Nucleation of β/α Phases in Undercooled Ti-48( at％ )Al Peritectic Alloy

采用电磁悬浮熔炼的深过冷凝固技术实现了Ti-48( at％) Al合金的深过冷,获得的最大过冷度为295K.采用红外测温仪记录实验过程的温度变化数据,OM技术观察不同过冷度下凝固合金的显微组织,结合深过冷凝固过程中的再辉行为分析了该合金的非平衡凝固路径.运用负熵模型、经典形核理论及瞬态形核理论,研究了Ti-48( at％)Al包晶合金中初生相与次生相的形核与过冷度之间的关系.通过对该合金熔体的β(bcc)相和α(hcp)相的临界形核功、稳态形核率、形核孕育时间和瞬态形核率的计算,并结合实验结果对β,α两相的形核进行了综合分析.结果表明,在～10K/s的较低冷速情况下,在整个过冷度(≤295K)范围内B(bcc)相总能作为初生相首先形核.     

航天器介质盘环结构内带电特性三维仿真分析

针对航天器内特定结构的内带电（IDC）问题,以航天器典型复杂介质结构——太阳帆板驱动机构介质盘环为研究对象,开展了地球同步轨道恶劣充电环境（Flumic3）下介质内带电三维仿真分析。通过Geant4实现电荷输运模拟,根据电荷守恒定律数值计算得到电场,在此基础上,研究了屏蔽厚度对介质内带电的影响规律,提出了根据屏蔽厚度调整入射电子能谱能量下限以提高计算效率的方法。仿真结果表明,盘环结构内带电最严重部位是盘环最外圈上层介质与金属导电环接触的上边沿;增加屏蔽厚度可以减缓充电风险,但是随着温度降低,屏蔽效果会随之减弱。在地球同步轨道恶劣充电环境（Flumic3）下,当温度低至183 K时,由于辐射诱导电导率成为总电导率的主导部分,从而增大屏蔽的同时也会降低介质电导率,导致即使3 mm铝屏蔽下仍可能出现接近10⁷ V/m的场强峰值。     

超声速钝体空气和钾元素混合物喷流流场的数值研究

利用数值求解化学非平衡Navier-Stokes方程的方法,对有自由来流情况下喷流介质为空气和钾元素混合物的超声速钝体喷流流场进行数值模拟,分析喷流总温对模型周围电子密度分布的影响和喷流对模型气动力分布的影响,探讨利用喷流达到飞行器等离子体隐身方法的可行性。研究结果表明:在喷流总温T0j=2000K-3000K和喷流介质钾元素的质量分数为0.02和0.05的情况下,在模型周围可形成峰值电子数密度Ne=1011~5×1014/cm3的等离子体层,而且模型的阻力可降低35%。     

Si-Zr-B涂层结构稳定性及对DD3单晶高温合金深过冷的非催化形核惰性

以 Si O2 玻璃粉、分析纯 Zr O2 粉为粉料,分析纯 C2 H5OH,Zr OCl2 · 8H2 O,去离子 H2 O与分析纯Si(OC2 H5) 4按 4∶ 0.1 1∶ 3∶ 1的摩尔比配制的前驱液经 3 5℃,1 80 min水解 -聚合反应后形成的 Si O2 -Zr O2溶胶为粘结剂,H3 BO3 为软化剂,用熔模制壳技术在壳型内壁制备了 Si-Zr-B基底涂层。利用溶胶 -凝胶原理,以 Si O2 -Zr O2 溶胶中加入 H3 BO3 形成的 Si O2 -Zr O2 -B2 O3 溶胶为原料,经浸涂 -分级热处理和 80 0℃玻璃化热处理后,在基底涂层表面制备了适宜厚度的同成分玻璃薄膜涂层。将涂层在 1 5 0 0℃保温 3 0 min后,XRD结果表明,涂层的析晶量仅为 1 %～ 3 %,具有很高的高温结构稳定性。过冷实验表明,DD3单晶高温合金可在该 Si-Zr-B涂层壳型内获得最大 1 4 0 K过冷度,完全可以实现深过冷快速凝固,证明 Si-Zr-B涂层具有良好的非催化形核惰性。     

