双阳极霍尔推力器效率及推力的模拟研究

为确定双阳极霍尔推力器中阳极电压分布对推力器效率及推力的影响，采用PIC三维数值模拟对双阳极结构霍尔推力器的放电过程进行了模拟。基于垂直分布的双阳极结构，得到了放电后离子羽流分布以及电子在放电区域的分布。通过对放电状态的模拟，计算得到了不同工况下的推力以及效率。在双阳极结构中第二阳极的电压增加会增大离子能量，但将影响电子约束，而过高的电压将引起电子密度减小。推力随着霍尔电流的增大而增大，但效率会随之减小，导致在提升推力的同时损失效率。     

离子推力器推力密度特性

离子推力器推力密度分布对航天器轨道维持和修正具有重要影响.采用粒子模拟-蒙特卡罗碰撞方法模拟束流等离子体输运过程,分析束流多组分粒子喷出数量和速度等微观参数,并计算得到单孔束流推力,结合放电室出口等离子体密度分布,进一步对推力密度分布特性分析,最后开展实验验证.研究结果显示:束流中单价离子、双荷离子以及交换电荷离子的推力贡献比分别为84.63%,15.35%和1.82%,可见推力主要来源于束流中的单价离子和双荷离子,交换电荷离子对推力贡献很小;推力密度分布具有较好的中心轴对称性,从推力器中心沿着径向先快速下降后趋于缓慢;与实验结果对比,经验模型相对误差约为4.1%,数值模型相对误差约为2.8%,相比经验模型,数值模型具有更好的准确性.研究结果可为离子推力器推力密度分布均匀性等优化提供参考.     

脉冲等离子体推力器羽流沉积薄膜的结构与光学性质研究

为了研究脉冲等离子体推力器羽流污染特性,把握脉冲等离子体推力器羽流沉积薄膜性质,利用质谱仪对推力器羽流残余气体成分进行了分析,利用红外傅里叶光谱、光电子能谱及紫外可见光分光光度计对推力器羽流区阴极侧不同方位角处沉积薄膜的结构及光学性质进行了诊断研究结果表明,推力器羽流中主要存在C,F,CF,CF2和CF3气体分子;在各方位角处脉冲等离子体推力器羽流沉积生成了低氟碳比碳氟薄膜;沉积薄膜微观结构以30°角为界在羽流不同区域中具有不同的变化趋势;羽流沉积薄膜具有对波长小于500 nm的光低透射率及增强反射率的光学特性,受薄膜性质的影响,其光学特性具有极大的角度依赖性.     

离子推力器羽流特性的粒子模拟

采用粒子网格单元和蒙特卡罗碰撞方法,建立了离子推力器羽流场的2维轴对称模型,对其特性进行了数值模拟,并将模拟结果与实验测量数据进行了对比分析。研究结果表明:数值模拟结果与实验测量值基本一致,模型可以很好地评估离子推力器的羽流特性;返流区离子数密度达到1014m-3量级,会对航天器表面产生污染;背压对束流区域外电荷交换离子影响较为显著,不可忽略。     

栅极热变形对离子推力器工作过程影响分析

栅极热变形是影响离子推力器性能和寿命的重要因素. 采用三维粒子方法对栅极系统等离子体输运过程进行模拟, 对比、分析栅极热变形前后栅极系统的电子返流限制、导流系数限制、离子通过率和发散角损失. 结果表明: 栅极热变形增大了屏栅离子通过率和推力器推力值, 并由于加速栅截止电流阈值的提高拓展了推力器工作电流区间, 但电子返流阈值的明显降低对栅极系统可靠工作造成了不利影响.     

加速电压和阳极流率对离子推力器性能的影响

为了研究离子推力器输入参数对工作性能的影响,采用试验研究和理论分析的方法研究了离子推力器加速电压和阳极流率对离子推力器性能的影响.研究结果表明:一定范围内离子束流随着加速电压绝对值的减小不断减小,然后突然增大,大、小推力模式下的电子返流极限电压分别为–140 V和–115 V,放电电压、放电损耗随阳极流率减小单调增大,减速电流单调减小,通过调节阳极电流、栅间电压、工质气体流量,功率为300—4850 W下,推力为11—188 m N,比冲为1800—3567 s,效率为34%—67%,在3000 W时推力器最高效率达到67%,该转折点对推力器设计和应用有关键意义,应用要结合在轨任务剖面选择合理的工作参数区间.     

