Flow Of Dielectric Liquid Research Articles

Flow electrification characteristics of liquid hydrogen in pipe flow

Flow electrification occurs when a dielectric liquid flows along the wall surface, and it may cause electrostatic hazards such as spark discharges. Flow electrification issues of liquid hydrogen should be carefully treated since its low electrical conductivity and dangerous flammability. In this study, flow electrification characteristics of liquid hydrogen in pipe flow are numerically investigated. A theoretical model coupling the electric field and the flow field is introduced to govern the behaviour and distribution of charges, and an interface discharge model is adopted to describe the charge transfer boundary condition. The theoretical model is validated by comparing the simulation with experiment data, and the results show a good agreement with the averaged related deviation below 10%. The result of a common case, with flow velocity u of 1 m/s, pipe radius a 0.01 m, and length L 1 m, shows that the charge density, streaming current, and the electric potential of liquid hydrogen flow are in the magnitude of 10−12 C·m−3, 10−16 A and 10−2 V, respectively. Flow parameters, including flow velocity and pipe size, have a certain effect on the flow electrification characteristics of liquid hydrogen flow. By dimensionless parameterization, it is found that Reynolds numbers (Re), r/a, and x/a are exactly three orthogonal parameters determining the charge density distribution. Furthermore, the effects of electrical conductivity on the distribution of charges and charge dissipation have also been discussed. Finally, the safety assessment of the liquid hydrogen transportation system has been discussed, with extra consideration of the impurities and gas-liquid two-phase flow regime. To sum up, this work presents an in-depth and comprehensive insight into the flow electrification of liquid hydrogen flow, which is of great significance to the transportation safety of liquid hydrogen.

Virtual walls for dielectric fluid manipulation through controllable charge deposition

Fluid transportation is a basic physical process in various engineering applications, and solid walls are most commonly used to confine fluid flows. Solid walls, however, have some disadvantages, including large pressure drops and plaguing by fouling. Here we experimentally demonstrate a method of controlling dielectric fluid flows on planar surfaces by virtual (not solid) walls through a spatially controlled charge deposition. Acting on the three-phase contact line, an electrically induced pressure has the same function as solid sidewalls. Dielectric fluids can be confined within designated areas, or flow directionally with the aid of contactless masks. Dielectric fluids can also move on planar surfaces by moving contactless masks. Due to the virtual wall effect of this novel method, an open channel can be developed to continuously transport fluids. The method may provide a new perspective to manipulating dielectric liquid flows and can be an alternative way of open fluidic devices.

A High Voltage Nanogenerator Based on Electrification of the Dielectric Liquid Flow through the Glass Filter

A High Voltage Nanogenerator Based on Electrification of the Dielectric Liquid Flow through the Glass Filter

Numerical investigation of instability and transition to chaos in electro-convection of dielectric liquids between concentric cylinders

The two-dimensional (2D) electro-convection (EC) flow of dielectric liquids between two concentric cylindrical electrodes driven by unipolar injection of ions is investigated numerically. The finite volume method is used to resolve the spatiotemporal distributions of the flow field, electric field, and charge density. The flow transition routes from steady laminar to chaotic flow states are studied in various scenarios where the mobility parameter M of the dielectric liquids varies from 5 to 200. The dynamic characteristics and bifurcation routes of the EC flow depend on the electric Rayleigh number T, a ratio of the electric force to viscous force, and the mobility parameter M. For increasing T, three different transition routes from a convective steady-state to chaos via different intermediate states are observed. The flow states have been quantified by the power spectral density distribution and phase space trajectory of the velocity. The fractal dimensions and Lyapunov exponents are calculated to identify the chaotic flow. The increase in the mobility parameter M leads to a shorter and more direct route with fewer intermediate states when bifurcating to chaos. In addition, the power scale of charge transport that is defined by the electric Nusselt number Ne and T is discussed when the EC flow develops into electro-turbulence.

