This paper presents the experimental results and analyses of the structure velocity of air-water two-phase flow in a 3 × 3 rod bundle channel. A total of 56 flow conditions were tested and investigated for rod-gap, sub-channel, rod-wall and global regions of rod bundle geometry. The experimental tests were carried out under bubbly and cap-bubbly flow regimes with superficial gas and liquid velocities of 0–1 m/s and 1–1.7 m/s, respectively. The conductivity probes were set at different heights to measure the global and local void fractions. The structure velocity of air-water two-phase flow is the average bubble velocity calculated by the method in this study. The structure velocity were determined by utilizing the cross-correlation technique to analyze the time lags of the bubbles passing through the conductivity probes. The results of this study indicated that the structure velocity may increase with increasing superficial gas and liquid velocities. In low superficial gas velocity region, the structure velocity may first slightly increase and follow by a sudden jump which appear in most regions. After the sudden jump, the structure velocity may keep increasing mildly. The present structure velocities have been compared with the area-averaged gas velocities predicted by the drift flux model, and it appears that most structure velocities show a good agreement with the averaged gas velocities from the drift flux model after the jump.

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