Core Of Small Particles Research Articles

Numerical study of size-driven segregation of binary particles in a rotary drum with lower filling level

Discrete element method (DEM) simulations of size-driven segregation of binary particles in a rotary drum were conducted to investigate the influence of filling level, size ratio, and rotation speed on the segregation performance. Segregation experiments with different filling levels were used to verify the DEM model and analyze the influence of filling levels on segregation. The granular bed was divided into five layers to study the axial segregation in the rotary drum. The total velocity fluctuation was used to discuss the granular behavior from a mesoscopic perspective. A segregation index was adopted to quantify the segregation performance with different parameters. It was found that binary particles in the drum with different filling levels have different segregation patterns. A core of small particles was formed in the middle of granular bed for the case with a higher filling level, while there is no core formed for the case with a lower filling level. Results obtained indicate that the size-driven segregation in the drum with lower filling level increases with the increase of size ratio. Within the rolling regime, the rotation speed has little influence on the final segregation index of particles.

Continuum modelling of granular segregation by coupling flow rheology and transport equation

In granular flows, particles can mix or segregate owing to their difference in size/density. This behaviour is commonly understood as a size- or density-driven segregating flux at the macro scale and has been studied before in systems of shear cell and chute. This work aims to model the segregation in practical systems with complex geometries and flow modes, by coupling the fundamental flow rheology and the convection-diffusion-segregation transport equation. The model is validated against experimental measurements and discrete element simulations in various flow scenarios, shown able to capture the notable characteristics of particle segregation reported, such as the core of small particles in a rotating drum. The predictions can overall match the benchmark tests although the quantitative accuracy still varies with cases, indicating the need for further study. The proposed approach, not limited by operational or geometrical conditions, provides a useful tool for the design and control of mixing processes.

Sensitivity of granular segregation of mixtures in quasi-two-dimensional fluidized layers.

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 periphery). 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: (i). the scaled profiles are nearly the same for different cylinder diameters, when the cylinder diameter to the cylinder length ratio is the same, and (ii). 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.

Particle mixing in a nearly filled horizontal cylinder through phase inversion

A procedure for homogeneously mixing two different sizes of particles in a horizontally rotating cylinder is presented. It was found that different cylinder rotation rates produce inverted segregation patterns. A radial core of small particles is formed at low rotation rates and a core of large particles is formed at high rotation rates. A simple force model is presented that can predict the particular segregation pattern for given rotation rates and particle sizes. A homogeneous mixture can be obtained by alternating between rotation rates at which particles occupy corresponding radial positions.