<p>The work is focused on experimental study and modelling of spacer grid influence on single- and two-phase flow. In the experimental study a mock-up of a realistic fuel bundle with five spacer grids of thin plate spring construction was investigated. A special pressure measuring technique was used to measure pressure distribution inside the spacer. Five pressure taps were drilled in one of the rods, which could exchange position with other rods, in this way providing a large degree of freedom. Laser Doppler Velocimetry was used to measure mean local axial velocity and its fluctuating component upstream and downstream of the spacer in several subchannels with differing spacer part. The experimental study revealed an interesting behaviour. Subchannels from the interior part of the bundle display a different effect on the flow downstream of the spacer compared to subchannels close to the box wall, even if the spacer part is the same. This behaviour is not reflected in modern correlations. The modelling part, first, consisted in comparing the present experimental data to Computational Fluid Dynamics calculations. It was shown that stand-alone subchannel models could predict the local velocity, but are unreliable in prediction of turbulence enhancement due to spacer. The second part of the modelling consisted in developing a deposition model for increase due to spacer. In this study Lagrangian Particle Tracking (LPT) coupled to Discrete Random Walk (DRW) technique was used to model droplet movements through turbulent flow. The LPT technique has an advantage to model the influence of turbulence structure effect on droplet deposition, in this way presenting a generalized model in view of spacer geometry change. The verification of the applicability of LPT DRW method to model deposition in annular flow at Boiling Water Reactor conditions proved that the method is unreliable in its present state. The model calculations compare reasonably well to air-water deposition data, but display a wrong trend if the fluids have a different density ratio than air-water.</p>

The work is focused on experimental study and modelling of spacer grid influence on single- and two-phase flow. In the experimental study a mock-up of a realistic fuel bundle with five spacer grids of thin plate spring construction was investigated. A special pressure measuring technique was used to measure pressure distribution inside the spacer. Five pressure taps were drilled in one of the rods, which could exchange position with other rods, in this way providing a large degree of freedom. Laser Doppler Velocimetry was used to measure mean local axial velocity and its fluctuating component upstream and downstream of the spacer in several subchannels with differing spacer part. The experimental study revealed an interesting behaviour. Subchannels from the interior part of the bundle display a different effect on the flow downstream of the spacer compared to subchannels close to the box wall, even if the spacer part is the same. This behaviour is not reflected in modern correlations. The modelling part, first, consisted in comparing the present experimental data to Computational Fluid Dynamics calculations. It was shown that stand-alone subchannel models could predict the local velocity, but are unreliable in prediction of turbulence enhancement due to spacer. The second part of the modelling consisted in developing a deposition model for increase due to spacer. In this study Lagrangian Particle Tracking (LPT) coupled to Discrete Random Walk (DRW) technique was used to model droplet movements through turbulent flow. The LPT technique has an advantage to model the influence of turbulence structure effect on droplet deposition, in this way presenting a generalized model in view of spacer geometry change. The verification of the applicability of LPT DRW method to model deposition in annular flow at Boiling Water Reactor conditions proved that the method is unreliable in its present state. The model calculations compare reasonably well to air-water deposition data, but display a wrong trend if the fluids have a different density ratio than air-water.

