PIV measurements inside a wire-wrapped hexagonal rod bundle: From experiments to governing equations

The core of a Liquid Metal Fast Reactor (LMFR) has wires wrapped helicoidally around the fuel pins. This solution prevents damage of the cladding by fretting, and pushes the coolant flow through the gaps between the pins, thus, enhancing the heat exchange. The wires make the flow through the core very complex to understand and model. The goal of this study is, thus, twofold: it aims to obtain insight into the physics of the flow field near the wire and to provide high-fidelity data for benchmarking numerical simulations. Water at room temperature flowing inside a 7 rods, wire-wrapped hexagonal bundle is studied within a Reynolds number range of 4000−14000 using the Particle Image Velocimetry (PIV) technique. The measured quantities are the axial and lateral velocity components, and their root mean square. The measurement area is close to the wire around the central rod. Fluorinated ethylene propylene (FEP), which is a refractive index-matching (RIM) material, provides optical access to the measured region without disturbance of the light beam. The flow direction below the wire follows the wrapping path, as expected. However, if the flow is measured at the front of the wire, the streamlines bend in the opposite direction. The Euler equations applied to the streamlines show that this effect is caused by the pressure gradient across the wire. These findings deepens the physical understanding of the rod bundle flow in the presence of wire spacers, improving the LMFR designs. Moreover, the generated data will support validation of numerical codes for Gen-IV nuclear reactors.