Conduction Pumping Research Articles

Heat transfer enhancement and flow resistance reduction characteristics of non-uniform electrohydrodynamic conduction pumping in microchannels

In this study, the thermo-hydraulic performance of non-uniform electrohydrodynamic (EHD) conduction pumping in rectangular and straight microchannels was experimentally investigated. Electrodes are arranged in two ways: dimensions constant, spacing varies (Cases 1–3); both dimensions and spacing vary (Cases 4–6). The results show that EHD conduction pumping significantly enhances heat transfer performance, increasing the average Nusselt number (Nu) by 17.9 %-87 % compared to a smooth microchannel. As Re increases, the inertial force of the fluid dominates, diminishing EHD-induced vortices and limiting heat transfer enhancement. EHD conduction pumping enhances heat transfer by creating vortices that mimic solid pseudo-roughness microelements, increasing fluid disturbance and mixing near the wall. However, these vortices also impede the flow, resulting in a trade-off with increased pressure drop resistance. This paper innovatively adopts an arrangement of increasing electrode size (non-uniform), which effectively reduces the pressure drop resistance in the microchannel while maintaining the advantage of enhanced heat transfer. Cases 4, 5, and 6 exhibit significantly higher comprehensive heat transfer performance (η) than Case 0 (without electrodes), especially at low Re. While the inertial force at high Re suppresses vortex formation and heat transfer enhancement, η remains superior to Case 0. Notably, Case 5 maintains a minimum η of 1.18 even at Re = 1000. Furthermore, this paper proposes an equivalent slip drag reduction mechanism to elucidate the drag reduction effect of the increasing electrode size arrangement. Heat transfer augmentation and pressure drop reduction, as mentioned here in microchannels, are crucial for electronics cooling and microfluidic devices.

Demonstration of heat switch function of loop heat pipe controlled by electrohydrodynamic conduction pump

Loop heat pipes (LHPs) are attractive devices to cool electronic devices in spacecraft. The LHP operation must be shut down in some cases to avoid considerable reductions in the temperature of such electric devices. A new active technology capable of heat switching is developed for LHPs using an electrohydrodynamic (EHD) conduction pump that operates to facilitate liquid flow in the reverse direction against the normal LHP flow. To demonstrate our proposed approach, an EHD conduction pump with rod–ring electrodes was developed and evaluated to obtain generated pressure with varying applied voltages. The LHP with R134a as a working fluid was shut down successfully when a sufficiently high voltage was applied. The LHP shutdown continued until the EHD conduction pump was turned off. A comparison with a test in which a manually controlled mechanical valve caused the capillary limit showed that the EHD pump worked differently from the valve: the shutdown by the EHD pump involved a reverse flow. The consumed electric power for the EHD pump was 10–30 mW for 6–8 kV of applied voltage during the LHP shutdown. Furthermore, pressure loss through the EHD pump during normal LHP operation was approximately 0.5 % of the capillary pressure.

Electrohydrodynamic conduction pumping of viscoelastic dielectric liquids on the microscale

Electrohydrodynamic conduction pumping of viscoelastic dielectric liquids on the microscale

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.

Magneto-hydrodynamic characteristics of molten salts in a model-based high temperature superconducting conduction pump

The molten salt reactor which normally uses fluoride salts, chlorides and potassium salts as fuel and coolant has advantages in operation sustainability, economic value and security. It is regarded as an advanced nuclear reactor and is currently under development and design. One of the challenging issues for the reactor is their pumping system. Characteristics of molten salts allow them to be pumped by using electromagnetic force, but their low electrical conductivity requires high and compact magnetic field environment. In consequence, a DC electromagnetic pump with superconducting coils was designed and analyzed in this article. In order to study the dynamic behavior of molten salts under the interaction of the compact magnetic flux generated by superconducting coils with the iron yoke. A three-dimensional pumping system has been built. The superconducting coils are modeled by using the E-J constitutive law together with the T-A formulation based finite element method, while the studies of magneto-hydrodynamic characteristics of liquid molten salts are analyzed through the coupling of electromagnetism and fluid dynamics. Studies show that the electromagnetic pump has superior magneto-hydrodynamic properties with the operation of superconducting coils, under which condition liquid molten salts are driven smoothly through the channel of the pump.

