Experimental investigation of turbulent flow characteristics in cross-flow planes of a 5×5 rod bundle with a spacer grid

Abstract

This study summarizes experimental investigations of the flow field characteristics in the cross-flow planes of a 5×5 rod bundle with spacer grid and split-type mixing vanes. Under the isothermal and atmospheric conditions, the velocity fields in the cross-flow planes are obtained by applying the matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of Re1=14,000 and Re2=28,000. The cross-flow planes axially start from the tips of the mixing vanes of the spacer grid and end at 11.73Dh (Dh is the hydraulic diameter of the rod bundle). From the acquired TR-PIV velocity fields, flow statistics such as the in-plane mean velocity and vorticity, turbulent kinetic energy and Reynolds stress are computed and presented for four cross-flow planes of interest axially located at 0,2Dh,5Dh, and 11Dh. Two counter rotating vortices are observed near the spacer grid. After a change in elevation to z/Dh=5, the vortices merge into a single vortex centered in the middle of the subchannel. In addition, the turbulence flow characteristics at the elevations of interest are discussed through calculations of integral length scales and spectral analysis. No dominant frequency is observed, and the turbulence characteristics are strongly influenced by the proximity to the spacer grid. The mass transfer between the subchannels and the associated turbulent mixing flow rate is calculated for all elevations; hence, this study is a step toward improving the correlations currently used in subchannel codes. Results for secondary flow intensities, lateral velocity profiles and velocity contours are compared with large-eddy simulation results from the literature for Re1=14,000, and a reasonable agreement is observed.