Flow characteristics of a single stage EHD gas pump in circular tube

Characteristics of flow induced by electrohydrodynamic (EHD) gas pumps in circular pipe have been experimentally evaluated. Two tube diameters (61.8 mm and 127.8 mm) and two electrode gap distances (25 mm and 50 mm) have been considered. The gas pumps use eight evenly spaced emitting electrodes which are flush mounted on the tube wall. As such, flows induced by the pumps have a profile with a higher velocity near the wall and a lower velocity at the tube center. Experiments are conducted using positive corona discharge with voltage varying from 17.5 kV to 30 kV. The results show that the volume flow rate increases with the applied voltage but approaches an asymptotic value before sparkover takes place. From the present results, several important implications for the practical engineering applications are presented.     

Numerical investigation of using various electrode arrangements for amplifying the EHD enhanced heat transfer in a smooth channel

Forced convection heat transfer enhancement with electrohydrodynamic (EHD) technique of turbulent flow inside a smooth channel has been numerically investigated. A two dimensional numerical approach has been chosen to evaluate the local and average heat transfer coefficient. In addition, the swirling flow pattern in the presence of an electric field has been studied. To achieve higher enhancement while using multiple electrodes, variety of electrode arrangements have been examined for specified values of Reynolds number, applied voltage, and wire radius. The results demonstrate that different electrode arrangements cause significant improvement of the heat transfer coefficient.     

Computer simulation of EHD flows in a needle-plane electrode system

Complex investigation of the structure of electrohydrodynamic (EHD) flows in a needle-plane electrode system is carried out on the basis of analysis of experimental data and the results of computer simulation. An algorithm of iterative simulation of the volume charge distribution in a fluid is developed. Simulation is carried out using the ANSYS system. The fields of velocities and pressures, as well as electric characteristics of EHD flows, are calculated. Analysis of the results reveals a number of features of EHD flows in the electrode system under investigation. Peculiarities of the band structure are determined, and the characteristic size of the low-pressure zone near the active electrode, as well as the sizes of the acceleration and deceleration zones of the fluid in the electrode gap, is determined.     

Mechanism of electrohydrodynamically induced flow in a wire-non-parallel plate electrode type gas pump

An experimental investigation and one-dimensional modeling have been conducted to study the mechanism of net flow direction induced by electrohydrodynamic (EHD) forces in a wire-non-parallel plate electrode type EHD gas pump. The experiments were conducted with various different locations of corona wire electrode for negative and positive applied voltage from 0 to 14 kV at atmospheric pressure and room temperature, where air was used as the working fluid. A one-dimensional cross-sectional averaged model based on mass and momentum conservation as well as Poisson electric field and ion transport equations was also developed. The results show that the net flow direction of electrohydrodynamically induced gas flow in a wire-non-parallel plate electrode system significantly depends on the location of the corona wire electrode relative to the grounded electrode position. The effect of conversion angle of non-parallel plate electrode on the net flow direction and pressure drop also was investigated and discussed in detail.     

Computation of the gas mass and heat fluxes in a rectangular channel in the free molecular regime

The problem of heat- and mass transfer in a long rectangular channel of a constant cross section is solved in the free molecular regime. The distributions of the mass flow rate and the heat flux vector over the channel cross section are calculated. The specific gas mass flux and heat flux are calculated. The results are compared with those obtained for nearly free molecular flows.     

EHD atomization characteristics of a point-plate system with a porous point electrode

An electrohydrodynamic (EHD) atomization from a point-to-plate system, with a wet porous point as a corona electrode, has been studied. And the atomized water droplets from the wet porous point, as well as the water droplet traces, the water droplet charge-to-mass ratios, and the water droplet number concentrations, were investigated. It was observed that the wet porous point can atomize abundant amounts of water droplet, 2.8, 2.6 and 2.2 mg/min for negative, AC and positive corona, respectively. The migrated water droplet traces were observed. The positive wet porous point atomized very fine water droplets than those obtained with the negative wet porous point. Moreover, the water droplets atomized from the AC corona showed granular-like larger traces. A weak corona discharge can atomize water droplets very effectively. On the other hand, an intensive corona discharge can eject more water droplets. As a result with the wet porous point, the maximum corona-current-based and corona-power-based water droplet atomization yields of Y_C = 3.34, 3.32 and 3.25 μg/μAs and Y_P = 0.35, 0.40 and 0.27 mg/Ws have been obtained for the negative, AC and positive corona discharges.     