Utilizing a porous-electrode for the flushing fluid in electrical discharge machining

In traditional electrical discharge machining (EDM), abnormal machining phenomena, such as short circuits and arc drawing, are often induced by inter-electrode erosion particles. These result in low machining efficiency and poor surface quality. In this paper, a porous electrode is prepared by high-temperature sintering. The dielectric liquid flows down through the pores in the electrode and then out of its sides to flush the eroded particles rapidly out from between the electrodes, thus reducing the occurrence of these abnormal machining phenomena in EDM and improving machining efficiency. A computational fluid dynamics simulation was used to analyze and compare the flow between the electrodes and the particles flushed out by a single-channel electrode and a porous electrode. The results show that the flow field for the porous electrode is distributed, and it can effectively and quickly remove eroded particles out of the inter-electrode discharge gap. Consistent with the simulation, the experimental results show that porous-electrode EDM can achieve a higher machining efficiency than traditional EDM, and the machining speed is not affected by the machining depth. Moreover, the use of a flushing fluid can avoid the generation of carbon pillars. With the increase of the flow rate of the flushing fluid, due to the dual effects of the flushing fluid reducing the concentration of eroded particles and the flushing fluid interfering with the plasma discharge channel, the material removal rate has a double peak.

Numerical investigation of injection-induced electro-convection in a dielectric liquid between two eccentric cylinders

Numerical analysis of the 2D radial and azimuth electro-convection (EC) flow of dielectric liquid between two eccentric cylindrical electrodes driven by unipolar injection of ions is presented. The finite volume method is used to resolve the spatiotemporal distributions of the flow field, electric field, and charge density. The flow instability is studied in various scenarios where the radius ratio Γ = Ri/Ro ranges between 0.1 and 0.7 and the eccentricity η between 0.1 and 0.5. The bifurcation of the flow patterns depends on the electric Rayleigh number T, a ratio of the electric force to viscous force, and the two geometric parameters Γ and η. For an increasing T, the EC system develops from a weak steady convective state to chaos via different intermediate states experiencing pitchfork and Hopf bifurcations. The influence of Γ and η on the bifurcation behavior is also investigated. When Γ lies between 0.1 and 0.3, a novel periodic oscillation of the flow patterns has been observed.

Flow structure and heat transfer of electro-thermo-convection in a dielectric liquid layer

Electrothermohydrodynamic flow of dielectric liquid in a square cavity driven by the simultaneous action of Coulomb and buoyancy forces is studied numerically. A high resolution upwind scheme is applied to study the flow bifurcations and heat transfer. We focus on the strong injection case with the nondimensional injection parameter C fixed at C = 10. Two Prandtl numbers Pr = 1 and Pr = 10, two Rayleigh numbers Ra = 105 and Ra = 106, as well as two mobility numbers M = 10 and M = 20 are considered to evaluate the dependence of flow structure and heat transfer on these parameters. Multistates are found to coexist in a wide range of parameter regimes. Various flow patterns such as one-cell, two-cell, four-cell, and multicell flow are observed. The electric field is found to enhance heat transfer more efficiently for large Prandtl number fluid at low mobility parameter and relatively low Rayleigh number.

Hydrodynamical flows in dielectric liquid in strong inhomogeneous pulsed electric field

We consider a hydrodynamical flow of dielectric liquid near a high voltage needle-shaped electrode in a strong inhomogeneous pulsed electric field. It was shown that under a small rise time, a negative pressure area (pressure is less than critical pressure) appears near the electrode leading to the formation of a cavity in which electric breakdown can develop. A comparison of the dependence of the velocity of fluid near an electrode for two cases (taking into account the dependence of dielectric permeability of the liquid on the electric field and without taking it into account) was made. A field-dependent dielectric coefficient leads to the appearance of two local maximums of the velocities and increases the minimum pressure, thus lowering the possibility of cavitation. While under the constant value of dielectric permeability only one local maximum appears.

