高压交流电场中单液滴变形度影响因素

采用不同的白油和水为实验介质对高压交流电场作用下单液滴的变形特性进行了研究。研究了电场强度、油品黏度、界面张力以及液滴直径等因素对液滴变形度的影响规律,并利用显微高速摄像系统结合图像处理技术对单液滴的变形规律进行了观察和分析。结果表明：液滴变形度主要受电场强度、液滴直径以及界面张力等的影响。与电场强度的N（1.7<N<2.4）次幂呈正比;与液滴直径近似呈线性关系;界面张力较低时,液滴更容易变形。油品黏度对液滴变形度没有明显影响。在变形度较小时,离心率和Weissenberg数具有较好的一致性,但We不适用于变形度较大的情况。     

The effect of temperature on coalescence of liquid drops at liquid/liquid interfaces

Drop/interface coalescence times measured at 293–343K are reported for three oil/water systems (benzene, paraffin oil and 1,1,2,2-tetrabromoethane) with different interface ages. The anomalous coalescence behavior of water droplets is explained by considering the static electrical double-layer residing at the interface which influences the film thinning and the film rupture processes. Analyses using simplified coalescence models reveal that the incorporation of temperature dependence on the physical properties such as density difference between phases, viscosity of the continuous medium and interfacial tension, does not produce satisfactory agreement with the measured coalescence times. The effect of mutual saturation in contrast to unsaturated systems on coalescence times is illustrated. The reproducibility of the drop/interface coalescence times is examined and explanations are offered, relating the method and the conditions of the experimentation. Finally the importance of both the coalescence time and the film thickness to drop stability analysis is demonstrated.     

Electrocoalescence of binary water droplets falling in oil: Experimental study

The approaching movement and consequent coalescence of binary water droplets falling in stagnant oil and exposed to an external electric field are investigated using a high speed camera. Different situation of the droplets and electric field intensities are applied in the experiments. The qualitative results of the experimental observations are exhibited through the scaled images of the binary droplets snapshots in milliseconds. Furthermore, different approaching trends of the droplets are presented as quantitative plots and discussed based on the theoretical electrostatic and hydrodynamic models. The effect of the applied voltage amplitude, initial distance of the drop pair, and skew angle of the electric field are investigated. The experimental results prove the electrostatic theories; as acceleration in electrocoalescence demonstrated using a stronger electric field as well as closer distance between the droplets. It was also revealed that the skew angle of the electric field decelerates the electrocoalescence until alignment of the droplets.     

流花油田老化油高频/高压脉冲交流电场破乳脱水研究

在测定老化油乳化液粘度-温度曲线、含水率反相点曲线的基础上,采用静态静电聚结破乳实验装置研究了油水反相特性对电场破乳脱水效果的影响,使用自主搭建的动态破乳脱水特性快速评价装置研究了高频/高压脉冲交流电场下电场强度和频率对老化油乳化液破乳脱水效果的影响. 结果表明,流花油田老化油反相点含水率约为40%,油水反相过程中乳化液粘度增加,电场破乳脱水难度增大;老化油乳化液含水率为30%时,最优电场强度1.25 kV/cm、电场频率2.5 kHz下破乳后的离心脱水率为97.8%,远高于工频电场下的离心脱水率(4.2%),高频/高压电场破乳比工频/高压电场破乳优势明显.     

非均匀电场下乳化油中液滴变形动力学行为

外加电场下液滴的变形动力学行为是乳化液电脱水机理研究的重要内容。基于Cahn-Hilliard方程的相场方法,建立了液滴在非均匀电场下的仿真模型,研究了电场作用下乳化液中液滴在形变、移动和聚结过程中电荷密度和电场力的分布规律,以及流场和电场的耦合作用。仿真分析了液滴粒径、电场强度以及电场非均匀系数对液滴运动行为的影响。利用实验室小型脱水系统开展了乳化液脱水实验,并通过高速摄像机对乳化液中液滴的运动行为进行了观测与分析。研究结果表明,在非均匀电场中液滴表面的极化电荷分布不均,由液滴中部向两端逐渐增大,在靠近电场集中方向处的电荷密度和Maxwell应力值最大;在一定范围内增大电场强度、电场非均匀系数或液滴粒径,可使液滴形变量增大,液滴向电场集中区域的移动速度以及液滴间的聚结速度增加。     