加入纳米颗粒的相变悬浮液黏性和热导率特性

提出了在相变悬浮液中添加TiO2纳米颗粒以提高其热导率的概念,并使用旋转流变仪和热物性仪测量了这种新型悬浮液流变特性和热导率。研究结果表明,对于体积浓度为10%的相变悬浮液,当加入的纳米颗粒质量浓度不超过5%(体积浓度1.18%)时,低剪切率下悬浮液表现出非牛顿流体的特性,高剪切率下仍可视为牛顿流体。相变悬浮液的黏性随纳米颗粒浓度增加而增加,当纳米颗粒质量浓度为5%时,相变悬浮液的黏性提高约23%。相变悬浮液的热导率随着纳米颗粒浓度的增加而增加,当纳米颗粒质量浓度为5%时,相变悬浮液热导率提高约7.3%。     

高焓流场球头外形气动热试验研究

再入地球大气层时,飞行器的再入速度极高,面临严重的气动加热问题。为了研究高焓流动导致的热化学非平衡现象,在高焓膨胀风洞FD-14X中开展了球头外形的热流测量试验以及CFD仿真模拟。FD-14X为中国空气动力研究与发展中心新建成的高焓膨胀风洞,速度模拟能力达到第二宇宙速度,总温模拟能力超过10 000 K,能够产生总焓70 MJ/kg的试验气体。试验来流总焓16.9～63.5 MJ/kg,球头直径20～50 mm,流场采用自发光拍照,同时CFD仿真采用Park双温非平衡模型计算球头绕流流场。试验与仿真结果表明：来流总焓大于5 MJ/kg时,球头绕流场存在显著的热化学非平衡现象;304钢模型壁面在来流总焓小于20 MJ/kg时表现为非催化壁面特性,在来流总焓大于30 MJ/kg时表现为催化壁面特性;当球头表面镀氧化锆膜、来流总焓49.5 MJ/kg时,球头壁面表现为非催化壁面特性。     

进口速度分布对钝体稳定器贫油点熄火性能的影响研究

为了探究燃烧室不均匀进口速度分布对燃烧性能的影响,采用试验的方法,开展了0.101MPa,700K进口条件下四种进口速度分布对钝体稳定器贫油点熄火性能的影响研究,结合流场特性分析了贫油点熄火性能变化的原因,四种进口速度分布依次为:h/H=0,3/6, 4/6和5/6 (其中h为速度峰值距离通道底部的高度,H为试验通道的高度)。结果表明:整体而言,均匀进口贫油点熄火性能优于非均匀进口。当h/H=3/6时,贫油点熄火性能最差;而当h/H=5/6时,贫油点熄火性能最优。随着h/H由3/6增大到5/6时,贫油点火当量比下降7.39%,贫油熄火当量比下降18.45%。当进口速度分布为h/H=0和3/6时,钝体稳定器下游回流区基本对称;而当h/H=4/6和5/6时,钝体下游回流区不对称,且每个涡出现主副双涡心,存在流体由一个涡心向另一个涡心流动的现象。同时,当进口速度分布由h/H=3/6增大到h/H=5/6时,钝体下游回流区变长,其特征长度Lvc_A增大14.56%,Lvc_B增大50.70%。     

