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。     

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。     

直流电压下电场对植被火焰影响的机理分析

为研究直流电压对植被火焰温度、高度和离子浓度等燃烧状态的影响,提出了化学反应-温度-流体-电场多物理场耦合模型．首先确定流体、温度等之间相互关系,然后确定植被燃烧相关化学反应过程及化学反应表达式,最后利用多物理场耦合模型仿真分析不同电场作用下火焰的温度、流速以及离子浓度的变化规律．结果表明,负极性条件下随着电压升高,靠近电极火焰的最高温度依次增高,且-10 kV、-20 kV和-30 kV分别比无电压条件下高139℃、215℃和219℃;正极性条件下随着电压升高,靠近电极火焰的最高温度呈先下降后上升的趋势,且10 kV、20 kV和30 k V分别比无电压条件下高200℃、159℃和286℃．火焰达到稳定时,10 kV与-10 kV相较于无电压条件下火焰高度增幅分别为11.31%和31.98%．在一定电压范围内,正极性电压越高,火焰向上传播速率越快,而负极性电压越高,火焰向上传播速率越慢．通过分析离子变化规律,20 s时刻-10 kV火焰H₃O⁺、HCO⁺摩尔分数分别比无电压情况下增加34.5%和45.7%.     

麻风树油在极不均匀场中的雷电冲击放电现象

文章利用针-板电极产生的极不均匀场强,对非来源于食物的麻风树植物油和矿物绝缘油在极不均匀场中雷电冲击电压作用下的放电现象进行了研究。研究结果显示,在具有相同峰值的极不均匀正、负雷电冲击电压作用下,麻风树油中的放电形状、放电长度、放电量和放电发展速度均优于或等同于矿物绝缘油,这说明麻风树油可替代矿物绝缘油用于高电压设备的绝缘保护。     

点火电极附近离子信号的形成机理

通过在定容燃烧弹中同时进行点火电极附近离子信号和室内压力的测量,以及通过高速摄影仪在燃烧中拍摄的纹影照片与相应的离子信号对比,分析了离子信号由点火、火焰前锋和火焰后区三部分组成。阐述了点火阶段的离子信号主要与火花放电时所产生的感应电势有关;火焰前锋的信号与电极附近的火核有关,其值主要取决于火焰中由化学反应形成的CHO^＋速度和自由电子浓度;火焰后区的信号发生在已燃区,主要与该区内由高温所导致的激态NO^＋数量和电子浓度有关。     

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。     

定容燃烧弹中离子信号的特征分析

在定容燃烧弹中同时进行离子信号和压力的测量,并通过高速摄影仪在燃烧过程中拍摄的纹影照片与相应的离子信号进行对比,分析了点火和燃烧中离子信号的点火、火焰前锋和火焰后区三个阶段．阐述了点火阶段的离子信号主要与电极间空气隙被击穿瞬间火花放电时产生的感应电压有关,火焰前锋的信号主要与电极附近的火核有关,以及火焰后区的信号主要与已燃区内各种成分的离子和电子有关的观点;论述了离子信号三个阶段的主要特点以及产生的主要原因．     

高电压下浓/稀天然气燃烧特性的对比研究

利用定容燃烧装置研究了高电压作用下浓/稀天然气火焰的燃烧特性,并探讨了以自由电子为主的阴离子对火焰燃烧的作用机理。试验中加载电压为0kV、5kV和10kV,浓/稀混合气的过量空气系数λ分别为0.8和1.4。在加载电场作用下,球形膨胀火焰在与电场相反方向上的火焰传播明显加快,火焰传播速度和火焰拉伸率随加载电压的增强而增大,稀燃混合气的火焰传播的增强程度最为明显。加载10kV电压时,λ为0.8和1.4火焰的平均速度比没有电场作用时分别增加了36.4%和49.5%。火焰传播速度的增强,促进了燃烧过程的进行,使得混合气燃烧的滞燃期缩短,压力升高率增加,且压力峰值时间提前。稀燃混合气的燃烧压力峰值有较大提高,而浓燃混合气的压力峰值变化不大。试验中加载电场对球形膨胀火焰燃烧的作用机理可用火焰中的阴离子,尤其是O_2~-主导的离子风效应来说明。同时,电场在一定程度上也促进了火焰燃烧化学反应的进行。     

