The structural variation of the gas diffusion layer and a performance evaluation of polymer electrolyte fuel cells as a function of clamping pressure

Interfacial contact resistance between gas diffusion layers (GDLs) and bipolar plates (BPs) has a substantial effect on the performance loss of polymer electrolyte fuel cells (PEFCs). Particularly during the final manufacturing process of a fuel cell stack, an externally applied clamping load determines the extent of electrical contact between those two solid components. In order to have the least electrical contact loss, it is highly necessary to keep all PEFC components close each other without causing structural failure of fuel cell stacks. In the present work, we investigated the effect of the clamping pressure on extrinsic properties such as porosity and permeability, which is closely related to mass transfer of reactants. Also, the variance of interfacial electrical resistance was analyzed as a function of the stack clamping pressure or the compressed GDL thickness, which reflects the external clamping load. Then with these experimentally obtained material properties of GDL, computational efforts were made to account for the effect of the clamping pressure on the fuel cell performance.     

Effects of microporous layer penetration ratio and substrate carbonization temperature on the performance of proton exchange membrane fuel cells

The gas diffusion layer (GDL) is composed of a microporous layer (MPL) and a substrate; this substrate is generally fabricated from carbon fiber, carbonized resin, and polytetrafluoroethylene. When the MPL penetrates deeper into the substrate, the porosity and pore size of the GDL decrease, and the tortuosity increases; this leads to a reduction in the water discharge capability of the GDL. In this study, the MPL penetration ratio over the total GDL thickness was controlled using three different substrate manufacturing methods. These manufacturing methods for preventing the MPL from penetrating deeper into the substrate were based on the carbon fiber content within the substrate, the amount of carbonized resin coating on the substrate, and the approach used for loading the MPL. Furthermore, the GDLs were manufactured at different carbonization temperatures to investigate the effects of the carbonization temperature of the substrate on the performance of the proton-exchange membrane fuel cell.

     

Supercooling release of micro-size water droplets on microporous surfaces with cooling

The gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells plays a key role in controlling moisture in these cells. When the GDL is exposed to a cold environment, the water droplets or water nets in the GDL freeze. This work observed the supercooling and freezing behaviors of water droplets under low temperature. A GDL made of carbon fiber was coated with a waterproof material with 0%, 40%, and 60% PTFE (polytetrafluoroethylene) contents. The cooling process was investigated according to temperature, and the water droplets on the GDL were supercooled and frozen. Delay in the supercooling release was correlated with the size of water droplets on the GDL and the coating rate of the layer. Moreover, the supercooling degree of the droplets decreased as the number of freeze-thaw cycles in the GDL increased.

     

Thin-walled pipe expansion by a punch with a curvilinear profile

A model is developed for the nonstationary process of thin-walled pipe expansion by a punch with a curvilinear profile generator by taking into account hardening, normal anisotropy, wall thickness measurement, and contact friction according to the membrane theory of rigid-plastic shells with a quadratic plasticity condition. The computational program of the model determines the distributions of thickness, meridian stress, contact pressure, and hardened material yield load along the generating line of the deformed tube and the expansion force variation from the punch displacement.     

Prediction of plastic deformation under contact condition by quasi-static and dynamic simulations using explicit finite element analysis

We compared the quasi-static and dynamic simulation responses on elastic-plastic deformation of advanced alloys using Finite element (FE) method with an explicit numerical algorithm. A geometrical model consisting of a cylinder-on-flat surface contact under a normal load and sliding motion was examined. Two aeroengine materials, Ti-6Al-4V and Super CMV (Cr-Mo-V) alloy, were employed in the FE analysis. The FE model was validated by comparative magnitudes of the FE-predicted maximum contact pressure variation along the contact half-width length with the theoretical Hertzian contact solution. Results show that the (compressive) displacement of the initial contact surface steadily increases for the quasi-static load case, but accumulates at an increasing rate to the maximum level for the dynamic loading. However, the relatively higher stiffness and yield strength of the Super CMV alloy resulted in limited deformation and low plastic strain when compared to the Ti-6Al-4V alloy. The accumulated equivalent plastic strain of the material point at the initial contact position was nearly a thousand times higher for the dynamic load case (for example, 6.592 for Ti-6Al-4V, 1.0 kN) when compared to the quasi-static loading (only 0.0072). During the loading step, the von Mises stress increased with a decreasing and increasing rate for the quasi-static and dynamic load case, respectively. A sudden increase in the stress magnitude to the respective peak value was registered due to the additional constraint to overcome the static friction of the mating surfaces during the sliding step.

