Outer Vortex Research Articles

Microstructure and characterization of Ti–Al explosive welding composite plate

The interface characteristics and microstructure formation of TA2-5083 composite plate after explosive welded were studied. Optical microscope and electron microscope were used to analyze the microstructure of intermetallic compounds. Furthermore, ANSYS/AUTODYN was adopted to calculate the characteristics of interface microstructure simulated by the smooth-particle hydrodynamics (SPH) method. The results show that most molten metal in the wave front stays in the wave-waist region. There was a relative velocity difference between the vortex of molten metal and the titanium tissue, resulting in that broken titanium particles being scoured by vortexes. Ti3Al was generated in the vortex, whose antioxidant capacity wound lead to the formation of cracks. Consider soldering in a vacuum environment or adding an intermediate layer to reduce interfacial defects. The temperature of the outer vortex was higher than that of inner vortex, and the vortex has a transition layer of 5 μm, which is thinner than the transition layer of 10 μm between the fly and base plate. The jet of molten metal was mostly composed of aluminum, with the jet velocity of interface reaching 3000 m·s−1 and the interface temperature rising up to 2100 K. Compared with the molten metal in the wave-back vortex, the jet temperature at the interface was higher, resulting in a thicker transition layer at the bonding surface.

The formation process of annular starting jets

The formation and evolution process of the outer vortex ring (denoted as OV0) in annular starting jets has been studied numerically at stroke ratio $3\leqslant L_m/D\leqslant 8$ , $\beta \leqslant 0.8$ and Reynolds number $400\leqslant Re_{\triangle R}\leqslant 1400$ , where $D$ is the outer diameter of orifice, $L_m$ is the length of the fluid column, $\beta$ and $\triangle R$ are the ratio and difference between inner and outer radii of the annular orifice, respectively. The initial formation process of OV0 is found to be similar to that of the leading vortex ring in a circular starting jet, but the circulation strength is relatively lower and decreases gradually with increasing $\beta$ . The pinch-off process will also appear after OV0 grows to a certain size but is different from that of the circular starting jet. On one hand, OV0 is stretched and deformed by the strain field generated by the trailing jet's instability, and vorticity would be shed. On the other hand, OV0 accelerates the irrotational fluids around it under viscous action so as to form the induced velocity field, which would consume energy causing it to shed vorticity further under the limit of the Kelvin–Benjamin variational principle. Furthermore, OV0 would eventually become a stable vortex ring whose non-dimensional circulation ( $\varGamma /(U_{0max}D$ )) is found to be between 0.59–0.62 and it would not change with different $L_m/D$ at $\beta =0.5$ , which is much less than the 2–2.5 of circular starting jets.

Study on the global and spatial pulsation frequencies of rectangular syngas jet fire with different aspect ratios

Flame pulsation is a significant feature for both laminar and turbulent jet fires. In this study, the pulsation behaviors and the dominating mechanism of rectangular jet fires were explored. A series of experiments were conducted on the rectangular syngas jet fires with fuel flow rates ranging from 5 L/min to 45 L/min and aspect ratios ranging from 2 to 8. Both the global and spatial pulsation frequencies of syngas jet flame were obtained based on the time variations of image correlation coefficient and flame width. The local pulsation frequency of jet flame near the nozzle exit, which is defined as the natural frequency, increases slightly with the fuel flow rate due to the enhanced interaction of the inner vortex with the flame sheet and outer vortices. A transition from natural pulsation to subharmonic pulsation was found to happen when the local Richardson number (RiZ) of the jet flame is close to 1. The global pulsation frequency is dominated by the natural frequency at the nozzle exit for Qf<20 L/min, however, the subharmonic frequency for higher fuel flow rates. Non-axisymmetrical development of buoyancy-induced instability is considered to play a significant role in the rectangular jet fire, leading to the varied spatial pulsation frequency and the elongated flame height at side edges. In addition, the dimensionless global pulsation frequency (St) was found to correlate well with the Froude number (Fr) at the nozzle exit, St=0.42(1/Fr)0.46 for Fr > 10 and St=0.40(1/Fr)0.56 for Fr < 10.

