Abstract In this study, the vertical upward air–water two-phase flow tests have been carried out to investigate the distribution of local and global void fractions in a rod bundle channel under low-flow conditions. The height of the rod bundle channel is about 5 m, and the channel cross section is comprised of a 52 × 52 mm rectangular casing and 3 × 3 circular rods. The rods with a diameter of 11.5 mm and a pitch of 15.4 mm were set as a rectangular array. Four sets of non-intrusive conductivity sensors were built to measure the local and global void fractions of various gaps within the rod bundle channel. The tests were operated at low flow conditions: j g = 0.016–0.92 m/s and j f = 0.02–0.05 m/s, which covered the possible flow regimes of bubbly, cap-bubbly and cap-turbulent flows. The measured global void fraction were benchmarked with existing one-dimensional drift-flux model, and the local void fractions have been summed up by regions and compared with the global void fraction for validation. The dynamic characteristics of local/global void fractions have been analyzed with probability density function (PDF) and fast Fourier transform (FFT), and the distributions and variations of averaged void fraction against test conditions have been presented. Furthermore, the empirical correlations for estimating the local void fractions have been proposed, which can successfully estimate the local void distribution from global void fraction with limited errors. The present results and correlations can be a reference for future development and modification of detailed two-phase flow models in the rod bundle geometry.

中文翻译：

---

低流量条件下3×3棒束通道中气水两相流局部/全局空隙分布的实验研究

摘要 在这项研究中，已经进行了垂直向上的气-水两相流测试，以研究低流量条件下棒束通道中局部和全局空隙率的分布。杆束通道高度约为5 m，通道截面由52×52 mm矩形套管和3×3圆杆组成。将直径为 11.5 mm、节距为 15.4 mm 的杆设置为矩形阵列。构建了四组非侵入式电导率传感器来测量棒束通道内各种间隙的局部和全局空隙率。测试在低流量条件下进行：jg = 0.016–0.92 m/s 和 jf = 0.02–0.05 m/s，其中涵盖了气泡、帽状气泡和帽状湍流的可能流动状态。测量的全局空隙率与现有的一维漂移-通量模型进行基准测试，局部空隙率已按区域汇总并与全局空隙率进行比较以进行验证。用概率密度函数（PDF）和快速傅立叶变换（FFT）分析了局部/全局空隙率的动态特性，并给出了平均空隙率相对于测试条件的分布和变化。此外，已经提出了用于估计局部空隙率的经验相关性，其可以成功地从具有有限误差的全局空隙率估计局部空隙分布。目前的结果和相关性可为未来开发和修改棒束几何中详细的两相流模型提供参考。