The entrainment rate of Ferrosilicone (FeSi) particles was measured in a 140 mm perspex column with air as the fluidising medium. Two different types of FeSi were used, namely atomised FeSi, which is mostly spherical in shape with smooth surfaces, and milled FeSi, which is irregular with rough surfaces. Both the FeSi mixtures had the same solid density and the similar average particle diameters ranging from 38 µm to 50 µm. The size and density of these particles put them on the border between Geldart A and Geldart B powders, similar to the high temperature Fischer-Tropsch catalyst. The atomised FeSi had a slightly higher concentration in fines (8.6% vs 1.8%), but except for the difference in particle shape, the two mixtures had otherwise very similar physical properties. A substantial difference in entrainment rate was measured between the atomised and milled FeSi, where the atomised had an entrainment rate of about six times higher than the milled FeSi throughout the range of superficial velocities tested. It was shown that the higher entrainment rate cannot be attributed only to the higher fines concentration, but that the difference in particle shape had a significant effect on the entrainment rate. Several two dimensional shape characterisation techniques were used in attempt to quantify the difference between the atomised and the milled FeSi. Of these the particle circularity managed to differentiate the best between the two particle mixtures. The circularities of the atomised and the milled FeSi were found to be 0.782 and 0.711 respectively. The measured circularity was used instead of a sphericity to adjust for the effect of particle shape on the terminal velocity of the particles. The adjusted terminal velocity was then used in the elutriation rate constant correlations to see which of the popular correlations in literature predicts the entrainment rate of the FeSi the best. All of the correlations gave a poor performance in predicting the measured entrainment rates. The two correlations that performed the best were that of Choi et al. (1999) (AARE = 72.6%) and Geldart et al. (1979) (AARE = 79%). It was concluded that single particle drag and single particle terminal velocities are not adequate to incorporate the effect of particle shape on entrainment rate. The method i by which shape affects entrainment rate therefore deserves further investigation. Further studies should also be done to develop a three dimensional shape descriptor that predicts bulk behaviour better. / Dissertation (MEng)--University of Pretoria, 2008. / Chemical Engineering / unrestricted

Parmi les centrales solaires thermiques à concentration, la technologie des centrales à tour offre l'un des rendements les plus importants de production d'énergie. Néanmoins, l'efficacité et la sécurité de ces centrales sont améliorables. En effet, les sels fondus, généralement utilisés comme fluide de transfert thermique, présentent une plage limitée d'utilisation (200-550°C), à l'origine des limites d'efficacité de la conversion thermique-électrique, ainsi que de consommations parasites d'énergie de chauffage. De plus, leurs caractères corrosif et comburant sont à l'origine de sévères contraintes de sécurité. Un nouveau concept de récepteur solaire, dont les caractéristiques permettent de s'affranchir des contraintes associées aux sels fondus, est présenté dans ce manuscrit. Il utilise des suspensions denses de particules fluidisées par un gaz comme fluide de transfert et de stockage de l'énergie thermique. Ce concept, et la technologie de récepteur associée, a été brevetée par Flamant et Hemati dans le cadre d'une collaboration entre le Laboratoire CNRS-PROMES d'Odeillo, et l'Institut National Polytechnique de Toulouse. Son développement a reçu le soutien financier du CNRS, puis de la Commission Européenne. Les propriétés thermiques du carbure de silicium ont déterminé le choix de ce solide. Le diamètre moyen des particules utilisées avoisine 60 micromètres (groupe A). Ces particules présentent d'excellentes propriétés de fluidisation pour des vitesses de gaz faibles. La construction et l'exploitation d'une maquette froide transparente ont permis de démontrer la faisabilité hydrodynamique du concept. Cette maquette est un échangeur à deux passes. Chaque passe est constituée de deux tubes verticaux en parallèle. L'une est traversée par un débit vertical ascendant de solide, l'autre descendant. Un débit de solide continu, stable et équitablement réparti a été obtenu à l'intérieur des tubes. La caractérisation hydrodynamique détaillée de l'écoulement, et du comportement globale de la maquette, en fonction des conditions