Numerical simulation on ionic wind in circular channels

Ionic wind induced by direct-current corona discharge has attracted considerable interest because of its low energy consumption, low noise emission, flexible designs, and lack of moving parts. The purpose of this study is to investigate the configuration parameters to improve the velocity of the ionic wind. Accordingly, this study develops a three-dimensional(3 D) model of circular tube with multi-needle-to-mesh electrode configurations, in this model, the influences of various parameters were explored,such as the mesh gap, the distribution of needle electrodes, the number of needle electrodes,and the radius of the circular channel. The numerical research results showed that the mesh gap, the distribution of needle electrodes, and the radius of the circular tube significantly affected the velocity of the ionic wind. When mesh gap is 12 mm,which indicates that there is an optimal mesh gap which can enhance the velocity of the ionic wind. What is more, changing the distribution of needle electrodes and increasing the number of needle electrodes can effectively improve the velocity of the ionic wind, the optimum distribution α of needle electrodes is 0.7–0.9, which greatly increase the velocity of the ionic wind. However, for multi-needle-to-mesh structure, the improvement of the radius of the circular channel is conducive to enhance the velocity and improve the velocity distribution.

Characteristics and underlying physics of ionic wind in dc corona discharge under different polarities

During a dc corona discharge, the ions’ momentum will be transferred to the surrounding neutral molecules, inducing an ionic wind.The characteristics of corona discharge and the induced ionic wind are investigated experimentally and numerically under different polarities using a needle-to-ring electrode configuration.The morphology and mechanism of corona discharge, as well as the characteristics and mechanism of the ionic wind, are different when the needle serves as cathode or anode.Under the different polarities of the applied voltage, the ionic wind velocity has a linear relation with the overvoltage.The ionic wind is stronger but has a smaller active region for positive corona compared to that for negative corona under a similar condition.The involved physics are analyzed by theoretical deduction as well as simulation using a fluid model.The ionic wind of negative corona is mainly affected by negative ions.The discharge channel has a dispersed feature due to the dispersed field, and therefore the ionic wind has a larger active area.The ionic wind of positive corona is mainly affected by positive ions.The discharge develops in streamer mode, leading to a stronger ionic wind but a lower active area.

Pressure Distribution Induced by Ionic Wind in Needle-to-water Corona Discharge

The negative DC corona discharge in air at atmospheric pressure was investigated in a needle-to-water system to obtain the pressure distribution of corona ionic wind.The deformation of water surface was measured and the distribution of wind pressure over the water surface was calculated.The effects of varying discharge parameters,such as applied voltage,gap spacing,tip radius of needle,and the shape of grounded electrode,on the wind pressure were studied.The measured wind pressure ranges from several Pa to several tens of Pa and up to 33 Pa over a small area;the pressure is comparatively large in the center and decreases quickly outwards.In the experiment system,a higher voltage on a 3 mm gap resulted in a stronger pressure of the ionic wind;around the onset voltage,using a needle with tip radius of 50μm obtained a larger wind pressure than using a needle with 100μm tip radius,but the latter one can produce larger pressure at higher voltages.Plus,the shape of the grounded electrode only influences the wind pressure a little.

Wire-to-Plate Surface Dielectric Barrier Discharge and Induced Ionic Wind

The electrical and mechanical characteristics of the wire-to-plate surface dielectric barrier discharge and the induced ionic wind are investigated experimentally.The different temporal behaviors in positive and negative half-cycles are studied by time-resolved images.It is shown that the discharge and the light emission are generally stronger in the positive half cycle.The discharge is inhomogeneous and propagates in streamer mode;however,in the negative half-cycle,the discharge appears visually uniformly and operates in the diffuse mode.The surface discharge can produce ionic wind about several m/s above the dielectric surface.There exists an optimal width of the grounded electrode to produce a larger plasma area or active wind region.Increasing of the applied voltage or normalized dielectric constant leads to a larger wind velocity.The performance of ionic wind on flow control is visualized by employing a smoke stream.

