Laser Doppler measurement and CFD validation in 3 × 3 bundle flow

• Five-beam LDV is operated in the three-beam mode to measure 3 × 3 bundle flow.
• Correlation and FFT techniques are applied to analyze the flow structure.
• Large coherent structure is observed in gaps between different subchannels.
• The Reynolds stress models predict weak mixing between different subchannels.

The five-beam three-component laser Doppler system is operated in the three-beam two-component mode to measure the 3 × 3 bundle flow with simple grid spacer. Experiment has been conducted at Re = 15,200 and 29,900. According to the experiment result, the root mean square (RMS) of axial velocity fluctuation shows large value in the gap and the near-wall region of the edge sub-channel which is induced by the axial velocity gradient. Significant intensity of lateral velocity fluctuation is observed which indicates the strong lateral mixing in a 3 × 3 rod bundle. Through the correlation analysis coherent structures have been observed in the gap region. The spectral analysis shows that the LDV measurement complies to the Komogorov spectrum law, f−5/3, well. The low-frequency peak spectral density of the axial velocity fluctuation has been observed in the gap region connecting sub-channels with velocity difference. The performance of the SSG model and the baseline Reynolds stress model are investigated based on the experiment result. The models predict higher axial velocity in the interior sub-channel and lower in the edge and corner ones than the experiment result. Large discrepancy between the calculated and measured axial flow velocity is resulted from failure in calculating the strong negative u′w′¯ in the gap region connecting different sub-channels.