregation typically occurs with the small particles falling into voids between the large particles and concentrating in a central core, with the large particles in the periphery. This mechanism is called the percolation effect (Chou et al., 2010, Jain et al., 2013, Gray, 2018, Huang et al., 2021). Segregation may also occur with different densities due to the buoyancy effect, where dense particles tend to sink to a low level in the flowing layer to form a core at the drum center (Huang et al., 2012, Tripathi and Khakhar, 2013, Jones et al., 2018).

In a horizontal (quasi) two-dimensional (2D) rotating drum, the behavior of particles of different sizes segregated radially perpendicular to the axis of rotation by the percolation mechanism was affected by gravitational and centrifugal forces. Different centrifugal forces caused by the rotational speed will cause different degrees of segregation or even different segregation patterns and structures (Gui et al., 2013, Zhang et al., 2017, He et al., 2019, Hou and Zhao, 2020, Wang et al., 2021). He et al. (2019) investigated the effects of the rotation speed and aspect ratio of ellipsoids on the extent of segregation in the equilibrium state and observed that coarse particles tended to segregate to the periphery of the bed, whereas fine particles were trapped in the central area. They also indicated that in a rolling or cascading regime, an increase in the rotation speed could reduce the extent of segregation for spheres and ellipsoids. Hou and Zhao (2020) used experimental results and numerical simulations to analyze the radial segregation phenomena of bidisperse particles of the same density but unequal size in a rotating drum. The results showed that the degree of segregation generally decreased with an increase in rotation speed, whereas the filling degree exhibited different influences on segregation in different flow regimes. They also clarified the reverse segregation mechanism in the cataracting regime. Wang et al. (2021) used the discrete element method (DEM) to investigate the radial segregation characteristics of a Gaussian-dispersed mixture in a horizontal rotating drum, and the segregation behaviors were examined using the gyration degree and mixing index. The results showed that the large standard deviation, smooth particle shape, and small rotation speed were conducive for segregation in granular systems.

The filling degree is another simple and important experimental parameter that influences the segregation of granular matter in rotating drums (Arntz et al., 2008, Pereira et al., 2013, Santos et al., 2016, Widhate et al., 2020, Brandao et al., 2020). Different filling degrees represent the number of particles to be processed in the system and induce different flow behaviors and transport properties. Arntz et al. (2008) reported a simulation in which various flow regimes and radial segregations were observed by systematically varying the filling degree and angular velocity. Strong correlations between the flow regime and segregation pattern were summarized in two bed behavior diagrams. Widhate et al. (2020) studied the mixing phenomena of a rotating drum at different angles of inclination and observed that the mixing index was related to the area ratio of the active region to the whole bed and filling degree. An increase in the volumetric fill could lead to a decrease in the mixing index. Brandao et al. (2020) used experiments and numerical simulation tests to investigate the mixing/segregation behavior of particles in an unbaffled rotating drum. They employed DEM in their simulation test, in which they input parameters by experimental measurements and calibration. They observed the effect of the diameter and density differences on radial segregation and quantified the kinetic constant of mixing for different binary mixtures as a function of the filling degree and rotation speed.

Although mixing or segregation based on the size effect of particles has received considerable attention in the past few decades, several physical mechanisms are still unknown or poorly understood. Specifically, they are typically focused within a low or medium range of filling degrees; therefore, we attempted to understand the effect of the rotation speed and filling degree on the behavior of size-induced particle segregation in an almost fully filled double-walled rotating drum. We determined the segregation phenomena, including the segregation degree, pattern, and even distribution through an image-processing method.

Section snippets

Experimental procedure

Fig. 1(a) illustrates the experimental apparatus. A 2D circular double-walled rotating drum made of 5mm Plexiglas with transparent glass front and back faceplates was used to collect the experimental images. The inside and outside wall diameters of the drum were Di=180mm and Do=300mm, respectively. The axial length (δ) of the drum and the disk of the inside wall were both 5mm, which was virtually the same as the diameter of the big particle. To reduce the electrostatic effect, the rear

Results and discussion

The two operational parameters of this system (the rotation speed and filling degree) had an important impact on the segregation process and particle flowing behavior. The other system parameters remained fixed (Table 1).

Fig. 2 shows the time evolution of the center position for the big particles (circles) and small particles (squares) relative to the inside wall of the drum. The drum was rotated clockwise at rotation speed ω=4.71rad/s and filling degree f=0.941. The high-resolution raw

Conclusions

Here, the impacts of the rotation speed and filling degree of a granular bed in a practically full double-walled rotating drum were investigated. Certain key physical quantities and dynamic behaviors were measured and calculated during the segregation process. As observed, the rotation speed significantly influenced the segregation of binary-size mixture granular flows at different filling degrees.

The results showed that different segregation patterns (BNE, MS, and RBNE) can be quantified using

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors would like to acknowledge the support from the Ministry of Science and Technology of the R.O.C. for this work through Grants: MOST 109-2223-E-008-001-MY3 and MOST 109-2221-E-008-011-MY3.

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    FAQs

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    The "moveability" is determined by parameters like particle size, density, surface roughness etc, there is a chance that with increased mixing time the indivual ingredients accumulate due to the individual movement of each component. This effect is called segregation or demixing.

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    Segregation is made up of two dimensions: vertical segregation and horizontal segregation.

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    Segregation of powders - Demixing.

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    Through so-called Jim Crow laws (named after a derogatory term for Blacks), legislators segregated everything from schools to residential areas to public parks to theaters to pools to cemeteries, asylums, jails and residential homes.

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    Genes come in different versions, or alleles. A dominant allele hides a recessive allele and determines the organism's appearance. When an organism makes gametes, each gamete receives just one gene copy, which is selected randomly. This is known as the law of segregation.

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    A segregation coefficient can be defined to assess the magnitude of the effect. Segregation effects tend to become more severe for small segregation coefficients since larger concentration differences need to be equalized.

    What causes banding in steel? ›

    Banding is caused by segregation of the alloying elements during solidification. Subsequent hot-working operations result in segregation aligned in the direction of working, which results in the banded appearance delineated in the microstructure.

    Who developed the law of segregation? ›

    Mendel proposed the Law of Segregation after observing that pea plants with two different traits produced offspring that all expressed the dominant trait, but the following generation expressed the dominant and recessive traits in a 3:1 ratio.

    What is grain boundary segregation? ›

    The segregation of impurities to grain boundaries (GBs) has a significant influence on the cohesive properties, atomic arrangements and properties of such interfaces. The segregation strongly depends on the structural units of the GB as well as on the impurity atom itself.

    Videos

    1. Particle Segregation in Dense Granular Flows: Supplemental Video 6
    (Annual Reviews Extra)
    2. Particle Segregation in Dense Granular Flows: Supplemental Video 3
    (Annual Reviews Extra)
    3. Particle Segregation in Dense Granular Flows: Supplemental Video 8
    (Annual Reviews Extra)
    4. IMA Conference on Dense Granular Flows, University of Cambridge
    (Amin Haeri)
    5. Lu Jing - A particle-scale force approach to granular segregation - 19/01/22
    (Programa de Pós-graduação em Matemática da UnB)
    6. Particle Segregation in Dense Granular Flows: Supplemental Video 9
    (Annual Reviews Extra)

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