离子信号与空燃比关系的探讨

在定容燃烧弹中通过对点火电极附近离子信号的测量,表明了该信号主要由火焰前锋和火焰后区两部分组成．理论分析证明,火焰前锋的信号主要与电极附近火核中的H3O 离子和自由电子浓度有关,火焰后区的信号主要与已燃区中高温下激态的NO 和自由电子浓度有关,特别是火焰前锋区的信号与空燃比有直接关系．实验研究发现火焰前锋和后区的信号峰值随过量空气系数的变化趋势相同,当过量空气系数为1时均达最大值．因此,利用该信号可以实现空燃比的探测．     

变风向通风对池火火焰长度的影响

通过改变风速以及纵向风与燃料表面之间的风向角,研究了风速和风向变化对池火火焰长度的影响.研究结果表明,随着风速的增加,酒精池火和正庚烷池火表现出迥然不同的特征.在本研究工况条件下,酒精池火的火焰长度随着风速的增大先增长,而后逐步缩短;正庚烷池火的火焰长度随着风速的增大首先增长,而当风速增大到一定值后,其火焰长度保持相对稳定.实验发现,酒精池火和正庚烷池火的火焰长度均随着风向角的增大而减小,相同面积的燃料盘,其火焰长度与燃料盘顺风面边长成正比,与背风面等无关.     

低温等离子体对甲烷/氧反扩散火焰影响的实验研究

为探究低温等离子体对甲烷/氧反扩散火焰的影响,通过对同轴式喷注器环缝甲烷射流施加介质阻挡放电产生甲烷等离子体,综合采用多种测量手段实验研究了多种工况下该低温等离子体特性及火焰关键参数的变化。结果显示,放电击穿电压随混合比增大而减小,电流脉冲数量和幅值则随混合比增大而先增加后减小;甲烷等离子体呈灰白色,低电压下提高气体流量则放电有所减弱;受等离子体气动效应作用,放电后甲烷射流角有所增大,且电压越高射流角越大,增幅则逐渐减小,过高激励强度下射流发生失稳;等离子体通过改变燃料和氧化剂的掺混而影响甲烷/氧反扩散火焰的形态,使得火焰中心高度总体有所下降,特征长度缩短,释热强度则有所增加,其中小流量、低混合比条件下作用效果更明显;喷注器功率则随混合比上升而先增大后减小。     

Effect of electric fields on the propagation speed of tribrachial flames in coflow jets

The effect of electric fields on the propagation speed of tribrachial (or triple) flames has been investigated in a coflow jet by observing the transient flame propagation behavior after ignition. The propagation speed of tribrachial edges when no electric fields were applied showed typical behavior by having an inverse proportionality to the mixture fraction gradient at the flame edge. The behavior of flame propagation with electric fields was investigated by applying high voltage to the central fuel nozzle, thereby having a single-electrode configuration. The enhancement of propagation speed has been observed by varying the applied voltage and frequency for ac electric fields. The propagation speed of tribrachial flames was also investigated by applying positive and negative dc voltages to the nozzle, and similar improvements of the propagation speed were also observed. The propagation speeds of tribrachial flames in both the ac and dc electric fields correlated well with the electric field intensity, defined by the applied electric voltage divided by the distance between the nozzle electrode and the edge of the tribrachial flame.     

Effect of electric fields on the stabilization of premixed laminar bunsen flames at low AC frequency: Bi-ionic wind effect

The stabilization characteristics of laminar premixed bunsen flames have been investigated experimentally by applying AC electric fields at low frequency below 60 Hz together with DC in the single electrode configuration. The blowoff velocity has been measured for varying AC voltage and frequency. A transition frequency between low and high frequency regimes has been identified near 40–50 Hz, where AC electric fields have minimal effect on flame stabilization. In the low frequency regime, the blowoff velocity decreased linearly with AC voltage such that the flames became less stable. This was consistent with the DC result, implying the influence of the ionic wind effect. The variation of blowoff velocity with AC frequency showed a non-monotonic behavior in that the velocity decreased and then increased, exhibiting minimum blowoff velocity near 6–8 Hz. Based on the molecular kinetic theory, the developing degree of ionic wind was derived. By considering the ionic wind effects arising from both positive and negative ions in a flame zone, the bi-ionic wind effect successfully explained the non-monotonic behavior of blowoff velocity with AC frequency in the low frequency regime.     