Experiments investigating complex flows in rod bundles with spacer grids that have mixing devices (such as flow mixing vanes) have mostly been performed using single-point measurements. Although these measurements allow local comparisons of experimental and numerical data they provide little insight because the discrepancies can be due to the integrated effects of many complex flow phenomena such as wake-wake, wake-vane, and vane-boundary layer interactions occurring simultaneously in a complex flow environment. In order to validate the simulations results, detailed comparison with experimental data must be done. This work describes an experimental database obtained using Time Resolved Particle Image Velocimetry (TR-PIV) measurements within a 5 x 5 rod bundle with spacer-grids. Measurements were performed using two different grid designs. One typical of Boiling Water Reactors (BWR) with swirl type mixing vanes and the other typical of Pressurized Water Reactors (PWR) with split type mixing vanes. High quality data was obtained in the vicinity of the grid using the multi-scale approach. One of the unique characteristic of this set-up is the use of the Matched Index of Refraction (MIR) technique employed in this investigation. This approach allows the use of high temporal and spatial non-intrusive dynamic measurement techniques to investigate the flow evolution below and immediately above the spacer. The experimental data presented includes explanation of the various cases tested such as test rig dimensions, measurement zones, the test equipment and the boundary conditions in order to provide appropriate data for comparison with Computational Fluid Dynamics (CFD) simulations. Turbulence parameters of the obtained data are analyzed in order to gain insight of the physical phenomena. The shape of the velocity profile at various distances from the spacer show important modifications passing the grid which delineates the significant effects of the presence of the grid spacer. Influence of the vanes wake in the global velocity was quantified to be up to a distance of 4 hydraulic diameters from the edge of the grid.Spatial and temporal correlations in the two measured dimensions were performed to quantify the time and length scales present in the flow in the vicinity of the grids and its influence in the flow modification induced by the vanes. Detection of vortex cores was performed using the vorticity, swirl strength and Galilean decomposition approach. The resulted cores were then tracked in time, in order to observe the evolution of the structures under the influence of the vanes for each grid. Vortex stretching was quantified in order to gain insight of the energy dissipation process normally associated with the phenomena. This work presents data in a single-phase flow situation and an analysis of these data for understanding complex flow structure. This data provide for the first time detailed temporal velocity full field which can be used to validate CFD codes.

Les vibrations induites par l'écoulement dans le coeur du REP peuvent provoquer une usure par frottement des crayons combustibles par friction au niveau des contacts entre la cellule de grille et les crayons des assemblages combustibles. Cela peut entraîner des dommages irréversibles de la gaine du crayon combustible et compromettre la première barrière de sûreté du réacteur. Assurer l'intégrité de la gaine est une préoccupation majeure dans la sûreté du réacteur. Cependant, les spectres d'excitation des forces fluides agissant sur les crayons ne sont pas bien connus. Le but de cette thèse est d'utiliser des éléments géométriques simples pour reproduire des cellules de grilles d'un REP. SGE ont été effectuées sur une conduite annulaire avec différents maillages en utilisant le code TrioCFD. Une étude de sensibilité de maillage a été réalisée afin de proposer un maillage reproduisant correctement les résultats dans la littérature. Ces informations de résolution de maillage ont été utilisées lors de la réalisation des simulations en utilisant divers obstacles géométriques intérieurs à la conduite, i.e., des ailettes de mélange, une grille circulaire et une combinaison de grille carrée et d'ailettes de mélange. Un maillage hybride a été utilisé dans le cas des ailettes de mélange et dans le cas de cellule de grille carrée. Les caractéristiques hydrauliques ainsi que la pression pariétale ont été analysées dans chaque cas. Il apparaît que la grille carrée est une combinaison approximative du cas des ailettes de mélange et du cas de la grille circulaire. Les simulations ont été comparés avec des mesures réalisées au CEA / Flow-induced vibrations in the pressurized water reactor core can cause fretting wear in the fuel rods. Due to friction, wear occurs at the contact locations between the spacer grid and the fuel rod. This may compromise the first safety barrier of the nuclear reactor by damaging the fuel rod cladding. In order to ensure the integrity of the cladding, it is necessary to know the random fluctuating forces acting on the rods. However, the spectra for these fluid forces are not well known. The goal of this thesis is to use simple geometrical elements to check the reproducibility of realistic PWR spacer grids. As a first step, LES were performed on annular pipe for different mesh refinements using the CFD code TrioCFD. A mesh sensitivity study was performed to propose a good mesh for reproducing standard literature results. This information on mesh resolution was used when carrying out simulations using various geometric obstacles inside the pipe-mixing vanes, circular spacer grid and a combination of square spacer grid with mixing vanes. Structured mesh was generated for the annular pipe case and circular grid case. An innovative hybrid mesh was used for the two remaining cases of the mixing vanes and the square grid; keeping unstructured mesh around the obstacles and structured mesh in the rest of the domain. Both hydraulic and wall pressure characteristics were analyzed for each case. The results for the square grid case were found to be an approximate combination of the mixing vane case and circular grid case. Simulation results were compared with experiments performed at CEA Cadarache. Some preliminary comparisons were also made with classical empirical models.