20A发射电流空心阴极小孔区等离子体特性研究

为了获得30cm口径离子推力器20A额定发射电流空心阴极工作时小孔区的等离子体特性参数,并验证现有阴极小孔结构设计下的阴极电流发射能力,采用数值模拟及有限元分析方法研究了空心阴极小孔区的等离子体特性参数。结果显示:空心阴极小孔区的中性原子密度基本在4×10~(21)~6×10~(21)/m~3,分布较为均匀且越靠近小孔出口区域的原子密度越低;当阴极发射体温度为1800K时,采用等离子体零维扩散模型得到阴极小孔区轴向平均电子温度约为2.66e V,且靠近阴极顶小孔出口方向电子温度相对较高,从小孔区入口至出口电子温度增幅在1~2e V;通过离子连续性方程得到阴极孔区内,等离子体密度约在1×10~(21)~1.4×10~(21)/m~3,靠近出口处的等离子体密度降低较为明显;通过电子连续性方程,得到小孔区入口处的电子电流约为7.2A,而出口处的电子电流约为11.6A,与性能测试试验结果一致,电子电流增益系数约60%;离子电流密度峰值约为6.16×106A/m~2,出现在距离小孔入口约0.5mm处。通过理论分析认为,阴极孔区的腐蚀特点是靠近出口处的直径在离子腐蚀作用下不断地扩张,并在扩张到一定程度后,孔区出口处被腐蚀后的直径将不会再发生变化,理论分析腐蚀趋势与兰州空间技术物理研究所研制的LHC-5阴极小孔区寿命试验腐蚀情况基本一致。     

动态舱门对空腔复杂流动和噪声特性影响分析

为研究运动舱门对内埋弹舱（空腔）非定常流场和舱内噪声特性的影响,开发了应用于运动问题的动态嵌套重叠网格组装方法,采用改进的脱体涡模拟方法对亚声速流场（Ma=0.6）进行了高精度数值模拟。首先采用空腔标准模型（M219）验证所用的高精度数值格式的有效性,然后应用发展的方法对干净空腔（C201）、带静态舱门（30°、60°、90°和120°）的空腔以及运动舱门的空腔进行模拟,并分析静、动态舱门对空腔湍流流场和腔内气动噪声的影响。针对运动舱门的非稳态非定常流动问题,采用经验模态分解方法分析空腔中的湍流脉动特征和声压级。通过分析研究结果发现,与干净空腔相比,舱门小开度（30°）时,舱门会限制法向和展向的流动,从而降低腔内流场与外部流场的流动掺混和交换,腔内壁面总声压级比干净空腔低5～8 dB,但是两者变化趋势一致,且二阶Rossiter模态频率偏高;在打开角度较大（60°以后）时,舱门对腔内流动的影响主要表现在展向,此时空腔上方的剪切层涡结构运动的高度更高,舱门阻碍噪声的展向传播,使得腔内的总声压级升高（3～10 dB不等）,二阶Rossiter模态的强度增大。然而舱门开启过程中,腔内总声压级...     

粗糙壁热交换实验研究

 本文介绍了来流马赫数5的条件下,典型球锥模型的粗糙壁热交换实验结果。模型头部半径R为27.4毫米,底部直径D为60毫米,对五个不同粗糙度的模型进行了实验。模型表面粗糙颗粒直径d分别为0、0.3、0.5、0.7、0.9毫米。风洞前室总压P_t为10～45公斤/厘米。,相应的来流雷诺数R_eD为（O.8～3.6）×10⁶。 实验结果表明:光滑壁模型表面是层流加热,驻点热流与层流理论计算值较一致。粗糙度的影响,在低总压条件下（10公斤/厘米）主要在于促使边界层的转捩和发展。随着风洞总压的提高,物面静压和局部雷诺数的相应增大,粗糙度对热流的影响才明显增强,而严重的区域在端头。在实验最大粗糙度和最大总压条件下（d=O.9毫米、p_t=45公斤/厘米。）,除驻点值外,热流与光滑壁层流驻点值相比(q_i/q_so)的峰值在音点区域且接近4,而在驻点,此模型有别于其它模型,较为特殊,比热流最大值接近6,看来这可能与驻点局部外形变化有关。     

Atmospheric Aerosol and Cloud Condensation Nuclei Formation: A Possible Influence of Cosmic Rays?