电子回旋共振离子推力器放电室等离子体数值模拟

电子回旋共振离子推力器属于静电型推力器,具有寿命长、比冲高、结构简单、可靠性高等优点,适用于深空探测等长时间空间飞行任务.放电室是一个关键部件,其内部通过电子回旋共振产生等离子体.针对放电室内等离子体流场建立飘移-扩散近似模型,采用迎风格式有限差分法对该模型进行数值求解,得到了放电室内不同时刻的等离子体流场分布及其演化规律.数值模拟结果可以为推力器的设计和实验研究提供有用信息.     

使用不同工质的会切磁场等离子体推力器

会切磁场等离子体推力器是一种利用磁镜约束等离子体产生推力的新型推力器,具有寿命长、推力大范围连续可调等优点,在无拖曳控制领域具有较大的应用前景.分别采用Xe,Kr和Ar三种不同工质,开展了会切磁场等离子体推力器实验.首先,对所用的推力器进行了简要的原理和设计介绍;然后,对三种工质的点火电压分别进行了测试,发现Xe是最容易点火成功的,Kr和Ar点火难度较大.在阳极电流、推力、效率和比冲等性能方面,三种工质在同等条件下也存在明显的区别.分析发现,三者的工质利用率高低导致了性能上的差别,通过提升通流密度能够大幅提升Kr和Ar的工质利用率.在羽流结构方面,法拉第测量结果表明三者都存在明显的空心羽流,且离子电流密度峰值出现的角度随着原子量的减小而逐渐减小.     

二次曝光全息术对微波等离子体推进器羽流的诊断及计算机模拟

二次曝光全息术是诊断等离子体电子数密度的一个有效测量方法,具有非接触式测量及不受等离子体发光干扰等优点.根据阿贝尔变换公式,通过计算机应用matlab程序数值模拟了各种不同电子数密度状态下的等离子体干涉图.同时拍摄了航天器中微波等离子体推进器羽流的二次曝光全息图,再现了不同状态下的干涉图样.通过分析和对比两次干涉图,得出了羽流的电子数密度.     

外磁场作用下的磁等离子体动力学过程仿真

磁等离子体动力学推力器是空间高功率电推进装置的典型代表,磁等离子体动力学过程是其核心工作机制.为深入理解外磁场对其工作特性的影响,本文采用粒子云(particle in cell,PIC)方法结合基于自相似准则的缩比模型,进行外加磁场作用下磁等离子体动力学推力器工作过程的建模仿真,通过与实验结果对比验证模型和方法的可靠性,并重点分析推力器点火启动过程的等离子特性参数分布,以及外磁场和阴极电流对推力器工作性能的影响.研究结果表明:阴阳极放电电弧构建是推力器启动和高效工作的关键步骤;外磁场强度较低工况不利于构建稳定放电电弧,等离子体束流集中于轴线附近,推力主要产生机制是自身场加速;外磁场强度较高时,阴阳极放电电弧稳定,推力产生主要机制是涡旋加速,推力、比冲随外磁场强度线性增大;推力器效率随阴极电流和外磁场强度增大而增大;放电电压随阴极电流增大而增大,但随外磁场强度的增大表现出先减小后增大的趋势.     