opératoires, a été effectué sur la partie ascendante de l'écoulement dans l'échangeur. La construction et l'exploitation d'une maquette chaude, constituée d'un seul tube traversé par une suspension dense en écoulement ascendant, chauffé par 3 fours d'une puissance totale de 5,6 kW, a permis d'estimer la capacité de transfert thermique de ce nouveau type d'échangeur. Le contrôle et la stabilité des conditions opératoires a permis d'évaluer l'effet de ces dernières sur le transfert thermique entre l'échangeur et la suspension dense de fines particules le traversant. La modélisation par 3 approches du transport ascendant de la suspension dense a également été réalisée. Une approche corrélative 1D basée sur le formalisme du modèle Bulle-Emulsion, adapté afin de tenir compte de l'entraînement des particules dans le sillage des bulles. Ce modèle permet de représenter la structure diphasique de l'écoulement. Une autre approche 1D a été utilisée. Elle repose sur la résolution des équations locales de conservation de masse et de quantité de mouvement sur chaque phase gaz et solide. Cette méthode permet de s'affranchir des hypothèses du modèle Bulle-Emulsion. Enfin, la simulation numérique 3D a été réalisée sur un maillage complet du système, de telle sorte que les conditions aux bornes imposées son identiques à celle imposée par l'opérateur (débit de fluidisation, débit d'aération, débit de solide, pression de la nourrice). Cette dernière apporte des informations sur la structure locale de l'écoulement, dont les caractéristiques permettent d'expliquer l'efficacité du transfert thermique entre la suspension et la paroi observé expérimentalement. / Among concentrating solar power plants, solar tower technology is one of the more power efficient. Nevertheless, their efficiency and safety can be improved. Indeed, molten salts, commonly used as heat transfer fluid, have a limited range of operating temperature (470-820K), thus lowering the thermal-electrical conversion efficiency, and increasing parasitic power consumption. Moreover, they are corrosive and combustion agent, leading to severe safety constraints. A new concept of solar receiver is presented in the present study, the characteristics of which avoid most of the molten salts drawbacks. It uses dense gas-particle suspension as heat transfer and storage fluid. This concept and the associated technology has been patented by Flamant et Hemati in the frame of a collaboration between the PROMES-CNRS Laboratory of Odeillo and the Polytechnic National Institute of Toulouse. Its development has been first supported by the CNRS, and later by the European Commission. Thermal properties of silicon carbide have determined the choice of this solid. The mean diameter of particles is around 64 microns (A group). These particles have excellent fluidisation properties at low gas velocities. The construction and the operation of a transparent cold mockup allowed demonstrating the hydrodynamic feasibility of this concept. This mockup is composed of two passes. Each pass is composed of two tubes in parallel. One pass is upward flow of solid, the other is downward flow. A steady, stable and evenly distributed solid flow has been set inside the tubes. The global behaviour of the system and the hydrodynamics of the suspension has been evaluated as a function of operating parameters on the upward pass. The construction and the operation of a hot mockup allowed estimating the heat transfer efficiency of this new kind of exchanger. On this mockup, the dense suspension flows upward inside a single tube, heated by three ovens of 5.6 kW total power. Thanks to the control and stability of the operating parameters, their effects on the heat transfer between the tube and the dense gas-solid suspension has been accurately determined. Modelling of the suspension upward flow has been performed using 3 approaches. The first one is based on the 1D Bubble-emulsion formalism, adapted to take into account the solid entrainment by the bubble wakes. It allows modelling the diphasic structure of the flow. The resolution of the local mass and momentum balance equation on each phase has also been performed. It allows to sidestep the Bubble-Emsulion assumptions, and to study the effects of drag models. 3D simulation has been performed on a complete mesh of the system, so that the boundary conditions are the same as those imposed by the operator (fluidisation flow rate, aeration flow rate, solid flow rate, dispenser pressure). These simulations give information on the local structure of the suspension flow, influencing on the heat transfer efficiency between the exchanger wall and the suspension.