Enhanced ionic wind generation by graphene for LED heat dissipation

High voltage is an obstacle when applying electrohydrodynamics technology. Nanomaterials are good candidates for its extraordinary electrical properties and heat‐conducting characteristics. An originally designed ionic wind cooling system was secured with graphene using the dip‐coating method to study the cooling effect for the high‐power light‐emitting diodes. The experiment results indicated that the graphene on the needles' surface acted as new emitting electrode with a smaller curvature radius. The corona discharge current density increased, and the ionic wind inception voltage decreased, because of the high aspect ratios and the field emission characteristic of graphene. The maximum ionic wind volume velocity was improved by 41.3% when the discharge gap was 10 mm, which was attributed to the local electric field enhancement and the electron field emission effect that the graphene had. The best cooling performance was obtained when the needles and the heat sink were both coated with graphene. The junction temperature decreased 21%, and the luminous flux increased 5.6% at the discharge gap of 15 mm, taking the electrostatic screening effect into consideration.     

线-板结构离子风发生器强化通道内对流换热的数值研究

离子风发生器是利用电晕放电引起空气流动的一种装置。利用COMSOL Multiphysics软件建立了基于线-板电极结构离子风发生器的计算模型,对离子风发生器强化通道内对流换热的能力开展了数值模拟研究。针对入口风速、发射极电压、初速度方向以及电极水平间距这4个关键因素进行研究,分析了4种因素对强化换热效果的影响。模拟结果表明:在入口风速较小时,离子风对换热的强化效果更好;当发射极电压较高,离子风射流和主流方向相反时,离子风的扰动作用更剧烈,对换热过程有更加显著的增强作用。而电极水平间距则存在最优值,可以使换热效果达到最优。     

Optimized electrode arrangement in solar air heater

Laminar forced convection inside the solar air heater with various wire electrode arrangements are numerically examined for heat transfer enhancement using electrohydrodynamic technique. The electric field is generated by the wire electrodes charged with DC high voltage ranging from 7.5 to 17.5 kV. Reynolds number corresponding to the flow considered is between 100 and 2000. The numerical modeling of computational fluid dynamics includes the interactions among electric field, flow field, and temperature field. It is found that the enhancement of heat transfer coefficient with the presence of electric field increases in relation with the supplied voltage but decreases when the Reynolds number and the distance between electrode and channel surface are augmented. The optimized electrode arrangement, which obtains the best heat transfer enhancement is investigated incorporating with the pressure drop consideration. The heat transfer enhancement is also depended on the number of electrodes per length and the channel dimension.     

Effect of electric field on heat transfer performance of automobile radiator at low frontal air velocity

The effect of electric field on the performance of automobile radiator is investigated in this work. In this experiment, a louvered fin and flat tube automobile radiator was mounted in a wind tunnel and there was heat exchange between a hot water stream circulating inside the tube and a cold air stream flowing through the external surface. The electric field was supplied on the airside of the heat exchanger and its supply voltage was adjusted from 0 kV to 12 kV.From the experiment, it was found that the unit with electric field pronounced better heat transfer rate, especially at low frontal velocity of air. The correlations for predicting the air-side heat transfer coefficient of the automobile radiator, with and without electric field, at low frontal air velocity were also developed and the predicted results agreed very well with the experimental data.     

电晕离子风湿烟羽治理机理研究及计算分析

为分析电晕离子风除湿的机理,探究不同电压下等离子场与流场的分布以及影响收水效果的因素,利用COMSOL软件进行数值模拟,计算了电除雾器一维等离子场,得到了电子与离子的分布规律和空间电荷密度;通过附加体积力的方法,计算了不同条件下离子风的分布情况和变化规律;选取内置层流混合物模型进行两相流模拟,分析电场对电除雾器收集液滴的效果。研究表明:阳离子大量集中在电晕极附近,而阴离子则充满电晕外区,使得电除雾器内大部分区域充满了负电荷;由于阴离子受电场力而形成的离子风,在电除雾器内将空气吹卷至筒壁,而且随着电势差的增大,空间电荷密度越大,离子风速度越大。通过对比施加电场前后除雾器出口液滴浓度,发现离子风使得出口水汽量减少近25.8%,验证了电晕离子风除湿的可行性。     

Flow control with electrode bank arrangements by electrohydrodynamics force for heat transfer enhancement in a porous medium

