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The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region. Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated. / MSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region. Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated. / MSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

Le traitement des conditions aux limites et la modélisation fine des interactions de sous-maille ont été abordés dans cette thèse. La formulation caractéristique des conditions aux limites a été analysée et une nouvelle procédure 3D-NSCBC est proposée qui autorise la prise en compte de l’évolution de la vitesse et de la pression dans le plan des frontières, afin d’introduire le caractère tridimensionnel de l’écoulement dans les conditions limites. Des nouvelles formulations pour resoudre le couplage des ondes caractéristiques au niveau des arêtes et des coins ont été développées. Dans le cadre de la Simulation des Grandes Échelles, pour reproduire correctement la dynamique de la turbulence à la paroi et pour mieux prendre en compte l'anisotropie du tenseur des contraintes de sous-maille, un modèle structural fondé sur l'hypothèse de similarité est développé pour des écoulements modérément compressibles et validé sur la simulation d'un jet rond en impaction sur une paroi plane. / The treatment of boundary conditions and sub-grid scale interactions’ modeling, with particular attention to the asymptotic behavior near the wall, were addressed in this thesis. The characteristic formulation of boundary conditions has been analyzed and a novel procedure 3D-NSCBC is proposed, which, accounting for the evolution of velocity and pressure on the boundary planes, allows a better representation of the three-dimensional character of the flow at the boundary. New formulations to solve characteristic wave coupling on edges and corners are developed. Within the framework of the Large-Eddy Simulation, in order to give a correct reproduction of near-wall turbulence dynamics and in order to better account for the sub-grid scale stress tensor’s anisotropy, a structural model based on the similarity hypothesis has been developed for weakly compressible flows and validated on the simulation of a round jet impinging over a flat plane.