Experimental Studies of the Enhanced Heat Transfer from a Heating Vertical Flat Plate by Ionic Wind

The effects of the ionic wind off the heat transfer rate from a heated vertical flat plateare described.The ionic wind is induced by three different types of discharge,corona discharge,dielectric barrier discharge (DBD) and dc glow discharge.The heat transfer coefficients for theheated copperplate under free convection conditions with and without an ionic wind are obtainedby measuring the temperature and the heating power of the copper plate.It has been proved thatthe convective heat transfer coefficients increase by several times with the help of the ionic wind.With the ionic wind induced by a uniform dc glow discharge,the heat transfer coefficient of theheated copper plate is highly enhanced compared with those induced by a corona discharge orDBD.With the use of DBD,the breakdown voltage is increased significantly,which is helpful inavoiding a breakdown when heat transfer is enhanced by the ionic wind.In addition,it makes theapplication of the ionic wind much safer.

吸气式多级电流体推进器性能研究

吸气式电流体动力(Electrohydrodynamic,EHD)推进器通过多级加速的方式可以有效提高推力,缓解航空器在空间环境下燃料不足的问题。为了探索不同参数下推进器的性能变化规律,分别研究了海拔、电压、电极间距和推进器级数等对电流体推进器的影响,通过求解电流体力学基本方程获得了放电、流动及推进特性相关的数据。研究发现,推进器在海拔0~20 km存在较大的性能差异,当海拔增大时,电极可施加的极限电压明显减小,电流及空气流速均有不同程度的降低。单级加速腔在海拔0 km处产生的最大推力是海拔20 km处的18.4倍,对应的推功比是海拔20 km处的3.1倍。多级推进器的推进性能并不等同于多个单级加速腔的代数相加,其前后两级电极的相互干扰会导致电场强度和电荷密度均发生变化,产生的反向电场将阻碍气体的流动。当推进器级数由1提升至5时,总推力由1.7 mN提升至6.53 mN,增大3.84倍;而推功比由0.94 N/kW降至0.58 N/kW,减小61.7%。

低频和高频交流电场助燃对比及燃烧特征参数预测研究

为探究不同频率交流电场助燃的内在机制,利用定容燃烧试验平台比较了低频(40、60、80、100 Hz)和高频(15、20、25、30 kHz)交流电场对甲烷/空气稀薄燃烧(过量空气系数为1.2、1.4、1.6)火焰的影响,并利用机器学习方法对混合气的燃烧特征参数进行了预测研究。结果表明:低频和高频交流电场下,火焰在电场方向上均被拉伸,不同频率交流电场对火焰传播的促进效果处于同一量级,但低频交流电场下的火焰锋面更加稳定;高频交流电场对燃烧特性参数的影响更加显著,其对应的压力峰值和压力升高率峰值均比低频交流电场下变化更大;采用支持向量机方法构建的平均火焰传播速度和燃烧压力峰值的预测模型相关系数均高于0.998,平均绝对百分比误差和希尔不等系数分别小于1.093%和0.007,均具有良好的预测性能和泛化能力。研究进一步验证了低频和高频交流电场的助燃机制,证实了机器学习方法应用于基础燃烧特征参数预测的可行性,丰富了电场助燃理论。

Passive attitude stabilization of ionic-wind-powered micro air vehicles

The ionic-wind-powered Micro Air Vehicles(MAVs)can achieve a higher thrust-toweight ratio than other MAVs.However,this kind of MAV has not yet achieved controlled flight because of the unstable thrust produced by the ionic wind and the dynamic instability related to the small size.In this paper,a passive attitude stabilization method of the ionic-wind-powered MAV using air dampers is introduced.The key factors that influence the performance of the air dampers,including the layout,position,and area of the air dampers,are theoretically studied.The appropriate optimal position of the air dampers is also obtained by Monte Carlo stochastic simulations.Then the proposed passive attitude stabilization method is applied to the ionic-wind-powered MAVs of different wingspan(2 cm and 6.3 cm).Finally,the experimental results show that using the proposed method,attitude stabilization is achieved for the first time for the ionic-wind-powered MAV.Moreover,the altitude control of an ionic-wind-powered MAV with a wingspan of 6.3 cm is also demonstrated.