Numerical simulation and design optimization of an electrohydrodynamic pump for dielectric liquids

With an ever increasing demand for more effective heat sinks, liquid based electronic cooling has become a new prospect in the field. The present study introduces an electrohydrodynamic (EHD) pump with a simple design for dielectric liquids which have potential applications for electronic cooling. The pump consists of an eccentrically sandwiched wire electrode placed at the horizontal centerline between two parallel flat-plate electrodes. The EHD flow of dielectric liquid induced by the space charge generated due to the Onsager effect was obtained by the numerical solution of the Poisson–Nernst–Planck equations for ion transport and the Navier–Stokes equations for fluid flow. Good agreement obtained in the comparison of the numerical and the experimental results of velocity for the centrally sandwiched wire electrode case confirmed the validity of the numerical results. For a fixed voltage, the pump flow rate depends on the eccentricity of the wire electrode with respect to the plate electrodes and also with the electrode dimensions. By using the Taguchi method an optimum design for the EHD pump is obtained considering the wire electrode diameter, the flat plate electrode length and the eccentricity (the horizontal distance between the centers of wire and flat-plate electrodes) as the design parameters for fixed channel dimensions.

Electrohydrodynamic Analysis of Dielectric Guide Flow Due to Surface Charge Density Effects in Breakdown Region

A fully coupled finite element analysis (FEA) technique was developed for analyzing the discharge phenomena and dielectric liquid flow while considering surface charge density effects in dielectric flow guidance. In addition, the simulated speed of surface charge propagation was compared and verified with the experimental results shown in the literature. Recently, electrohydrodynamics (EHD) techniques have been widely applied to enhance the cooling performance of electromagnetic systems by utilizing gaseous or liquid media. The main advantage of EHD techniques is the non-contact and low-noise nature of smart control using an electric field. In some cases, flow can be achieved using only a main electric field source. The driving sources in EHD flow are ionization in the breakdown region and ionic dissociation in the sub-breakdown region. Dielectric guidance can be used to enhance the speed of discharge propagation and fluidic flow along the direction of the electric field. To analyze this EHD phenomenon, in this study, the fully coupled FEA was composed of Poisson’s equation for an electric field, charge continuity equations in the form of the Nernst?Planck equation for ions, and the Navier-Stokes equation for an incompressible fluidic flow. To develop a generalized numerical technique for various EHD phenomena that considers fluidic flow effects including dielectric flow guidance, we examined the surface charge accumulation on a dielectric surface and ionization, dissociation, and recombination effects.

Effect of charge mobility on dielectric liquid flow driven by EHD conduction phenomenon

Abstract Electrohydrodynamic (EHD) conduction phenomenon takes advantage of the electrical Coulomb force exerted on a dielectric liquid generated by externally applied electric field and dissociated charges from electrolytes. EHD conduction generated flow relies primarily upon the asymmetry of the electrodes where the flow is always directed toward the broken symmetry regardless of the electrodes polarity. This paper studies the effects of unequal positive and negative charge mobilities on the heterocharge layer structure and generated flow with symmetric and asymmetric electrode designs. The numerical simulations are conducted for a 2-D rectangular channel with the electrodes embedded against the channel wall.

Electrohydrodynamic flow of dielectric liquid around a wire electrode - effect of truncation of onsager function

Electrohydrodynamic flow of a dielectric liquid around a wire electrode sandwiched between two parallel flat-plate electrodes is studied both numerically and experimentally. The Poisson-Nernst-Planck equations for the ion transport and the Navier-Stokes equations for the fluid flow are solved numerically by using a commercial code. Charge generation in a dielectric liquid (dodecane + 0.5% wt Span 80) due to Onsager effect is considered in the ion transport equations. Numerical results well predict the experimental ones both qualitatively and quantitatively. At high electric field the numerical approach slightly over-predicts the flow velocity measured from the experiment, but the level of discrepancy can be further lowered by using a truncated series for the Onsager function.