Heat transfer intensification by EHD conduction pumping for electronic cooling applications

Liquid based cooling has become a new prospect in the thermal management of electronic devices. This work numerically investigates the potential of EHD conduction pumping to remove heat from a high density flux heat source, as commonly encountered in electronic cooling applications. A 2-D horizontal plate configuration with a heated patch releasing uniform heat flux has been considered. EHD conduction flow of dielectric fluid generated by the space charge developed due to the Onsager effect has been considered. The coupled set of governing equations are incorporated into the finite-volume framework of OpenFOAM®. The numerical implementation in OpenFOAM® is validated against experimental and numerical benchmark results. Present results exhibit good agreement with the benchmark solutions. Combined interaction of natural convective flow and EHD conduction pumping induced flow is studied. The effects of Rayleigh number and electric Reynolds number are investigated. The flow morphology of thermal convection is altered by the EHD conduction pumping flow. Flow strength is intensified and lead to heat transfer augmentation. Lower temperature distribution and higher mean Nusselt numbers are observed along the heated patch. The heat transfer intensification obtained by EHD conduction pumping is more prominent at lower Rayleigh numbers and higher electric Reynolds numbers. A maximum heat transfer enhancement of up to 31.2% is obtained for the configuration and parameters considered in this study.

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.

Plasma sprayed alumina-yttria composite ceramic coating for electrical insulation applications

A composite ceramic coating consisting of 50:50 weight fraction of alumina (Al2O3)-yttria (Y2O3) blend was deposited as an electrically insulating topcoat on Oxygen-Free Electronic Copper (OFEC) substrates with a cermet interlayer of 30 wt% NiAl-70 wt% Al2O3 blend, both deposited by Atmospheric Plasma Spray (APS) process, and with a NiCrAlY bond coat deposited by High-Velocity Oxy-Fuel (HVOF) process for application in Direct Current (DC) conduction pump of the sodium-cooled fast reactor. The cermet interlayer in between topcoat and bond coat is beneficial for a reduction in the thermal expansion mismatch stresses and further contribute to the improvement in the thermal fatigue life of the multilayer coating system. The coated samples were subjected to a thermal fatigue cycle between 600 °C and 200 °C, simulating sodium-cooled fast reactor temperature patterns in regular operation and transient conditions. The multilayer composite ceramic coating with cermet interlayer successfully endured up to 1000 thermal cycles with no cracking, spallation, or delamination of the topcoat. Surface cross-sectional micrographs before and after the thermal cycle revealed neither significant changes in the splat and lamellae morphology nor distribution of phases, except for the densification effects. X-ray Diffraction (XRD) analysis revealed neither phase change nor the formation of any new phase upon thermal cycling. The adhesion test as per ASTM-C633 revealed that the bonding strength of the coatings lower by ~20% after thermal cycling. The insulation resistance of thermal cycled samples showed a two order reduction in the magnitude, compared to that of the as-coated samples.

Numerical simulation and optimization of natural convection heat transfer enhancement in solar collectors using electrohydrodynamic conduction pump

This study investigated the application of electrohydrodynamic conduction pumping for heat transfer enhancement between two 45-degree diagonal plates containing a dielectric fluid, while there is a constant volume heat density inside the inner wall. A numerical simulation was performed to determine the optimum dimensional configuration of electrodes and to clarify how the heat transfer in the fluid between the electrodes is influenced by the electric field magnitude and volume heat density. At low applied voltages, a small flow vortex near the electrodes traps the heat and creates an obstacle blocking the natural convection, which leads to decreasing the efficiency of heat transfer. When the applied voltage is increased to 16kV, the heat trap begins to separate from the wall, which decreases the wall temperature and leads to a faster heat transfer. Therefore, the heat transfer efficiency suddenly increases. In comparison with the case without the electric field, the use of EHD technique can increase the efficiency of heat transfer through the operating fluid almost twice. This configuration of the electrohydrodynamic pump can be used for increasing the efficiency of solar collectors.

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.

Effects of working temperature and fluid physical properties on heat transfer enhancement in conduction pumping: An experimental study

In the current study, heat transfer behavior in electrohydrodynamic conduction pumping of two different dielectric liquid films using flush electrodes has been experimentally studied, and effects of working temperature and fluid physical properties on heat transfer enhancement are investigated. The study is conducted for different liquid film thicknesses and working temperatures, while changing the magnitude of the applied electrical voltage. Results show that higher ion mobility difference and working temperature of the dielectric liquid, as well as lower density and viscosity increase flow rate and heat transfer of the pump. Also, it is seen that heat transfer coefficient is intensively enhanced for all different film thicknesses and working temperatures by using the conduction pump, which is due to the turbulent flow created in the vicinity of the electrodes. For 8 mm film thickness of n-hexane at35°C, the significant enhancement of heat transfer coefficient equal to 1040.9% was observed.