High-fidelity velocity measurements in a totally blocked interior subchannel of a wire-wrapped 61-pin hexagonal fuel bundle
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2019 Elsevier B.V. Liquid metal fast reactors typically utilize a tightly packed triangular lattice of fuel pins helically wrapped with a wire spacer and enclosed in a hexagonal duct. During reactor operation partial or total flow blockage of coolant channels may occur at different locations of the fuel assembly. Due to potential isolated or combined causes, these blockages may include collection and accumulation of debris, and cladding deformation. The complexity of the flow behavior and heat transfer phenomena within a wire-wrapped fuel assembly accompanied with the effects of channel blockage has motivated the research community around the world, initiating extensive experimental and numerical investigations. The 61-pin wire-wrapped experimental bundle at Texas A& M University, with its clear, fully accessible test section, has been designed, constructed, and operated to conduct high-resolution measurements of the flow characteristics at different locations within the bundle. High spatial and temporal resolution measurements of the velocity fields within vertical and horizontal planes at different locations in test bundle have been produced using advanced laser-based techniques. The pressure at different axial and azimuthal locations in the bundle has been measured within a wide Reynolds number range to investigate laminar, transitional, and turbulent flow regimes. In this article new high-resolution measurements of the velocity fields and turbulent characteristics within a totally blocked interior subchannel are presented. The time-resolved particle image velocimetry (TR-PIV) measurements have been performed within the interior subchannel of the wire wrapped fuel bundle, under the presence of a localized, total blockage of one of the subchannels near the center pin. From the obtained velocity fields the first and second-order flow statistics, such as mean velocity, root-mean-square fluctuating velocity, and Reynolds stress, are computed and presented. Spectral analysis was performed to the fluctuating velocity and the vortex shedding frequency was found at St=0.16. Finally, proper orthogonal decomposition (POD) analysis was applied to the instantaneous velocity fields to extract the coherent flow structures in the flow region downstream of the blockage. The experimental results produced not only provide a better understanding of the flow behaviors under a channel blockage, but also support the validation of commercial and advanced Computational Fluid Dynamics (CFD) codes with a unique set of experimental data.