Viscous Rankine vortices

A generalized version of the Rankine vortex is taken as initial state in an infinite fluid domain in two dimensions. The inner rigid-body motion of the Rankine vortex is surrounded by an outer irrotational vortex, where the inner flow and outer flow are allowed to have different amplitudes. We study the time evolution in a viscous fluid, where the kinematic viscosity scales the unit of dimensionless time. By an inverse Hankel transform, the Rankine vortex appears as an integrated continuous spectrum of axisymmetric Fourier–Bessel modes with radial wave numbers k. Each of these velocity modes is represented by a Bessel function J1(kr) where r is the radial distance from the vortex center. The amplitude of each individual mode will decay exponentially at a rate of νk2, where ν is the kinematic viscosity. The decaying continuous Fourier–Bessel spectrum is calculated analytically, while the evolution of the flow field is found by numerical evaluation of an inverse Hankel transform. We also consider the presence of a free surface in the gravity field, where pressure distribution in the vertical direction is hydrostatic. The far field remains irrotational during the viscous decay.

Experimental investigation of the helical mode in a stratified swirling flame

In a centrally-staged combustor, the stratified swirling flame often manifests a helical mode, which is characterized by the emergence of helical vortices, including the precessing vortex core (PVC) and the outer helical vortex (OHV). To understand the flame-vortex interaction mechanism responsible for the coherent heat release rate (HRR) fluctuation of such a helical mode, this work employs 20 kHz simultaneous measurements of PIV/CH2O PLIF and PIV/CH* chemiluminescence to resolve the unsteady flow field and flame motion. Through a combination of time-resolved and phase-averaged analysis, together with the three-dimensional reconstruction of the coherent flow and flame structures, we arrive at the qualitative understanding that in the helical mode the pilot flame dominated by PVC is a major source of coherent HRR fluctuation, while the main flame only slightly modifies the fluctuation intensity. Quantitative analyses further demonstrate the different effects of PVC and OHV on the coherent HRR fluctuation. Specifically, the PVC affects the primary heat-release process by dictating the convective transport of the reactants upstream of the flame front. And the OHV contributes to additional heat release by inducing stretching and roll-up of the pilot flame tip; it happens during the OHV's second growth, which is likely caused by the thermal expansion resulting from the primary heat release. This explains our finding that the OHV-induced heat release occurs at a slightly delayed phase angle and a downstream location compared with the primary PVC-induced heat release.

A novel hydrocyclone for use in underground DNAPL phase separation

Halogenated organic solvents are the most commonly detected pollutants in groundwater and are particularly toxic and harmful. How to separate these dense nonaqueous phase liquid (DNAPL) pollutants efficiently from groundwater has become an important research question. Here, a novel hydrocyclone with annular overflow structure was designed, which eliminated the short-circuit flow of the traditional hydrocyclone and solved the problem of overflow entrainment caused by the enrichment of droplets near the locus of zero vertical velocities (LZVV) into turbulence. The flow field characteristics of this novel hydrocyclone were studied using Computational Fluid Dynamics (CFD) simulation and compared with the traditional hydrocyclone. It was found that the annular gap structure of the novel hydrocyclone increased the tangential velocity of the outer vortex. Moreover, the radius of the LZVV was expanded outward by 0.17 mm, which reduced the possibility of droplets with small particle sizes in the second phase escaping from the overflow pipe. The collective effect was to eliminate the short-circuit flow. This novel hydrocyclone was able to separate DNAPL pollutants with low consumption and high efficiency, across a range of inlet velocity from 4 to 6 m/s. The maximum separation efficiency was 99.91 %. In addition, with trichloroethylene (TCE) as the target pollutant, the maximum volume fraction of the dispersed phase in the hydrocyclone was located on the side wall of the hydrocyclone. Taken together, we believe that this work will provide a low-cost, efficient separation method for the separation of groundwater- contaminated liquid mixtures. Furthermore, it has broad application prospects in the field of heterotopic remediation of groundwater.

