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


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


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


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.


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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    What is segregation of granular materials? ›

    Granular materials have a tendency to segregate. Segregation occurs due to small differences in either size or density when they flow or are shaken or vibrated. But this is not true with liquids and gases. Fluids often mix with each other after agitation in industrial processes.

    What is segregation in mixing? ›

    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.

    What is particle size segregation? ›

    In particle segregation, particulate solids, and also quasi-solids such as foams, tend to segregate by virtue of differences in the size, and also physical properties such as volume, density, shape and other properties of particles of which they are composed.

    What does granular flow mean? ›

    A granular flow is essentially a mixture of a granular (particle) material and one fluid - the latter might be e.g. air or water. Examples are e.g. circulating fluidized beds or sand transport in river beds.

    What are the different types of segregation? ›

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

    What do you mean by segregation? ›

    1 : the act or process of segregating : the state of being segregated. 2a : the separation or isolation of a race, class, or ethnic group by enforced or voluntary residence in a restricted area, by barriers to social intercourse, by separate educational facilities, or by other discriminatory means.

    What is the process of segregation of unwanted particle from powder called? ›

    Segregation of powders - Demixing.

    How does particle size affect segregation? ›

    Recent work showed that adding an intermediate particle size to an equal mass fraction binary mixture of small and large particles reduces segregation and enhances mixing [20].

    What causes particle segregation? ›

    Separation of a mixture is caused by the differences in size, shape or density of the powder particles. Particle size is probably the most significant source of segregation.

    How many categories of particle clump size are there? ›

    Particles can be placed into one of three categories according to their diameter: Coarse particles (also called particulate-matter or dust) Fine particles.

    What is granular used for? ›

    Granular materials are commercially important in applications as diverse as pharmaceutical industry, agriculture, and energy production. Powders are a special class of granular material due to their small particle size, which makes them more cohesive and more easily suspended in a gas.

    What is the function of the granular system? ›

    Granule cells are the smallest and most numerous type of neurons in the brain. They are involved in functions ranging from processing visual and motor information to learning and memory. Brain Bytes showcase essential facts about neuroscience.

    What is the meaning of granular structure? ›

    Soil structure on which the individual grains are grouped into spherical aggregates with indistinct sides. Highly porous granules are commonly called crumbs.

    What is an example of segregation? ›

    Segregation is the act of separating, especially when applied to separating people by race. An example of segregation is when African American and Caucasian children were made to attend different schools.

    What is the law of segregation? ›

    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.

    What is a sentence for segregation? ›

    There continues to be de facto racial segregation in schools. Other research confirms that school segregation is higher than it has been in decades.

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    A segregated account is a company's account that is separate from the company's money. The clients' funds are held in a separate account so that there is no relationship between their accounts and the company's bank account.

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    A mixture made of solid particles of different sizes, for example sand and gravel, can be separated by sieving.

    What are the 5 separation techniques? ›

    What are chemical methods of separation? Distillation, crystallisation, adsorption, membrane procedures, absorption and stripping, and oxidation are the typical chemical engineering methods of isolation and purification.

    How can we reduce segregation? ›

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    What is segregation in pharmaceutical industry? ›

    Segregation occurs when particles separate due to differences in their size, shape, or density. It can be caused by the way in which a powder blend is handled throughout the production process.

    What does the shape of a particle effect? ›

    The choice of shape affects the region from which particles can be launched, but also the initial direction of the particles. For example, a Sphere emits particles outward in all directions, a Cone emits a diverging stream of particles, and a Mesh emits particles in directions that are normal to the surface.

    What is ordered mixing? ›

    The concept of ordered mixing involves the mixing of small cohesive particles to a considerable degree of homogeneity. A basic principle is that fine particles will adhere, especially to larger particles; the adhesional forces involved may be electrostatic or surface tensional.

    Why do nanoparticles have a high surface area to volume ratio? ›

    It means given volume is divided into smaller piece, the surface area increases. So, we conclude that surface area to volume ratio is very large for nano-particles. Since nano-particles have a very small size, r is small, so the ratio of surface area to volume is very large.