Non-coalescence behavior of neutral droplets suspended in oil under a direct current electric field

The excessive electric field gives rise to droplet non-coalescence and droplet chains in the electric dehydrator, which severely deteriorates oil–water separation efficiency and even leads to short circuit. To reveal the underlying mechanism of droplet non-coalescence, dynamic behavior of two neutral droplets in silicone oil under a direct current electric field is investigated by using high-speed photography. The experimental results show that there exists a critical electric field strength above which two droplets will bounce off after the contact. The critical electric field strength of droplet non-coalescence is affected by the initial separation distance between droplets, the radius of droplet, and the surfactant concentration. Whether the non-coalescence behavior occurs in the electric field is determined by the competition of electric force and capillary force, which dominates the evolution of tiny connection channel.     

COALESCENCE OF SINGLE DROPS AT A LIQUID-LIQUID INTERFACE IN THE PRESENCE OF SURFACTANTS/POLYMERS

The stability of single oil droplets at a liquid-liquid interface in the presence of surfactants/polymers was investigated and the coalescence time of oil droplets at an oil-aqueous surfactant solution interface was measured in a specially constructed coalescence cell. A unique feature of the experimental set-up was that the drop size could be controlled quite accurately and was independent of the interfacial tension and the difference in the densities of the two phases used. The coalescence time of oil droplets correlated favorably with interfacial viscosity of the system. However, no correlation was found between interfacial tension and coalescence time or the rate of coalescence. Both the coalescence time and interfacial viscosity were found to be strong functions of the surfactant concentration.     

Coalescence behaviour of water droplets in water‐oil interface under pulsatile electric fields

In this research, the deformation of water droplets in sunflower oil-interface under pulsatile electric field was studied experimentally. Three types of coalescence were observed:(i) complete coalescence, (ii) incomplete coalescence and (iii) no-coalescence. The first type is desirable because of leaving no secondary droplets. The second type that produced secondary droplets which caused by necking process, due to extreme elongation of droplets (mostly small droplets), was undesirable; because the small droplets were more difficult to coalesce and remove. The no-coalescence was caused by very fast coalescence and extensive pushing of droplet into the continuous phase. In this work the process was operated with the utilization of a batch cylindrical separator with high voltage system. The lower part of the cylinder was filled with the aqueous phase and its top part was filled with sunflower oil to form an interface between the two phases. The effects of electric field strength, frequency, and waveform types were investigated. It was found that, the ramp-ac waveform was the best waveform, avoiding the production of secondary droplets and in this case the frequency also played an important role.     

液滴在高压静电场中分散现象的实验研究

液滴在高压电场的作用下将分散成粒径细小的均匀雾滴。用涂有硅油的玻璃板对分散后的雾滴进行收集,通过统计学的方法即可测出雾滴平均粒径的大小。实验测量了不同液体的雾滴在不同电压下的平均粒径以及丙三醇在不同流量下雾滴平均粒径随电压升高的变化趋势,意在考察电压和流量这两大因素对液滴分散的影响。实验发现每种液体都存在一个临界电压,只有当电压超过临界值时,液滴才会发生大面积的分散。而在其它条件不变的情况下,流量的升高将导致临界电压的升高及平均粒径的增大。     

Numerical Study of the Collision and Coalescence of Water Droplets in an Electric Field

The coalescence of binary water droplets in oil exposed to an external electric field is simulated using a model including both electrostatic and hydrodynamic sections. Available mathematical models for electric dipole‐dipole force are presented in the first part of the model. Volume of Fluid approach is applied in the second part of the model. The simulation results were in good agreement with the published experimental observations. The results indicated that an improvement in electrocoalescence speed could be achieved. It was also revealed that the skew angle of the electric field, the oil viscosity, and the initial drops distance influence the electrocoalescence. Moreover, a correlation was developed to predict electrocoalescence kinetic as a function of the participant parameters.     

