Study Of Hydrodynamics Behaviors Of Bubbles In Gas-Liquid Systems

The hydrodynamic behaviors of bubbles not only have important effect on the velocity distributions of gas-liquid flow, but also play an important role in determining the interfacial area, and thereby affect the mass and the heat transfer between the two phases. Several typical hydrodynamic behaviors of bubbles in gas-liquid system were investigated in this study, including the direct coalescence and the conjunct coalescence between two in-line bubbles at low Reynolds numbers, behaviors and dynamics of two bubbles in conjunct condition in high-viscosity liquids, and the breakup of individual bubbles in a horizontal jet flow.In the research of bubble coalescence, the co-axial coalescence between two in-line bubbles at low Reynolds numbers were studied in six different systems. This study mainly focused on pairs of bubbles with equivalent diameters of 6.5～9.5 mm for the leading bubble and 7.0～10 mm for the trailing bubble, and the Reynolds numbers of bubbles were less than 2. The behaviors of the interacting bubbles were obtained by high speed camera and digital image processing technology. VOF model was used to simulate the transient process of direct coalescence between in-line bubbles, and the maximum scale of the structured grid was 50 μm, so the flow field surrounding two bubbles and the change rule of liquid film between bubbles were obtained. The co-axial coalescence of bubbles at low Reynolds numbers (lower than 2) can be divided into two forms based on whether bubbles undergo a conjunct stage. During the direct coalescence process, the velocity of the trailing bubble and the draining speed of the liquid between bubbles both reached the maximum at the contact moment, and then decreased with time. The central area of the liquid film changed from being arched to flat and then crinkled, and finally the film rupture occurred near the center of liquid film. During the conjunct coalescence process, bubbles became more difficult to coalesce and easy to slide as the liquid viscosity decreased or a surfactant was added. The ratio of the Reynolds numbers of the two bubbles at the contact moment can be used to predict the coalescence or non-coalescence. The total coalescence time can be expressed as the sum of the drainage time (film thinning) and conjunct time (film rupture). The drainage time was directly proportional to the liquid viscosity, while the liquid viscosity and the presence of the surfactant both affect the conjunct time. Accordingly, a model based on the hydrodynamic stability theory can be used to predict the film rupture time of conjunct bubbles.In the research of two bubbles in conjunct condition, conjunct bubbles were made by direct collision of two in-line bubbles without slide, and the bubble size ratio was in the range 1.0～1.2. The formation, the motion characteristics, and the force conditions of conjunct bubbles were studied. The conjunct bubbles rose vertically with an unchanged shape and a constant velocity during the conjunct stage. Models for the prediction of the projected area diameter and the rising velocity were provided respectively for the conjunct bubble and the single bubble. The rise velocity of the bubbles could be determined from the drag coefficient and leading bubble projected area diameter. A succinct algorithm was presented to calculate the drag forces on the conjunct bubbles and the interaction force between the two bubbles. The drag force mainly acted upon the leading bubble, and was directly proportional to the equivalent diameter of the whole conjunct bubble. The main role of the interaction force between the two bubbles was to balance the buoyancy force of the trailing bubble, and the interaction force was directly proportional to the equivalent diameter of the trailing bubble.In the research of bubble breakup, the single bubble behaviors of rising, deformation and breakup in a horizontal jet flow were investigated. By changing the experimental parameters such as the initial size of bubble size, the downstream location of bubble generation, and the continuous phase flow rate, the breakup process was discussed. High speed camera was employed to visually determine bubble behavior, and PIV method was used to measure the speed and turbulent kinetic energy of flows.2D and 3D VOF model were both used for the simulation of the break process of bubbles. The position where the bubble gets the maximum acceleration in the horizontal direction was defined as the shear position. The shear rate and the kinetic energy dissipation rate both reached the maximum at the shear position, and the forces on the bubble at the shear position played a crucial role in determining the breakup of the bubble. Bubbles became easier to break up as the increase of the liquid flow rate, the decrease of the formation distance of bubble, or the increase of bubble diameter. Ne, which was defined as the expected bubble number, was the ratio of the number of bubbles after and before breakup. The expected bubble number was directly proportional to the Ca number. It was found that 3D numerical simulation better fitted the flow filed of the liquid phase, while 2D numerical simulation was more suitable and accurate for the simulation of the distortion and breakup of bubbles.