Active flow control with electrohydrodynamics (EHD) force in the channel flow has been numerically investigated for enhancing heat transfer. This study focuses on the effect of electrode bank arrangements and the number of electrodes on corona wind and fluid flow for heat transfer onto a porous medium. Aligned and staggered configurations of electrode banks are compared. The numerical results show that electric field intensity depends on electrical voltage and the number of electrodes. Shear flow is increased with larger numbers of electrodes and in the aligned configuration, resulting in the enhancement of vortex strength. The swirling flow from staggered configurations spread wider than that of aligned configurations, but the aligned configuration produced more turbulence. In addition, the temperature distribution in the channel flow is increased with increasing numbers of electrodes. With the effect of swirling flow, airflow above the porous sample surface is faster leads the heat to more transfer to the porous sample surface. This causes the temperature of porous medium to increase rapidly so the convective heat transfer coefficient on porous medium surface is increased. Finally, the modified case of the numerical results is validated against the experimental results. The experimental flow visualization is based on the incense smoke technique, in order to verify the accuracy of the swirling flow pattern subjected to the electric field. It is shown that the comparison results in both techniques are in good agreement.     

On the enormous enhancement in water evaporation rates obtained by employing a different polar ambient

Enhancing evaporation rates are of great practical interest in many technological applications, such as water desalination or drying in industry. Evaporation mass flux is a function of ambient temperature, relative humidity, and velocity of the air passing over it. Since the conventional methods of increasing evaporation, namely increasing temperature, surface area, and so on, may not be always practically feasible or economical, novel methods of evaporation enhancement are necessary. The current study proposes that the introduction of a polar gas ambient above an evaporating liquid can drastically enhance its evaporation rate when the dipole moments of the evaporating liquid and the ambient gas are comparable. Thus, we explore the possibility of evaporation enhancement of water by introducing a polar Tetrafluoroethane gas ambient. The weight loss of water due to evaporation as well as the ambient temperature and air circulation conditions were measured. For each case, a companion experiment was carried out simultaneously under the same ambient conditions but without the presence of polar gas ambient. Each experiment lasted for 4 h. The evaporation rate has been found to be crucially dependent upon the polar gas ambient, the temperature as well as circulation conditions. Depending upon the temperature and circulation conditions, an enhancement of 59%–373% in evaporation rate has been recorded by the introduction of tetrafluoroethane gas ambient over the water surface. Some insights into the underlying mechanisms have been suggested in light of the physics of the evaporation process. Finally, some comparisons in the measured enhancement in evaporation rates under different experimental conditions are observed to concur with the suggested physical mechanism.     

Three-Dimensional Electrohydrodynamic Enhanced Water Evaporation Using Needle-Arrayed Electrodes

Abstract

A study was made of the electrohydrodynamic (EHD) effect on the water evaporation rate of a channel flow with a needle-arrayed electrode system using both numerical and experimental methods. The local charged density, flow, and concentration distributions and their growth along the flow stream of a rectangular channel were described in detail. The results showed that the EHD effect on the evaporation rate increased with increases in applied voltage and electrode pitch and decreased in electrode height. For example, at an applied voltage of 16?kV, the mass transfer enhancement was 1.5 times than that of the electrode pitch from 50 to 100?mm at an electrode height of 20?mm. The mass transfer enhancement was twice as great for electrode height from 20 to 15?mm at an applied voltage of 12?kV and pitch of 100?mm. The present needle-arrayed electrode system results were compared with a five-wire electrode system based on the same geometric model and boundary conditions. It was concluded that the needle electrode cases achieved better mass transfer gains than the wire electrode cases for the present evaluated cases. Finally, the numerical results obtained satisfactory consistency with experimental data with a maximum error of 25%.     

基于高压电晕除湿的烟气消白技术参数优化设计研究

为了探究离子风除湿机理,获得影响除湿效率的关键参数,基于ANSYS CFX均质成核理论建立了除湿过程的三维空间多场耦合多相流数值模型,分析了集电极筒结构、流动、物性等参数变化对除湿效果的影响。基于可视化后处理软件,分析了电/流场耦合作用下流场速度、液滴的分布规律,并进一步阐明离子风除湿机理和除湿过程。结果表明：渐扩形集电极筒效果最佳但难以并联组合;随着进口流速的增加除湿效果迅速减弱;温差的增大显著提高了饱和蒸汽凝结的效率;相比于入口饱和蒸汽,气液两相入口明显改善了圆柱形集电极筒内的整体凝结程度,且核心凝结区域不仅限于出口处的U形区域;而电压的增强可以促进气相凝结,但是电压超过38 kV之后,液相平均质量分数变化幅度很小;增加电极长度可以提高除湿效果,但当电极长度达到0.37 m时,继续增加电极长度液相平均质量分数没有显著变化。     

