Xin Huang,Limin He,Xiaoming Luo,Haoran Yin,Donghai Yang
Deformation and coalescence behaviors of water droplets in viscous fluid under a direct current electric field are investigated with experiments and numerical simulations. An electro-hydrodynamic model is developed based on fluid dynamics equations and electrostatic equations. A coupled level set and volume of fluid method is proposed to capture the dynamic interface. The numerical results are validated to be in good accordance with classic results in the literature. The effects of electric capillary number Ca and viscosity ratio λ on droplet deformation and coalescence are systematically analyzed. Two different droplet response modes are obtained, namely overdamping response and underdamping response. The conversion of response modes is discussed with the ( Ca, λ ) phase diagram. Moreover, the distributions of electric field and flow field before and after the droplets contact are compared and analyzed. Numerical results show the time evolution of liquid bridge is dominated by the interfacial tension and viscosity of continuous phase. The electric stress is extremely weak at the liquid bridge and has a slight influence on evolution of liquid bridge. The correlation formula between dimensionless liquid bridge radius and conical angle during the growth of liquid bridge is regressed. These results are helpful for the optimization of electro-coalescence process.
Xin Huang,Limin He,Xiaoming Luo,Ke Xu,Yuling Lü,Donghai Yang
The dynamic behaviors of two droplets and droplet cluster under an alternating current (AC) electric field are investigated. Two droplets generally undergo transformation from complete coalescence to partial coalescence and finally to non-coalescence as the electric capillary number Ca p increases. The critical electric capillary number Ca pc for complete coalescence in the AC electric field remains unchanged and is twice as large as that in the direct current (DC) electric field when the frequency f ≥ 250 Hz. Charge transfer and reversal of electric field result in the reversal of the direction of electric force, which is the fundamental mechanism of non-coalescence of two droplets and chain formation in droplet cluster. The number of rebounds dramatically increases as f increases, promoting the stability of droplet chain. The droplet chains in the high-frequency AC electric field are longer and more stable than those in the low-frequency AC electric field.
Xin Huang,Mindan Chen,Yunyun Huang,Xiaoming Luo,Lin Teng,Longqiang Xiao,Jiaqing Li
Excessive electric field strength causes the droplets to contact but not coalesce, severely deteriorating the oil–water separation efficiency. The investigation of the non-coalescence of nonidentical droplets in oil under a DC electric field is conducted using high-speed photography. The effects of droplet volume ratio and interfacial tension ratio on the characteristics and critical parameters for droplet non-coalescence are systematically explored. The results show the critical electrocapillary number for the non-coalescence of nonidentical droplets and critical contact cone angle decreases as the volume sum increases or the interfacial tension ratio decreases, which is related to the competition between the resultant electric field force and the interfacial tension force. These results contribute to optimizing the operating parameters of the electric coalescer and improving the oil–water separation efficiency.
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