Single bubble rising behavior in narrow rectangular channels with finite channel width

Narrow rectangular channels (NRCs) are widely used in industrial applications due to their compact structure and high heat transfer efficiency. Accurate thermal-hydraulic analysis of two-phase flow in NRCs requires a fundamental understanding of single-bubble dynamics. However, systematic investigations of single-bubble behavior in finite-channel-width NRCs remain limited, and reliable quantitative correlations for key parameters, such as bubble rising velocity, shape factor, and drag coefficient, are still lacking. In particular, the effects of confinement in the channel width directions on bubble rising velocity and geometric characteristics are not yet well established. In this study, systematic experiments were conducted to investigate the rising behavior of single bubbles in NRCs with finite widths. Bubble shape and rising trajectories were visualized, and the effects of channel dimensions on bubble dynamics were analyzed. The results show that when the bubble diameter was approximately 0.4 times the channel width, the rising velocity approached an asymptotic value that increases with both the channel width and gap. A new correlation for predicting the rising velocity was proposed, applicable to both infinite- and finite-width NRCs. Furthermore, the geometric characteristics of bubbles were quantitatively analyzed. For large bubbles, the bubble width was constrained by the walls in the width direction and approached 0.86 times the channel width. The drag diameter was primarily affected by channel width, whereas the Sauter diameter depended on channel thickness and asymptotically approached three times that thickness. The bubble shape factor decreased significantly with increasing bubble size. Based on these observations, new correlations were developed to predict bubble width, drag diameter, Sauter diameter, and shape factor. The resulting drag coefficient, calculated using the proposed correlations, showed good agreement with experimental data.