Experiments And Numerical Simulations Of Bubble Swarms Rising Motion In A Bubble Column

Bubble is the basic form of gas in the bubble column and its rising motion drives the two-phase flow. The research on the bubble motion is of great significance to understand the hydrodynamics of the bubble column and design the bubble column reactor. In this thesis, both experiments and CFD simulations were performed to study the bubble swarm motion in the two-dimensional bubble column, obtain the motion parameters, discuss the factors affecting the bubble rising motion, investigate the phenomenon of bubble clustering and establish the mathematical models, providing new contents for the study of hydrodynamics in gas-liquid bubble column.At first, high-speed digital video processing was used to measure the bubble swarm rising motion at low superficial velocity. Water, glycerol/water mixtures and xanthan gum solutions in a glycerol/water mixture were used to observe the bubble rising behavior in different Newtonian and shear-thinning fluids. The experimental results indicated that the bubble swarm motion behaviors were greatly different in Newtonian and shear-thinning fluids. In shear-thinning fluids, multiple bubbles would aggregate together. However, in Newtonian fluids, the bubbly flow was dominated by the dispersed bubbles and obvious bubble clusters were not observed. The effects of Gas holdup, Reynolds number, distributor aperture, liquid viscosity and the power-law index on bubble size, shape, rising velocity, drag coefficients and bubble clusters were examined. The results showed that as the gas holdup increased, the bubble drag coefficients increased, the bubble velocity decreased and the deformation of bubbles increased; as Reynolds number increased, the bubble drag coefficients decreased and the bubble velocity increased. Smaller distributor aperture resulted in the reduction of bubble size and bubble velocity. It was easier for bubbles to aggregate together in fluids with high viscosity and low power-law index.Then, the cell model and flow around multiple bubbles model were applied to simulate the bubble swarm motion in Newtonian and shear-thinning fluids. The cubic cell model was proposed to calculate the bubble swarm velocity and drag coefficients and the results were in good agreement with the experimental values. This indicated that the cubic cell model is an effective tool to simulate the bubble swarm motion under uniform state in Newtonian fluids. The flow around multiple bubbles model was built to simulate bubble clusters containing different numbers of bubbles.It was found that the drag forces of bubbles in different positions were different and the drag forces of central bubbles were smaller since the bubble shielding effects. The drag forces of the bubbles at top layer along the flow direction were found to be larger than those at bottom layer and as the bubble spacing increased, the bubble drag coefficients increased, which revealed the aggregation of bubbles made the drag force largely decrease. Compared with Newtonian fluids, the drag force, attractive force and repelling force of bubbles in shear-thinning fluids were smaller, resulting in more stable bubble clusters. The model discussed the causes of bubble aggregation in shear-thinning fluids, offering a feasible simulation method to study the mechanism of bubble clustering.