Metal hydride batteries research using nanostructured additives

We describe here, our recent research efforts to improve the capacity of metal hydride batteries using nanostructured additives. Nanostructured additives of palladium, copper and nickel were incorporated separately into the negative electrode of the metal hydride batteries. The nanomaterials were synthesized by template-based methods and characterized by scanning electron microscopy. These nanomaterials were incorporated in the negative electrode of the metal hydride battery and the electrochemical performance at 2 C rate was studied. The nanomaterial-incorporated negative electrodes all showed increased cell voltage and negative electrode potential compared to that of a pristine cell. The increase in discharge capacity for a cut-off voltage of 1 V depends on the nanomaterial incorporated and a comparative analysis of the performance of the different batteries is presented.     

Effect of pulsed,sub-breakdown applied electric field on propane/air flame through simultaneous OH/acetone PLIF

The effects of chemi-ion current induced flow perturbations in a premixed, laminar propane/air flame at atmospheric pressure have been measured with 30 ms-wide applied pulsed voltages. Single-shot OH and acetone planar laser-induced fluorescence (PLIF) images have been collected to measure the spatio-temporal structural changes to a laminar flame with incoming flow speed of 2 m/s in response to positive polarity voltage pulses of 2.8 kV over a 20 mm electrode gap. OH and acetone PLIF are specifically chosen to measure reaction zone modification as the flame undergoes large-scale, stochastic changes. These large-scale changes of flame structure are observed after the flame becomes fully crushed and unstable behavior occurs lasting until the end of the applied voltage pulse. The experimental results of combined OH and acetone PLIF presented in this paper show a significant widening of the reaction zone observed during this unstable behavior. This widening of the reaction zone is indicative of a flame brush normally observed in turbulent flames, demonstrating the ability of the sub-breakdown applied voltage to cause a laminar flame to a transitioning-to-turbulent behavior.     

A simple method for measuring positive ion concentrations in flames and the calibration of a nebulizer/atomizer

A simple and accurate (<±10%) method has been developed for measuring absolute concentrations of positive ions in a flame. The method involves only the measurement of a saturation current (∼μA) when collecting the total flux of cations in a flame when it impinges on a conducting plate or electrode perpendicular to the flame axis. A fixed negative bias (∼50 or 100 V) is applied to this electrode with respect to the metallic burner; such a voltage is more than adequate to stop all negative charges from reaching the electrode. The required bias voltage for this separation of positive and negative charges varies roughly as the one-third power of the concentration of positive ions. One application of the method involves the accurate (<±10%) calibration of the delivery factor f for a nebulizer or pneumatic atomizer used to dope a flame with a metal (<5 × 10⁻⁶ mole fraction); the metal is introduced by spraying an aerosol of an aqueous salt solution into the gas mixture feeding the burner. When a dilute solution (∼10⁻⁴ molar) of a Cs salt is sprayed into a fairly hot flame (>2300 K), the Cs added is almost completely ionized (>99%); accordingly, f can be determined by measuring the absolute ion density of Cs⁺. Calibration difficulties can be encountered if a more concentrated solution is employed, because Cs⁺ can, quite surprisingly, achieve a superequilibrium concentration early in the flame. However, the method was used to show that the delivery of a pneumatic atomizer is essentially linear when the strength of the salt solution varies by at least three orders of magnitude. Of great utility is the ease with which ion concentration profiles can be measured along a flat flame for studying the kinetics of reactions involving ions. Such ion profiles have revealed the very rapid production of Cs⁺ ions and free electrons in the reaction zone of a flame doped with Cs; two possible chemi-ionization reactions are discussed to explain this phenomenon. In addition, the rate constant of the recombination reaction H₃O⁺ + e^- → H + H + OH is confirmed to be (3.2 ± 0.3)×10⁻⁷ cm³ ion⁻¹ s⁻¹ at 2400 K. Confirmation of this rate coefficient provides verification of the simple method presented here for the measurement of absolute concentrations of positive ions in flat flames.     

