Olukayode Isaiah Imole

Experiments and discrete element simulation of the dosing of cohesive powders in a simplified geometry

We perform experiments and discrete element simulations on the dosing of cohesive granular materials in a simplified geometry. The setup is a simplified canister box where the powder is dosed out of the box through the action of a constant-pitch screw feeder connected to a motor. A dose consists of a rotation step followed by a period of rest before the next dosage. From the experiments, we report on the operational performance of the dosing process through a variation of dosage time, coil pitch and initial powder mass. We find that the dosed mass shows an increasing linear dependence on the dosage time and rotation speed. In contrast, the mass output from the canister is not directly proportional to an increase/decrease in the number coils. By calibrating the interparticle friction and cohesion, we show that DEM simulation can quantitatively reproduce the experimental findings for smaller masses but also overestimate arching and blockage. With appropriate homogenization tools, further insights into microstructure and macroscopic fields can be obtained. This work shows that particle scaling and the adaptation of particle properties is a viable approach to overcome the untreatable number of particles inherent in experiments with fine, cohesive powders and opens the gateway to simulating their flow in more complex geometries.

Deformation Modes for Assemblies of Frictionless Polydisperse Spheres

The challenge of dealing with cohesive powders during storage, handling and transport are widely known in the process and pharmaceutical industries. Simulations with the discrete element method (DEM) provide further insight into the local microstructure of bulk materials. In this work, the DEM approach is presented to investigate the flow behavior of granular systems subjected to different modes of deformations. When uniaxial compression is applied of frictionless, polydisperse spheres above jamming (transition from fluid-like state to solid-like state), the evolution of coordination number (average number of contacts per particle) and pressure as functions of the volume fraction are, astonishingly, identical to results obtained for purely isotropic compression. Analytical predictions for the evolution of pressure and coordination number under isotropic strain can thus be separated from different deformation modes, as applied in this study. After two different modes of volume-conserving deviatoric shear, the results still compare quite well with results for purely isotropic compression. The difference between the two deviatoric modes and uniaxial deformation is examined with respect to the anisotropic stress response as a function of deviatoric strain.

Force correlations, anisotropy, and friction mobilization in granular assemblies under uniaxial deformation

We study dense, frictional, polydisperse 3D granular assemblies under uniaxial deformation with Discrete Element Method (DEM) simulations. The overall goal – beyond the scope of the present study – is to link microscopic parameters and observations with the macroscopic behavior, for different elementary deformation modes. At present, we focus on the behavior of the force/contact network during uniaxial deformation, for different coefficients of friction. We discuss the stress and structural anisotropy and the relationship between force intensity weighted by contact state (sticking or sliding, at the Coulomb limit) or force strength. Furthermore, we study the orientational distribution of contacts and forces and the contribution of friction to structural anisotropy. We find that initial isotropic states are irrecoverable, since the structural anisotropy is independent of the deviatoric stress behavior both with and without friction. Contacts display an interesting anisotropy of order four in the presence of friction.

Behavior of cohesive powder in rotating drums

We present experimental findings on the flowability and avalanching behavior of cohesive powders in a rotating drum. The main goal – beyond the scope of the current study – is to develop a method to understand and predict phenomena that precede the occurrences of events like avalanches and then to simulate this with the Discrete Element Method. In the present study, we focus on the characterization, classification, and description of the various events possible in cohesive powders – other than in non-cohesive particle systems – during rotation in a drum. Events are categorized based on their nature and we speculate on their relation to the micro-structure and properties of the powder. As main result, we show that repeatable and consistent results can be obtained in the characterization of cohesive powders when angle–based (e.g. local surface and global center-of-mass) parameters are used. Different events can be distinguished, especially for strong cohesion, bulk shear sliding is often replaced by other events like slumping.

Discrete element simulations and experiments: toward applications for cohesive powders

Granular materials are omnipresent in nature and widely used in various industries ranging from food and pharmaceutical to agriculture and mining – among others. It has been estimated that about 10% of the world’s energy consumption is used in the processing, storage and transport of granular materials. In this thesis, we couple experiments and particle simulations to bridge this gap and link the microscopic properties to the macroscopic response for frictionless, frictional and cohesive granular packings, with the final goal of industrial application. The procedure of studying frictionless, frictional and cohesive granular assemblies independent of each other allows to isolate the main features related to each effect and provides a gateway into the use of discrete element methods to model and predict more complex industrial applications. For frictionless packings, we find that different deformation paths, namely isotropic/uniaxial over-compression or pure shear, slightly increase or reduce the jamming volume fraction below which the packing loses mechanical stability. This observation suggests a necessary generalization of the concept of the jamming volume fraction from a single value to a “wide range” of values as a consequence of the modifications induced in the microstructure, i.e. fabric, of the granular material in the deformation history. With this understanding, a constitutive model is calibrated using isotropic and deviatoric modes. We then predict both the stress and fabric evolution in the uniaxial mode. By focusing on frictional assemblies, we find that uniaxial deformation activates microscopic phenomena not only in the active Cartesian directions, but also at intermediate orientations, with the tilt angle being dependent on friction, and different for stress and fabric. While a rank-2 tensor (representing a second order harmonic approximation) is sufficient to describe the evolution of the normal force directions, a sixth order harmonic approximation is necessary to describe the probability distributions of contacts, tangential forces and the mobilized friction. As a further step, cohesion is introduced. From multi-stress level uniaxial experiments, by comparing two experimental setups and different cohesive materials, we report that while stress relaxation occurs at constant volume, the relative relaxation intensity decreases with increasing stress level. For longer relaxation, effects of previously experienced relaxation becomes visible at higher stress levels. A simple microscopic model is proposed to describe stress relaxation in cohesive powders, which accounts for the extremely slow force change via a response timescale and a dimensionless relaxation parameter. In the final part of the thesis, we compare results from experiments and discrete element simulations of a cohesive powder in a simplified canister geometry to reproduce dosing (or dispensing) of powders by a turning coil in industrial applications. Since information is not easily accessible from physical tests, by scaling up the experimental particle size and calibrating material parameters like cohesive strength and interparticle friction, we obtain quantitative agreement between the mass per dose in simulations and experiments for different dosage times. The number of doses, for a given total filling mass is inversely proportional to dosage time and coil rotation speed, as expected, but increases with increasing number of coils. Using homogenization tools, we obtain the exact local velocity and density fields in our device.

Evolution of the effective moduli for anisotropic granuar materials during pure shear

We analyze the behavior of a frictionless dense granular packing sheared at constant volume. Goal is to predict the evolution of the effective moduli along the loading path. Because of the structural anisotropy that develops in the system, volumetric and deviatoric stresses and strains are cross coupled via four distinct quantities, the classical bulk and shear moduli and two anisotropy moduli. Here, by means of numerical simulation, we apply small perturbations to various equilibrium states that previously experienced different pure shear strains and investigate the effect of the microstructure (2 nd rank fabric tensor) on the elastic bulk response. Besides the expected dependence of the bulk modulus on the isotropic fabric, we find that both the isotropic density of contacts and the (deviatoric) orientational anisotropy affect the anisotropy moduli. Interestingly, the shear modulus of the material depends also on the actual stress state, along with the (isotropic and anisotropic) contact configuration.