Electrohydrodynamic Conduction Research Articles

Upstream Electrohydrodynamic Conduction Pumping for Flow Distribution Control of Parallel Microchannel Evaporators

Abstract Flow boiling in mini and microchannels has become an attractive option for many applications, such as compact and low charge heat exchangers. Microchannel heat exchangers, however, are more susceptible to maldistribution between parallel flow channels. When operating during uneven heat load conditions, the maldistribution becomes even more severe. Electrohydrodynamic (EHD) conduction pumping technology offers an innovative way to redistribute flow between parallel branches in a microchannel heat exchanger and is also being explored as a next generation mechanism of microgravity heat transport. In EHD conduction pumping, a strong electric field interacts with dissociated electrolytes in dielectric fluid to generate a net body force, and thus, a net flow, with no moving parts, no acoustical noise, lower power consumption, and the ability to operate in microgravity. An EHD conduction pump was designed, fabricated, and tested for upstream flow distribution control of a parallel microchannel evaporator in an opposing configuration. Flow redistribution capability was measured at system flowrates up to 6 ml/min. The EHD conduction pump was capable of completely blocking and reversing the flow in its branch. Recovery from near critical heat flux conditions up to a maximum heat flux of 77.5 W/cm2 was also demonstrated for the operating conditions and design of this study. This was achieved in the absence of enhanced surfaces. The working fluid is HFE 7100. The results show that EHD conduction is able to effectively control the flow distribution of the microchannel evaporator, however, its effectiveness decreases with increasing heat flux and flowrate.

Working Regime Criteria for Microscale Electrohydrodynamic Conduction Pumps.

We investigated the microscale electrohydrodynamic (EHD) conduction pumps in a wide range of working regimes, from the saturation regime to the ohmic regime. We showed that the existing macro- and microscale theoretical models could not accurately predict the electric force of microscale EHD conduction pumps, especially for the cases of a strong diffusion effect. We clarified that the failure is caused by a rough estimate of the heterocharge layer thickness. We revised the expression of heterocharge layer thickness by considering the diffusion effect and developed a new theoretical model for the microscale EHD conduction pumps based on the revised expression of heterocharge layer thickness. The results showed that our model can accurately predict the dimensionless electric force of the microscale EHD conduction pumps even for the cases of a strong diffusion effect. Furthermore, we developed a working regime map of microscale EHD conduction pumps and found that the microscale EHD conduction pumps more easily fall into the saturation regime compared with the macroscale EHD conduction pumps due to the enhanced diffusion effect; in other words, the microscale EHD conduction pumps have a wider saturation regime. We showed that the conduction number C0 could not distinguish the working regime of the microscale EHD conduction pumps because it does not take the diffusion effect into account. By employing the revised expression of heterocharge layer thickness, we proposed a new dimensionless number, C0D to distinguish the working regimes of microscale EHD conduction pumps.

Effect of ionic diffusion on microscale electrohydrodynamic conduction pumps

In this work, we investigate microscale electrohydrodynamic (EHD) conduction pumps of dielectric liquids in a wide working regime range from the Ohmic to the saturation regime. We show that the electric force of microscale EHD conduction pumps differs from that of macroscale EHD conduction pumps owing to the scale effect. We reveal that the scale effect of microscale EHD conduction pumps is triggered by the enhanced ionic diffusion. When the characteristic length of the system reduces from millimeter to micrometer, the maximum order of magnitude of α increases from O(10−5) to O(10−1), resulting in significantly enhanced ionic diffusion; however, the ionic diffusion can be neglected for macroscale EHD conduction pumps, where α denotes the ratio of ionic diffusion velocity to ionic migration velocity. We found that the existing macroscale theoretical models do not work for the microscale EHD conduction pumps due to the rough estimation of heterocharge layer thickness. We show that the heterocharge layer thickness of microscale EHD conduction pumps not only depends on ionic migration but also relies on ionic diffusion. By modifying the expression of the heterocharge layer thickness with respect to ionic migration and diffusion, we develop a theoretical model of dimensionless electric force for the microscale EHD conduction pumps.

