This work investigated a numerical approach to the search of a maximum heat transfer rate density (the overall heat transfer dissipated per unit of volume) from a two-dimensional laminar multiscale array of cylinders in cross-flow under an applied fixed pressure drop and subject to the constraint of fixed volume. It was furthermore assumed that the flow field was steady state and incompressible. The configuration had two degrees of freedom in the stationary state, that is, the spacing between the cylinders and the diameter of the smaller cylinders. The angular velocity of the cylinders was in the range 0 ≤ ϖ, ≤ 0.1. Two cylinders of different diameters were used, in the first case, the cylinders were aligned along a plane which lay on their centrelines. In the second case, the cylinder leading edge was aligned along the plane that received the incoming fluid at the same time. The diameter of the smaller cylinder was fixed at the optimal diameter obtained when the cylinders were stationary. Tests were conducted for co-rotating and counterrotating cylinders. The results were also compared with results obtained in the open literature and the trend was found to be the same. Results showed that the heat transfer from a rotating array of cylinders was enhanced in certain cases and this was observed for both directions of rotation from an array which was aligned on the centreline. For rotating cylinders with the same leading edge, there is heat transfer suppression and hence the effect of rotation on the maximum heat transfer rate density is insignificant. This research is important in further understanding of heat transfer from rotating cylinders, which can be applied to applications ranging from contact cylinder dryers in the chemical processes industry and rotating cylinder electrodes to devices used for roller hearth furnaces. / Dissertation (MEng)--University of Pretoria, 2011. / Mechanical and Aeronautical Engineering / unrestricted

We report on low resolution (R~3000) spectropolarimetry of the A0 supergiant star HD 92207. This star is well-known for significant spectral variability. The source was observed on seven different nights spanning approximately 3 months in time. With a rotation period of approximately 1 year, our data covers approximately a quarter of the star's rotational phase. Variability in the continuum polarization level is observed over this period of time. The polarization across the Halpha line on any given night is typically different from the degree and position angle of the polarization in the continuum. Interestingly, Hbeta is not in emission and does not show polarimetric variability. We associate the changes at Halpha as arising in the wind, which is in accord with the observed changes in the profile shape and equivalent width of Halpha along with the polarimetric variability. For the continuum polarization, we explore a spiral shaped wind density enhancement in the equatorial plane of the star, in keeping with the suggestion of Kaufer etal (1997). Variable polarization signatures across Halpha are too complex to be explained by this simple model and will require a more intensive polarimetric follow-up study to interpret properly.

L’enrobage à sec par action mécanique permet de formuler des particules composites dont les fonctionnalités et les propriétés physiques, comme l’écoulement, sont améliorées et cela sans l’ajout de solvant ou de liant. Actuellement la plupart des procédés d’enrobage à sec sont conduits en mode discontinu. L’objectif de ce travail est donc de développer et d’étudier une opération d’enrobage à sec par un procédé continu innovant, une extrudeuse bi-vis corotative sans filière, constituant une rupture technologique dans son domaine. Dans le cadre de ce travail, des sphères de cellulose microcristalline, appelées particules hôtes sont enrobées avec soit des talcs de différentes granulométries soit du stéarate de magnésium, nommées particules invitées. Dans un premier temps, le comportement des particules hôtes est étudié dans le procédé. Les particules sorties du procédé sont analysées par microscopie électronique à balayage, par granulométrie laser et par voluménométrie. Pour la configuration de vis présentant que des éléments de transport, la vitesse de rotation des vis (25 à 200 rpm) et le débit d’alimentation (0,5 à 2 kg/h) choisis influencent le taux de remplissage dans le fourreau, et pour un taux supérieur à environ 14 % : la quantité de particules endommagées est négligeable. Un taux de remplissage faible favorise les frictions particules-particules ou particules-métal au niveau de l’entrefer. L’ajout de zones de mélanges dans la configuration des vis accentue ce phénomène de brisure par l’augmentation des contraintes de cisaillement. Par ailleurs, des masses retenues de la poudre dans le fourreau évoluent linéairement en fonction de la masse de poudre transportée en un tour de vis pour différentes configurations de vis. Ces relations laissent penser à l’existence d’un volume mort. Les mesures de distribution de temps de séjour mettent en évidence par l’application d’un modèle d’association de réacteurs idéaux, la présence d’écoulement piston et d’un volume mort. Dans un second temps, la faisabilité d’une opération d’enrobage à sec par le procédé étudié est analysée. Une couche d’enrobage continu de talc modifiant le comportement hydrophile des particules hôtes, est obtenue pour une vitesse de rotation de vis de 50 rpm et une configuration de vis cisaillante. Le type de particules invitées influence la morphologie de la couche d’enrobage : de type film avec le stéarate de magnésium et de type discret avec le talc micronisé. Néanmoins les propriétés de taille, d’écoulement, de compressibilité des particules composites sont semblables à celle des particules hôtes, seule la propriété de mouillabilité est modifiée. La présence de stéarate de magnésium diminue le temps de séjour des particules dans le procédé grâce à son caractère lubrifiant. / Dry particle coating with mechanical action allows the production of composite particles whose functionalities and physical properties, such as flowability, are improved thanks to the absence of solvent or binder. Currently, most of dry particle coating processes are carried out in a discontinous mode. The objective of this work is thus to develop and study a dry particle coating operation with a continuous innovative process, twin screw co-rotating extruder without die, constituting a technological rupture in this application field. For this, spherical particles of microcristalline cellulose, as host particles, are coated with either talc particles of two sizes or magnesium stearate particles (MgSt), as guest particles. Firstly, the host particles behaviour is studied in the process. The exiting particles are analyzed by scanning electronic microscopy, laser granulometry and tapping test. Regarding the transport configuration, the chosen screw speed (between 25 an 200 rpm) and the feed rate (between 0.5 and 2 kg/h) have an important influence on the filling level in the barrel. For a filling level superior to approximatly 14 %, the quantity of broken particles is not significant. A low filling level facilitates the particle-particle or particle-metal friction in the barrel gap. When more mixing zones are added to the screw configuration, the shear stresses increase and, therefore, the breakage particle phenomenon becomes more prominent. Futhermore, the retained powder mass increases linearly when the transported mass in one screw turn increases for all the screw configurations. These evolutions suggest the existence of a dead volume. The residence time distribution mesurements highlight, thanks to the application of flowing model, the presence of plug-flow and a dead volume. Secondly, the feasibility of using the process for dry particle coating is analyzed. A continuous coating layer of talc modifiying the hydrophilic behaviour of the host particles is obtained for a low screw rotation speed (50 rpm) and a screw configuration with one mixing zone. The type of guest particles influence the morphology of the coating layer: film layer with the MgSt particles and discontinous layer with the micronized talc. However, size, flow and compressibility of the coated particles are similar to the ones of the initial host particles, only the wettability properties are modified. The presence of MgSt decreases the particle mean residence time in the process thanks to its lubricant action.