Experimental study of coherent structures downstream mixing vaned spacer grid of different inclination angles in a 5 × 5 rod bundle by TR-PIV

Abstract

Experimental study of coherent structures downstream of a spacer grid with different inclination angles (20°, 25°, 30°) of split-type mixing vanes in a 5 × 5 rod bundle was conducted by time-resolved particle image velocimetry (TR-PIV) under Reynolds number from 6.6 × 103 to 3.96 × 104. The fluorinated ethylene propylene (FEP) tubes and water are used as the matched index of refraction (MIR) materials. The maximum standard uncertainty of time-averaged velocity is less than 2 % of bulk flow velocity (Wb), and that of Reynolds stress is less than 2 % Wb2.The secondary flow structures downstream spacer grid with mixing vanes of different inclination angles change with several similar patterns, including vortices shedding from mixing vanes and fast dissipation near spacer grid, counter-current shear-flow development and shear-flow induced vortices formation, shear-flow induced vortices decaying and dissipation. The development trends of spanwise time-averaged velocity, Reynolds stress, integral spatial length scales and integral temporal length scales are determined by coherent structures. With Reynolds number increasing, smaller coherent structures are generated and the time-averaged velocity, Reynolds stress, spatial and temporal length scales decrease. Larger inclination angle generates larger coherent structures, and spatial and temporal length scales increase with inclination angle increasing.The experimental findings provide valuable information for understanding sub-channel mixing phenomena caused by mixing vaned spacer grids. The experimental data can be used for sub-channel mixing model developments and validations of turbulent models.