PIV measurement of turbulent flow downstream of mixing vane spacer grid in 5×5 rod bundle

Abstract

Axial and cross flow downstream a split-type mixing vane spacer grid in a 5×5 rod bundle are measured with particle image velocimetry (PIV) at sub-channel Reynolds number (Resc) of 6600, 13,200, 26,400 and 39,600. The cross flow in the sixteen inner sub-channels are obtained at five elevations, i.e. 3Dh, 5Dh, 10Dh, 15Dh and 20Dh downstream of the spacer grid. The axial flow measurement is conducted in a plane parallel to the bundle axis. The satisfactory convergence of turbulent statistics is achieved with the sampling number of 2000. The maximum standard uncertainty of statistical quantities normalized by the bulk velocity for all the test conditions is less than 0.6%. The flow characteristics and structures were determined by the mixing vanes located on the upper edge of support grids in rod bundle, and the flow downstream the spacer grid can be divided into the advection zone, the reorganization zone and the dumping zone. And the maximum lateral velocity decreases from 44% of the bulk velocity (Wb) at 3Dh to 16%Wb at 20Dh. The mixing vanes generate drastic turbulence flow downstream the spacer grid, and the lateral root mean square (RMS) fluctuating velocity decays in exponential way. The maximum lateral RMS fluctuating velocity decays rapidly from 20%Wb at 3Dh to 9%Wb at 20Dh. The maximum lateral Reynolds stress decreases from 0.6%Wb2 at 3Dh to 0.3%Wb2 at 20Dh. The normalized mean velocity and turbulent statistics show similarity for the Reynolds above 13,200, while the low Reynolds number effect at Re = 6600 is observed for the axial mean velocity and axial RMS fluctuating velocity.