Academic literature on the topic 'Interparticle forces'

The lack of a suitable system for hot gas cleaning is the greatest obstacle to the development of gasification combined-cycle power generation. In this work, a shallow 0.15 m diameter fluidised bed of 1 mm sand particles was used as a filter for 0.5-10 mum gasifier char particles redispersed in nitrogen at 700-900°C and 1 bar a. Earlier work has suggested that, for a correctly-designed low free-area distributor, initial collection efficiencies greater than 99% at 2 could be realised but that poor retention limits the overall filtration efficiency. As an aid to retention, heavy fuel oil was sprayed continuously into the bed using a concentric-tubs atomiser. With this arrangement, theoretically-predicted filtration efficiencies were approached for particles below; 7 mum in size, but the overall performance was seriously marred by secondary aerosol originating from the addition of the oil itself. The results suggested that two modes of filter operation are possible; at lower gas and retention-aid flowrates the challenging aerosol is retained on the collector; at higher gas and retention-aid flov/rates the challenging aerosol is agglomerated and re-emitted from the bed in a distribution with a larger mean size. The operation of various instruments for particle size measurement below 10 mum has been reviewed, and their capabilities have been compared by experiment. The method selected (collection in a liquid impinger, followed by off-line analysis by "Coulter Counter") is described in detail. The addition of a liquid retention-aid to a fluidised bed can cause modification of its fluidisation behaviour, leading eventually to catastrophic defluidisation. Theoretical and experimental aspect of these effects are discussed, leading to the development of an experimental method for direct measurement of interparticle forces, and an expression for the transition condition between Geldart's (1973) groups A and B.

Les suspensions—un type de matériau qui comporte des particules solides dispersées dans un milieu liquide—sont omniprésentes dans notre vie quotidienne et dans l’industrie. Leur caractéristique-clé est la contrainte requise pour les mettre en écoulement à une vitesse désirée : cet attribut est le centre d’intérêt de la rhéologie. Récemment, il émerge que le frottement entre les particules se répercute sur la rhéologie des suspensions concentrées. Cette interaction microscopique peut être altérée en modifiant la surface des particules ou, notamment, en changeant le milieu liquide. Dans cette thèse, nous cherchons à démontrer et caractériser l’effet du frottement inter-particulaire sur des comportements rhéologiques des suspensions dans le régime dense. Nous trouvons que des suspensions de mêmes particules se comportent de façons différentes (newtonienne ou rhéofluidifiante) en dépendant des solvants utilisés. En outre, leur courbe d’écoulement peut être connectée à la mesure de coefficient de frottement en fonction de la force normale appliqué sur les particules. Notre travail expérimental aide ouvrir la voie aux études sur des effets de forces à l’échelle microscopique sur la rhéologie en bulk<br>Suspensions - a type of material consisted of solid particles dispersed in a liquid medium— are omnipresent in our daily life and in industry. Their key characteristic is the shear stress required to make them flow at a desire shear rate: this attribute is the area of interest of Rheology. Recently, it emerged that the friction between the particles impact the rheology of concentrated suspensions. This microscopic interaction can be altered by modifying the particle surface or, especially, by changing the liquid medium. In this thesis, we are looking to evidence and characterize the effect of interparticle friction on the rheological behaviors of suspension in the dense regime. We found that suspensions of same particles behave differently (Newtonian or shear-thinning) depending on the solvents utilized. Furthermore, their flow curve can be connected to the measurement of friction coefficient as a function of the normal force applied on the particles. Our work help paving the way for studies on effects of forces at microscopic scale on the bulk rheology

