Abstract: Wire-wrapped spacer is a type of locating device, which has been planned to be used in the design of fuel assembly of Gen-Ⅳ advanced nuclear reactors, e.g., sodium-cooled fast reactor and supercritical water-cooled reactor. As the coolant sweeps the wrapped wires, directional mixing and periodic lateral flow can be encountered under the interaction of wires. The cross flow and mixing significantly affect the fuel cladding temperature, which is a concerned issue in the thermal-hydraulic design of the fuel assembly. In this paper, the cross flow of water in a 7-pin wire-wrapped rod bundle was studied by means of particle image velocimetry (PIV) and computational fluid dynamics (CFD). The diameter and wall thickness of each tube were 30 mm and 0.5 mm, respectively. The wrapped wires were made of fluorinated ethylene propylene (FEP) whose index of refraction is close to water at room temperature. This meets the requirement of MIR (matching index of refraction) in performing PIV experiment. The diameter, wall thickness and axial pitch of each wire were 5 mm, 0.3 mm and 500 mm, respectively. A continuous-wave laser with a power of 8 W was utilized as the laser source. Hollow glass microspheres of 10-14 μm in mean diameter and 1.04 g/cm³ in density were used as the seeding particles. The movement of the particles was captured by a Fastec TS3 high-speed camera with a resolution of 1 024 pixels×1 024 pixels. The flow-visualization experiment was carried out under three Reynolds numbers, i.e., 3 000, 6 000 and 9 000. This corresponds to streamwise mean velocity of 0.2, 0.4 and 0.6 m/s, respectively. The cross-section of the rod bundle was divided into 18 sub-areas which were photographed separately. Vectors in each sub-area were merged together eventually to obtain the cross flow on the entire cross-section. Based on the experimental measurements, it is found that small-scale turbulent vortex and large-scale circulation flow exist inside the rod bundle. The former usually appears at the windward of the wire, which is caused by the fluid-wire collision. The latter generally appears at the outer region of the bundle, which is mainly resulted from the blockage effect of the rod and consequent flow diversion. With the increase of Reynolds number, the distribution of cross flow stays universal, but the magnitude of velocity increases almost linearly. In addition to the experimental measurement, CFD software STAR-CCM+ was utilized as well to study the characteristics of cross flow inside the wire-wrapped rod bundle. Sensitive study of turbulence models reveals that the prediction given by elliptical hybrid Reynolds stress model agrees well with experimental data. For the selected bundle geometry, the maximum cross flow is about 25% of the axial velocity, and changes abruptly when the wire passes the rod gap.