High‐quality imaging in environmental scanning electron microscopy – optimizing the pressure limiting system and the secondary electron detection of a commercially available ESEM

In environmental scanning electron microscopy applications in the kPa regime are of increasing interest for the investigation of wet and biological samples, because neither sample preparation nor extensive cooling are necessary. Unfortunately, the applications are limited by poor image quality. In this work the image quality at high pressures of a FEI Quanta 600 (field emission gun) and a FEI Quanta 200 (thermionic gun) is greatly improved by optimizing the pressure limiting system and the secondary electron (SE) detection system. The scattering of the primary electron beam strongly increases with pressure and thus the image quality vanishes. The key to high‐image quality at high pressures is to reduce scattering as far as possible while maintaining ideal operation conditions for the SE‐detector. The amount of scattering is reduced by reducing both the additional stagnation gas thickness (aSGT) and the environmental distance (ED). A new aperture holder is presented that significantly reduces the aSGT while maintaining the same field‐of‐view (FOV) as the original design. With this aperture holder it is also possible to make the aSGT even smaller at the expense of a smaller FOV. A new blade‐shaped SE‐detector is presented yielding better image quality than usual flat SE‐detectors. The electrode of the new SE detector is positioned on the sample table, which allows the SE‐detector to operate at ideal conditions regardless of pressure and ED.     

Unsteady stagnation flow and heat transfer towards a shrinking sheet

The unsteady stagnation flow towards a shrinking sheet is investigated. With assumptions that the sheet is shrunk impulsively from rest, and simultaneously the surface temperature is suddenly increased from that of surrounding fluid, the boundary layer equations are transformed to a set of nonlinear partial differential equations by means of a similarity transformation. The highly accurate analytical approximations are given, which match the numerical results given by the Keller–Box scheme.     

Impact of the Cattaneo‐Christov thermal and solutal diffusion models on the stagnation point slip flow of Walters' B nanofluid past an electromagnetic sheet

The present study investigates the effect of Cattaneo‐Christov thermal and solutal diffusion on the stagnation point flow of Walters' B nanofluid past an electromagnetic sheet subject to velocity, thermal, and solutal slips. The study analyzed the role of electromagnetic fields. In addition, the authors introduced the heat transfer aspect due to Brownian motion and thermophoretic force. The numerical solution of the transformed governing equations employed the spectral local linearization method. Comparisons showed an excellent agreement with the numerical data presented in previous notable works. The study reveals that the developed electromagnetic field due to the arrangement of the sheet causes accelerated fluid motion, and diminution of nondimensional temperature and concentration. In addition, augmented velocity, thermal, and solutal slips develop the corresponding descending boundary layers. The augmented short memory coefficient enhances the skin friction coefficient. The Cattaneo‐Christov thermal and solutal diffusion upsurge the heat and mass transfer rates from the electromagnetic sheet, respectively.     

轴流转桨式水轮机桨叶卡阻原因分析和故障处理

2006年8月,长湖发电公司1号机转轮桨叶发生卡阻故障,经现场转轮动作试验,并结合机组检修,分析查找轴流转桨式水轮机转轮桨叶卡阻的原因,制定相关处理方法,最终解决卡阻故障。     

偏导射流式压力伺服阀污染卡滞机制及影响研究

偏导射流式压力阀属于典型的复杂电液伺服系统,服役期间油液污染会引起油液等级增高,造成整阀性能的渐进劣化甚至结束耐久寿命。针对此问题,阐述偏导射流式压力阀工作原理并建立整阀的数学模型;分析污染卡滞机制并建立了偏导射流式压力阀污染卡滞摩擦力模型,同时分析GJB420B中污染等级颗粒数量、直径分布并计算了相应摩擦力数值;搭建整阀模型,进行了不同污染等级下偏导射流式压力阀性能劣化仿真。结果表明:阀芯卡滞摩擦力随着油液污染等级的增高而逐渐提高,并最终引起偏导射流式压力阀输出压力幅值的衰减及响应速度减慢。研究方法和结果对压力伺服阀污染卡滞故障的定性分析、预测及恶劣工况下延寿具有重要参考价值。     

添加难熔金属碳化物提高C/C复合材料抗烧蚀性能的研究

高性能固体火箭发动机（SRM）对喉衬材料的烧蚀率提出了更高的要求,传统的炭／炭（C／C）材料有待于进一步发展,讨论了含有难熔金属碳化物（ZrC,TaC) 的多元基体C／C复合材料及其耐烧蚀性能,通过微观结构分析,探讨该种材料抗烧蚀性能提高的机理,展示了这种材料作为高温烧蚀材料的良好前景。     

小间距两喷嘴对置撞击流流场的数值模拟与实验研究

运用热线风速仪和CFD软件对小喷嘴间距下两喷嘴对置撞击流时均流场进行了实验研究和数值模拟,并和文献中的实验结果和近似解析式进行了比较。研究结果表明：由于边界层存在,单股喷嘴出口速度分布为“礼帽”形状分布;在L<2D(L为喷嘴间距,D为喷嘴直径)时,喷嘴出口速度剖面出现中间低、两边高的“双峰”形状, L=2D时,“双峰”形状消失。随着喷嘴间距的增大,相同气速比导致的撞击面驻点的偏移量增大。相同气速比下,喷嘴出口为“礼帽”分布时驻点的偏移量比均匀分布时大。文献中的撞击流流场的近似解析式对喷嘴出口速度分布为均匀分布有很好的精度,当喷嘴出口速度为“礼帽”分布时,文献中近似解析式的预报精度变差。     

多尺度绿地景观格局对滞蓄能力的影响研究

城市绿地系统作为构建城市海绵系统中的重要组成要素,通过优化绿地景观格局以提升城市整体滞蓄能力的方式更符合可持续发展的要求.以沈阳市三环内建成区为研究范围,利用景观格局指数方法,对城区、场地2个研究尺度的绿地景观格局进行评价,并利用相关性分析与回归分析等数理统计方法对2个研究尺度的绿地景观格局与内涝风险之间的关系进行分析...     

基于进化停滞周期的局部变异PSO算法及其收敛性分析

为了克服粒子群优化算法容易陷入局部最优而发生早熟收敛的问题,提出一种基于进化停滞周期的局部变异粒于群优化算法.算法引入进化停滞周期和近期全局最优位置的概念,使粒子的飞行受近期全局最优位置影响,并在种群进化停滞时对随机选中的局部粒子执行变异操作,增加种群多样性,扩大搜索范围,提高求解质量.算法用种群进化停滞周期代替多样性度量,避免了多样性计算引起的高计算复杂度.对于几个常用基准函数的仿真结果验证了算法的合理性和有效性.     

OPTIMAL NOZZLE-TO-PLATE SPACINE FOR CONVECTIVE HEAT TRANSFER IN NONISOTHERMAL, VARIABLE-DENSITY IMPINGING JETS

The optimal nozzle-to-plate spacing is experimentally studied for the case when a free jet of high temperature gas impinges normally on a large flat plate. The stagnation-point heat transfer becomes maximal when the flat plate is placed at the end of the potential core region. A generalized model is proposed which permits prediction of the optimal nozzle-to-plate spacing and stagnation-point heat flux over a wide range of the initial density ratio.