Quantum phases of spin-orbital-angular-momentum–coupled bosonic gases in optical lattices

Spin-orbit coupling plays an important role in understanding exotic quantum phases. In this work, we present a scheme to combine spin-orbital-angular-momentum (SOAM) coupling and strong correlations in ultracold atomic gases. Essential ingredients of this setting is the interplay of SOAM coupling and Raman-induced spin-flip hopping, engineered by lasers that couples different hyperfine spin states. In the presence of SOAM coupling only, we find rich quantum phases in the Mott-insulating regime, which support different types of spin defects such as spin vortex and composite vortex with antiferromagnetic core surrounded by the outer spin vortex. Based on an effective exchange model, we find that these competing spin textures are a result of the interplay of Dzyaloshinskii-Moriya and Heisenberg exchange interactions. In the presence of both SOAM coupling and Raman-induced spin-flip hopping, more many-body phases appear, including canted-antiferromagnetic and stripe phases. Our prediction suggests that SOAM coupling could induce rich exotic many-body phases in the strongly interacting regime.

Origin of enhanced skin friction at the onset of boundary-layer transition

Boundary-layer transition is accompanied by a significant increase in skin friction whose origin is rigorously explained using the stochastic Lagrangian formulation of the Navier–Stokes equations. This formulation permits the exact analysis of vorticity dynamics in individual realizations of a viscous incompressible fluid flow. The Lagrangian reconstruction formula for vorticity is here extended for the first time to Neumann boundary conditions (Lighthill source). We can thus express the wall vorticity, and, therefore, the wall stress, as the expectation of a stochastic Cauchy invariant in backward time, with contributions from (a) wall vorticity flux (Lighthill source) and (b) interior vorticity that has been evolved by nonlinear advection, viscous diffusion, vortex stretching and tilting. We consider the origin of stress maxima in the transitional region, examining a sufficient number of events to represent the increased skin friction. The stochastic Cauchy analysis is applied to each event to trace the origin of the wall vorticity. We find that the Lighthill source, vortex tilting, diffusion and advection of the outer vorticity make minor contributions. They are less important than spanwise stretching of near-wall spanwise vorticity, which is the dominant source of skin-friction increase during laminar-to-turbulent transition. Our analysis should assist more generally in understanding drag generation and reduction strategies and flow separation in terms of near-wall vorticity dynamics.

Numerical simulation on acoustic streaming characteristics in boiler tube array

Strong sound can produce acoustic streaming around the tube, which can change the flow state around the tube, and then affect the heat exchange between the tube and the surrounding medium. In order to obtain the influence law of sound wave on the heat transfer characteristics in the tube array, firstly, we study the acoustic streaming and heat transfer characteristics around a single cylindrical tube by the scheme of theoretical derivation and simulation. On this basis, we study the acoustic streaming and heat transfer characteristics in the periodic tube array. The results show that the acoustic streaming structure outside the tube is axially symmetric, and the flow direction of the inner vortices and the outer vortices are opposite. When the frequency is constant, sound pressure level (SPL) only changes the acoustic streaming velocity, not the acoustic streaming distribution. As the frequency continues to increase, the velocity peak gradually shifts from the inner vortex to the outer vortex, so the outer vortex velocity gradually becomes the main body of flow. Low frequency and high intensity sound waves can form strong acoustic streaming movement and enhance heat transfer outside the tube. In the tube array, the acoustic streaming distribution should consider the effect of different incident directions on crystal plane spacing. For incident angle φ = 0°∼34°, the ratio of maximum velocity of outer vortex to the one of inner vortex an Nusselt number (Nu) are increasing with φ increasing. In this stage, the heat transfer effect changes greatly with the change of angle. For incident angle φ = 35°∼45°, the inner and outer vortex flow velocity and Nu increases together, but the scale of outer vortex decreases. In this stage, with the change of angle, the heat transfer effect changes less due to the reduction of the scale of outer vortex dominating heat transfer between tubes. In tube arrays, the optimal flow field incidence angle is φ = 34 °and the optimal heat transfer incidence angle is φ = 45°. The sound band gap and sound pressure level enhancement exist in the tube array, which affects the acoustic streaming distribution. The results are helpful to reveal the mechanism of acoustic streaming more clearly and provide a basis for the augmentation of heat transfer in tube arrays by acoustic waves.