Le dépassement de la chute de pression du lit à la vitesse minimale de fluidisation, qui se produit pendant la transition d'un état de lit fixe à un état de lit fluidisé, est un phénomène courant pour les particules fines classées dans le groupe A selon la classification de Geldart. Ces particules présentent une hystérésis entre les courbes de chute de pression pour les trajectoires de vitesse de gaz décroissante et croissante. Cette étude utilise deux modèles de pression de particules adhésives dans des simulations de modèles à deux fluides pour incorporer l'influence de la force de Van der Waals interparticulaire, dans le but de prédire le dépassement de la pression. Le premier modèle de pression adhésive, développé dans le cadre de la théorie cinétique des écoulements granulaires rapides, n'a pas réussi à capturer le dépassement en raison de la prévalence de contacts multiples et prolongés dans les lits fixes. Nous avons proposé une fermeture alternative basée sur le nombre de coordination, générant une contribution adhésive significativement plus élevée que le modèle de la théorie cinétique et reproduisant avec succès le dépassement de la chute de pression.En outre, nous avons construit une base de données numériques CFD-DEM (Computational Fluid Dynamics-Discrete Element Method) pour prédire l'hystérésis dans la chute de pression. Cette base de données peut guider la formulation d'une équation de transport eulérienne pour le nombre de coordination, permettant l'incorporation des effets de l'historique des déformations. Nous avons étudié l'impact de la force de Van der Waals et de la friction statique sur la fluidisation des solides fins à l'échelle moyenne en utilisant des simulations CFD-DEM et leur rôle dans l'apparition du phénomène de dépassement de pression. Notre analyse examine des paramètres tels que la chute de pression du gaz, le vide du lit, le nombre de coordination, les pressions répulsives et adhésives des solides, le gradient vertical de vitesse des solides, le tenseur de tissu et la contrainte de cisaillement particule-paroi tout au long des processus de défluidisation et de fluidisation. Nous avons démontré qu'il est nécessaire de prendre en compte l'adhésion de Van der Waals pour prédire l'expansion homogène du lit sur toute la gamme des vitesses, du minimum requis pour la fluidisation au minimum pour le bullage. L'ensemble de données CFD-DEM généré peut guider le développement de fermetures de contraintes solides pour les modèles à deux fluides afin d'incorporer les effets de l'adhésion de Van der Waals et de la friction statique sur l'hydrodynamique de la fluidisation, ce qui permet de prédire l'hystérésis dans la chute de pression du lit à l'échelle macroscopique. Dans ce travail, nous avons incorporé un modèle de frottement statique-dynamique dans le code CFD-DEM massivement parallèle YALES2 à l'aide d'un algorithme en deux étapes, afin de remédier aux lacunes du modèle de frottement dynamique de Coulomb, qui est pratique pour les écoulements granulaires rapides mais ne s'applique pas aux lits stationnaires. Nous avons validé notre mise en œuvre par une série de tests à macro- et micro-échelle. En outre, nous avons introduit dans YALES2 les forces de Van der Waals entre particules et entre particules et parois, et validé cet ajout à l'échelle microscopique. En outre, nous avons postulé une expression de relaxation pour le terme source dans l'équation de transport des nombres de coordination et déterminé le temps de relaxation des nombres de coordination à l'aide de données de simulation CFD-DEM. En outre, nous avons utilisé une technique de pénalisation pour coupler de manière semi-implicite les phases gazeuse et solide, en particulier par le traitement implicite des forces de traînée et d'Archimède. Cette approche vise à résoudre les problèmes de stabilité rencontrés lorsque le couplage interphase est explicite<br>The overshoot in bed pressure drop at the minimum fluidization velocity, occurring during the transition from a fixed to a fluidized bed state, is a common phenomenon for fine particles categorized under Group A according to Geldart's classification. These particles exhibit hysteresis between the pressure drop curves for the decreasing and increasing gas velocity paths. This study employs two adhesive particle pressure models within two-fluid model simulations to incorporate the influence of interparticle Van der Waals force, aiming to predict the pressure overshoot. The first adhesive pressure model, developed within the kinetic theory of rapid granular flows framework, failed to capture the overshoot due to the prevalence of multiple and prolonged contacts in fixed beds. We proposed an alternative closure based on coordination number, generating a significantly higher adhesive contribution than the kinetic theory model and successfully reproducing the pressure drop overshoot.In addition, we constructed a Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) numerical database to predict hysteresis in pressure drop. This database can guide the formulation of an Eulerian transport equation for the coordination number, enabling the incorporation of deformation history effects. We explored the impact of Van der Waals force and static friction on the fluidization of fine solids at the mesoscale using CFD-DEM simulations and their role in causing the pressure overshoot phenomenon. Our analysis examines parameters such as gas pressure drop, bed voidage, coordination number, repulsive and adhesive solid pressures, vertical solid velocity gradient, fabric tensor, and particle-wall shear stress throughout the defluidization and fluidization processes. We