Experiments and modeling of head-on collision of a pair of bubbles driven by buoyancy in quiescent water are presented. The experiments showed that two bubbles after contacting for a few milliseconds may coalesce or bounce back depending on the collision velocity. As theoretical comparisons, the influence of velocity boundary conditions on the liquid film drainage and the characteristics of two film drainage models with the assumptions of "partially" and fully mobile interfaces under variablevelocity approach condition are studied. The "partially" mobile model is found to be easier to predict rebound case and longer drainage time, and could not obtain the drainage process where the film is thinned to the thickness below 10 −7 m at both low and relatively high Re. As a whole, the fully mobile model with variable-velocity approach shows more reasonable prediction results for the coalescence and rebound of bubbles in the ultrapure water.
A coupling framework for modeling the non-constant-velocity approach of two fluid particles and the curved film drainage was developed, and an improved model was presented to predict the variable-velocity approach. Using this framework, the effect of the constant-velocity and variable-velocity approach on liquid film drainage was investigated. Two film drainage models based on immobile interface and fully mobile interface were adopted. The simulation results showed that the film thinning rate of the former is much less than that of the latter. In the case of constant-velocity approach, the immobile interface model showed a relatively flat curved film, while in the case of variable-velocity approach, three types of film, wimple, pimple and dimple, can be found. The different combinations of the drainage models and the approach velocity boundary conditions were compared with the experiments. The fully mobile interface model with variable-velocity approach can reasonably predict the coalescence and rebound of bubbles.
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