A physical mechanism which may have a potential to connect climate with cosmic rays (CR) involves aerosol particle formation by CR generated atmospheric ions followed by new particle growth. Only grown particles can scatter sunlight efficiently and can eventually act as cloud condensation nuclei (CCN) and thereby may influence climate. Moreover grown particles live longer as they are less rapidly scavenged by pre-existing larger particles. The present paper discusses aerosol particle formation and growth in the light of new measurements recently made by our MPIK Heidelberg group. Emphasis is placed upon the upper troposphere where very low temperatures tend to facilitate new particle formation by nucleation. The new measurements include: laboratory measurements of cluster ions, aircraft measurements of ambient atmospheric ions, and atmospheric measurements of the powerful nucleating gas H₂SO₄ and its precursor SO₂. The discussion also addresses model simulations of aerosol formation and growth. It is concluded that in the upper troposphere new aerosol formation by CR generated ions is a frequent process with relatively large rates. However new particle formation by homogeneous nucleation (HONU) which is not related to CR also seems to be efficient. The bottleneck in the formation of upper troposphere aerosol particles with sizes sufficiently large to be climate relevant is not nucleation but growth of small particles. Our recent upper troposphere SO₂ measurements suggest that particle growth by gaseous sulphuric acid condensation is at least occasionally efficient. If so CR mediated formation of CCN sized particles should at least occasionally be operative in the upper troposphere.     

Influence of Solar Wind on the Global Electric Circuit,and Inferred Effects on Cloud Microphysics,Temperature, and Dynamics in the Troposphere

There are at least three independent ways in which the solar wind modulates the flow of current density (Jz) in the global electric circuit. These are (A) changes in the galactic cosmic ray energy spectrum, (B) changes in the precipitation of relativistic electrons from the magnetosphere, and (C) changes in the ionospheric potential distribution in the polar caps due to magnetosphere-ionosphere coupling. The current density J _z flows between the ionosphere and the surface, and as it passes through conductivity gradients it generates space charge concentrations dependent on J _z and the conductivity gradient. The gradients are large at the surfaces of clouds and space charge concentrations of order 1000 to 10,000 elementary charges per cm³ can be generated at cloud tops. The charge transfers to droplets, many of which are evaporating at the cloud-clear air interface. The charge remains on the residual evaporation nuclei with a lifetime against leakage of order 1000 sec, and for a longer period the nuclei also retain coatings of sulfate and organic compounds adsorbed by the droplet while in the cloud.The charged evaporation nuclei become well mixed with more droplets in many types of clouds with penetrative mixing. The processes of entrainment and evaporation are also efficient for these clouds. The collection of such nuclei by nearby droplets is greatly increased by the electrical attraction between the charge on the particle and the image charge that it creates on the droplet. This process is called electroscavenging. Because the charge on the evaporation nuclei is derived from the original space charge, it depends on J _z, giving a rate of electroscavenging responsive to the solar wind inputs.There may be a number of ways in which the electroscavenging has consequences for weather and climate. One possibility is enhanced production of ice. The charged evaporation nuclei have been found to be good ice forming nuclei because of their coatings, and so in supercooled clouds droplet freezing can occur by contact ice nucleation, as the evaporation nuclei are electroscavenged. Although quantitative models for the all the cloud microphysical processes that may be involved have not yet been produced, we show that for many clouds, especially those with broad droplet size distributions, relatively high droplet concentrations, and cloud top temperatures just below freezing, this process is likely to dominate over other primary ice nucleation processes. In these cases there are likely to be effects on cloud albedo and rates of sedimentation of ice, and these will depend on J _z.For an increase in ice production in thin clouds such as altocumulus or stratocumulus the main effect is a decrease in albedo to incoming solar radiation, and in opacity to outgoing longwave radiation. At low latitudes the surface and troposphere heat, and at high latitudes in winter they cool. The change in meridional temperature gradient affects the rate of cyclogenesis, and the amplitude of planetary waves. For storm clouds, as in winter cyclones, the effect of increased ice formation is mainly to increase the rate of glaciation of lower level clouds by the seeder-feeder process. The increase in precipitation efficiency increases the rate of transfer of latent heat between the air mass and the surface. In most cyclones this is likely to result in intensification, producing changes in the vorticity area index as observed. Cyclone intensification also increases the amplitude of planetary waves, and shifts storm tracks, as observed.In this paper we first describe the production of space charge and the way in which it may influence the rate of ice nucleation. Then we review theory and observations of the solar wind modulation of J _z, and the correlated changes in atmospheric temperature and dynamics in the troposphere. The correlations are present for each input, (A, B, and C), and the detailed patterns of responses provide support for the inferred electrical effects on the physics of clouds, affecting precipitation, temperature and dynamics.     