Electrohydrodynamic flow of dielectric liquid around a wire electrode - effect of truncation of onsager function

Electrohydrodynamic flow of dielectric liquid around a wire electrode - effect of truncation of onsager function

Asymptotic and numerical analysis of electrohydrodynamic flows of dielectric liquid

We perform an asymptotic analysis of electrohydrodynamic (EHD) flow of nonpolar liquid subjected to an external, nonuniform electric field. The domain of interest covers the bulk as well as the thin dissociation layers (DSLs) near the electrodes. Outer (i.e., bulk) equations for the ion transport in hierarchical order of perturbation parameters can be expressed in linear form, whereas the inner (i.e., DSL) equations take a nonlinear form. We derive a simple formula in terms of various parameters which can be used to estimate the relative importance of the DSL-driven flow compared with the bulk-driven flow. EHD flow over a pair of cylindrical electrodes is then solved asymptotically and numerically. It is found that in large geometric scale and high ion concentration the EHD flow is dominated by the bulk-charge-induced flow. As the scale and concentration are decreased, the DSL-driven slip velocity increases and the resultant flow tends to dominate the domain and finally leads to flow reversal. We also conduct a flow-visualization experiment to verify the analysis and attain good agreement between the two results with parameter tuning. We finally show, based on the comparison of experimental and numerical solutions, that the rate of free-ion generation (dissociation) should be less than the one predicted from the existing formula.

Dielectrophoresis flow control in microchannels

Abstract The control of flow in microscale is one of the most important problems in microfluidic devices, which in particular, are used as micro heat exchangers. The use of electric field is one of the efficient methods of control of dielectric liquid flow in microscale. The electric field influences liquid flow by the EHD force which affects liquid behaviour in terms of the flow rate and pressure. The EHD force consists of three components: the first is the electrostatic force due to free charges present in the liquid, the next one is the force due to the gradient of permittivity of material, and the third one is caused by the change in the electric field intensity. The EHD force is used also in many commercial devices, for example EHD pumps or dielectrophoretic separators. An own approach to apply the EHD force to control the liquid flow rate is presented in this paper. Authors paid a close attention to the dielectrophoresis effect. Dielectric liquid in a non-uniform electric field tends to drift/migrate towards the region of high electric field intensity. With decreasing the electrode dimensions, the dielectrophoresis force becomes relatively stronger. For the dimensions under 400 μm the dielectrophoresis phenomenon can be used for control and actuation of the liquid flow in microchannels. The originally developed design of such flow controller is presented in this paper. The experimental investigations covered flow rate measurement of 2-propanol in microchannel flow controller with application of AC field. It was showed that the dielectrophoresis phenomenon could effectively control the flow. The results for distilled water are also comparatively discussed in the paper.

Fully Coupled Finite Element Analysis for Cooling Effects of Dielectric Liquid Due to Ionic Dissociation Stressed by Electric Field

A fully coupled Finite Element Analysis (FEA) technique was developed and tested to validate the cooling effects of a dielectric liquid stressed by an electric field, resulting in the ionic dissociation phenomenon. Recently, electrohydrodynamics (EHD) techniques have been applied widely to enhance the cooling performance of electromagnetic systems by introducing gaseous or liquid media. The main advantage of EHD cooling is non-contact and low-noise resulting from smart control using an electric field. In addition, in some cases, the flow can be achieved using only a main electric field source not an extra one. The driving sources in EHD flow are ionization in the breakdown region and ionic dissociation in the sub-breakdown region. This study focused on dielectric liquid flow driven by the ionic dissociation phenomena, resulting in a cooling effect of the heat source. To build on this EHD phenomenon, fully coupled FEA, which consisted of the Poisson's equation for an electric field, Nernst-Planck equations for ions, and the Navier-Stokes equation for incompressible fluidic flow, was performed. To confirm the cooling effects, the developed velocities of fluidic flow were tested with the different applied voltages. In the sub-breakdown region, the effective velocity was approximately 2 m/s in the tip-sphere electrodes and a temperature drop of approximately 40 $^{\circ}$ C was obtained in a numerical analysis model with a fluidic velocity of 1.96 m/s from the inlet.