Experimental and numerical analyses of electrohydrodynamic force according to air pressure

Electrohydrodynamic(EHD) force produced by corona discharge is considered as a new thrust for solar-powered aircraft and stratosphere balloons in near space. However, its performance at low air pressures remains to be clarified.An experiment of measuring the EHD force at 0.02 atm–1.0 atm(1 atm = 1.01325×10^(5)Pa) is carried out with the wireto-cylinder geometric structure. The ion distribution is analyzed by using the drift-diffusion model with two-dimensional numerical simulation. The experimental result shows that the EHD force is not linearly related to the corona discharge current at low air pressures. Numerical simulation finds that the proportion of ions in the counter-direction electric field increases from approximately 0.36% to 30% when the pressure drops from 1.0 atm to 0.2 atm. As a result, the EHD force with a constant power supply drops faster than the previous theoretical prediction in the ground experiment, suggesting that the consideration of counter-direction EHD force is necessary to improve the ionic wind propulsion efficiency in near-space applications.

低气压离子风推进器仿真与实验

离子风动力具有无需机械旋转部件、低功耗及低噪声等优点,在平流层飞艇和太阳能飞机上具有重要的应用潜力。模拟临近空间的低气压环境,采用“线-柱”电极结构电晕放电装置产生离子风,实验测量不同放电条件下离子风推进器产生的推力和推功比,研究了气压、放电电压、电极间隙对离子风动力的影响。仿真和实验结果表明,气压的降低会导致推进器推功比的下降,具体表现为:当电极间隙为2.0 cm,最大推功比由1 atm(1 atm=101325 Pa)时的17.70 mN/W降低至0.02 atm时的0.24 mN/W;而推功比的损失可以通过增大电极间隙来补偿,当电极间隙增加至14 cm,在0.02 atm的气压环境下,推功比由0.24 mN/W升至0.70 mN/W。通过优化离子风推进器结构,增加电极间隙,离子风动力具备低气压环境下的应用潜力。

电极结构对离子风速度与分布影响的仿真研究

为研究电极结构对电晕放电产生的离子风速度与分布的影响,基于COMSOL软件构建了针—网、针—板、针—环—网和针—环—板等电极结构下电晕放电产生离子风的仿真模型。模拟结果显示:当电压、电极间距、高压电极针曲率等参数相同时,针—板结构比针—网结构下离子风分布更集中,当在高压电极和地电极之间放置相同的环状屏蔽地电极时,针—环—板结构与针—环—网结构下离子风分布相似;对比针—板与针—网结构,其对称轴线上离子风轴向速度最大值分别约为2 m/s和1 m/s,地电极表面离子风径向速度最大值分别约为1.5 m/s和0.7 m/s;增加环形电极后,两种结构对称轴线上离子风轴向速度最大值比之前分别增加约2.5和4倍,地电极表面离子风径向速度最大值比之前分别增加约1.8和4倍。

离子风强化水蒸气与液滴捕获数值研究

面向工业雾气排放过程的水回收需求,提出一种将空冷式冷凝与静电除雾过程集成的静电水回收方法,可实现水蒸气与液滴的协同捕获。构建水蒸气冷凝与静电捕雾过程相耦合的数学模型,数值分析离子风强化水蒸气冷凝的机理及静电水回收器的性能。研究表明环境空气侧自然对流换热系数仅为雾气侧对流换热系数的10%,通过针状放电极产生离子风在空气侧形成局部冲击射流,可使空气侧的局部对流换热系数呈脉冲状分布,最大强化幅度约13倍。相比空气侧为自然对流的情况,在空气侧设置7层放电极所产生的离子风可使水蒸气收集效率强化1.5倍。在定义的雾气和空气条件下,静电水回收器的水蒸气和液滴收集效率最高分别可达9.2%和97.7%。