Thermal boundary layer depletion in minichannels by electrohydrodynamic conduction pumping

The article presents a numerical analysis of two-dimensional laminar flow and heat transfer in a minichannel with flat plate electrode pairs periodically flushed to the bottom wall. The two-way coupled governing equations of flow, heat transfer, electric potential and charge conservation are solved using the opensource finite-volume framework of OpenFOAM. Two cases of the minichannel at completely horizontal (case A) and 90° bent (case B) configurations are considered. The inlet laminar flow with Reynolds number varying from 10 to 1000 has been taken into account. The applied electric potential is varied in three steps 0, 0.5 and 1 kV. Electrohydrodynamic (EHD) conduction pumping due to the residual conductivity of the working fluid and the electric-field enhanced dissociation of free charges generate vortices in the proximity of electrodes. The flow and thermal performance of the minichannel in the presence of EHD conduction induced vortices are quantified in terms of pressure drop, Nusselt number and a performance factor (which is a combined function of pressure drop and Nusselt number). The vortices generated by EHD conduction pumping phenomenon deplete the thermal boundary layer, aid mixing and enhance the heat transfer. The mean and local Nusselt numbers are notably increased with only a minimal raise in pressure drop. In both the configurations of the minichannel, the performance factor is greater than 1 for the entire parameter space considered in this study. However, the thermal boundary layer depletion and heat transfer enhancement is prominent at low Reynolds number flows. The maximum drop in performance factor from case A to case B is only 18.5%. Results of this study indicate that EHD conduction pumping is a potential technique to enhance heat transfer in minichannels with minimum additional power consumption.

Dielectrophoretically-Assisted Electrohydrodynamic-Driven Liquid Film Flow Boiling in the Presence and Absence of Gravity

Abstract The ongoing development of modern electronic systems leads to smaller, more powerful devices that are expected to operate in complex environments. Due to this, advanced thermal management technologies are required to meet the growing demand, especially in space where two-phase thermal systems are limited by the absence of gravity. Electrohydrodynamic (EHD) and dielectrophoretic (DEP) forces can be used to sustain stable liquid film flow boiling in the absence of gravity, which is otherwise impractical due to the lack of a required buoyancy force to initiate bubble departure. EHD is a phenomenon that is represented by the interaction between electric fields and fluid flow. The DEP force is characterized by its ability to act on liquid/vapor interfaces due to a high gradient of electrical permittivity. This study investigates the heat transfer characteristics of EHD conduction pumping driven liquid film flow boiling coupled with DEP vapor extraction during a microgravity parabolic flight and on the ground. The results of this study show that EHD and DEP raise the critical heat flux, lower heater surface temperature, and successfully sustain boiling in both microgravity and on the ground with low power consumption. Additionally, the heat transfer data captured in terrestrial, microgravity, and 1.8 g conditions compare well, indicating that combining these mechanisms can provide thermal enhancement independent of gravity. This study provides fundamental understanding of electrically driven liquid film flow boiling in the presence of phase change, paving the way toward developing next-generation heat transport devices for space and terrestrial applications.

Electrohydrodynamic conduction induced convection in a film with interface charge

Film transportation driven by Electrohydrodynamic(EHD) conduction pumping has become a new prospect in many micro-scale industrial applications. This work numerically studied the electro-convection in a film induced by EHD conduction pumping. The effect of the interface charge on the heterocharge layer and the flow pattern has been carefully investigated. A 2-D geometry with flush electrodes situated at the bottom of a cavity was considered, and the EHD conduction flow was generated by both space charge and interface charge with the Onsager-Wien effect. Results show that the height of the heterocharge layer decreases due to the attraction by the interface charge. The interface charge also affects the flow pattern and contributes to the enhancement of the overall flow intensity. The average velocity of the flow field increases by 24.1% and the total kinetic energy increases by 44.1% within the parameters studied in this paper.

Overview of electrohydrodynamic conduction pumping

<p indent="0mm">This article describes an emerging fluid-flow generation and control technology, namely, electrohydrodynamic (EHD) conduction pumping. This pump is a nonmechanical-type pump based on the direct interaction of electric field and dielectric liquid. The driving force of the pump is the Coulomb force, and its free charge comes from the dissociation process of impurities or weak electrolytes in the liquid. In addition to the inherent advantages of EHD conduction pumps (such as intelligent active control via an electric field, absence of mechanical moving parts, simple design, light weight, suitability for special environments such as low-gravity spaces, and low energy consumption), EHD conduction pumps provide stable performance, long working life, and other unique advantages. In addition to traditional applications such as flow control, heat-transfer enhancement, and enhanced mixing, these conduction pumps have found a variety of innovative applications in the fields of micro-robot drive, wearable devices, and other fields in recent years. Currently, conduction-pump technology in the international community has gradually developed from macro-scale single-phase fluid to micro-scale multi-phase flows and has started to be tested in a low-gravity environment in orbit, which is expected for application in future spacecraft space-thermal management and other fields. However, research on electrohydrodynamics in China has started late, and studies on conductance-pump technology have been lacking. Therefore, this review introduces the EHD conduction-pump technology in detail from the aspects of its working principle, theoretical model, mathematical equations, research status, and recent progress, as well as in terms of future perspectives.

