Coalescence dynamic analysis of water droplets in oil in electric field

Abstract: In order to reveal the characteristics of the droplet coalescence in electric demulsification process, an experiment on electric coalescence between 2 adjacent water droplets in oil in a uniform electric field is conducted. The uniform electric field is generated by 2 parallel plate electrodes. One electrode is connected to the electrostatic voltage generator and the other is grounded. Test medium is sunflower oil as well as water. The viscosity and the density for sunflower oil are 0.085 Pa·s and 892 kg/m3, respectively. The density and the surface tension for water are 998 kg/m3 and 0.069 N/m, respectively. Different electrostatic voltage can be applied by electrostatic voltage generator to the electrode so that the desired electric field strength can be obtained. In experiment, an MD55 type microscopic camera, which was made by Guangzhou Mingmei Optoelectronic Technology Co., Ltd., is used to record the positions and shapes of droplets under different time and different electric field strengths. The effect of electric field strength on droplet movement and deformation is analyzed based on the obtained experiment data. Furthermore, considering droplet deformation, the equations of droplet motion are established according to electric dipole model. By means of the droplet motion equations, the electric coalescence process between 2 adjacent droplets is numerically simulated. A typical Runge-Kutta algorithm is adopted in simulation and the time step is 0.000 2 s. Through the simulation, the distances between 2 adjacent water droplets under different time and different electric field strengths are obtained, and they are used to analyze the effects of medium physical properties, droplet size as well as electric field strength on droplet coalescence efficiency. The simulating and experimental results show that the prediction of the present model on the evolution of the distance between 2 droplets is basically consistent with the experiment, and the average relative error of prediction values is about 25.95%. Further analysis reveals that the droplet coalescence is mainly affected by the mutual movement velocity of 2 droplets due to electrostatic attraction and the droplet deformation due to accumulation of surface charge on droplet. When the electric field strength increases, the droplet mutual movement velocity increases. Meanwhile, the drop deformation is also enhanced. This will lead to a faster contact between 2 droplets. Thus, a higher field strength is extremely beneficial to significantly improve the demulsification efficiency. But, it should be also noted that a too large electric field strength may produce droplet electrostatic breakup and this is obviously conducive to demulsification. Therefore, the applied field strength in electric demulsification process cannot reach a critical value in order to avoid droplet electric breakup. The results also show that the effects of continuous phase viscosity and droplet size on coalescence are significant. The continuous phase viscosity increases, the resistance on droplet increases correspondingly, and it will take a long time to achieve contact between 2 droplets. In addition, for larger droplets, an intense electric attraction between them can be produced. Therefore, the droplet coalescence will be more efficient for the emulsion with larger size dispersed droplets. The present results provide an important reference for the design of electric demulsifier and the optimization of operating parameters.