Experimental investigations on coherent flow structures in acoustically excited swirling flames using temporally-separated dual-plane Stereo-PIV

A temporally-separated dual-plane stereoscopic particle image velocimetry (PIV) technique is utilized to investigate the dynamics of periodically-excited vortices in a swirling flame under four experimental conditions. The time-averaged results under the unexcited condition are analyzed to verify the accuracy of the method. For the excited conditions, Q criteria is used to extract and classify the vortex structures within the swirling flame. It is found that under different excitation frequencies of the same amplitude, the outer vortex rings (OVRs) and the inner vortex rings (IVRs) show opposite rotational characteristics. While the excitation frequency has little influence on the central vortices induced by the swirler, the amplitude does have a great influence on the development trajectory of the central vortices. Furthermore, the amplitude and phase of oscillation velocities in three directions are successfully decomposed by using sine curve fittings at each point. A periodic acceleration and deceleration process is observed only in the axial direction. Besides the acoustic wave, it is found that the vortex rings (both OVRs and IVRs) also contribute to the axial velocity oscillations.

On the generalized Beltramian motion of the bidirectional vortex in a right-cylindrical cyclone with a hollow core

In this work, an exact inviscid solution is developed for the incompressible Euler equations in the context of a bidirectional, cyclonic flowfield in a right-cylindrical chamber with a hollow core. The presence of a hollow core confines the flow domain to an annular swirling region that extends into a toroid in three-dimensional space. The procedure that we follow is based on the Bragg–Hawthorne framework and a judicious assortment of boundary conditions that correspond to a wall-bounded cyclonic motion with a cylindrical core. At the outset, a self-similar stream function is obtained directly from the Bragg–Hawthorne equation under the premises of steady, axisymmetric, and inviscid conditions. The resulting formulation enables us to describe the bidirectional evolution of the so-called inner and outer vortex motions, including their fundamental properties, such as the interfacial layer known as the mantle; it also unravels compact analytical expressions for the velocity, pressure, and vorticity fields, with particular attention being devoted to their peak values and spatial excursions that accompany successive expansions of the core radius. By way of confirmation, it is shown that removal of the hollow core restores the well-established solution for a fully flowing cylindrical cyclone. Immediate applications of cyclonic flows include liquid and hybrid rocket engines, swirl-driven combustion devices, as well as a multitude of heat exchangers, centrifuges, cyclone separators, and flow separation devices that offer distinct advantages over conventional, non-swirling systems.

The effects of injector size on the dynamics and instabilities of lean premixed swirling flame