Numerical investigation on electrohydrodynamic conduction pumping with an external flow

As an advanced flow control and generation technology, electrohydrodynamic (EHD) pumping has received extensive attention in recent years. EHD conduction pumping is a type of EHD pumping that makes use of electric Coulomb force produced by an applied electric field and heterocharge layers formed near the electrodes in dielectric liquids. In this paper, we numerically investigated the flow characteristics of EHD conduction pumping with an external flow. A two-dimensional flushed electrode configuration with the external flow was considered. The coupled series of governing equations, which includes the charge conservation equations and the Poisson equation for the electric potential, are integrated into the finite-volume framework of the open-source OpenFOAM. The effects of external flow's strength and flow direction on the heterocharge layers and the flow field are investigated. The strength of external flow is shown by the Reynolds number varying between 0 and 40. It is found that with the strong external flow, the structure of heterocharge layers is seriously deformed toward the downstream. In addition, the maximum velocity generated in the flow field of the co-current flow is greater than that of the countercurrent flow.

Electrohydrodynamic conduction pumping of a viscoelastic dielectric fluid with the Onsager–Wien effect

Due to the Onsager–Wien effect (enhanced electric field dissociation), electrohydrodynamic (EHD) conduction pumping problems in the Oldroyd-B viscoelastic dielectric liquid are solved using the finite volume method. Fully coupled nonlinear equations, including the Naiver–Stokes (N–S) equation, the Nernst–Plank equation of the ion conduction mechanism, and the constitutive equation of viscoelastic fluid, are thoroughly implemented. Our numerical model is fully verified with two benchmark cases, and simulations are performed under different conduction numbers C0 (0.01–103), Weissenberg numbers Wi (0.01–50), and viscosity ratios (0.3–0.9). Two limiting operating states of EHD conduction pumping, the Ohmic and the saturation regimes, are discussed in detail. Elastic instability is more likely to occur in the Ohmic regime, and the thickness of the heterocharge layer (called the dissociation layer) remains sensitive to C0 and the elasticity values. Current density and pressure generation show crucial degrees of dependence on the fluid elasticity and viscosity ratio.

The interaction between two electrohydrodynamics phenomena when an electric field affects a two-phase immiscible liquid

Electrohydrodynamic (EHD) phenomena in two-phase immiscible liquids—droplet electrodeformation and electrical coalescence—underpin several technologies, e.g., electrostatic phase separation of water-oil emulsions. The corresponding devices' development calls for a deeper understanding of the underlying physics and reliable mathematical and numerical models. However, another EHD phenomenon that has to take place under the same conditions and can affect the results is unfairly overlooked. The phenomenon is the emergence of near-interface dissociation–recombination layers and EHD conduction pumping inside the dielectric phase. The present paper considers the electrical deformation of a conducting droplet suspended in a low-conducting liquid. A numerical simulation is used to show an EHD conduction pumping (like that emerging near an electrode in low-conducting fluid) to emerge near the interface of two immiscible liquids when one of them is a low-conducting medium. The space charge that causes this EHD flow also affects the Coulomb force applied to the interface and therefore changes droplet electrodeformation. Moreover, both the increase and decrease of electrodeformation are possible outcomes, which can be explained by a concurrent effect on the electric field distribution of charged layers emerging near both the interface and electrodes.

Numerical study of micro-scale EHD conduction pumping: The effect of pump orientation and flow inertia on heterocharge layer morphology and flow distribution control

Innovative technological solutions for thermal management of components at small scales, such as electrohydrodynamic (EHD) conduction pumping, are being developed to enable the next generation of miniaturized high density electronics. In EHD conduction, a strong electric field is applied via asymmetric submerged electrodes in a dielectric liquid. The field enhances the dissociation of electrolytic impurities present within the fluid, generating ions that migrate to form heterocharge layers over each electrode. The subsequent abundance of asymmetrically distributed space charge generates a net Coulomb force that is applied on the fluid, producing a net flow in the direction of the force. EHD conduction pumps have no moving parts, consume very little power, and have simple, flexible designs that can be easily miniaturized to the micro-scale. This study numerically investigates the heterocharge layer morphology of EHD conduction pumping used in flow distribution control between parallel micro-scale branches, using different pumping orientations and accounting for flow inertia effects. The results are qualitatively compared with available experimental data and serve to explain observed behaviors.