Study on electrochemical and thermal characteristics of lithium‐ion battery using the electrochemical‐thermal coupled model

The higher specific energy leads to more heat generation of a battery, which affects the performance and cycle life of a battery and even results in some security problems. In this paper, the capacity calibration, Hybrid Pulse Power Characteristic (HPPC), constant current (dis)charging, and entropy heat coefficient tests of chosen 11‐Ah lithium‐ion batteries are carried out. The entropy heat coefficient increases firstly and then decreases with the increase of the depth of discharge (DOD) and reaches the maximum value near 50% DOD. An electrochemical‐thermal coupled model of the chosen battery is established and then verified by the tests. The simulation voltage and temperature trends are in agreement with the test results. The maximum voltage and temperature error is within 2.06% and 0.4°C, respectively. Based on the established model, the effects of adjustable parameters on electrochemical characteristic are systematically studied. Results show that the average current density, the thickness of the positive electrode, the initial and maximum lithium concentration of the positive electrode, and the radius of the positive electrode particle have great influence on battery capacity and voltage. In addition, the influence degree of the internal resistance of the solid electrolyte interface (SEI) layer, the thickness of negative electrode, and the initial and maximum lithium concentration of the negative electrode on the capacity and voltage is associated with certain constraints. Meanwhile, the influences of adjustable parameters related to thermal characteristic are also systematically analyzed. Results show that the average current density, the convective heat transfer coefficient, the thickness, and the maximum lithium concentration of the positive electrode have great influence on the temperature rise. Besides, the uniformity of the temperature distribution deteriorates with the increase of the convective heat transfer coefficient.     

Electric fields effect on liftoff and blowoff of nonpremixed laminar jet flames in a coflow

The stabilization characteristics of liftoff and blowoff in nonpremixed laminar jet flames in a coflow have been investigated experimentally for propane fuel by applying AC and DC electric fields to the fuel nozzle with a single-electrode configuration. The liftoff and blowoff velocities have been measured by varying the applied voltage and frequency of AC and the voltage and the polarity of DC. The result showed that the AC electric fields extended the stabilization regime of nozzle-attached flame in terms of jet velocity. As the applied AC voltage increased, the nozzle-attached flame was maintained even over the blowout velocity without having electric fields. In such a case, a blowoff occurred directly without experiencing a lifted flame. While for the DC cases, the influence on liftoff was minimal. There existed three different regimes depending on the applied AC voltage. In the low voltage regime, the nozzle-detachment velocity of either liftoff or blowoff increased linearly with the applied voltage, while nonlinearly with the AC frequency. In the intermediate voltage regime, the detachment velocity decreased with the applied voltage and reasonably independent of the AC frequency. At the high voltage regime, the detachment was significantly influenced by the generation of discharges.     

Electrodeposited behavior of lead on the negative electrode in the neutral lead-carbon hybrid capacitor: Influence and inhibition

Because negative electrode of the neutral lead-carbon hybrid capacitor is composed by the active carbon and graphite, it is general believe that only the absorption-desorption process occurs in the negative electrode. However, in this work, we find that there is a second reaction going on the negative electrode: During charging, lead ions diffused from the positive electrode can be reduced to lead crystals when the charging cut-off voltage of the negative electrode is negative to the reduction potential of lead ions. These electrodeposited lead crystals can be converted into lead sulfate crystals during discharging, and these lead sulfate crystals cannot be completely dissolved in the next charging process. When lead sulfate crystals are accumulated on the surface of the carbon material gradually, the performance of the negative electrode is inhibited. Current densities have a great influence on this phenomenon because the charging cut-off voltage of the negative electrode is negatively shifted with the decrease of current density. The anion exchange membrane can completely inhibit this phenomenon by preventing the free diffusion of lead ions. After cycling at 2 A g^?1 for 3400 times, the energy retention rate of the neutral lead-carbon hybrid capacitor increases from 34% to 81%.     

Analysis of pulse and relaxation behavior in lithium-ion batteries

A mathematical model of a lithium-ion cell is used to analyze pulse and relaxation behavior in cells designed for hybrid-electric-vehicle propulsion. Predictions of cell voltage show good agreement with experimental results. Model results indicate the ohmic voltage loss in the positive electrode is the dominant contributor to cell overvoltage in the first instances of a pulse. The concentration overvoltage associated with the reduced lithium in the solid phase of the positive is of secondary importance through pulse duration, but dominates after current interruption. Effects of anisotropy in the particle diffusion coefficient are also studied. Heaviside mollification functions are utilized to describe the thermodynamic open-circuit voltage of lithiated graphite, and the “pleated-layer model” is extended to realize the phase behavior of primary-particle aggregates during cell operation. The negative electrode contributes little to the cell overvoltage, and two-phase behavior results in a reaction front within the electrode. No voltage relaxation is associated with the negative electrode, and after full relaxation, a stable composition gradient of lithium exists throughout the solid phase. Internal galvanic coupling removes the composition gradients in the positive electrode during relaxation.