PIV measurement of cross flow in a rod bundle assisted by telecentric optics and matched index of refraction

Spacer grids induce local flow disturbance in fuel assembly including lateral cross flow and elevated turbulence fluctuation. In order to investigate the cross flow downstream of spacer grids and to provide the high-fidelity data for the CFD validation of rod bundle flow, a flow measurement method has been proposed to measure the flow in the full cross section of the 5 × 5 rod bundle. A two-dimensional (2D) particle image velocimetry (PIV) system is utilized for the flow measurement. A telecentric lens is employed to minimize the perspective angle when the CCD camera is located distant from the measured cross section. The matched refractive index technique is also introduced to eliminate the non-uniform laser illumination behind the rods which is resulted from the laser refraction on the convex rod surface. Such configuration is examined for the cross flow measurement of a 5 × 5 rod bundle in a small scale test facility. It has been shown that the cross flow pattern can be well depicted in the entire cross section when the measuring plane is within 10 Dh downstream of a spacer grid. The obtained distribution of sampled flow velocity appears in good agreement with the normal distribution.