Competition between the bulk and the dissociation layer in electrohydrodynamic flow of dielectric liquid around coplanar electrodes

An experimental and numerical study has been conducted on the electrohydrodynamic flow around coplanar electrodes with a dielectric liquid: dodecane mixed with the surfactant Span 80. It is shown that the flow is asymmetric, although the electrode is symmetrically arranged, and numerically, we have shown that a difference in the ionic size can reproduce such asymmetric patterns. It is also found that the dissociation layer effect becomes more important in determining the flow pattern than is predicted from the conventional theory where the Langevin formula is used for the recombination constant. In numerical simulations, reducing the recombination constant to 0.035-0.055 times the Langevin value turned out to produce good comparisons between the experimental and the numerical results for the electrode pairs with 1 and 0.2 mm gaps.

Numerical simulation of the electro-convective onset and complex flows of dielectric liquid in an annulus

We conducted a numerical study on the onset of electro-convection as well as the complex flow phenomena of dielectric liquid subjected to unipolar autonomous charge injection in the annular gap between two concentric circular cylindrical electrodes. The Nernst-Planck equations governing the charge density transport, the Poisson equation for the electric potential and the Navier-Stokes equations for the fluid flow are solved numerically using the finite volume method. The developed code is validated by comparing the critical stability parameter values for the onset of electro-convection with those obtained from the linear stability analysis. We identify in a parameter space the stable hydrostatic state and the electro-convection state. The electro-convection is again divided into three regimes: stationary, oscillatory and chaotic. For inner cylinder radius ri ≥ 1.0, we observed an increase in the number of charged plumes and vortex pairs with stability parameter T before the electro-convection becomes chaotic. For outer injection, although the onset of electroconvection starts at T higher than the inner injection, the onset of chaotic motion occurs at lower T.

Three-dimensional study of heat and fluid flow of air and dielectric liquids filling containers partially heated from below and entirely cooled from above

In this work, a numerical procedure based on the finite volume method coupled with a full multigrid acceleration is utilized to study three-dimensional flow structures and heat transfer characteristics in partially heated 3-D containers having a heated strip on the lower horizontal wall. The opposite horizontal wall is cooled to a uniform low temperature and the four remaining walls are insulated. Two different fluids, one air ( Pr = 0.7) and the other a dielectric liquid ( Pr = 25) are employed encompassing descriptive Rayleigh numbers Ra that range three orders of magnitude from 10 3 to 10 6. Typical plots of streamlines and isotherms are presented to analyze the circulatory flow patterns set up by the buoyancy force of the fluids. At low Ra, the flow structures and average Nusselt numbers are similar for both fluids under consideration. On the contrary, for higher Ra, a systematic comparison illuminates heat transfer augmentation when the Prandtl number is switched from air to the dielectric fluid. At high Ra = 10 6, the air flow ( Pr = 0.70) becomes unsteady whereas the dielectric liquid flow ( Pr = 25) remains steady. At the end, monomial correlations are presented for the quantification of the heat transfer that emanates from the heated bottom strip in harmony with the various Rayleigh number associated with the two dissimilar fluids.

Electrization and liquid glow in a coaxial channel with dielectric walls

Dielectric-liquid flows in narrow channels with walls composed of different dielectrics are investigated experimentally. It is revealed that at a certain flow velocity, as a result of electrization, on the interface between the dielectrics the liquid begins to glow. The glow is discrete in the form of frequent pulses and is accompanied by electromagnetic noise on the radio-frequency range. It is shown that with decrease in the channel thickness the glow appears at smaller flow velocities. The glow is accompanied by heating of the liquid in the wall region to temperatures that may reach several tens of degrees. The electrization potential reaches more than 100 kV. The mechanisms of electrization and liquid glow are discussed.