工程计价模式与建筑技术发展关系探讨

 本文针对我国建筑技术发展缓慢的现状,对目前新技术、新工艺、新材料、新设备的推广方式、推广通道以及推广应用过程中的问题进行了分析。文章认为,现阶段计价模式存在着一定的缺陷,没有反映市场经济条件下“谁受益、谁付出”的费用承担原则,影响了施工单位对“四新”的推广应用的积极性。文章建议在计价模式中增设“技术应用支持费”计费口,使工程造价的高低与“四新”应用相联系,发挥施工单位在“四新”应用方面的主动性,加快我国“四新”研发、推广、应用的步伐。     

Water Evolution in Direct Methanol Fuel Cell Cathodes Studied by Synchrotron X‐Ray Radiography

Water evolution, distribution, and removal in the cathodes of a running direct methanol fuel cell were investigated by means of synchrotron X‐ray radiography. Radiographs with a spatial resolution of around 5 μm were taken every 5 s. Special cell designs allowing for through‐plane and in‐plane viewing were developed, featuring two mirror‐symmetrical flow field structures consisting of one channel with the through‐plane design. Evolution and discharge of water droplets and the occurrence of water accumulations in selected regions of the channels were investigated. These measurements revealed a nonuniform distribution of water in the channels. Both irregular and periodic formation of water droplets were observed. In‐plane measurements revealed, that the droplets evolve between adjacent carbon fiber bundles of the gas diffusion layer. The water distribution within the channel cross‐section fits very well to the pressure difference between cathode channel inlet and outlet. The quick discharge of water droplets causes sudden decreases of the pressure difference up to 4.5 mbar.     

Flow‐Field Designs of Bipolar Plates in PEM Fuel Cells: Theory and Applications

A generalized model developed by Wang was modified for flow field designs of the most common layout configurations with U‐type arrangement, including single serpentine, multiple serpentine, straight parallel, and interdigitated configurations. A direct and quantitative relationship was established among flow distribution, pressure drop, configurations, structures, and flow conditions. The model was used for a direct, systematic, and quantitative comparison of flow distributions and pressure drops among the most common layout configurations of interest. The straight parallel configuration had the lowest pressure drops but suffered the most possibility of the uneven flow distribution across the channels. The single serpentine had the best flow distribution but had the highest pressure drops. The flow distribution and the pressure drop in the multiple serpentine was between the straight parallel and the single serpentine. Finally, we suggested basic criteria of the flow field designs of bipolar plates for the industrial applications. This provides a practical guideline to evaluate how far a fuel cell is from design operating conditions, and measures how to improve flow distribution and pressure drop.     

认知无线电中基于多址接入信道的多用户联合频谱感知算法

针对多用户联合感知场景问题,考虑次用户至决策中心之间有损信道的情况,提出了基于多址接入信道（MAC）的联合频谱感知算法。在系统结构和数学建模基础上,分析了传统MAC算法渐近性、中断概率等性能。在次用户平均发射功率约束下,以最大化检测概率为目标,对基于MAC算法中的发射增益进行优化;并考虑了一定服务质量情况下最小化次用户数目的问题。仿真结果表明,MAC算法能够保证良好的检测性能,以决策中心错误概率为例,提出的优化算法取得了指数级的性能提升。     

Case Study: Equivalent Widths of the Middle Rio Grande, New Mexico

Successive reaches of the Rio Grande have maintained equivalent channel widths of 50 and 250?m, respectively, over long periods of time. It is hypothesized that alluvial channels adjust bed slope to match the long-term changes in channel width. Analytical relationships show that wider river reaches develop steeper slopes. A modeling approach using daily water and sediment discharges simulates the transient evolution of bed elevation changes. The analytical and numerical models are in very good agreement with the longitudinal profile measurements of the Bosque del Apache reach of the Rio Grande, NM, from 1992 to 1999. The slope of the 50?m wide reach was 50?cm/km and the slope of the 250?m wide reach of the same river increased to 80?cm/km. This unsteady daily transient model compares well with a steady transient solution at a constant discharge close to the mean annual flow. The transient slope adjustments can also be approximated with an exponential model. Accordingly, it takes about 20–25?years for the Rio Grande to achieve about 90% of its slope adjustment.     

Turbulent unsteady flow and heat transfer in channels with periodically mounted square bars

A numerical investigation was conducted to analyze the unsteady turbulent flowfield and heat transfer characteristics in a channel with streamwise periodically mounted square bars arranged side-by-side to the approaching flow. The transverse separation distance between the bars is varied, whereas the bar height to channel height (d/H) are 0.152 and 0.2, the Reynolds number Re based on channel height is 2×10⁴ and the periodicity length is 2H. The channel walls are subjected to a constant wall temperature. The k-ε turbulence model was used in conjunction with the Reynolds-averaged momentum and energy equations for the simulations. A finite volume technique is applied with a fine grid and time resolution. Complex periodic vortex shedding develops in the channel due the interaction between the two streamwise periodically mounted square bars. Results show that the unsteady flow behavior, pressure drop and heat transfer are strongly dependent of the transverse separation distance of the bars.     

正弦波相位差的一种精确评价方法

介绍了正弦波相位差的一种简单实用的精确评价方法 ,即加数字滤波器的正弦波拟合法 ;它可以对通道间延迟进行修正。讨论了评价过程的误差来源以及减小其评价误差的几种对策 ;同时 ,给出了评价过程的几个典型实验结果     

空气流经光滑平行平板通道的换热特性

本文对空气流经光滑平行平板通道时的换热工况进行了理论求解,得出了理论表达式。同时也给出了不同通道高度的实测换热特性曲线,对理论值与实测数值进行了分析比较,提出了适合于工程应用的经验公式。     

生命信息专家诊断系统的设计与实现

本文详细讨论了生命信息专家诊断系统的设计与实现方法,介绍了系统的总体结构、各模块的功能、特点以及具体的实现方案。     

Restoring Coastal Wetlands: The Californian Experience

Coastal wetlands are an important natural resource, providing habitat for fish and wildlife, particularly many rare or endangered species. These wetlands also serve important functions for the treatment of urban runoff and effluent discharges, the dissipation of wave energy, and shore protection. The US Fish and Wildlife Service estimates that approximately 45 000 km² of US wetlands have been destroyed in the past two centuries.
The importance of wetlands in the ecosystem has only begun to be understood during the past two decades, and there has been a major legislative effort to preserve, protect, and enhance existing coastal wetlands. The productivity of tidal wetlands is related directly to the hydrologic characteristics of the system, and some considerations for the hydrologic design for the enhancement or construction of tidal wetlands are outlined. Recent attention has focused on the use of dredged spoil materials from harbours or shippping channels to restore historic tidal wetlands. The hydrologic design criteria are illustrated through a series of case studies in California.