Atmospheric Ions and Aerosol Formation

This paper discusses atmospheric ions and their role in aerosol formation. Emphasis is placed upon the upper troposphere where very low temperatures tend to facilitate new particle formation by nucleation. New measurements addressed include: Laboratory measurements of cluster ions, aircraft measurements of ambient atmospheric ions, atmospheric measurements of the powerful nucleating gas H₂SO₄ and its gaseous precursor SO₂. The paper also discusses model simulations of aerosol formation and growth. It is concluded that in the upper troposphere new aerosol formation via ions is a frequent process with relatively large rates. However new particle formation by homogeneous nucleation which does not involve ions also seems to be efficient. The bottleneck in the formation of upper troposphere aerosol particles with sizes sufficiently large to be climate relevant is mostly not nucleation but sufficient growth of new and still very small particles. Our recent upper troposphere SO₂ measurements suggest that particle growth by gaseous sulphuric acid condensation can be efficient in certain circumstances. If so, cosmic ray mediated formation of CCN sized particles should at least occasionally be operative in the upper troposphere.     

铝硅共晶柱状晶凝固

 将强制凝固的理论和试验结果应用到铝硅共晶铸锭柱晶凝固过程,得到了共晶片间距同凝固速度、形核过冷间的关系。柱晶凝固时共晶片间距同平均凝固速度因子V~(-2/3)成正比,还同凝固时形核过冷密切相关,形核过冷增加使共晶片间距减小。文中讨论了影响形核过冷的诸因素和凝固组织均匀性问题。     

Fast Dust in the Heliosphere

The dynamics of dust particles in the solar system is dominated by solar gravity, by solar radiation pressure, or by electromagnetic interaction of charged dust grains with the interplanetary magnetic field. For micron-sized or bigger dust particles solar gravity leads to speeds of about 30 to 40 km s^–1 at the Earths distance. Smaller particles that are generated close to the Sun and for which radiation pressure is dominant (the ratio of radiation pressure force over gravity F _rad/F _grav is generally termed ) are driven out of the solar system on hyperbolic orbits. Such a flow of -meteoroids has been observed by the Pioneer 8, 9 and Ulysses spaceprobes. Dust particles in interplanetary space are electrically charged to typically +5 V by the photo effect from solar UV radiation. The dust detector on Cassini for the first time measured the dust charge directly. The dynamics of dust particles smaller than about 0.1 m is dominated by the electromagnetic interaction with the ambient magnetic field. Effects of the solar wind magnetic field on interstellar grains passing through the solar system have been observed. Nanometer sized dust stream particles have been found which were accelerated by Jupiters magnetic field to speeds of about 300 km s^–1.     

Numerical study of transient interchamber processes when reaching the SPRE operational conditions

A comprehensive numerical study of transient interchamber processes occurring when reaching the SPRE operational conditions is carried out. A conjugate problem under consideration includes nonstationary operation of an igniter heating, ignition and subsequent nonstationary and turbulent burning of a solid-propellant charge; nonstationary three-phase homo-heterogeneous flow of combustion products in the combustion chamber, nozzle and downstream of the nozzle block of a rocket engine; motion of a nozzle block cover. The calculation results are presented.     