    How do you find the surface area of nanoparticles? ›

    The aggregation of nanoparticles have a impact on their surface area. S=σ/d ρ where d is diameter of particle , ρ density of particle.In this method assumption is to consider all particles are spherical in shape.

    What size are nanoparticles? ›

    A nanoparticle is a small particle that ranges between 1 to 100 nanometres in size. Undetectable by the human eye, nanoparticles can exhibit significantly different physical and chemical properties to their larger material counterparts.

    What are the characteristics of granular materials? ›

    Granular materials, which can be either wet or dry and range in size from nanometers to centimeters, are widely encountered as particles or powder in industries and in nature. As with solids, they can withstand deformation and form heaps; as with liquids, they can flow; as with gases, they exhibit compressibility.

    What is granular nature? ›

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    Is soil a granular material? ›

    Soil that is primarily composed of finegrained material is cohesive material. Soil composed primarily of coarse-grained sand or gravel is granular material.

    What is types of granules? ›

    Several categories of granules may be distinguished: – effervescent granules; – coated granules; – gastro-resistant granules; – modified-release granules.

    What are the four types of granules? ›

    There are four types of granulocyte; basophils, eosinophils, neutrophils and mast cells.

    Where are granules located? ›

    Granule cells are found within the granular layer of the cerebellum, the dentate gyrus of the hippocampus, the superficial layer of the dorsal cochlear nucleus, the olfactory bulb, and the cerebral cortex.

    What is the opposite of granular? ›

    A smooth, shiny floor is the opposite of granular.

    What is granular layer? ›

    The stratum granulosum (or granular layer) is a thin layer of cells in the epidermis lying above the stratum spinosum and below the stratum corneum (stratum lucidum on the soles and palms). Keratinocytes migrating from the underlying stratum spinosum become known as granular cells in this layer.

    What is the meaning of glandular layer? ›

    1 : of, relating to, or involving glands, gland cells, or their products. 2 : having the characteristics or function of a gland glandular tissue.

    What do you mean by segregation? ›

    1 : the act or process of segregating : the state of being segregated. 2a : the separation or isolation of a race, class, or ethnic group by enforced or voluntary residence in a restricted area, by barriers to social intercourse, by separate educational facilities, or by other discriminatory means.

    What are the different types of segregation? ›

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

    What is segregation in metallurgy? ›

    In materials science, segregation is the enrichment of atoms, ions, or molecules at a microscopic region in a materials system.

    What is segregation microstructure? ›

    When casting an alloy, segregation occurs, whereby the concentration of solute is not constant throughout the casting. This can be caused by a variety of processes, which can be classified into two types: Microsegregation; which occurs over distances comparable to the size of the dendrite arm spacing.

    What is an example of a segregation? ›

    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.

    What is the law of segregation? ›

    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.

    What is a sentence for segregation? ›

    There continues to be de facto racial segregation in schools. Other research confirms that school segregation is higher than it has been in decades.

    What is the cause of segregation? ›

    America's separate and unequal neighborhoods did not evolve naturally or result from unfettered market forces. Rather, they resulted from plans, policies, and practices of racial exclusion and disinvestment that primarily targeted Black people and laid the foundation for the segregation of other people of color.

    What was the purpose of segregation quizlet? ›

    Integration is an act to bring together blacks and whites, segregation is an act to separate blacks and whites.

    What causes social segregation? ›

    Social segregation exists whenever the proportions of population rates of two or more populations are not homogenous throughout a defined space. This takes place in many formats, it happens with ethnic groups, social classes and gender groups.

    What causes segregation in metal casting? ›

    Of the many causes of segregation, the feeding of solidification shrinkage, thermal and solutal gradients in the liquid, buoyancy-driven force, flow during pouring, magnetic stirring, rotation, vibration, movement of equiaxed grains due to heterogeneous nucleation, etc.

    What is mechanical segregation mean? ›

    Mechanical separation means to separate a material by a mechanical method such as removal of screws, cutting, grinding or crushing. For example, a covered electrical cable can be mechanically separated into an electrical cable and a cover.

    What is segregation coefficient? ›

    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.


    1. Particle Segregation in Dense Granular Flows: Supplemental Video 6
    (Annual Reviews Extra)
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