Aspect Ratio and Drag Coefficient of Coalescing Bubbles in a Narrow Channel with Bottom Gas Injection

Abstract The drag coefficient and bubble morphology are essential in influencing the adjacent flow field and the properties of heat and mass transfer. Nonetheless, precisely forecasting these parameters for numerous bubbles or bubble clusters continues to be a significant challenge. This study systematically investigates the impact of varying gas flow rates and NaCl solution concentrations on the bubble aspect ratio and the drag coefficient. Employing high-speed videography alongside sophisticated image processing methods, therefore, the equivalent bubble diameter, the ascent velocity, and the aspect ratio of bubbles produced in restricted vertical channels are accurately quantified. The principal findings indicate that the aspect ratio converges to approximately 0.35 as the bubble diameter increases. In dilute NaCl solutions, the aspect ratio exhibited greater variability as the gas flow rate increased from 50 to 300 mL/min. A strong correlation, with a coefficient of determination of 0.915, is shown between the experimental aspect ratios and predictions based on the Weber and Eötvös numbers. Moreover, the pace of bubble ascent escalates with both the solution concentration and the equivalent bubble diameter. The drag coefficient decreases as the Reynolds number increases. A novel empirical correlation, integrating bubble deformation and the Reynolds number, is introduced to forecast the drag coefficient with a maximum relative error of 16.5%. These discoveries offer essential insights into dynamics of gas–liquid two-phase heat and mass transport.