Experimental investigation of turbulent wake flows in a helically wrapped rod bundle in presence of localized blockages

Journal Article · Wed Jul 01 00:00:00 EDT 2020 · Physics of Fluids

DOI:https://doi.org/10.1063/5.0008589· OSTI ID:1801264

Nguyen, T.  ^[1]; Vaghetto, R. ^[1]; Hassan, Y. ^[1]

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1. Texas A & M Univ., College Station, TX (United States)

In nuclear sodium fast reactors, bundles of rods are tightly packed into a triangular lattice, enclosed in a hexagonal duct, and each pin is spirally wrapped with a thin wire. Flow blockages can potentially impact the local flow characteristics and heat transfer mechanisms in the bundle due to its small subchannel size. The effects of the blockage on the flow structures and heat transfer mechanisms are important aspects that require an accurate investigation. In this study, the flow-field characteristics in the vicinity of a blockage located in the exterior subchannel of rod bundles with helically wrapped wires were experimentally investigated. The velocity fields in the exterior subchannel were acquired by applying matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of Re₁ = 4000 and Re₂ = 17 000, i.e., equivalent to Rew₁ = 19 600 and Rew₂ = 83 200, respectively, based on the blockage width. The results from the TR-PIV measurements revealed an arch-shaped vortex with a large flow recirculation and a pair of counter-rotating vortices in the wake region downstream of the blockage, which is commonly observed in the wake flow of bluff bodies. The relative lateral distance and angle between the two vortices decreased when the Reynolds numbers increased. Profiles of maximum turbulence intensity along the shear layers illustrated the transition process including the growth, peak, and decay along the flow direction. From the spectral analysis of the turbulent velocities extracted at points along the shear layer, the Strouhal numbers (St) representing the vortex shedding frequency were found to be St = 0.25 and St = 0.56 for the left and right shear layers, respectively. Characteristics of shear layers generated by the blockage in the exterior subchannel were investigated via the two-point cross correlation of fluctuating velocities. The patiotemporal cross correlations of turbulent velocities, computed at points in the region where the left shear layer exhibited rolling effects and vortex breakdowns, were considerably wider and longer. The convection velocity U_c was estimated to be ~0.82U_m to 0.93U_m. Proper orthogonal decomposition (POD) analysis was applied to the instantaneous velocity fields to extract the statistically dominant flow structures. It was found that POD modes 2–3 and 4–5 formed the pair modes when the corresponding POD temporal coefficients depicted sinusoidal shapes and exhibited nearly circular orbits in the phase space. Spectral analysis of the POD temporal coefficients confirmed the vortex shedding frequencies detected in the analysis of turbulent velocities.

• Research Organization: Texas A & M Univ., College Station, TX (United States). Texas A & M Engineering Experiment Station
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE)
• Grant/Contract Number: NE0008652
• OSTI ID: 1801264
• Alternate ID(s): OSTI ID: 1638772
• Journal Information: Physics of Fluids, Vol. 32, Issue 7; ISSN 1070-6631
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

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