Heat transfer of air-water dispersed flow in a vertical pipe

Heat transfer of air-water dispersed flow in a vertical heating pipe and its enhancement have been studied. The axial and circumferential wall temperature distributions were measured using various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular streamwise location, with a notable variation in the circumferential profile. This sharp increase was conceivably caused by a breakdown of the water film rather than by its dryout. A separate unheated experiment was carried out to estimate the droplet deposition velocity and the water-film flow rate. A numerical analysis, taking into account heat and mass transfer from the water film to the bulk flow, was performed in order to estimate the mean wall temperature. Good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times larger than that for single phase air flow. This enhancement was shown to be due mainly to evaporation of the water film. The mechanism of heat transfer enhancement is discussed in detail using the numerical analysis results. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 255–270, 1998     

等离子点火系统数值模拟计算

本文用等效的内部加热区代替产生欧姆热的电弧,采用k—ε双方程湍流模型进行数值模拟,对等离子点火装置内的气体流场进行计算。通过改变不同的工况,分析了进气速度,电功率对电极斑点附近的电极表面温度及流场温度的影响。计算表明增加气流量可以有效提高电极的冷却效果,也是提高电极冷却效果的最佳途径;但增加气流量不但使喷嘴出口气流平均温度降低,而且还会增加电源功率,同时也容易使等离子弧进入湍流区。通过对数值模拟结果的分析,提出了等离子发生器的结构设计的合理建议。     

Electric-field-induced enhancement of vapour condensation heat transfer in the presence of a non-condensable gas

This paper presents the results of an experimental investigation of heat transfer in film vapour condensation from a vapour-gas mixture on a vertical plate under the influence of an electric field. It is shown that with a gas concentration in vapour below 10% a uniform electric field should be applied, and at higher concentrations a corona discharge should be used. The heat transfer augmentation is found to be determined by the electric hydrodynamic processes just as on the surface of a condensate film in the form of a rearranging wave structure and condensate splashing, that decrease the condensate film thickness, so over the volume of the vapour-gas mixture which is stirred by condensate droplets and, to a greater extent, by the corona discharge electric wind. The effects of gas concentration in vapour, of medium pressure, temperature difference between a vapour-gas mixture and a wall, difference of potentials, electric current strength, physical properties of a liquid phase and of a vapour-gas mixture on the degree of heat transfer enhancement are investigated. A seven-fold increase of the relative heat transfer coefficient in the conditions of corona discharge effect is obtained the development of which is favoured by the maximum gas concentration and minimum temperature differences.     

A computational fluid dynamics modeling of natural convection in finned enclosure under electric field

Numerical modeling of the electric field effect on natural convection in the square enclosures with single fin and multiple fins is investigated. The interactions between electric, flow, and temperature fields are analyzed using a computational fluid dynamics technique. The parameters considered are the supplied voltage, Rayleigh number, size of enclosure, electrode arrangement, number of fins, and fin length. It can be concluded that the flow and heat transfer enhancements are the decreasing function of Rayleigh number. Moreover, the heat transfer coefficient is substantially improved by the electric field effect especially at the high number of fins and long fin length. Surprisingly, the maximum average velocity and heat transfer enhancement occur at the different electrode arrangements for the single fin and multiple fins.     

电除尘器内电晕放电流体场的数值模拟

在静电除尘器电晕放电场中,电晕放电将空气中原子、分子电离,并将小颗粒荷电,在电场和其他附加场的作用下形成电流体流动．在阴极电晕放电下,阴离子向阳极移动,最终离开电晕区．在电晕区外。阴离子朝收尘极运动,沿途不断撞击中性气体分子．通过这种碰撞,动量从离子传递给了中性气体分子,从而引起越来越强的气体运动．因此,就形成了电场、离子流场、带电气体粒子气流场相互作用的电流体场。讨论如何用FEMLAB软件建立平板型电除尘器内电流体场理论模型。利用三种应用模式建立耦合方程系统,并选择恰当的边界条件：在离子漂移区（电晕外区）,应用静电模型描述空间电势;应用PDE方程系数模型描述电荷流动;利用不可压缩Navier-Stokes模型描述流体运动。     

EHD enhanced heat transfer in wavy channel

Heat transfer enhancement with electrohydrodynamic (EHD) technique in laminar forced convection inside a wavy channel with different wire electrode arrangements is numerically investigated. The electric field is generated by the wire electrodes charged with DC high voltage. The mathematical modeling includes the interactions among electric field, flow field, and temperature field. The simulation is firstly conducted with the experimental data in case of rectangular flat channel and the results agree very well. Then the modeling is carried out in the case of wavy channel. It is found that the heat transfer coefficient with the presence of electric field increases with the supplied voltage but decreases when the Reynolds number and the distance between the wire electrodes and the wall surface are augmented. The heat transfer enhancement is also dependent on the number of the wire electrodes, the number of wave per length, and the wave aspect ratio.     