A spark channel plasma electrode for a CO2 laser gas discharge

A plasma electrode potentially suitable for dc discharge pumped lasers has been developed based on the repetitive creation of a spark channel. The high-density plasma is a source of charge carriers for the dc discharge, thereby largely eliminating the cathode and anode falls. Potential reduction improves with increasing spark repetition frequency until about 9 kHz, at which point the falls are virtually eliminated. Fortuitously, the most beneficial regime of plasma electrode operation also appears to coincide with the optimum E/N range for CO₂ laser vibrational excitation.     

Effect of process parameters on the EHD airflow

This work deals with the assessment by numerical investigations of the electrohydrodynamic enhancement of heat transfer in a channel. In order to get a better understanding of the phenomena, we developed an original numerical approach. The model handles the couplings between a non-isothermal turbulent flow and corona discharges from multiple wires. The typical configuration consists in a channel where air enters at a given velocity. In the channel, the ionic wind is produced by multiple coronating wires. In this study, the model is used in order to analyse the effect of some process parameters (inter-electrode distance, wire radius, applied voltage) on the flow and on the convective heat transfer enhancement. We firstly confirm that the model operates well and permits selection of the best parameters in a specific configuration. The model reveals that similar flows could be obtained with different combinations between the inter-electrode distance and the applied voltage. In a second stage, the distance between adjacent wires is discussed. Synergetic effects are obtained for close wires; they locally lead to a tenfold increase of the heat transfer enhancement.     

电极材料对气体火花开关静态性能的影响

在工作气压和火花间隙固定的条件下,针对稍不均匀场的圆饼形电极开关开展了不同电极材料下开关自击穿实验,开关间隙距离为5 mm,工作气压为0.25 MPa,击穿电压平均值为40 kV。分别选取了不锈钢、黄铜、钨铜合金和石墨材料作为实验对象,对比了不同电极材料下电极质量损失、电极表面形貌和开关静态特性的差异。实验结果表明,石墨电极质量损失速率略高于金属电极,但是由于石墨电极烧蚀产物多为气体,因此石墨电极绝缘子污染程度远小于金属电极。石墨电极开关在低欠压比下自击穿概率也远小于金属电极开关。三种金属电极开关,其静态特性差异不大,但钨铜电极烧蚀程度显著低于不锈钢和黄铜电极开关。     

电极材料对气体火花开关静态性能的影响北大核心CSCD

在工作气压和火花间隙固定的条件下,针对稍不均匀场的圆饼形电极开关开展了不同电极材料下开关自击穿实验,开关间隙距离为5mm,工作气压为0.25 MPa,击穿电压平均值为40kV。分别选取了不锈钢、黄铜、钨铜合金和石墨材料作为实验对象,对比了不同电极材料下电极质量损失、电极表面形貌和开关静态特性的差异。实验结果表明,石墨电极质量损失速率略高于金属电极,但是由于石墨电极烧蚀产物多为气体,因此石墨电极绝缘子污染程度远小于金属电极。石墨电极开关在低欠压比下自击穿概率也远小于金属电极开关。三种金属电极开关,其静态特性差异不大,但钨铜电极烧蚀程度显著低于不锈钢和黄铜电极开关。     

Effect of number density on velocity distributions in a driven quasi-two-dimensional granular gas

The motion of mono-disperse spherical steel particles in a vibration driven quasi-two-dimensional (2D) square cell is studied. The cell is horizontally vibrated to eliminate the effect of gravity compaction. The velocity distributions at different particle number densities are studied and found to obey the form exp[-β(|vy|/σy)α], in which v y and σ y are velocity and its variance in the transverse direction, and α and β are fitting parameters. The value of α is found to decrease with the number density of particles increasing. To investigate the effect of the bottom plate, the molecular dynamics simulation without considering any bottom friction is performed. The accordance between the simulation result and the experimental result shows that the influence of bottom plate friction force on the high energy tail of the velocity distribution can be neglected.     

