Discrete element simulations and experiments : towards applications for cohesive powders

Abstract

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.