Measurement of bubble velocity profiles and turbulent diffusion coefficients of the gaseous phase in rectangular bubble column using image processing

The flow in a rectangular bubble column with a cross-section of 100 × 20 mm and a height of 1500 mm was studied using a high-speed video-system. Series of the images were taken at a frequency of 500 Hz at different elevations. The images were treated by means of a bubble recognition algorithm. It allows the tracking of most of the individual bubbles in a bubble swarm. Time-averaged velocity profiles and turbulent diffusion coefficients were derived as a function of the superficial gas velocity. To measure the turbulent diffusion coefficient of the gaseous phase the lateral displacement of bubbles traveling over a certain vertical distance was transformed into a probability density distribution. It was found that the shape of distributions fits well to a Gaussian standard distribution. Dispersion coefficients were found to grow nearly proportionally to the square root of the vertical distance. Diffusion coefficients were calculated from the proportionality factor and were compared to correlations from the literature. Experiments were performed at superficial gas velocities ranging from 0.5 to 4 mm/s. The bubbles were generated either by a porous sparger or a set of capillaries placed at the bottom of the column. Measurements were taken at heights between 1 and 1.5 m, where the bubble cloud was occupying the entire cross-section.