A lattice Boltzmann modelling of electrohydrodynamic conduction phenomenon in dielectric liquids

In this work, we propose a lattice Boltzmann method to model the electrohydrodynamic (EHD) conduction phenomenon in dielectric liquids. To incorporate the effect of the source term appeared in the charge conservation equations, some unified force terms that can be implemented locally are delicately designed in the charge evolution equations. We further validate the present model against some analytical solutions as well as experimental data for several problems, and a good agreement is obtained. Then it is used to study the fluid circulation in a square cavity generated by EHD conduction phenomenon, and the numerical results indicate that the geometric scale and the difference in the mobilities of the positive and negative charges could play an important role in studying EHD conduction phenomenon. Further, to analyze the characteristics of the EHD conduction mechanisms in the ohmic and saturation regimes, the influences of the conduction number W0 as well as the parameter β, which is related to Onsager effect, are also investigated, and the numerical results fit well with previous work.

Experimental Study of Flexible Electrohydrodynamic Conduction Pumping for Electronics Cooling

Abstract As modern-day electronics develop, electronic devices become smaller, more powerful, and are expected to operate in more diverse configurations. However, the thermal control systems that help these devices maintain stable operation must advance as well to meet the demands. One such demand is the advent of flexible electronics for wearable technology, medical applications, and biology-inspired mechanisms. This paper presents the design and performance characteristics of flexible electrohydrodynamic (EHD) pumps, based on EHD conduction pumping technology in macro- and mesoscales. Unlike mechanical pumps, EHD conduction pumps have no moving parts, can be easily adjusted to the microscale, and have been shown to generate and control the flow of refrigerants for electronics cooling applications. However, these pumping devices have only been previously tested in rigid configurations unsuitable for use with flexible electronics. In this work, for the first time, the net flow generated by flexible EHD conduction pumps is measured on a flat plane in various configurations. In this study, the results show that the flexible EHD conduction pumps are capable of generating significant flow velocities in all size scales considered in this study, with and without bending. This study also proves the viability of screen printing as a manufacturing method for these pumps. The selection of working fluid for EHD conduction pumping is also a topic of discussion. Novec Engineered Fluids have been a popular choice for EHD pumping; however, long-term testing has shown that some Novec fluids degrade over time.

In-depth description of electrohydrodynamic conduction pumping of dielectric liquids: Physical model and regime analysis

In this work, we discuss the fundamental aspects of Electrohydrodynamic (EHD) conduction pumping of dielectric liquids. We build a mathematical model of conduction pumping that can be applied to all sizes, down to microsized pumps. In order to do this, we discuss the relevance of the Electrical Double Layer (EDL) that appears naturally on nonmetallic substrates. In the process, we identify a new dimensionless parameter related to the value of the zeta potential of the substrate-liquid pair, which quantifies the influence of these EDLs on the performance of the pump. This parameter also describes the transition from EHD conduction pumping to electro-osmosis. We also discuss in detail the two limiting working regimes in EHD conduction pumping: ohmic and saturation. We introduce a new dimensionless parameter, accounting for the electric field enhanced dissociation that, along with the conduction number, allows us to identify in which regime the pump operates.

Influence of Electrode Spacing on a Symmetrical Washer-Type Electrohydrodynamic Conduction Pump

Electrohydrodynamic (EHD) pumping is an attractive method that can be applied to drive a fluid in mass transport and heat transfer enhancement applications, and more particularly for microfluidic applications. EHD pumps are very interesting because they do not require moving parts. The electric energy is directly converted into kinetic energy via the Coulomb force. Three modes can generate this fluid motion: conduction, induction, and injection. This paper presents the experimental results of a parametric investigation on EHD conduction pumps with washer-type electrode geometry. Only symmetrical electrode configuration is investigated in order to highlight the influence of the positive/negative charge mobility ratio. The working fluid is a dielectric liquid (HFE-7100). The pumping mechanism is examined with several washer-type electrode geometries and different types of spacers. For each configuration, pressure and current time variations are recorded and compared.