demonstrated that it is necessary to consider the Van der Waals adhesion to predict the homogeneous expansion of the bed across the range of velocities from the minimum required for fluidization to the minimum for bubbling. The generated CFD-DEM dataset can guide the development of solid stress closures for two-fluid models to incorporate the effects of Van der Waals adhesion and static friction on fluidization hydrodynamics, allowing for the prediction of hysteresis in bed pressure drop at the macroscale.In this work, we incorporated a static-dynamic friction model into the massively parallel CFD-DEM code YALES2 using a two-step algorithm, aiming to address the shortcomings of the Coulomb dynamic friction model, which is practical for fast granular flows but not applicable to stationary beds. We validated our implementation through a series of macro- and microscale tests. Furthermore, we introduced interparticle and particle-wall Van der Waals forces into YALES2 and validated this addition at the microscale. Additionally, we postulated a relaxation expression for the source term in the coordination number transport equation and determined the coordination number relaxation time using CFD-DEM simulation data. Moreover, we employed a penalization technique to semi-implicitly couple gas and solid phases, specifically through the implicit handling of drag and Archimedes forces. This approach aimed to resolve the stability issues encountered when the interphase coupling is explicit

Forces acting between individual grains in a powder can have a critical and controlling effect on powder bulk behaviour. Operations such as powder flow, fluidisation, compaction, agglomeration and mixing are all influenced significantly by the intensity of interparticle forces. This is especially true when the particle size falls below around 100 mum at which point the surface forces outweigh the force due to gravity acting on a single particle. Studies of cohesion using bulk powder samples are of limited use because it is difficult to decouple the fundamental mechanisms of interparticle force from other contributions to cohesion such as variations in the powder microstructure, or geometric interlocking of individual particles. A review of the relevant literature has unearthed conflicting evidence associated with the influence of relative humidity (RH) on both bulk powder cohesion and interparticle force. Therefore there is a need for experimental force studies at the scale of the individual particle to identify the fundamental mechanisms that prevail and resolve some of the apparent uncertainty that currently exists. A custom built force instrument, incorporating Atomic Force Microscope (AFM) technology, was designed, constructed and commissioned. This instrument was used to quantify the interactions between particles of around 40 mum in diameter and flat surfaces as a function of the relative humidity of the surrounding air. Interactions between soda-lime glass surfaces, gold surfaces and amorphous quartz surfaces were studied. Striking results were obtained on soda-lime glass surfaces upon decreasing the RH from > 70% to around 40%. At this point the glass surfaces suddenly exhibited a strong repulsion upon approach. The range of this repulsion was observed at separation distances as great as 250 nm. Once the surfaces were brought into contact the strong repulsion was accompanied by a very large force of adhesion. This strong repulsion and associated peak value of adhesion was not observed at other RH values and was specific to desorption rather than adsorption. Force curves for gold and quartz surfaces showed no such repulsion and peak adhesion. It is thought that the critical humidity coincides with the formation of a complete monolayer of adsorbed water molecules. A number of possible explanations have been offered for the effect and its uniqueness to soda-lime glass in the present experiments. Theoretical calculations of adhesion force have been performed based on the concept of capillary meniscus formation. Calculations give values of around 17000 nN for a sphere 40 mum in diameter and a contact angle of 20&deg;. These values are somewhat larger than measured values in all cases apart from peak adhesion. It is thought that at low humidities there is insufficient water adsorbed to overcome the effect of surface roughness. Contact occurs at asperities, which reduces the expected contact area and hence leads to an adhesive force that is lower than predicted. At humidities > 80% the experiments show evidence of capillary elongation upon surface separation. This implies that the surface adsorbed film is mobile with bulk liquid being drawn into the bridge under the action of the surface tension force. The associated increase in bridge volume and the change in bridge curvature with I elongation will tend to equalise the Laplace pressure inside the bridge and therefore give a value of adhesion that is lower than predicted.