无机盐浓度及种类对电脱水过程水滴极化的影响

为了深入探究无机盐对电脱水过程水滴极化的影响机理,分别改变无机盐浓度及种类,对高频高压脉冲电场作用下水滴的极化变形进行显微实验研究。结果表明,不同NaCl浓度条件下水滴的变形度随电场频率和占空比的增加呈先增大后降低,由于电导率增大、离子冲击效应等的影响,随着NaCl浓度的增大,水滴的变形度随之增大。高价无机盐离子所受的电场力是一价离子的数倍,有利于其冲击速度和冲击动量的提升,水滴变形度随离子价位升高而增大。另外,由于离子活泼程度以及水解等原因,在同一高压脉冲电场中,不同钠盐溶液水滴变形度顺序为：磷酸钠 >碳酸钠 >硫酸钠 >氯化钠 >硝酸钠。不同氯盐溶液水滴变形度顺序为：氯化镁 >氯化钙 >氯化钾 >氯化钠 >氯化铵。研究成果为高压高频脉冲静电破乳机理的深入探讨奠定了基础。     

电场作用下液滴的变形及库仑分裂模式

基于高速成像技术,本文对电场作用下甲醇液滴的显微形貌特征进行了可视化研究,精确捕捉了两相流体系中不同生长阶段的荷电液滴基于时间分辨特性的变形及库仑分裂演变行为,得到不同工况下荷电液滴的变形分裂过程及行为演化细节。基于液滴所受库仑力和介电泳力与周围流域的耦合作用,揭示了电场作用下不同生长阶段的液滴库仑分裂形成机理。结果表明,电场强度和液滴粒径是决定液滴变形及库仑分裂模式的主要因素,荷电液滴的变形及库仑分裂模式可以分为推压变形、顶部破碎、顶部-边端破碎、伞状破碎。结合量纲为1参数对液滴的变形及破碎特征进行了定量分析,随着电场强度的增大及液滴粒径的减小,液滴变形及顶部破碎的程度更加剧烈,液滴临界伞状破碎长度减小。     

Simulation and experiments of droplet deformation and orientation in simple shear flow with surfactants

The deformation and orientation behavior of three-dimensional (3D) viscous droplets with and without surfactants is studied in simple shear flow using simulations and experiments. Two added amounts of surfactants are considered, along with a range of viscosity ratios and capillary numbers. The numerical method couples the boundary integral method for interfacial velocity, a second-order Runge-Kutta method for interface evolution, and a finite element method for surfactant concentration. The algorithm assumes a bulk-insoluble, nonionic surfactant, and uses a linear equation of state to model the relationship between the interfacial tension and the surfactant concentration on the drop surface. The algorithm was validated by comparison with other numerical results and good agreement was found. The experiments are performed in a parallel-band apparatus with full optical analysis of the droplet. The simulated and measured 3D steady-state shape of the ellipsoidal drops and their orientation are in reasonably good agreement. It was found that the surfactants have a greater effect on drop geometry for smaller viscosity ratios and that the deformation increases as the transport of surfactant becomes more convection dominated. It was also found that surfactants cause the drops to align more in the flow direction and that, for both clean and surfactant-covered drops, this alignment increases with viscosity ratio. Finally, simulations showed a wider distribution of surfactant on the interface for smaller viscosity ratios.     