     

深沟球轴承的载荷分布与刚度特征研究

采用有限元法计算了"奇压"与"偶压"两种受载形式下深沟球轴承的静力学响应。对比讨论了轴承内接触应力分布、轴承变形及刚度的变化特征。结果表明当外载荷达到某一值时,关键滚动体与外圈滚道接触中心的接触应力明显大于与内圈滚道接触中心的接触应力,并随着载荷的增加,这个趋势越发明显;两种受载形式下,轴承的位移与刚度表现出明显的差异,奇压形式下,轴承的径向位移小于偶压形式下的径向位移,而奇压形式下的轴承刚度则大于偶压形式下的刚度;相同载荷作用下,静力学方法计算的轴承径向位移小于有限元法计算结果。     

Prediction of flow crossover in the GDL of PEFC using serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for Polymer electrolyte fuel cells (PEFCs) since it ensures the removal of water produced in the cell with an acceptable parasitic load. The operating parameters such as temperature, pressure and flow distribution in the flow channel and gas diffusion layer (GDL) has a great influence on the performance of PEFCs. It is desired to have an optimum pressure drop because a certain pressure drop helps to remove excess liquid water from the fuel cell, too much of pressure drop would increase parasitic power needed for the pumping air through the fuel cell. In order to accurately estimate the pressure drop precise calculation of mass conservation is necessary. Flow crossover in the serpentine channel and GDL of PEFC has been investigated by using a transient, non-isothermal and three-dimensional numerical model. Considerable amount of cross flow through GDL is found and its influence on the pressure variation in the channel is identified. The results obtained by numerical simulation are also compared with the experimental as well as theoretical solution.     

滚动轮胎接地性能有限元分析

为深入理解轮胎滚动运动学机理，建立了轮胎在平面上滚动的三维非线性有限元模型。在模型中考虑了轮胎的几何非线性、材料非线性以及轮胎接触非线性，计算了子午线轮胎195／60R14在不同速度滚动时的变形情况、接地区压力分布、摩擦力分布、带束层应力分布等滚动特性。重点对轮胎胎面的变形情况进行了分析，为轮胎结构设计、振动与噪声分析提供了依据。     

质子交换膜燃料电池密封系统热力耦合仿真研究

温度影响质子交换膜燃料电池（PEMFC）的密封性能和力学行为,因而影响其使用寿命和可靠性。为研究PEMFC在热力耦合下的密封性能和力学行为,建立PEMFC单电池和多电池结构的二维模型,研究密封系统在不同工作温度下的应力-应变分布,讨论橡胶密封圈压缩比、双极板错位和密封垫尺寸对PEMFC密封性能和力学性能的影响。结果表明：温度对密封圈的Mises应力和膜电极组件（MEA）框架接触压力有很大影响;在不同工作温度下单电池和多电池结构的密封性能相似,应力和接触压力分布差别也不大,因此单电池结构的研究结论可以推广到多电池结构;随着橡胶密封圈压缩比和密封圈尺寸的增加,燃料电池密封性能得到改善;而双极板错位会加剧MEA框架的变形;高应力区出现在橡胶密封圈的横截面内部,容易导致局部应力集中和密封失效。     

Fabrication of micro-channel arrays on thin stainless steel sheets for proton exchange membrane fuel cells using micro-stamping technology

Bipolar plates are one of the crucial components of proton exchange membrane fuel cells. Because of the expensive production costs of traditional graphite bipolar plates, which require a few millimeters thickness over the space, the resulting metal bipolar plate reduces the cost, and the thickness can be reduced to the micron range. This study explored the application of micro-stamping technology to produce thin metal bipolar plates with the relevant process parameters. In this study, the channel design was 0.8?×0.75 mm for the use of a rigid punch on a 50-μm-thick stainless steel sheet (SUS 304) for micro-channel stamping processes. The finite element method and the experimental results were used to analyze the main parameters of the micro-stamping process. The traditional material model and the scale factor modified material model were used for simulation. The experimental results verified that the modified material model is more realistic for products and has superior similarity because the punch load is relatively small. This study used updated Lagrangian formulation concept to establish an elastic–plastic deformation finite element analysis model and scale factor to modify the calculation to effectively simulate the micro-stamping process for metal bipolar plates.     

