Rhéologie des poudres cohésives : expériences et modélisation

Les milieux granulaires constituent une des ressources primaires les plus utilisées dans le monde, particulièrement dans l’industrie. Parmi la diversité de matériaux granulaires existants, les poudres cohésives constituent l’un des matériaux les plus délicats à manipuler. Comprendre le comportement de ces poudres en écoulement et développer des outils adaptés à leur manipulation constitue donc un enjeu industriel majeur. Cependant la difficulté d’étudier ces poudres cohésives réside dans la diversité des interactions cohésives entre les grains qui les composent. Dans le but de comprendre les effets de la cohésion sur les écoulements de poudres, une première étape consiste donc à concevoir un milieu cohésif modèle dont on peut contrôler la cohésion, et qui soit peu sensible aux conditions expérimentales. Le travail proposé dans cette thèse est de développer des méthodes permettant de caractériser et quantifier la cohésion inter-particules ainsi que la cohésion macroscopique d’un milieu granulaire cohésif modèle (CCGM), puis de concevoir des dispositifs expérimentaux permettant d'étudier le comportement du CCGM en écoulement. Les résultats expérimentaux seront comparés à des résultats obtenus grâce à des simulations numériques, basées sur un modèle de rhéologie granulaire continue à laquelle une contrainte seuil de mise en écoulement dépendante de la cohésion est ajoutée, réalisées par Pierre-Yves Lagrée et Anaïs Abramian, ainsi que des simulations de dynamiques des contacts réalisées par Lydie Staron et Sandip Mandal. La pertinence de cette modélisation continue sera discutée à travers les chapitresGranular media is one of the most widely used primary resources in the world, particularly in industry. Among the diversity of existing granular materials, cohesive powders are one of the more challenging material to handle. Understanding the behaviour of these powders and developing adapted tools to their handling is therefore a major industrial challenge. However, the difficulty of studying these cohesivepowders lies in the diversity of cohesive interactions between the grains. In order to understand the effects of cohesion on powder flows, a first step is to design a model cohesive medium whose cohesion can be controlled and which is weakly sensitive to experimental conditions. The work proposed in this thesis is to develop methods to characterize and quantify the inter-particle cohesion as well as the macroscopic cohesion of a model cohesive granular medium (CCGM), and then to design experimental devices to study the flow behaviour of the CCGM. The experimental results will be compared to results obtained through numerical simulations, based on a model of continuous granular rheology enhanced with a yield stress, carried out by Pierre-Yves Lagrée and Anaïs Abramian, as well as contact dynamics simulations by Lydie Staron and Sandip Mandal. The relevance of this modelisation will be discussed in the different chapter

A cohesion-controlled granular material

International audienceWe present a simple method to prepare a granular material with a controlled cohesion between particles. The granular material is made of spherical glass beads coated with a polyborosiloxane polymer. This material is proved to be stable in time and non-sensitive to temperature and humidity. The inter-particle force is measured and related to the size of the grain and the polymer coating thickness. Classical measurements (packing fraction, repose angle, macroscopic cohesion), are performed with this cohesion-controlled granular material. This model material opens many perspectives to study in a controlled manner the flow of cohesive grains

The effect of cohesion on the discharge of a granular material through the orifice of a silo

We present the results of both experimental and numerical investigations of the silo discharge for a cohesive granular material. In our study, thanks to a cohesion-controlled granular material (CCGM) we propose to investigate the effect of the cohesive length lc, on the discharge of a silo for two different configurations, one axisymmetrical, and one quasi-2D rectangular silo. In both configurations, an adjustable bottom is used to control the size of the orifice. As observed for cohesionless granular material by previous studies, the mass flow rate and the density through an orifice are mostly controlled by the diameter of the orifice D. The experimental results of the quasi-2D silo are compared with continuum numerical simulations

