Study Of Hydrodynamics Behavior Of Two Bubbles Coalescence

Bubble coalescence behavior can significantly change the flow field structure and the interface between gas and liquid; thereby can affect the characteristics of the whole system deeply. Two bubbles in the conjunct condition were often encountered in high viscosity system and very conducive to know the mechanism of coalescence, but very little research had focused on this area. In this study the behaviors of conjunction and coalescence between two in-line bubbles in high viscous liquids at low Reynolds number were systematically studied both experimentally and numerically, especially for the morphological characteristics and dynamic behaviors of conjunct bubble during conjunct coalescence process.In this study, the hydrodynamics behavior of two in-line bubbles were studied in six different systems (3 kinds of pure glycerin with different temperatures, one kind of 95% water-glycerin solution,2 kinds of glycerin solution with CDEA) by using high speed camera system which can record the whole coalescence process clearly from bubble generation to liquid film break-up. Moton number ranges from 1.69 to 661.83; bubble Reynolds number ranges from 0.190 to 2.998 and bubbles’spherical equivalent diameter is around 6-10.5 mm. The conjunct coalescence process can be divided into three stages:contact stage, conjunct stages and drainage stage. The characteristics of conjunct bubbles and the whole process of contact and conjunction were studied in detail, including the effects of bubble volume, approaching velocities, liquid viscosity and surface tension. The critical condition of bubble coalescence under different systems was determined, and the timescales of different stages during conjunct coalescence process was proposed and investigated. It was found that the conjunct bubble rose vertically with unchanged shape and constant velocity during the drainage stage before the coalescence or slide happened, not merely showing some characters of single bubble or a pair of bubbles simultaneously but also some unique properties that its morphological features of conjunct bubble could be used to describe the characteristics of coalescence process. Models for conjunct bubble and single bubble were provided separately for calculating the drag coeffcient and the rising velocity. Accordingly, a model based on dynamic force analysis was established to describe the interaction force as well as drag force on conjunct bubbles during liquid film drainage process. The conjuct time was mainly dependent on the viscosity while the drainage time shows a conspicuous randomness. With the decrease of viscosity or the presence of surfactant, bubbles had a tendency to slide rather than coalesce.For the part of numerical simulation, VOF model was used with 2D structured grid (the minimum scale of the grid is 50 μm) to simulate the transient process of direct coalescence between a pair of in-line identical bubbles, in order to obtain the flow field surrounding two bubbles and the rule of liquid film thickness changing with the time. The simulated results showed that both the shape and dynamic behavior of two bubbles were consistent with experiment results. During the process from approaching to contact, the flow fields surrounding especially between two bubbles were analyzed deeply. It was found that low speed zones on both sides of two bubbles were caused by four little vortexes, and narrowed down gradually and merged with circumjacent overall circular flow into a whole. In the meantime, the value of liquid velocity surrounding bubbles increased gradually and the distribution tended to be more uniform. For liquid velocity, the maximum value appeared at the center axis, decreased gradually to the minimum value (at the center area of two vortexes on both sides) in the process of horizontal extension, and then developed again at the overall circular flow region outside both vortex areas. What’s more, the change rule of liquid film thickness after contact moment presented a phenomenon that liquid film attenuated rapidly at first and then thined slowly until break-up.