Multi-dimensional characteristics of transitional bubbly flow in a tight lattice subchannel

Tight lattice rod (or tube) bundles offer a more compact structure, enhancing heat transfer efficiency and equipment compactness. Subchannel analysis codes are often utilized in thermal–hydraulic analyses for rod bundles. Constitutive correlations in subchannel analysis codes are based on flow regimes, including the typical interfacial structure of each regime. However, rarely research focused on the applicability of these constitutive correlations in transition bubbly flow, which was observed at the transition between bubbly and beyond bubbly flow. This paper focuses on the local flow characteristics and axial evolution of gas–liquid two-phase flow in transitional bubbly flow in tight lattice subchannels. Dual WMSs were employed as the main device to measure two-phase flow in the tight lattice subchannels, and three different parameters (superficial gas velocity, subchannel-averaged void fraction, and IAC) were utilized to verify the experiments. The detailed profiles of two-phase flow parameters, including group-wised void fraction, bubble velocity, and IAC, were examined. Due to the relatively large size of G2 bubbles, the G2 velocity profile was relatively flat in a tight lattice subchannel. The peak of IAC appeared closer to the gap region than the void fraction due to the geometric effects of the narrow gap. The applicability of group-wised void fraction covariance and IAC correlations are also evaluated. For G1 and intergroup void fraction covariance, the deviation between the Swearingen correlation and the experimental data was within 10% and 30%, respectively. G2 void fraction covariance was significantly high (around 3.0) due to the geometry of the tight lattice subchannel, and it decreased rapidly as the void fraction increased. The deviations from the Yu-Hibiki correlation were 6.05 %, 8.41 %, and 5.92 % for G1, G2, and one-group bubble Sauter diameter, respectively. Since the proportion of G2 IAC was small in transitional bubbly flow, one-group IAC was similar to G1 IAC, with a mean deviation between the correlation and experimental data of 15.5%. © 2024 Published by Elsevier Ltd.