考虑硬度变化的结合面法向接触刚度分形模型*

基于分形几何理论,考虑微凸体因应变硬化而造成弹塑性变形阶段硬度随变形量变化而变化,建立结合面第一、第二弹塑性变形阶段单次加载刚度分形模型。推导出在计入硬度变化的情况下,单个微凸体在弹塑性变形阶段法向接触刚度与接触面积之间的关系式,进而得出结合面在弹塑性变形阶段法向接触刚度与接触面积、接触载荷之间量纲为一的关系式,并通过仿真分析得出相关参数对结合面法向接触刚度的影响。仿真结果显示:考虑硬度变化时,结合面量纲一法向接触刚度的值与法向实际接触载荷、实际接触面积之间存在关系;结合面法向接触刚度随着分形维数D的增大而增大;分形维数一定时,结合面法向接触刚度随表面长度尺度参数G值增大而增大。     

Effect of bipolar plate materials on the stress distribution and interfacial contact resistance in PEM fuel cell

In polymer electrolyte fuel cell (PEFC) system, ideal structural integrity design takes into account the end-to-end load transfer mechanism. Hence, structural durability of cell is dependent of individual layers response to the external clamp loading. Since failure modes of each layer differ from another, multidisciplinary approach is needed to innovative designs. Bipolar plate (BPP) of PEFC is multi-functional layer that has a significant potential for research to avoid its structural failure as well as neighboring layers. To this end, present work investigates the effect of BPP materials on the stress transfer as well as distribution in cell layers based on theoretical investigation for clamping load. Gas-diffusion layer (GDL) / BPP interface has drawn considerable interest among researchers due to its susceptibility for damage failure and other related losses. Hence, investigating the interfacial behavior and relating this to the electrical contact resistance is the key feature in proposed investigation.     

机械结合面法向动态接触刚度理论模型与试验研究

机械结合面经常可能在动态条件下工作,使得结合面的接触状态偏离静态工作状态,导致接触刚度发生改变。为了揭示动态接触刚度的变化规律,考虑两个粗糙结合表面上单个微凸体由弹性变形向弹塑性变形以至最终向完全塑性变形转化的接触过程,建立一个振动周期内各变形阶段微凸体的平均接触刚度模型;在此基础上,基于高斯分布假设,建立整个粗糙结合表面的动态接触刚度解析模型。该模型揭示了接触面压、振动频率和相对位移振幅对动态接触刚度的影响规律,并与试验结果和静态接触刚度计算结果进行了比较。研究表明:法向动态接触刚度和静态接触刚度与接触面压之间的关系基本一致,但有一定偏离。这种偏离程度随动态位移幅值和振动频率的增加而分别呈线性和非线性增大。     

考虑基体变形的混合润滑结合面接触特性

为研究混合润滑状态下粗糙表面基体变形对结合面接触特性的影响,建立了考虑基体变形的结合面接触刚度模型。首先,通过单微凸体-基体系统模型分别求解微凸体和基体的接触刚度,利用不动点迭代法获得微凸体真实变形量;其次,基于分形理论建立结合面固体接触刚度模型,通过固体接触刚度获得液体介质的接触刚度。根据仿真结果分析了基体变形、粗糙表面形貌以及润滑介质对结合面接触特性的影响规律。结果表明:当真实接触面积一定时,通过新模型计算的法向载荷小于忽略基体变形的模型;在接触前期,结合面的接触刚度主要由液体介质接触刚度主导,随着真实接触面积的增加,液体接触刚度占总刚度的比率越来越小,最后转变为固体的接触刚度主导结合面的接触刚度。该模型为研究混合润滑状态下结合面的接触特性提供了理论基础。     

基于ANSYS的四列圆柱滚子轴承接触应力和刚度分析

对四列圆柱滚子轴承的接触应力、变形以及径向刚度进行了研究.建立了四列圆柱滚子轴承的有限元三维模型,基于ANSYS和Hertz接触理论,阐述了建模过程中的关键步骤,结合短应力线轧机的应用实例,计算了轴承滚子、内圈的接触应力与变形分布,直观表达了交错排列滚子之间的应力分布情况,计算结果和Hertz理论值相比较,具有较好的一致性.通过计算不同载荷下的轴承变形（轴承内孔轴线的相对位移）,获得了轴承刚度随径向载荷变化的规律.     

Three-dimensional numerical analysis of proton exchange membrane fuel cell

A full three-dimensional, non-isothermal computational fluid dynamics model of a proton exchange membrane fuel cell (PEMFC) with both the gas distribution flow channels and the membrane electrode assembly (MEA) has been developed. A single set of conservation equations which are valid for the flow channels, gas-diffusion electrodes, catalyst layers, and the membrane region are developed and numerically solved using a finite volume based computational fluid dynamics technique. In this research some parameters such as Oxygen consumption and fuel cell performance according to the variation of porosity, thickness of gas diffusion layer, and the effect of the boundary conditions were investigated in more details. Numerical results shown that the higher values of gas diffusion layer porosity improve the mass transport within the cell, and this leads to reduce the mass transport loss. The gas diffusion layer thickness affects the fuel cell mass transport. A thinner gas diffusion layer increases the mass transport, and consequently the performance of the fuel cell. Furthermore, the study of boundary conditions effects showed that by insulating the bipolar surfaces, hydrogen and oxygen consumption at the anode and cathode sides increase; so that the fuel cell performance would be optimized. Finally the numerical results of proposed CFD model are compared with the available experimental data that represent good agreement.     