不同气压下N_2~+离子迁移率对直流正电晕放电特性的影响

离子迁移率是高海拔地区电晕放电的关键参数之一,其取值大小对于考虑海拔因素的电晕计算模型具有重要意义,因此有必要研究不同气压条件下离子迁移率的变化对电晕放电的影响。采用5mm棒-板间隙模型,棒电极施以15kV直流电压,板电极接地,并在流体动力学模型的基础上加入9种粒子间化学反应。结果表明,N_2~+离子迁移率的增大会导致电晕头部离子数密度的增加,从而加强对间隙电场的畸变作用,轴向最大场强在接近板电极时会随之减小;N_2~+离子迁移率的增大也会降低电晕平均发展速率;海拔越高N_2~+离子迁移率对电场分布、平均发展速度和离子数密度的影响越明显。研究成果可为输电线路架设等工程实践提供指导。     

Water electrolysis using plate electrodes in an electrode-paralleled non-uniform magnetic field

Compared with other ways to produce hydrogen, water electrolysis is the best way to obtain ultra-pure hydrogen, but its low energy efficiency greatly limits its wide application. It was proved that external magnetic field can reduce energy consumption, thereby increase electrolysis efficiency. Most of the researchers are focused on the impact of uniform magnetic field but few on a non-uniform one. To address the industrial operation reality, in our work, water electrolysis was operated using alkaline solution and plate electrodes in a non-uniform Magnetic field. The results show that a rotational flow on the vertical plane was formed by Lorentz force within the entire cell range. Although the entrainment effect of rotating flow made the cell full of microbubbles, the cell voltage was still reduced. By measuring the voltage difference of cathode side and anode side, we think that the bubble layer in the vicinity of the electrode surface matters the most among the sources of electric resistance. And the velocity distribution near the electrode was measured by PIV, it reveals that MHD flow is the dominant effect on the flow field of the cell. The results show that non-uniform magnetic field has potential merit in industrial electrolysis process.     

Quantifying the water content in the cathode of enzyme fuel cells via neutron imaging

Neutron imaging was used to study cathode water content over time in a three-dimensional-cathode enzyme fuel cell (EFC). A porous carbon felt cathode allowed air to flow through the electrode. A solution with laccase and a mediator formed an aqueous layer on the electrode surface. Water loss was observed in situ via neutron imaging for varying experimental conditions, including flow rates of hydrogen and air, cathode inlet humidity, volume of enzyme solution, and its composition. Cathode water loss occurred for all experimental conditions, but the loss rate was noticeably reduced when a high-salt-concentration enzyme solution was used in the cathode in conjunction with increased humidity in the air feed stream. Results from neutron imaging and power density analysis were used in analyzing the causes that could contribute to EFC water loss. An increase in temperature due to the exothermic cathode reaction is considered a plausible cause of cathode water loss via evaporation. This is the first reported application of neutron imaging as a technique to study EFC water management. The results suggest that neutron imaging can be employed to provide a better understanding of EFC phenomena and thereby contribute to design and operational improvements of EFCs.     

Effects of inlet conditions on film evaporation along an inclined plate

The evaporation of falling water liquid film in air flow is used in different solar energy applications as drying, distillation and desalination, and desiccant systems. The good understanding of the hydrodynamics and heat exchange in falling liquid film and gas flow, with interfacial heat and mass transfer, can be applied in improving solar systems performance. The solar system performance is dependent on the operating conditions, system conception and related to several physical parameters, where the effects of some of these parameters are not completely clarified. In the present numerical study, we examine the effects of inlet conditions on the evaporation processes along the gas–liquid interface. The liquid film streams over an inclined plate subjected to different thermal conditions. Liquid and gas flows are approached by two coupled laminar boundary-layers. The numerical solution is obtained by utilizing an implicit finite-difference box method. In this analysis an air–water system is considered and the coupled effects of inclination, inlet liquid mass flow rate and gas velocity are examined. The results show that, for imposed heat flux or uniform wall temperature, the effect of inclination is highly dependent on the liquid mass flow rate and gas velocity. An increase in the liquid mass flow rate causes an enhancement of the effect of inclination on the heat and mass transfer. The inclination affects the heat and mass transfer, especially at lower gas velocities. In the range of inclination angles of 0–10°, an increase in the inclination improves the evaporation by increasing the vapor mass flow rate. The maximum effect of inclination is nearly achieved at an inclination angle of 10°.     

侧向风对自然通风直接空冷塔性能影响的数值分析

采用多孔介质简化模型分析了在不同风速情况下300 MW自然通风直接空冷系统空冷塔内的流场和各凝汽器换热量.结果表明:在没有侧向风时,流场是均匀对称的,此时塔内空气流量最大,而各凝汽器换热量均匀;当有侧向风时,由于空冷塔底部气流存在漩涡,使空冷塔内空气回流,空气流量减小;在风速大于4 m/s时,各凝汽器换热量差异较大,随着风速增加,这种现象加剧.