Two-phase Flow in Fuel Cells in Short-term Microgravity Condition

A visual observation of liquid-gas two-phase flow in anode channels of a direct methanol proton exchange membrane fuel cells in microgravity has been carried out in a drop tower. The anode flow bed consisted of 2 manifolds and 11 parallel straight channels. The length, width and depth of single channel with rectangular cross section was 48.0 mm, 2.5 mm and 2.0 mm, respectively. The experimental results indicated that the size of bubbles in microgravity condition is bigger than that in normal gravity. The longer the time, the bigger the bubbles. The velocity of bubbles rising is slower than that in normal gravity because buoyancy lift is very weak in microgravity. The flow pattern in anode channels could change from bubbly flow in normal gravity to slug flow in microgravity. The gas slugs blocked supply of reactants from channels to anode catalyst layer through gas diffusion layer. When the weakened mass transfer causes concentration polarization, the output performance of fuel cells declines.

燃烧物理学基础

Hybrid Qm/Mm Simulation Of The Hydration Phenomena Of Dipalmitoylphosphatidylcholine Headgroup

The polar headgroup of dipalmitoylphosphatidylcholine (DPPC) molecule both in gas phase and aqueous Solution is investigated by the hybrid quantum mechanical/molecular mechanical (QM/MM) method, in which the polar head of DPPC molecule and the bound water molecules are treated with density functional theory (DFT), while the apolar hydrocarbon chain of DPPC molecule is treated with MM method. It is demonstrated that the hybrid QM/MM method is both accurate and efficient to describe the conformations of DPPC headgroup. Folded structures of headgroup are found in gas phase calculations. In this work, both monohydration and polyhydration phenomena are investigated. In monohydration, different water association sites are studied. Both the hydration energy and the quantum properties of DPPC and water molecules are calculated at the DFT level of theory after geometry optimization. The binding force of monohydration is estimated by using the scan method. In polyhydration, more extended conformations are found and hydration energies in different polyhydration styles are estimated. (C) 2008 Elsevier Inc. All rights reserved.

Short-range-order effects on intrinsic plasticity of metallic glasses

Through a systematical analysis of the elastic moduli for 137 metallic glasses (MGs) and 56 polycrystalline metals, we use a simple model developed by Knuyt et al. [J. Phys. F: Met. Phys. 16 (1986) p.1989; Phil. Mag. B 64 (1991) p.299] based on a Gaussian distribution for the first-neighbor distance to reveal the short-range-order (SRO) structural conditions for plasticity of MGs. It is found that the SRO structure with dense atomic packing, large packing dispersion and a significant anharmonicity of atomic interaction within an MG is favorable for its global plasticity. Although these conditions seem paradoxical, their perfect matching is believed to be a key for designing large plastic bulk MGs not only in compression but also in tension.

两层液体Bénard-Marangoni对流的实验研究

采用粒子图像测速技术(PIV)对两层流体B(en)ard-Marangoni对流进行了实验研究.研究了各种不同厚度比下的临界对流模式,同时研究了在温差变大时向超临界对流模式转化过程.实验结果表明,界面张力对各种对流模式的形成和转变具有重要的作用.

基于偏振度检测的光纤电流传感方案

本文提出了一种将光学谐振腔中多次来回反射所产生的法拉第旋转积累效应转化为光学信号偏振度变化的光纤电流传感方案。这种方案的主要优点是可以克服环境等因素的扰动带来偏振态变化所产生的影响。文中对所提出方案的特性进行了简单的理论分析和模拟计算，并给出了相应的实验结果验证。

马赫-曾德尔型相移矢量剪切干涉仪

为了实现对激光波面的等光程测试，设计了一个新颖的相移矢量剪切干涉仪。该干涉仪以马赫一曾德尔结构为基础，为等光程干涉仪。通过在相干的两路光束中分别插入楔角方向正交放置的楔板实现矢量剪切；其中一块楔板分成两块（平板部分和楔板部分），楔板部分沿平板部分的表面移动，通过改变光程差来实现相移，2π相移所需的移动距离为几毫米的量级，该相移方法在相移精度控制上比较简单。文中还对楔板引入的横向剪切误差和相移误差进行了分析，最后给出了实验所得的一组相移干涉图。