An experimental investigation on turbulent flow mixing in a simulated 3 × 3 dual-cooled annular fuel bundle using particle image velocimetry

Abstract

The flow pulsation and mixing coefficient were investigated using the PIV (Particle Image Velocimetry) for a dual-cooled annular fuel assembly application. As the test sections, two kinds of simulated 3×3 rod bundles for dual-cooled annular and cylindrical solid fuel assemblies were prepared, and their P/Ds (Pitch-to-Diameter ratios) were 1.08 and 1.35, respectively. A MIR (Matching Index of Refraction) technique was applied to visualize and measure the flow structure in a rod bundle. For both the simulated dual-cooled annular and cylindrical solid fuel bundles, the time-average axial velocity in the center of the subchannel became faster than that in the rod gap center. On the other hand, the RMS (Root-Mean-Square) value of the lateral velocity and the lateral turbulence intensity in the rod gap center appeared higher than those in the subchannel center. A flow pulsation of the quasi-periodic oscillating flow motion occurred in the simulated dual-cooled annular fuel bundle, which was visualized clearly and successfully by the PIV and MIR techniques. The power spectral density and peak frequency of the flow pulsation increased with an increase in the bulk velocity. Based on the measured PIV data, as an alternative approach to estimate the turbulent mixing coefficient, the use of the RMS value of the lateral velocity was examined for various Reynolds number conditions for both rod bundles. Finally, the empirical correlation of the mixing coefficient for a dual-cooled annular fuel bundle was proposed.