Droplets–liquid layer coalescence in the coupling of rotating electric and rotating flow fields

The electric dehydration method offers strong controllability and high efficiency, providing a pathway to address the challenge of deep dehydration in crude oil. In this paper, the molecular dynamics method was employed to study the droplets–liquid layer (D–L) coalescence dynamics in the rotating electric and rotating flow coupled (RE&RF) fields. The effects of rotating flow (RF) field intensity, rotating electric (RE) field strength and frequency, nanoparticles (NPs), and surfactants on the D–L coalescence process were systematically analyzed. The results show that increasing the RF field intensity enhances the coalescence efficiency. However, excessively high angular velocities (ωf* ≥ 10.43) are not favorable for high water content (w ≥ 4.02 vol. %) systems. Compared with the direct current field, RE fields can destabilize water chains and promote complete coalescence by periodically altering the field direction. For low w (≤4.02 vol. %), high-frequency RE fields can be adopted to improve the coalescence efficiency; whereas, for high w (≥6.02 vol. %), low-frequency RE fields (ωE* ≤ 2.5) can effectively suppress the formation of water chains. The 45° angular interval RE fields (RE_45°) effectively promote the complete D–L coalescence at both low and high ωE*. Furthermore, the coupled RE_45°&RF fields exhibit superior performance in enhancing coalescence. The existence of Span-80 surfactants can effectively inhibit the effect of SiO2 NPs in reducing the coalescence efficiency. This study elucidates the microscopic mechanisms by which coupled RE&RF fields enhance the D–L coalescence and identifies the optimal conditions for electric dehydration. The findings provide theoretical guidance for addressing the challenges of optimizing high-efficiency and compact oil–water separators.