Computer simulations of the fission process of charged nanometre droplets

The process of fission of charged liquid droplets is an important stage of multiply charged ion formation in the electrospray ionization (ESI) process. ESI is currently the most powerful ionization method in the mass spectrometry of large molecules such as biopolymers. Fission plays a crucial role in the ion formation essentially in the range of nanosized droplets, since it determines the charge states of product ions. Usually the Rayleigh equation is used to determine critical conditions, at which the fission process takes place. This equation gives the value of the critical radius of the charged droplet as a function of its charge and specific surface energy. The Rayleigh equation does not give us the opportunity to determine charges and sizes of the fission fragments. In the present work we propose a mathematical model that describes the fission process of the charged liquid droplet in an external electrostatic field. The model is based on the quasi-equilibrium assumption that the most probable shape of the droplet corresponds to the minimum of the free energy of the system. The change in the internal kinetic energy and dissipative losses for the viscous liquid are also taken into account in the model.     

Rayleigh instability of charged droplets in the presence of counterions

The classical instability of a charged spherical droplet is reconsidered in the presence of counterions. An ensemble of such droplets is studied within a simplified cell model. Screening of the electric field by the counterions is found to increase the equilibrium droplet size. Furthermore, if the ions can enter the droplet, a first-order phase transition occurs upon increasing Bjerrum length, surface tension or droplet density, leading to a phase separation. Simple scaling properties of the free energy give the shape of the phase boundary and show the system to be scale-invariant there. Pearl-necklace structures of hydrophobic polyelectrolytes are discussed as an application. Received 30 August 2001     

A Numerical Calibration Approach to Obtain Cutting Fluid Droplet Sizes in a Turning Process via an Imaging System

Airborne inhalable particulate in the workplace can represent a significant health hazard, and one of the primary sources of particles is mist produced through the application of cutting fluids in machining operations. The atomization process is one of the principal mechanisms associated with cutting fluid mist formation and generates droplets from fifty to a few thousand micrometers in size. These particles subsequently undergo vaporization and settling effects resulting in an aerosol to which workers may be exposed. While a variety of equipment is available to characterize the fine particulate in the breathing zone, standard equipment to measure the size of the atomized droplets is not available. In this paper, an imaging system is employed to characterize the large droplets produced by atomization in turning. One of the drawbacks of such a system is the time‐consuming experimental calibration procedure that is required to improve the accuracy of the droplet size measurements and extend the depth of field of the imaging system. With this in mind, an approach is introduced to predict droplet diameter based on measurement data without physical system calibration. The relationship between the actual diameter and the measured diameter is established based on an imaging system simulation model that includes a three dimensional point spread function and an image formation relationship grounded in the principles of geometric optics. These two components are combined using convolution integral theory to derive an image intensity profile. The introduction of halo width into the simulation greatly extends the image depth of field, which is a critical factor in capturing more droplets in one image and also minimizing particle size distribution bias towards larger droplets. The model predicts droplet diameter as a function of measured diameter and halo width. Model behavior of predicted diameters from the simulation compares well with those from a physical calibration of the system. The numerical calibration model is then used in the study of cutting fluid atomization in a turning process, and the measured droplet size distribution compares favorably with droplet sizes predicted by a mechanistic atomization model.     

Investigations of Droplet‐Plasma Interaction using Multi‐Dimensional Coupled Model

Recently developed multi‐dimensional coupled fluid‐droplet model is used to investigate the behavior of complex interaction between the liquid precursor droplets and atmospheric pressure plasma (APP). The significance of this droplet‐plasma interaction is not well understood under diverse realm of working conditions in two‐phase flow. In this study, we explain the implication of vaporization of liquid droplets in APP which are subsequently responsible to control major characteristics of surface coating depositions. Coalescence of water droplets is more dominant than Hexamethyldisiloxane (HMDSO) droplets because of its sluggish rate of evaporation. A disparity in the performance of evaporation is identified in two independent mediums, such as gas mixture and discharge plasma using HMDSO precursor. The length of evaporation of droplets is amplified by an increment of gas flow rate indicating with a reduction in the gas temperature and electron mean energy. In particular, the spatio‐temporal density distributions of charged particles show a clear pattern in which the typical nitrogen impurity ions are primarily effective as compared to other helium ionic species along the pulse of droplets in APP. Finally, we contrast the behavior of discharge species in the pure helium and He‐N2 gas mixtures revealing the importance of stepwise and Penning ionization processes. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Water vapor condensation and the collapse of superheated water drops as possible causes of ball lightning