Experimental and numerical study of stratified viscous oil–water flow

Stratified two-phase flows of oil and water are important to the energy industry, and models capable of predicting this type of flow are primordial. Many studies focus on fluids with low viscosity, but a high viscosity oil in the mixture significantly changes its behavior. We gathered experimental data of pressure drop, volumetric fractions, and flow-pattern data of a stratified liquid–liquid flow with high viscosity ratio. In addition, a wire-mesh sensor provided tomographic views of the flow. The data were compared with computational fluid dynamics (CFD) models using OpenFOAM and a one-dimensional model. CFD simulations used an interface capturing method, and turbulence damping was introduced to avoid high eddy viscosity at the interface region. Reynolds Average Navier–Stokes and large eddy simulations were used to account for turbulence, and they showed significant differences. The comparisons showed good overall results for pressure drop, volumetric fractions, and phase distributions between CFD and experiments.     

高频脉冲电场参数对水滴极化变形的影响

在理论分析基础上进行显微实验,得到了高压高频脉冲电场参数对水滴极化变形的影响规律。结果表明,随电场强度的增加,水滴的变形度近似呈抛物线形增大,水滴的极化弛豫时间降低,极化电荷的迁移速度增加,水滴最大变形度所对应的电场频率增大。电场频率过低时,油水乳状液的电容较大,极化电压过小,极化程度较低;电场频率偏离水滴固有频率越多,水滴的振荡幅度及共振效应越差,水滴的变形受到抑制。随占空比的增加,作用于水滴的电场能随之增加,水滴变形度增加;占空比过大,部分电场能经油相发生泄漏,水滴变形度小幅降低。研究发现,高压高频脉冲电场作用下,各电场参数间的交互作用对水滴极化变形的影响不可忽略。上述研究成果为高压高频脉冲静电破乳机理的深入探讨奠定了基础。     

Deformation of an oil droplet on a solid substrate in simple shear flow

Displacement of immiscible fluids is important in sub-surface processes such as enhanced oil recovery, oil sand processing and detergency. In this study, simulation of an oil droplet deformation on a solid substrate in simple shear flow has been carried out using computational fluid dynamics tool (Fluent 6.3) and the shape of the oil droplet is compared with that of the experimental observation. The dynamic behavior of a two-dimensional oil droplet subject to shear flow in a closed channel is considered under the condition of negligible inertial and gravitational forces. The volume of fluid method is used in Fluent to determine the dynamics of free surface of the oil droplet during the fluid flow. The oil droplet deformation increases with the increase in capillary number, Reynolds number and size of the oil droplet. The deformation of an oil droplet attached to channel surface in simple shear flow is studied experimentally in laminar flow through visual observation using microscope (Ziess, SV11 APO) with high speed camera (PCO). Aniline and isoquinoline was used to form oil droplet and distilled water was used as shearing fluid. The deformation of aniline and isoquinoline droplets was recorded using a high speed camera connected to a PC. The recorded image was replayed and the deformation of aniline and isoquinoline droplets was analyzed using Axio Vision software and compared with the results obtained from CFD simulation. The deformation of different sizes of aniline and isoquinoline droplets at different flow rates of shearing fluid and with time are well predicted by the CFD simulation.     

电场对液滴界面张力及动力学特征影响的研究进展

电场是改变液滴界面张力的重要因素,施加不同类型的电场对驱动微流体运动、变形、分裂及合并等行为起着至关重要的作用,该技术广泛用于微液滴操控、电子显示和原油脱水等领域,并具有潜在的应用前景。文中综述了电场作用下微液滴气-液、气-固与液-液交界面张力的变化,分析了与界面张力对微液滴运动学行为影响相关的实验现象及模拟成果;指出电场作用下气-液交界面处表面张力的变化趋势,因实验及模拟方法不同目前仍存在较大争议;探讨了直流电和交流电对电润湿过程液滴铺展的不同影响和液滴形态振荡特征;分析了直流电、交流电及电脉冲对电破乳过程中液滴运动行为的重要作用。总结了电场对液滴界面张力影响研究中存在的问题,并提出了值得进一步深入研究的可能方向。     

Mass Transport from Single Droplets in Imposed Electric Fields

Abstract

The influence of electric charge phenomena on mass transfer in systems involving droplets in a continuous medium is of increasing importance. The enhanced transport behavior of droplets in imposed fields on mass transfer and the influence of naturally occurring charges on droplets, particularly in the atmosphere, are both subjects of current interest. The objective of this study is to elucidate the hydrodynamic and mass transfer effect of imposed fields on charged single droplets. Investigations of both static and pulsed electric fields are reported.

