Mechanism of bottom-blown bubble coalescence in narrow channels

•The coalescence phenomenon of bottom blown bubbles in narrow channel is studied.•The effects of gas flow rate and NaCl concentration on the bubbles were analyzed.•The mathematical model of bubble coalescence is established.•Suitable for laminar fluid, but not for turbulent fluid. The coalescence of bottom-blown bubbles moving in narrow channels was studied using digital image-processing techniques. The effects of the gas flow and NaCl concentration on the shape, size, motion orbits, velocity, and vertical distance between the bubbles were comparatively analyzed, and a mathematical model for the coalescence of irregularly shaped bubbles of different sizes was established. The mechanism of bubble coalescence under bottom-blown stirring was investigated, and the consistency between the model-predicted and experimental results was analyzed to determine the ranges where the model can be applied. At a high gas flow rate, low liquid concentration, and high velocities of the first and following bubbles, the gas-liquid two-phase flow was more chaotic and the probability of collision between the bubbles was larger; hence, coalescence occurred earlier. A mathematical model of bubble coalescence was established to analyze the motion of bubbles after coalescence. The results predicted using the model were 70–78% consistent with the experimental results. Our model exhibited higher computational accuracy than Li Shaobai's model and is applicable to laminar fluids but not turbulent fluids. These findings offer basic data for quantifying bubble strengthening and blending effects in narrow channels.