X-Ray and extreme ultraviolet emissions from comets

There is significant progress in the observations, theory, and understanding of the x-ray and EUV emissions from comets since their discovery in 1996. That discovery was so puzzling because comets appear to be more efficient emitters of x-rays than the Moon by a factor of 80000. The detected emissions are general properties of comets and have been currently detected and analyzed in thirteen comets from five orbiting observatories. The observational studies before 2000 were based on x-ray cameras and low resolution (E/E1.5–3) instruments and focused on the morphology of x-rays, their correlations with gas and dust productions in comets and with the solar x-rays and the solar wind. Even those observations made it possible to choose uniquely charge exchange between the solar wind heavy ions and cometary neutrals as the main excitation process. The recently published spectra are of much better quality and result in the identification of the emissions of the multiply charged ions of O, C, Ne, Mg, and Si which are brought to comets by the solar wind. The observed spectra have been used to study the solar wind composition and its variations. Theoretical analyses of x-ray and EUV photon excitation in comets by charge exchange, scattering of the solar photons by attogram dust particles, energetic electron impact and bremsstrahlung, collisions between cometary and interplanetary dust, and solar x-ray scattering and fluorescence in comets have been made. These analyses confirm charge exchange as the main excitation mechanism, which is responsible for more than 90% of the observed emission, while each of the other processes is limited to a few percent or less. The theory of charge exchange and different methods of calculation for charge exchange are considered. Laboratory studies of charge exchange relevant to the conditions in comets are reviewed. Total and state-selective cross sections of charge exchange measured in the laboratory are tabulated. Simulations of synthetic spectra of charge exchange in comets are discussed. X-ray and EUV emissions from comets are related to different disciplines and fields such as cometary physics, fundamental physics, x-rays spectroscopy, and space physics.     

凝结相变对低温风洞雷诺数试验的影响

为了研究凝结相变对低温风洞雷诺数试验能力的影响,基于Fluent软件建立了气-液两相凝结流动模型,相变模型采用考虑非等温效应修正的经典成核理论和Gyarmathy液滴生长理论;针对不同来流压力和不同试验雷诺数工况条件,对氮气绕流NACA 0012翼型进行了数值模拟。模拟结果表明:随着来流温度的降低,翼型附近区域气体膨胀越过气态饱和线并达到过冷状态,进一步降低来流温度则会在宏观层面上观测到凝结相变对当地流场的改变,与无凝结相变的流场相比,释放的潜热加热气流导致马赫数降低及压力系数的改变;在确保不破坏翼型气动性能试验的前提下,充分利用气体过冷区域来降低来流压力以此减少驱动功率和液氮喷入量是可行的,或者保持来流压力不变提升低温风洞的试验雷诺数。     

Charge Generation and Separation Processes

This paper presents a short overview of our current understanding of the generation of charged particles in different environments and circumstances (e.g. thunderclouds, dust storms, volcanic plumes, rings, and planetary surfaces) and the subsequent spatial separation that leads to the formation of electrical fields. Different mechanisms are involved on various scales, starting from the molecular level, through the single particle (droplet, crystal, solid) and finally the entraining volume (cloud, plume etc.). Encapsulated within a dynamic and turbulent medium, particles need to come into contact and to immediately separate, to be later transported away from each other. In order to explain the observed electrical fields and ensuing lightning or other forms of discharge, these processes need to be extremely effective. The section will briefly review laboratory results and modeling efforts of charge separation and electric field build-up in various planetary settings, and cite the appropriate observations of electrical activity on different planets.     

结冰风洞中液滴相变效应数值模拟

为明晰结冰风洞中液滴相变效应,发展了基于Euler法的气液两相传质传热耦合流动计算方法,探索了3 m×2 m结冰风洞主试验段构型相变效应对液滴传热过程的影响,开展了参数影响研究,评估了试验段内液滴过冷状态。结果表明：构型内液滴经历了先蒸发后凝结两个阶段,蒸发效应促进了准一维传热阶段液滴温度的下降趋势,使液滴温度趋于湿球温度,而凝结效应则抑制了三维收缩阶段液滴温度的下降趋势,进而增大了试验段液气温差,影响液滴过冷状态;增大初始相对湿度和试验段气流速度,会导致蒸发效应减弱而凝结效应增强,进而增强了相对湿度和气流速度对液滴过冷状态的影响程度,与此相反,初始液滴尺寸的增加,则会导致蒸发效应和凝结效应均减弱,进而减弱了液滴尺寸对液滴过冷状态的影响程度;在典型工况下,小尺寸液滴（液滴直径范围为40~100 μm）在高风速时（试验段气流速度大于164 m/s）将偏离过冷状态（液气温差超过3℃）。