The experimental approach is based on the suspension of single droplets in a precisely machined expanding channel. A stationary, saturated water droplet is fluidized by the upflow of 2-ethylhexanol. This arrangement allows precise photographic monitoring of droplet size and shape as well as the hydrodynamics of both phases in the region of the interface.

Baseline continuous-phase mass transfer studies with internally circulating droplets corroborate earlier studies with the same system. The effect of the accumulation of unavoidable surfactant on droplet circulation and mass transport rate at long exposure times is reported. The imposition of a static electric field in the axial direction caused distortion of droplets into prolate spheroidal shape.

An electric field pulsed at a tuned frequency is observed to cause oscillation of droplets between prolate-and oblate-spheroidal shapes. In addition to frequency and field amplitude, the interfacial properties of the fluid system as well as viscosities and densities of the two phases influence the oscillation. The effect of the pulsation of the field on mass transfer to the continuous phase is reported. These data are compared with results for both nonoscillating mass transfer and theoretical models.     

Numerical prediction of the electrical waveform effect on electrocoalescence kinetic

The effect of external electric field characteristics on the binary water droplet coalescence in stagnant oil has been studied using Computational Fluid Dynamics (CFD) simulations. Volume of Fluid (VOF) approach is applied as a multiphase model, in which the hydrodynamic equations consisting of Navier–Stokes and interface tracking equations are solved using finite volume discretization scheme. Dipole-induced-dipole (DID) model is utilized to calculate the electrostatic force on water droplets. Different types of electrical waveforms have been implemented using appropriate potential functions. The studied electrical voltages are sine, triangle, sawtooth, pulsed DC, and bipolar square waveforms. The predicted kinetic for electrocoalescence is in good agreement with the experimental data of the literature. The results demonstrated that a bipolar square wave provided the most efficient waveform, while sine and pulsed DC voltages gave lower efficiencies due to the time variation and off-time intervals, respectively. Triangle and sawtooth waveforms produced the least efficient waveforms. In addition, the effect of frequency has been studied for the aforesaid electrical waveforms in the adopted binary drop system. No significant differences have been proved in the approaching time of the drops from the studied frequencies. The results revealed that the electric fields with higher value of root mean squares (RMS) result in more efficient electrocoalescences. Furthermore, it was manifested that the higher voltage amplitude in every waveform improves this phenomenon.     

The effect of interfacial tension on secondary drop formation in electro-coalescence of water droplets in oil

The coalescence of water droplets in oils may be enhanced by application of an electric field. This approach is commonly used in the crude oil and petroleum industry to separate water from crude oil. However, the process could lead to the formation of fine secondary droplets during the process of coalescence. This is obviously undesirable, as it becomes more difficult to separate finer water droplets. In this work the effects of interfacial tension, manipulated by the use of surfactants, and electric field strength on the formation of secondary droplets are investigated. Two competing processes of necking and pumping determine whether secondary droplets are formed. The dimensionless groups Weber Number (describing droplet deformation and necking due to the electric field) and Ohnesorge Number (describing the pumping of water into the continuous phase in the process of coalescence) may be coupled to give a new dimensionless group WO, describing the volume fraction of secondary droplets that are formed. WO Number describes the ratio of the electrical stress energy that causes necking over the energy required for pumping the viscous fluid out of the droplets. For a wide range of interfacial tensions, brought about by the use of non-ionic and anionic surfactants and electric field strengths, a good unification of data is obtained. The outcome of this work will be useful for optimizing the design of the electro-coalescence systems.