Cell and stack‐level study of steady‐state and transient behaviour of temperature uniformity of open‐cathode proton exchange membrane fuel cells

The steady‐state temperature uniformity and thermal transients of open‐cathode proton exchange membrane fuel cell (PEMFC) at cell and stack level are researched experimentally in this study. The local temperatures are obtained by 30 thermocouples contacting the surfaces of cathode gas diffusion layers (GDL). The s temperature homogeneity under different load currents and air flow rates are investigated. The results reveal that the fluctuation of temperature distribution under different currents is small under the lowest air flow rate set in the experiments. Comparatively, the temperature is less uniform when the load current is higher under other air flow rates. The evaluation indicator, temperature uniformity index (TUI), varies nearly linearly with the current. And the maximum variation is 55.6% to 59.0%. This distinct behaviour is probably related to the existence of liquid water and its nonuniform distribution which can enlarge the temperature difference at high current. With respect to thermal transients, there is rapid deterioration in temperature uniformity when the load current is stepped up. It may arise from the uneven liquid water distribution which can lead to different temperature variation rates. Further, the research gives direction for optimization of cooling strategy and thermal management of open‐cathode PEMFC stack in application.     

大径厚比薄壁封头多约束旋压成形方法

大型薄壁封头具有大径厚比、弱刚性的特点,利用传统旋压加工方法易出现起皱等失稳现象。依据中心约束旋压法的成形缺陷,从能量法及金属塑性变形理论方面提出了多约束旋压新工艺方案,分析表明:多约束多道次配合作用能够增加工件成形的约束能,有效抑制法兰外缘起皱现象,成形件径厚比大,表面光洁度良好。通过逆行工程软件Poly Works进行模型重构,得到实际加工样件母线方向的壁厚数据,其壁厚分布规律表现为两端欠减薄,中间部分过减薄,旋压终端由于约束作用,壁厚增厚明显。正负壁厚偏差比稳定在15%以内,壁厚均匀性良好。     

Second order polynomial class of chip waveforms for band-limited DS-CDMA systems

The conventional frequency domain square-root raised cosine (Nyquist) chip waveform has much poorer anti-multiple-access-interference (anti-MAI) capability than the optimal band- limited waveform in direct sequence code division multiple access (DS-CDMA) systems. However, the digital implementation of the optimal chip pulse is very costly due to the slow decaying rate of the time waveform. In addition, its eye diagram and envelope uniformity are worse than the Nyquist pulse for a wide range of roll-off factor, which will incur performance degradation due to timing jitters and post non-linear processing. In this paper, based on an elementary density function of a second-order polynomial, a class of second-order continuity pulses is proposed. From this class of pulses, we can find some members having faster decaying rate, bigger eye opening, more uniform envelope and stronger anti-MAI capability than the Nyquist waveform. The normalized-band-width-pulse-shape-factor product, the decaying rate of the tail of the time waveform, the opening of the eye diagram, and the envelope uniformity of the second-order continuity pulses are addressed in the paper that provide the basic information for the selection of the chip pulse for CDMA systems.     

提高LED背光源质量的研究

随着LED产业的发展,LED光源在背光应用领域也不断拓展。文章主要参照IEC61747液晶和固态显示器件系列标准,从LED背光源的结构特点探讨如何提高亮度和均匀性,同时减少漏光现象,并指出LED背光源将具有良好的发展前景。     

混响室发射天线指向对场均匀性影响研究

采用基于矩量法的电磁仿真软件FEKO建立了混响室仿真模型,并对步进模式下的混响室模型进行了数值计算.将混响室数值计算与遗传算法相结合,通过仿真计算与理论分析研究了混响室发射天线指向对场均匀性的影响.研究结果表明:改变发射天线指向,其本质是改变混响室各模式前的加权系数,导致表征混响室工作区域场均匀性的电场标准偏差随之发生改变.发射天线指向处于最优配置时,工作区域各轴向电场标准偏差较初始状态有所下降,说明通过优化提高了工作区域场均匀性.     

一种改善LED背光亮度均匀性的新型反射膜研究

在LCD的LED背光腔体的底面设计了新型曲面形状及性能的反光膜,将扩散膜反射回灯腔底部的光重新反射回出光口以改善亮度均匀性能;在侧面设计了镜面反射膜,将位于LED矩阵边缘区域由LED射向灯腔侧壁的光反射重新射向出光口,增加了LED背光边缘区域的亮度,从而改善了背光边缘区域较暗的亮度不均匀状况.根据此类反射膜的设计完成了反射膜的制备,并应用于38 cm LED背光源样品.测试结果显示,设置反射膜的LED背光的亮度均匀性值为10.2%,明显优于未设置反射膜的值16.8%,亮度提高了29%左右,表明该种新型反射膜可以提高亮度并改善亮度均匀性.     

非均匀芯片布局的LED模组散热性能研究

芯片阵列均匀排列的常规大功率LED模组因温度场叠加会导致中心温度过高、温度分布不均匀。在理论分析面热源温度分布函数及多热源相互影响的基础上提出了一种LED芯片外密内疏排列的模组设计方案,并与ANSYS有限元仿真的结果进行了对比验证。当芯片按照优化后的对数和幂函数分布时芯片最高温度比常规均匀排列时均低约5℃,芯片温度的方差降低约56%。外密内疏的非均匀芯片排列方案在基板较小、较薄时对散热效果的改善更好。     

杂质分凝对FZ-Si单晶电阻率均匀性影响的研究

对区熔Si单晶样品的轴向、径向以及生长界面的电阻率进行了测试,分析了影响电阻率均匀性的主要因素.结果表明,通常认为的影响电阻率均匀性的因素,并不是决定性因素.本文从FZ单晶在生长过程中,杂质在轴向和侧向分凝的角度进行分析,认为由于晶体原子的侧向生长,产生的杂质侧向分凝,是电阻率分布不均匀的主要原因.大量生产实验表明,生长过程中改变常规生长参数对提高均匀性的作用并不明显,要想从本质上提高电阻率径向均匀性,只能尝试非常规的技术.     

高频驱动超磁致伸缩致动器的磁场设计与分析

针对超磁致伸缩致动器集中于静态与准静态领域的不足,开发设计出一款用于高频驱动的超磁致伸缩致动器。建立完善用于高精度励磁场的轴线磁场强度、三维空间磁场强度的ANSYS模型,完成励磁场均匀度分析,为高频磁场的设计提供理论基础。     

基于FEM的钢带均匀度交流励磁检测方法

该文为检测压延工序后钢带的分布形态,提出一种传感器探磁结构模型。采用矢量磁位有限元法建立系统分析模型进行磁场计算,研究励磁求解所得各项磁性参数。并通过ANSYS EMAG软件模拟在线检测,给出各种典型分布及缺陷状态下的特征信号曲线。试验结果表明:与钢带平面正交的磁密分量及绞线中的电流峰峰值可辨识钢丝分布,且后者更为直观、灵敏,该研究为钢丝帘线磁性检测仪器的研制奠定基础。