The external- and self-excited premixed swirling flames are experimentally studied to understand the thermoacoustic instabilities under different injector sizes. We use high-speed CH* chemiluminescence, formaldehyde laser-induced fluorescence and particle image velocimetry to measure the dynamic characteristics of the flame surface and flow field. Reducing the size of the injector from 20 mm to 16 mm changes the thermoacoustic system's stability. The flame transfer function shows that reducing the size of the injector significantly reduces the phase delay, enhances the response amplitude, and annihilates the local gain extrema. The low-order network model analysis is performed, and it is found that the phase variation caused by changing injector size is the main reason for the induced thermoacoustic instability. The pixel-by-pixel decomposition of CH* images is used to extract the weighted gain and phase maps of local heat release oscillation. The swirling flame with a small injector size of 16 mm has a short time delay and a slow phase velocity, which results in strong in-phase interference of the local heat release oscillators and enhanced gains. Reducing the size of the injector strengthens the inner and outer shear layers, thereby suppressing the heat release fluctuation of the flame base. The enhanced vorticity supply of the shear layer increases the size and strength of the outer vortex ring, resulting in strong heat release fluctuations at the flame tip, which is responsible for its large unadulterated fluctuation intensity. These changes in the flow field change the relative heat release intensity and fluctuation amplitude between the flame base and tip. The out-of-phase interference among multiple vortices causes weak flame response at high frequencies.

Evolution and Dynamics of the Coherent Vortices in Contraction and Premix Coaxial Jets

AbstractContraction and premix coaxial jets are commonly employed in gasification and combustion systems. Understanding the spatial development of coherent structures is a prerequisite to improve the mixing and stability characteristics. The evolution and dynamics of coherent vortices under the contraction and premix coaxial jets are visualized and discussed using flow visualization technology. The contraction configuration increases the instability of the outer shear layer corresponding to an organized rolling up of the outer vortex rings. In the premixed configuration, the earlier interaction between the inner and outer shear layers induces the precocious development of the inner Kelvin‐Helmholtz (K‐H) vortices. Two empirical equations are proposed to predict the transition length towards a fully turbulent flow.

Numerical investigation of semifilled-pipe flow

This study describes turbulent flow in a semifilled pipe with a focus on its secondary currents. To the authors’ knowledge, we provide the first highly resolved data-set for semifilled-pipe flow using direct numerical simulation. The flow parameters range from$Re_\tau =115$, just maintaining turbulence, to moderate turbulent flow at$Re_\tau =460$. Some of the main flow characteristics are in line with previously published results from experiments, such as the velocity-dip phenomenon, the main secondary flow and the qualitative distribution of the Reynolds stresses in the core of the flow. We observe some flow phenomena which have not yet been reported in the literature so far for this type of flow. Among those is the inner secondary cell in the mixed corner between the free surface and the pipe's wall, which plays a major role in the distribution of the wall shear stress along the perimeter. We observe that the position and extension of the inner vortex scale with the wall shear stress and those of the outer vortex scale with outer variables. For the first time, we present and discuss distributions of the complete Reynolds stress tensor and its anisotropy which gives rise to the generation of mean streamwise vorticity in a small region in the mixed corners of the pipe. Mean secondary kinetic energy, however, is generated at the free surface around the stagnation point between the inner and outer vortices. This generation mechanism is in line with a vortex dynamics mechanism proposed in the literature.

Numerical research on the instabilities of CLT propeller wake

Optimising contracted and load tip (CLT) propellers has proven difficult due to the lack of research on flow fields. In this study, the hydrodynamics and wake of the P1727 propeller, a CLT propeller, are investigated using large eddy simulation for three advance coefficients. Fluctuations in the thrust and torque are minimal, with a frequency of up to 1000 Hz, as indicated by the simulations, which are verified based on experimental data and study on spatial and temporal convergences. Instantaneous, phase-averaged, and azimuthal-averaged flow fields are used to analyse propeller wake, including the tip vortices of endplate, secondary tip vortices, merged and trailing sheet vortices and the hub vortex. Based on previous works, we present a novel mutual inductance mechanism for the instabilities in CLT propeller wake. The merged vortices formed by merging between the tip vortices of endplate and the secondary tip vortices are strengthened, stretched by the outer trailing vortices that are caused by the crack of trailing sheet vortices, then the reinforced merged vortices trigger the leapfrogging phenomenon and induce the outer instabilities. While the inner trailing vortices, or root vortices, remain coherent, they undergo contraction and approach the hub vortex. Thereafter, they intertwine with the spiraling hub vortex, acting as the dominant mutual inductance mode of the inner CLT propeller wake.