Ball lightning is modeled by the bulk of humid air heated to temperatures of 600–650 K and containing a multitude of charged drops and microbubbles of size 10⁻⁵ cm or less, as well as water vapor heated to a near-critical temperature. The condensation of the vapor in the microbubbles, followed by the cooling of resulting droplets, generates energy spent on the thermal radiation of ball lightning. The radiation of light and radio waves is explained by the motion of ions and electrons in the electric field of charged bubbles and droplets and by the thermal rotational motion of charged droplets. As a result of coagulation, the droplets overheat and tend to collapse. An external electric field, supersaturated water vapor condensation, and a number of other factors may contribute to the ball lightning explosion energy.     

Near infrared lights emission of <Superscript>40</Superscript>Ar<Superscript>16+</Superscript> impacting on metal surfaces

The highly charged ion ⁴⁰Ar¹⁶⁺ with the velocity (kinetic energy E _K=150 keV, velocity V=8.5×10⁵ m/s) smaller than Bohr velocity (V _Bohr=2.9×10⁶ m/s) was found to hove impacts on the surfaces of metals Ni, Mo, Au and Al, and the Ar atomic infrared light lines and X-rays spectra were simultaneously measured. The experimental results show that the highly charged ion that captures electrons is neutralized, and the multiply-excited hollow atom forms. The hollow atom cascade decay radiates lights from infrared to X-ray spectrum. The intensity of infrared lights shows that the metallic work functions play an important role in the neutralization process of highly charged ions during their interaction with metallic surfaces, which verifies the classical over-the-barrier model. Supported by the National Natural Science Foundation of China (Grant No. 10574132), the Natural Science Foundation of Shaanxi Province (Grant No. 2007A05) and the Talents Introduction Project of Xianyang Normal University (Grant No. 05XSYK103)     

Study on the algorithm of computational ghost imaging based on discrete fourier transform measurement matrix

We have developed a model and realized an algorithm for the calculation of the coefficient of coherent (direct) transmission of light through a layer of liquid crystal (LC) droplets in a polymer matrix. The model is based on the Hulst anomalous diffraction approximation for describing the scattering by an individual particle and the Foldy-Twersky approximation for a coherent field. It allows one to investigate polymer dispersed LC (PDLC) materials with homogeneous and inhomogeneous interphase surface anchoring on the droplet surface. In order to calculate the configuration of the field of the local director in the droplet, the relaxation method of solving the problem of minimization of the free energy volume density has been used. We have verified the model by comparison with experiment under the inverse regime of the ionic modification of the LC-polymer interphase boundary. The model makes it possible to solve problems of optimization of the optical response of PDLC films in relation to their thickness and optical characteristics of the polymer matrix, sizes, polydispersity, concentration, and anisometry parameters of droplets. Based on this model, we have proposed a technique for estimating the size of LC droplets from the data on the dependence of the transmission coefficient on the applied voltage.     

电场对协流式微流控装置中乳液液滴生成行为的调控机理

建立了直流电场作用下协流式微流控装置中单乳液液滴乳化生成过程的非稳态理论模型,并开展了数值模拟研究,揭示了电场对液滴乳化生成动力学行为的调控机理,阐明了流场/电场参数对液滴乳化生成特性的影响规律.研究结果表明:沿流体流动方向施加静电场可在电物性参数不同的两相流体界面法线方向上产生指向内相流体的电场力,进而强化了内相流体界面的颈缩和断裂,提升了液滴生成速率和形变程度,减小了液滴生成尺寸;在同一毛细数下,随着电毛细数的增大,乳液乳化流型由每周期仅有单一液滴生成的滴式流型转变为每周期有一个主液滴并伴随有卫星液滴生成的滴式流型;随着毛细数和电毛细数的增大,黏性拖曳力以及电场力作用增强,使内相流体颈缩过程后期更容易形成细长型液线,从而有助于诱发液线上产生Rayleigh-Plateau不稳定现象,继而促进卫星液滴的形成.