钢桥腹板间隙出平面变形影响因素敏感性分析

钢桥腹板间隙处的出平面变形破坏是钢桥破坏的常见形式之一。通过有限元数值模拟的方法,计算了不同腹板间隙大小,腹板厚度,以及肋板刚度的情况下,腹板间隙处范式等效应力和出平面位移量的分布规律;采用敏感性分析方法,计算了最大范式等效应力和最大出平面位移量对腹板间隙大小,腹板厚度,肋板刚度的敏感性程度。提出参数变化范围内,系统特性对影响因素平均敏感度的概念。研究表明,最大范式等效应力对腹板间隙大小最敏感,最大出平面位移量对腹板厚度最敏感。在实际应用中,采用合适的腹板间隙大小可以使最大范式等效应力最小,通过增加腹板厚度可以同时减小腹板间隙处的最大等效应力和最大出平面位移量,肋板采用较柔性的材料,能够有效减少腹板间隙处的出平面变形。     

多交叉曲梁簧片柔性铰链的力学建模与性能分析

针对直梁型多交叉簧片柔性铰链难以兼顾大转角、低刚度的问题,设计了一种多交叉曲梁簧片柔性铰链。采用圆弧曲梁簧片减小变形应力和转动刚度,在纯转矩驱动条件下具有高转动精度和大转角行程。推导了圆弧曲梁簧片的变形应力计算方程,建立了柔性铰链的大变形力学分析模型,并通过转动刚度实验和变形应力仿真计算验证了理论模型的准确性。进一步分析了半径系数、簧片曲率和位形角等设计变量对柔性铰链驱动转矩和变形应力的影响关系,提出了同时减小转动刚度和变形应力的设计方案,确定了使柔性铰链获得零刚度特性和负刚度特性的设计变量组合。所设计的柔性铰链结构及其性能分析结果可为新型大行程柔性铰链的设计提供参考。     

Thermo-mechanical analysis of angular contact ball bearing

The thermal-mechanical character, which is difficult to ensure because of the lack of a corresponding theory and tool, has a significant effect on the dynamics of bearings. It even leads to a sudden failure of bearings in a working situation. In this research, a thermal deformation model was established, based on the analysis of temperature effect on the basic size of angular contact ball bearing. And the transmission from rolling size to bearing axial stiffness was explicit. On the basis of the variation of Hertz contact stiffness and the change of initial contact angle of angular contact ball bearing caused by temperature rise, a “Thermo- mechanical” model of bearing was proposed. According to this model, using the corresponding calculation procedure programmed by MATLAB, the effect of bearing temperature on the axial stiffness has been studied. And the correctness of this model was verified with experiments. Some design suggestions have been made for the decision of bearing preload: to prevent the bearing failure caused by overheating.     

A finite element model of an idealized diarthrodial joint to investigate the effects of variation in the mechanical properties of the tissues

The stiffness of articular cartilage increases dramatically with increasing rate of loading, and it has been hypothesized that increasing the stiffness of the subchondral bone may result in damaging stresses being generated in the articular cartilage. Despite the interdependence of these tissues in a joint, little is understood of the effect of such changes in one tissue on stresses generated in another. To investigate this, a parametric finite element model of an idealized joint was developed. The model incorporated layers representing articular cartilage, calcified cartilage, the subchondral bone plate and cancellous bone. Taguchi factorial design techniques, employing a two-level full-factorial and a four-level fractional factorial design, were used to vary the material properties and thicknesses of the layers over the wide range of values found in the literature. The effects on the maximum values of von Mises stress in each of the tissues are reported here. The stiffness of the cartilage was the main factor that determined the stress in the articular cartilage. This, and the thickness of the cartilage, also had the largest effect on the stresses in all the other tissues with the exception of the subchondral bone plate, in which stresses were dominated by its own stiffness. The stiffness of the underlying subchondral bone had no effect on the stresses generated in the cartilage. This study shows how stresses in the various tissues are affected by changes in their mechanical properties and thicknesses. It also demonstrates the benefits of a structured, systematic approach to investigating parameter variation in finite element models.