不同构型电晕放电离子风强化传热的数值模拟研究

为了探索不同构型下离子风的传热性能,针对3种不同结构的离子风装置进行数值模拟,分析电压、电极高度与散热片宽度对平均风速、换热系数以及功率的影响规律。计算结果表明,电压的增大可以提升离子风的强化传热效果,但是消耗的功率也会逐渐增大,其中线-板型功率增大的幅度最大;电极高度越小,离子风的强化传热效果越好;当散热片的宽度比较小,散热需求比较低时,由于直接接地型电极的功率较小,所以其更加适用。研究结果对不同情况下的离子风结构选型提供了参考。

高压直流电场作用下的甲烷-氧气层流扩散火焰稳定性

设计开发了直流电场作用下层流火焰实验系统,通过对甲烷-氧气非预混层流火焰施加直流电场,改变电极间距及燃烧当量比(氧气与甲烷实际物质的量之比与氧气和甲烷完全燃烧时物质的量之比的比值),对高速相机下火焰脉动幅度受电场影响的变化规律进行分析,探究了直流电场对火焰稳定性的作用及电场约束火焰的可行性。结果表明:对于存在脉动的层流扩散火焰,当对其施加高压直流电场时,火焰受到离子风的作用,其脉动幅度会逐渐减弱直至趋于稳定状态,且火焰稳定时所对应的电场强度与其初始的脉动幅度有关,初始振幅越大则火焰稳定所需的电压越高。同时,电极间距的改变也会影响火焰稳定时所需的电场强度,当电极间距改变较大时,对同一当量比的火焰,间距越大则所需的稳定电压就越高。

电场对火焰特性的影响

电场辅助燃烧是利用外部电场,通过将带电物质分离到单极区域来产生离子驱动的风,从而驱动周围的中性物质,这通常被认为是一种强大的火焰控制机制,是提升燃料燃烧效率的一个重要手段。对不同形式、不同施加方式的电场下的火焰特性、碳烟颗粒的排放和仿真现状进行广泛研究,总结不同火焰参数测量方式导致的不同程度的误差,及完善的数值模拟对实验数据的预测作用。结果表明:当前的数值模拟大多是2维的,需要开发完善准确的3维模型来模拟和预测火焰在电场下的特性;也需要在高压力和温度条件下对不同燃料在电场下的测量结果,以便在电场下为火焰开发出更广泛、更精确的动力学和化学模型。

直流电晕放电中的离子风增强带电气溶胶沉积

为解决我国水资源短缺问题和开发大气水资源的急迫需求,突破之前单电极电晕放电离子源的离子密度较低、气溶胶荷电困难的难题,开发了一套基于离子风增强带电气溶胶沉积的负直流电晕放电系统.该系统利用多个针电极与接地的网状电极,实现了大面积、阵列式的负直流电晕等离子体,可以对流经其中的气溶胶进行高效的荷电和处理.虽然单电极放电产生的开放空间中的离子密度是双电极放电的20倍,但双电极放电区域的电场约为单电极放电的15倍,放电电流也更大.强电场推动离子流进行定向运动,同时推动中性分子,使该装置可以产生速度高达2 m/s的离子风,而与之相比,单电极电晕放电的离子风速低于检测限.双电极电晕放电产生的离子风带动了云室中空气的流动,可以有效促进气溶胶的荷电、碰并以及在接地网状电极上的沉积,可以快速减少空气中的气溶胶密度,所需的时间仅为单电极放电的1/4.在水雾气溶胶的沉积实验中,其水雾的总沉积量是单电极放电的8.3倍.因此,双电极放电系统是诱导降水或消除致病生物气溶胶的潜在有效方法.