Flows of cohesive granular media

Cohesive granular media have broad applications in industries. However, our understanding of their flow behavior is still limited compared to dry granular media, although rich knowledge about their static and plastic properties has been gained. In this paper, we provide some insights into the flow behavior of cohesive granular media from our recent numerical studies using an inclined plane and a plane shear cell. We evidence that the cohesive nature of flows is significantly affected by material properties of the particles like stiffness and inelasticity in addition to the inter-particle adhesion and introduce the concept of “effective” adhesion, which incorporates the effects of these three variables. We propose constitutive relations involving dimensionless inertial number and “effective” cohesion number, based on the “effective” adhesion to describe the rheology. We also show that increasing adhesion increases the hysteresis in granular media, evidencing the existence of a prominent shear weakening branch in the friction coefficient versus inertial number rheological curve. Moreover, we reveal that this increasing hysteresis gives rise to the increasing occurrence of shear banding instability, pointing to the increasing possibility of jamming in cohesive granular media. Finally, we present a promising experimental approach to investigate the flow behavior of cohesive granular materials, based on a simple method of preparing a long time stable medium with a controlled adhesion between particles

The effect of cohesion on the discharge of a granular material through the orifice of a silo

We present the results of both experimental and numerical investigations of the silo discharge for a cohesive granular material. In our study, thanks to a cohesion-controlled granular material (CCGM) we propose to investigate the effect of the cohesive length lc, on the discharge of a silo for two different configurations, one axisymmetrical, and one quasi-2D rectangular silo. In both configurations, an adjustable bottom is used to control the size of the orifice. As observed for cohesionless granular material by previous studies, the mass flow rate and the density through an orifice are mostly controlled by the diameter of the orifice D. The experimental results of the quasi-2D silo are compared with continuum numerical simulations

Flows of cohesive granular media

Cohesive granular media have broad applications in industries. However, our understanding of their flow behavior is still limited compared to dry granular media, although rich knowledge about their static and plastic properties has been gained. In this paper, we provide some insights into the flow behavior of cohesive granular media from our recent numerical studies using an inclined plane and a plane shear cell. We evidence that the cohesive nature of flows is significantly affected by material properties of the particles like stiffness and inelasticity in addition to the inter-particle adhesion and introduce the concept of “effective” adhesion, which incorporates the effects of these three variables. We propose constitutive relations involving dimensionless inertial number and “effective” cohesion number, based on the “effective” adhesion to describe the rheology. We also show that increasing adhesion increases the hysteresis in granular media, evidencing the existence of a prominent shear weakening branch in the friction coefficient versus inertial number rheological curve. Moreover, we reveal that this increasing hysteresis gives rise to the increasing occurrence of shear banding instability, pointing to the increasing possibility of jamming in cohesive granular media. Finally, we present a promising experimental approach to investigate the flow behavior of cohesive granular materials, based on a simple method of preparing a long time stable medium with a controlled adhesion between particles

Erosion of a cohesive granular material by an impinging turbulent jet

The erosion of a cohesive soil by an impinging turbulent jet is observed, for instance, during the landing of a spacecraft or involved in the so-called jet erosion test. To provide a quantitative understanding of this situation for cohesive soils, we perform experiments using a model cohesion controlled granular material that allows us to finely tune the cohesion between particles while keeping the other properties constant. We investigate the response of this cohesive granular bed when subjected to an impinging normal turbulent jet. We characterize experimentally the effects of the cohesion on the erosion threshold and the development of the crater. We demonstrate that the results can be rationalized by introducing a cohesive Shields number that accounts for the inter-particles cohesion force. The results of our experiments highlight the crucial role of cohesion in erosion processes

Erosion of a cohesive granular material by an impinging turbulent jet

The erosion of a cohesive soil by an impinging turbulent jet is observed, for instance, during the landing of a spacecraft or involved in the so-called jet erosion test. To provide a quantitative understanding of this situation for cohesive soils, we perform experiments using a model cohesion controlled granular material that allows us to finely tune the cohesion between particles while keeping the other properties constant. We investigate the response of this cohesive granular bed when subjected to an impinging normal turbulent jet. We characterize experimentally the effects of the cohesion on the erosion threshold and the development of the crater. We demonstrate that the results can be rationalized by introducing a cohesive Shields number that accounts for the inter-particles cohesion force. The results of our experiments highlight the crucial role of cohesion in erosion processes

Capillary filtering of particles during dip coating

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