Flame structure and fuel reaction on a non-premixed shuttlecock-like conical burner

To promote the efficiency of non-premixed combustion, we designed slot structures with a simple geometry on the side of a conical burner named a shuttlecock-like conical burner. The slot structure which can introduce the air co-flow naturally into the cone instead of using other and complicated geometry is inspired from the particular shape of a shuttlecock serving as a missile in badminton sport. Therefore, the air co-flow not only produces a recirculation zone on the edge of the burner but also moves directly into the burner. The combustion condition transforms from non-premixed to partially premixed combustion; the mixing and reaction of fuel and oxidant are hence greatly enhanced. We observed the structures of the flame and the flow of a shuttlecock-like conical burner, and examined its reaction characteristics with measurements of the chemiluminescence of OH* and the flame temperature. The observation of an isothermal flow field demonstrates that the air co-flow moves directly into the burner via the slots. Double pairs of vortex structures are observed above the exit of the burner because the introduced air co-flow decreases the strength of the outer vortex, allowing the central jet to form an inner vortex. The reaction zone of a shuttlecock-like conical burner hence approaches not only the central region of the flame but also the burner exit. The reaction intensity of the fuel of a shuttlecock-like conical burner is also stronger than that of an ordinary conical burner (without the slots). The designed shuttlecock-like conical burner is significantly useful to serve as a reference to improve the combustion efficiency of non-premixed combustion.

Flame flow field interaction in non-premixed CH4/H2 swirling flames

Alternative fuels and stocks like biomass or chemical and refinery waste, may potentially be used in gas turbines and industrial applications after gasification. Thus, understanding the role of hydrogen in these fuels is critical to the broader aim of utilising alternative fuels for power generation. In this work, the interaction between the flame and the flow field was studied in a quarl-stabilised swirl non-premixed flame burning CH4 and H2–enriched CH4. Simultaneous high-speed OH-PLIF/PIV imaging at 5 kHz was carried out on these flames to explore the flame-flow interaction. The instantaneous flow fields in the CH4 or CH4+H2 flames showed a small scale vortical structure near the shear layers, which were not apparent in the time-averaged flow fields. Increasing H2% in the fuel jet was observed to dampen the velocity fluctuations. The fuel composition affected the spatial location of the reaction zone; in the CH4 flames, the axial position of the reaction zone is seen to track the relatively large-magnitude axial velocity fluctuations while remaining in locally low-speed regions of the flow. In contrast, in H2-enriched flames, where the flame is more robust, the reaction zone was able to survive longer, in terms of axial distance, in the vicinity of high swirling jet velocity, with less sensitivity to velocity fluctuations. With increasing the H2%, the reaction zone steadily leaves the IRZ towards the swirling jet flow and localised between its outer and inner vortices. This acts as a stabilisation factor where the internal vortices convect hot product towards the fresh mixture. Moreover, the flame curvatures, the vorticity and compressive strain fields interactions with the reaction zone are presented and discussed. This article outlines results that yield more in-depth insight into hydrogen-enriched hydrocarbon non-premixed swirling flames' combustion, which is essential to accelerate the fuel switching from hydrocarbons to hydrogen.

