چكيده به لاتين
In the present thesis, the interaction between different oil droplets with air bubbles in various aquatic environments has been studied. Aqueous media used as substrate fluids include deionized water, brine solution, and a combination of deionized water with varying volumes of glycerin, while droplets were produced from fluids such as gasoline, crude oil, sunflower oil, and silicon oil with different viscosities (5-350 mPa.s). To perform the experiments, a special test device was designed and built, so that the interaction process between the bubble / drop could be observed and recorded by a high-speed camera. The results of these experiments in the quasi-static state and controlled contact show that the presence of salt in the continuous phase reduces both the drainage time of the intermediate film and its variations in the same repeated experiments. In other words, the discharge times calculated in brine are less dispersed and more concentrated than in pure water. The reason for this behavior is the reduction of electrostatic double layer repulsion due to the presence of NaCl ions. So the outcome of the interaction has lesser dependence on the random and microscopic fluctuations in the film. A similar behavior was observed in the dynamic state and the collision of the ascending bubble with the suspended drop. Estimation of encapsulation efficiency in 0.05, 0.1 and 0.6 mol brine solutions indicates a general increase in the number of successful engulfments relative to pure water. In this case, the ratio of the critical diameter of the bubble to the drop (Dcr = Db / Dd) for the engulfment is also reduced. In pure water, however, mainly bubbles larger than the droplet that had sufficient kinetic energy were able to overcome the surface energy of the droplet and pass through its interface with water. The results showed that in addition to the diffusion coefficient, parameters such as kinetic energy, velocity at the moment of impact and contact time affect the efficiency of this phenomenon. In the study of the effect of changing the properties of the substrate fluid, it was observed that the engulfment rate decreases for all oils by increasing the volume percentage of glycerol (increasing the viscosity of the medium). This observation is in line with the theorerical prediction based on the oil droplet spreading coefficient, which is determined by the interfacial tension between the three fluids, and the encapsulation efficiency inhances by increasing the spreading coefficient, ie decreasing the interfacial tension between the droplet and the continuous fluid. The graph of the collision in terms of two dimensionless numbers Weber (We) and capillary (Ca) shows that as Ca increases, the Weber number must also increase in order to form a capsulation. Also in this diagram, a quota line of order 3 was introduced, which separates the encapsulation and bounce areas. In the numerical study section, first the computational program for Phase-field LBM was developed in Fortran programming language. This computational code has the ability to consider high density ratios between bubble and water (O (1000)) as well as simulation of total spreading or wettability states, and therefore is among the pioneers works of three-phase LBM models with the mentioned capabilities. drop-bubble collision simulations were performed in two parts. At first section, the collision between two immiscible drops with relative initial velocity in the liquid medium was investigated. In addition to detailing the interaction and discussing the hydrodynamics of the flow field, the collision outcomes were presented in the form of Weber-collision parameter (We-B) maps in different states (changing density ratio, viscosity ratio and interfacial tension). The three collision regimes of Engulfment (I), Bounce (II) and Engulfment with the entrapment of a small drop of continuous phase (III) were identified in this section, and the transition / separation lines of these regimes were also specified in each state. In the second part of the numerical study, the process of collision and coalescence between ascending droplet-bubble was simulated using physical properties of fluids. To do so, parameter values were transferred from the lattice Boltzmann space to physical space. Various effects such as density, viscosity, surface tension, droplet-bubble size ratio, continuous phase viscosity, and off-center collisions were investigated, and the results of collision times and droplet’s spreading time on the bubble were estimated. The results of this section are in good agreement with experimental observations for the interaction stages and the spreading of droplets with different sizes and viscosities. These results show that the tcollision and tspreading times increase with increasing spreading coefficient, droplet density, droplet viscosity and continuous phase viscosity. These two characteristic times are inversely related to gravity acceleration and rise velocity. Also, for non-uniform bubble-droplet sizes, in the ratio of diameters Db/Dd greater than 1, the collision and spreading times have a downward trend. For Db/Dd0.5. for gravity constant of 9.81 m/s2 critical B for successful encapsulation was found to be B=0.75, while for accelerated gravity of 3.3*9.81, B=0.6 was observed.
