Wire-wrapped hexagonal fuel bundles have been extensively investigated due to their enhanced heat transfer and flow characteristics. Experimental measurements are important to study the thermal-hydraulic behavior of such assemblies and to validate and improve the predicting capabilities of specialized correlations and computational tools. Presently, very limited experimental data is available on the local subchannel pressure drop. Experimental measurements of subchannel pressure drop were conducted in a 61-pin wire-wrapped rod bundle replica, for Reynolds numbers between 190 and 22,000. Specialized instrumented rods were utilized to measure the local pressure drop and estimate the subchannels' friction factor. Three interior subchannels, one edge subchannel and one corner subchannel were selected to study the effects of location and flow regimes on the friction factor and hydraulic behavior. The transition boundaries from laminar to transitions regimes, and from transition to turbulent regimes were estimated for the subchannels analyzed. The results were found in agreement with the predictions of the Upgraded Cheng and Todreas Detailed Correlation (UCTD). The results of the experimental campaign provided a better understanding of hydraulic behavior of the subchannels of wire-wrapped bundles, in relation to its geometrical features
A determination of nominal flow phenomena in liquid metal fast reactor (LMFR) fuel assemblies is critical toward generation-IV reactor development. Axially positioned spacer grids are used to maintain the geometry of hexagonal rod bundles and simultaneously introduce perturbations in the flow. Three-dimensional (3D) printed asymmetric honeycomb spacer grids were installed in a prototypical 127-pin LMFR fuel assembly model to study complex fluid dynamics interactions induced by the spacer grid and rods. To characterize flow dynamics in this intricate geometry, time-resolved particle image velocimetry (TR-PIV) using the matched-index-of-refraction method was employed to obtain non-intrusive velocity measurements for three axial planes (one near-wall and two interior planes) at a Reynolds number of 6000. The statistical TR-PIV results compared sub-channel-dependent normalized time-averaged velocity, velocity fluctuations, Reynolds stress, vorticity, and turbulence kinetic energy distributions. TR-PIV line profiles characterized downstream spacer grid flow dynamics. Two-point spatial and spatial–temporal cross-correlation fields revealed local coherent structures and quantified convection velocities of traveling vortices. Spatial–temporal decomposition using dynamic mode decomposition (DMD) applied to the near-wall vorticity fields extracted turbulent structures and flow instabilities in the wake region of the spacer grid, along with their decay and frequency rates. Reduced-order velocity fields from DMD reconstructions identified the most energy-containing coherent structures persistent in the near-wall region. This research provides experimental data sets and analyses of flow behavior in rod bundles with hexagonal spacer grids. The results are critical toward LMFR design and geometry optimization, crucial for the validation of computational fluid dynamics and reduced-order flow models.
Hexagonal rod bundles arranged in a tightly packed triangular lattice are extensively used for heat transfer and energy generation applications. Staggered spacer grids are used to maintain the structural integrity of gas-cooled fast nuclear reactor (GFR) fuel assemblies, while inducing localized turbulence in flow. Damage to these spacer grids results in a disruption of flow fields within these hexagonal fuel bundles. Experimental flow visualizations are critical to identify the differences in local flow properties that the structural damage may cause. This experimental research investigates the flow-field characteristics at a near-wall and center plane in a prototypical 84-pin GFR fuel assembly. Newly installed typical spacers and spacers subject to naturally occurring damage, due to material degradation over prolonged experimentation, were investigated. Velocity fields were acquired by utilizing the matched-index-of-refraction (MIR) method to obtain time-resolved particle image velocimetry (TR-PIV) measurements for a Reynolds number of 12000. Reynolds decomposition statistical results divulged differences in the time-averaged velocity, velocity fluctuations, flow anisotropy and Reynolds stress distributions. Galilean decomposition demarcated the influence of spacer grid damage on the velocity fields. To extract turbulent structures and elucidate mechanisms of flow instabilities, proper orthogonal decomposition (POD) analysis was employed. Reduced order flow reconstructions enabled the application of vortex identification algorithms to determine the spatial and statistical characteristics of vortices generated. This research work provides unique experimental data on the spacer grid condition-dependent flow. The results offer a deeper understanding of fluid dynamics behavior to support GFR rod bundle design efforts and computational fluid dynamics model validation.
Potential accumulation of undesirable debris in a subchannel of a Liquid Metal Fast Reactor (LMFR) hexagonal fuel bundle presents accident conditions, which are crucial to investigate. Very limited experimental research persists in literature to understand the fluid dynamics effects of partially blocked subchannels, due to the presence of porous blockages. It is imperative to comprehend flow regime-dependent fluid response in the vicinity of porous blockages, to predict and counter abnormal conditions in an LMFR rod assembly. The presented experimental research investigates flow-field characteristics in a 61-pin wire-wrapped rod assembly with a three-dimensional (3D) printed porous blockage medium in an interior subchannel, at Reynolds numbers (Re) of 350, 5,000, and 14,000. Time-resolved velocimetry measurements were acquired yielding first- and second-order Reynolds decomposition flow statistics - revealing important fluid responses upstream and downstream of the porous blockage. Profiles of velocities, velocity fluctuations, Reynolds stresses, and vorticities uncovered the downstream blockage perturbation effects. Spatial cross-correlations of the velocity fluctuations displayed eddie structure elongations and quantified eddie integral scale lengths. A time-frequency analysis of the velocity fluctuations further detailed the mechanisms of flow instabilities via power spectral analysis. Application of a one-dimensional continuous wavelet transform revealed complex Re-dependent flow and characterized the temporal turbulence occurrences - caused by the trailing edge effects of the porous blockage. This research provides unique and novel experimental analyses on flow regime-dependent fluid physics due to a porous blockage medium and provides data sets vital for computational model benchmarking and development, towards the enhancement of LMFR rod bundle designs.
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