Theoretical and experimental modeling of EHD conduction in porous conductive material inside a tube

Laminar forced convection heat transfer in an annulus partially filled with copper foam is enhanced using electrohydrodynamics (EHD) technique. A mathematical model for simulation of EHD conduction mechanism in an electrically conductive porous zone is developed. The flow patterns of the induced flow and electroconvection heat transfer are investigated using numerical simulation. An experimental setup is constructed to investigate heat transfer enhancement in the double pipe with metal foams attached to the inner pipe. The insertion of metal foam results in an effectiveness of 4.2 and the applied voltage of 24 kV increased the effectiveness parameter to 5.1.

Enhancement of convection heat transfer using EHD conduction method

Laminar forced convection heat transfer in an annulus is enhanced using electrohydrodynamics (EHD) technique. A novel EHD-enhanced double pipe heat exchanger is designed and incorporated with six pairs of electrodes. An experimental setup is constructed to investigate heat transfer enhancement, pressure drop and power consumption in the double pipe. The finite volume method is employed to solve the governing equations of EHD conduction pumping, Navier-Stokes and energy. The numerical results are in agreement with the experimental results and demonstrated that in the considered range of the parameters, heat transfer enhances up to 50% with low electric power consumption and the method is more effective in lower Reynolds numbers. An effectiveness parameter is introduced which considers both heat transfer enhancement and pumping power consumption. Increasing applied voltage will improve performance until an optimum voltage which generates a cross-stream flow comparable to main-stream inlet flow. The flow patterns of the induced cross-stream flow are investigated using numerical simulation.

Electrohydrodynamic Conduction Pump with Asymmetrical Electrode Structures in the Microchannels

In this study, we have developed a microfluidic pump that utilizes the electrohydrodynamic (EHD) conduction of a working fluid. Ring-shaped pumps have been used in previous studies on EHD conduction, but these require a three-dimensional arrangement of electrode pairs, which makes it difficult to downsize the apparatus. Here, we propose a mechanism to achieve one-way fluid flow in the microchannels by arranging non-parallel electrodes in a plane to generate an asymmetric electric field. One advantage of this design is that it can be easily and precisely fabricated using microelectromechanical system (MEMS) processing techniques: this has allowed us to integrate a micropump and a microchannel into a single device. Moreover, the pressure generated by the pump is induced solely by electrochemical reactions; since mechanical components such as gears are not required, this helps reduce the noise generated by the device.

Flow Distribution Control in Meso Scale via Electrohydrodynamic Conduction Pumping

Electrohydrodynamic (EHD) conduction pumping technology offers an innovative way to control flow distribution in multiscale environments. In EHD conduction, the interaction between a strong electric field and dissociated electrolyte species in a dielectric fluid generates a net body force and therefore a net flow. EHD conduction pumps have simple designs with no moving parts, low power consumption, and the ability to operate in microgravity. These pumps perform better at smaller scales and have been shown to be effective for heat transfer enhancement, with possible applications in electronics cooling terrestrially and in space. Flow distribution control using EHD conduction pumps was previously examined only in macro scale. This study experimentally and numerically examines isothermal liquid flow distribution control between parallel tubes 1 mm in diameter, utilizing EHD conduction pumps in meso scale. The working fluid is Novec 7600 Engineering Fluid operated at ambient conditions.

Combined Dielectrophoretic and Electrohydrodynamic Conduction Pumping for Enhancement of Liquid Film Flow Boiling

This paper extends previous liquid film flow boiling studies by including the effect of an additional electrohydrodynamic (EHD) force, namely, the dielectrophoretic (DEP) force. Rather than using only EHD conduction pumping of the liquid film to electro-wet the heater surface, a localized nonuniform electric field above the heater surface is established to generate a DEP force for improved vapor bubble extraction during the nucleate boiling regime. The effects of liquid film height and applied potential are studied as a function of heater superheat and heat flux. A brief analytical study is also used to estimate the expected DEP force magnitude to explain the results. All of the above studies are also used to quantify the enhancement in heat transfer that can be achieved when heat transport systems are driven or augmented by these two EHD mechanisms. The results show remarkable enhancement of up to 1217% in boiling heat transfer coefficient at a given superheat when both mechanisms are used simultaneously. The experimental data are important for applications in thermal management in terrestrial and space conditions.