C-Band SAR Winds for Tropical Cyclone Monitoring and Forecast in the South-West Indian Ocean

Tropical cyclone (TC) monitoring and forecast in the South West Indian Ocean (SWIO) basin remain challenging, notably because of the lack of direct observations. During the 2018–2019 cyclone season, S-1 Sentinel SAR images were acquired, as part of the ReNovRisk-Cyclone research program, giving access to unprecedented detailed TC wind structure description without wind speed limitation. This paper assesses the quality of these data and the impact of their assimilation for TC forecasts. SAR observations are compared with analyses from a convection-permitting, limited area model AROME OI 3D-Var and with wind products used for operational TC monitoring. Their bias depends on the angle of incidence of the radar and the observation error is larger for extreme wind speed. The impact of SAR assimilation in AROME OI 3D-Var is assessed through two case studies. In the TC GELENA case, it leads to a better TC positioning and an improved representation of inner and outer vortex structures. The TC intensity reduction in the analysis propagates through subsequent analyses and it has an impact on forecasts for around 12 h. In the TC IDAI case, the 3D-Var does not manage to reproduce TC intensity captured by SAR. In both cases, the modification of the initial conditions has little influence on the intensification rate of the model forecasts. Sensitivity tests show that these results are robust to different observation errors and thinning.

Experimental study on the flow interaction between two synthetic jets emanating from a dual round orifice

Multiple orifice synthetic jet devices are becoming widely utilized for active flow control and jet impingement cooling, due to its mixing performance resulting from the vortices and jet interaction. Therefore, understanding the flow interaction between multiple synthetic jets is crucial in maximizing the potential for many industrial applications. In the present study, an experimental investigation on flow interaction between two synthetic jets generated from a dual-orifice device is performed. The influence of dimensionless orifice spacing (s/D = 1.2, 2.0, and 3.0) and stroke length (L0/D = 13.7, 18.6, and 27.5) is analyzed at a fixed Reynolds number of Re0 = 3700. Phase-locked particle image velocimetry (PIV) is used to obtain time- and phase-averaged flow fields. The jet interaction is enhanced as the orifice spacing and stroke length decrease, resulting in shorter distances for the merging and combining points. In addition, the inner vortices between the two jets are deformed and cancelled due to the jet interaction, which leads to the inner and outer vortices merging into a single vortex. The vortex interactions and merging are delayed as the orifice spacing increases, while the advection speed of vortices is increased without change of flow structure as the stroke length increases.

Coalescence dynamic response of an aqueous droplet at an oil-water interface under a steady electric field

The electrocoalescence of an aqueous droplet at the oil-water interface in the presence of externally direct current electric fields was numerically analyzed with the finite element method by solving the Navier-Stokes and charge conservation equations. The proprietary software Comsol Multiphysics was used for this purpose, and the interface motion was captured by the Level-Set method. Good agreement was obtained between numerical and experimental results in the literature. The numerical description of droplet-interface coalescence transition from the viewpoint of flow field evolution and bridge dynamics was systematically examined, analyzed, and discussed. In the early droplet-interface merging process of CC (complete coalescence) modes, the droplet vortex pair was more strongly developed than the interface vortex pair, which gave rise to the enlargement of the droplet-interface liquid bridge and the fluid motion from the droplet to the interface. The interfacial velocity at the liquid bridge was much larger than at the other regions of the droplet surface in the upheaval CC mode. The upward velocity of the outflow-natured outer vortex pair was observed in the PC (partial coalescence) mode, leading to the formation of secondary droplets. In the jet-like PC mode, multiple outflow-natured vortex pairs simultaneously existed, forcing the droplet tail to break up into tiny satellite droplets. In the NC (non-coalescence) mode, the outflow-natured outer vortex pair was always directed upward, and with the breakup of bouncing-off droplets in the NC mode, an abnormal flow field with an irregular outflow-natured outer vortex pair was formed. Conductivity played little or no part in determining the CC-PC transition, but influenced the PC-NC transition. With the whole liquid bridge dynamics involving liquid bridge growth and decay dynamics taken into consideration, a good quartic polynomial function trend was obtained between the non-dimensional liquid bridge width W* and the non-dimensional time t*. In addition, the liquid decay dynamics and the coupling effect of the electric field and fluid physical properties (such as interfacial tension and viscosity) determine the partial coalescence process. The outcome of this work is potentially useful for optimizing the design of compact and efficient oil-water separators.