Experimental study on flow pattern evolution characteristics of gas-liquid two-phase flow in a 3×3 rod bundle channel

To address the insufficient understanding of two-phase flow characteristics in the small-scale rod bundle channels, this study takes air-water two-phase flow as the research object and carries out a systematic experimental investigation in a 3×3 rod bundle channel. High-speed imaging is combined with a self-developed dual-fiber optical probe system. Flow pattern evolution, local phase distribution, and the time-domain and frequency-domain characteristics of probe signals are obtained under different operating conditions. Five typical flow patterns are identified from the optical probe signals, including bubbly flow, cap-bubbly flow, cap-turbulent flow, churn flow, and finely dispersed flow. A flow regime map is established using a combined identification criterion. Compared with existing transition criteria for vertical rod bundles, the general sequence of flow-regime evolution is consistent, while noticeable differences are observed in individual transition boundaries. The deviation of the finely dispersed-flow boundary from the existing criterion decreases as the gas superficial velocity increases. These differences are associated with rod-bundle geometry, bubble coalescence and breakup, and local disturbances. The local measurements show a core-peaked radial distribution of void fraction, while the radial profile of mean bubble velocity remains relatively uniform. Frequency-domain analysis of the probe signals reveals distinct spectral characteristics for different flow patterns. Bubbly and finely dispersed flows exhibit broadband spectra, whereas cap-bubbly, cap-turbulent, and churn flows show dominant low-frequency peaks with spatial variations.