The pulsating laminar flow in a rectangular channel

The finite difference method is used to solve the task of the developed pulsating laminar flow in a rectangular channel. The optimum of the difference scheme parameters was determined. Data on the amplitude and phase of the longitudinal velocity oscillations, the hydraulic and friction drag coefficients, the shear stress on the wall have been obtained. Using the dimensionless value of the frequency pulsations two characteristic regimes — the quasisteady-state regime and the high-frequency regime have been identified. In the quasi-steady-state regime, the values of all hydrodynamic quantities at each instant of time correspond to the velocity value averaged over the cross section at a given moment of time. It is shown that in the high-frequency regime, the dependences on the dimensionless oscillation frequency of oscillating components of hydrodynamic quantities are identical for rectilinear channels with a different cross-sectional form (round pipe, flat and a rectangular channels). The effect of the aspect ratio of the rectangular channel sides channel on the pulsating flow dynamics has been analyzed.     

Effect of ZnO film thickness on the performance of organic light emitting diodes

The electrical and optical properties of the ZnO layers and of the ITO/ZnO bilayers are investigated. We show that a ZnO layer of about 120 nm is the best compromise to obtain simultaneously a high transmittance and conductivity. Moreover an X-ray diffraction analysis underscores that an amorphous ITO deposited on a polycrystalline ZnO could change into a polycrystalline ITO. The modifications of the ITO layer by a preliminary deposition of a 120 nm thick ZnO underlayer enables us to decrease the threshold voltage of organic light emitting diodes.     

微小型矩形槽道平板热管传热性能的数值模拟

在充分考虑槽道内汽-液界面剪切摩擦力下,建立了微小型矩形槽道平板热管的数学模型,通过迭代计算得出工质质量流量、流体和蒸汽平均流速、汽一液界面弯月面半径及蒸汽和液体压力沿轴向分布规律,最后对热管外壁面温度值的计算结果和实验结果进行了比较,两者数值基本一致,证明了所建数学模型与实际相符,对热管的理论分析具有指导意义。     

Effect of micropillar electrode spacing on the performance of electrohydrodynamic micropumps

Electrohydrodynamic (EHD) micropumps with three-dimensional 50 μm × 50 μm micropillar electrodes were fabricated and tested in this study. Two basic electrode configurations were investigated: (i) micropillar emitter and collector electrodes (symmetric) and (ii) micropillar emitter and planar collector electrodes (asymmetric). The micropumps were fabricated by integrating chromium/gold planar electrodes with electroplated 3-D Nickel micropillars on a glass substrate with a 100 μm high PDMS microchannel. The effect of the spanwise micropillar spacing on the pump performance was determined. The pumps were tested using HFE-7100 as the working fluid for the maximum pressure generation under a no flow condition. The micropumps with the asymmetric electrode design generated a significantly higher pressure head than the corresponding micropumps with symmetric electrode configuration for the same applied voltage, with lower power consumption. A decrease in the spanwise spacing of the micropillar electrodes increased the pump performance for the symmetric configuration, while the performance decreased for the asymmetric configuration.     

Numerical modeling of the effect of number of electrodes on natural convection in an EHD fluid

Numerical modeling of the electrohydrodynamic effect on natural convection in enclosures is investigated. The interactions between electric field, flow field, and temperature field are analyzed by using the computational fluid dynamics technique. Flow pattern and temperature distribution are substantially affected by the voltage supplied at the wire electrodes, especially at low Rayleigh number. It can be concluded that the fluid velocity and heat transfer coefficient in the presence of an electric field are significantly increased with when a large number of electrodes is used. Surprisingly, a minimum value of augmented heat transfer occurs with an intermediate number of electrodes. The optimized condition between the aspect ratio of an enclosure and number of electrodes which leads to maximum heat transfer enhancement is expressed in terms of the parameters concerned.     

EHD flow in a wide electrode spacing spike–plate electrostatic precipitator under positive polarity

In this work, results of two- and three-dimensional particle image velocimetry (PIV) measurements of the flow velocity fields in a wide spacing spike–plate electrostatic precipitator (ESP) under positive polarity are presented. A DC voltage of positive polarity (up to 28 kV) was applied to the spike electrode. The average gas flow velocity was 0.6 m/s. The PIV measurements were carried out in four planes perpendicular to the plate electrodes. Three parallel planes passed along the ESP while one plane passed across the ESP duct. The results show that electrohydrodynamic (EHD) secondary flow with relatively strong vortices exist in the ESP. The EHD secondary flow pattern depends on applied voltage and measuring plane position in respect to the spike tip. The strongest vortices occur in the plane passing through the tip of the upstream-directed spike. These relatively strong EHD vortices may hinder collection of the particles in the diameter range of 0.1–1 μm in the wide electrode spacing spike–plate ESPs.