Non-Intrusive Experiemental Investigation of Multi-Scale Flow Behavior in Rod Bundle with Spacer-Grids
Dominguez Ontiveros, Elvis Efren
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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.