针电极正负电晕离子风反推力的对比研究

搭建了空气中针电极电晕放电离子风的反推力测试平台,实验测量了在不同电压极性和电极曲率下离子风反推力的大小,得到了针电极所受反推力与各变量之间的关系,并结合相关仿真进行分析。结果表明针电极曲率半径较小时的反推力大于曲率半径较大时的反推力,负极性针尖的反推力大于正极性针尖的反推力。分析得出尖电极前方空间电荷对局部电场及电晕电流的巨大影响是针电极反推力出现极性现象的原因。

电极参数对尖-板电极空气放电离子风特性的影响

空气放电离子风技术是一门新颖的技术,在实际应用方面具有现有技术手段不可比拟的优势。文章采用PIV(Particle Image Velocimetry)粒子测速技术,获得了清晰准确的离子风速度场图像。研究结果显示:减小尖电极曲率半径,可以在同样的电压等级条件下有效地提高离子风速度;接地电极采用金属网,可以克服板状电极对于离子风发展的部分束缚作用,在较低电压下能得到较高的风速,且可获得较为广阔、流场较为均匀的离子风作用区域。

空气放电离子风特性的研究

空气放电离子风技术在能源、环境、材料、军事等领域具有广阔的应用前景,采用粒子图像测速(particle image velocimetry,PIV)粒子测速系统得到了空气中尖-板电极放电产生的离子风速度场的图像,研究了相同电压等级正、负极性直流和工频交流电源下的离子风速度特性和流场分布情况并用离子碰撞、电流体动力学(electrohydrodynamic,EHD)顺电效应以及流体动力学相关理论进行了分析。最后采用红外热像仪测得了离子风对于发热体强化传热的作用,分析了离子风作用下发热体的温度分布特性。

电除尘技术发展与应用

电除尘技术是目前主要的颗粒物分离和捕集技术之一,为此,根据目前的研究热点和难点,对现代电除尘技术的现有进展与应用进行了综述。研究结果表明:1)在比电阻与电除尘方面,面对细颗粒物控制的需求,需要通过燃煤特性、高压电源、电除尘器(ESP)本体选型等多方面优化来实现超低排放目的;颗粒物特性如形状、粒径和比电阻等都会影响电收尘过程,其中比电阻影响最为显著;比电阻随温度增加而呈先增后降的趋势,中国煤种粉煤灰的比电阻可以采用Yan&Li模型通过锂、钠、铁的质量分数、湿度和电场强度来进行预测。2)在电除尘器中的PM2.5与PM10方面,电除尘器出口粒径低于2.5μm的颗粒物(PM2.5)与粒径低于10μm的颗粒物(PM10)的质量浓度比值一般在6%~40%范围内;而其后端脱硫塔在捕集PM10的同时还会产生次生颗粒物,因此其出口PM2.5与PM10质量浓度比值在28%~60%范围内;电除尘器出口PM2.5还与振打形式相关,断电振打时出口质量浓度可比带电振打约高6 mg/m3。3)在电除尘器收尘效率预测方法方面,为了预测分级效率和提供设计依据,以平均电场强度、峰值电场强度和比收尘面积的乘积作为电除尘指数;电除尘器出口质量浓度限值低于10mg/m3时,该指数值应大于1 300。4)在电除尘器供电形式方面,电除尘指数要求较高的运行电场强度,不仅三相电源的平均工作电压可以接近单相电源的峰值电压,而且三相电源的收尘效率也可以比单向电源提高10%以上。5)在电除尘技术路线和本体选型方面,电除尘器本体则应采取小分区差异供电,前端应对高粉尘负荷,后端应对细颗粒物二次扬尘;同时运行温度应控制在130℃以下以提高收尘效率并满足能量回收要求。6)在离子风、电凝并及粉煤灰高值化方面,电晕放电产生的离子风及次生湍流、电凝并器引起的颗粒荷电和凝并都对细颗粒物控制有益;为降低电除尘器运行成本,粉煤灰还可以进行高值化应用,用于废气废水处理、矿物聚合物合成等用途。这些研究结果的总结和梳理可以对电除尘技术的发展有所裨益。