Effects of porosity gradient in gas diffusion layers on performance of proton exchange membrane fuel cells

A three-dimensional, two-phase, non-isothermal model has been developed to explore the interaction between heat and water transport in proton exchange membrane fuel cells (PEMFCs). Water condensate produced from the electrochemical reaction may accumulate in the open pores of the gas diffusion layer (GDL) and retard the oxygen transport to the catalyst sites. This study predicts the enhancement of the water transport for linear porosity gradient in the cathode GDL of a PEMFC. An optimal porosity distribution was found based on a parametric study. Results show that a optimal linear porosity gradient with ?₁ = 0.7 and ?₂ = 0.3 for the parallel and z-serpentine channel design leads to a maximum increase in the limiting current density from 10,696 Am⁻² to 13,136 Am⁻² and 14,053 Am⁻² to 16,616 Am⁻² at 0.49 V, respectively. On the other hand, the oxygen usage also increases from 36% to 46% for the parallel channel design and from 55% to 67% for the z-serpentine channel design. The formation of a porosity gradient in the GDL enhances the capillary diffusivity, increases the electrical conductivity, and hence, benefits the oxygen transport throughout the GDL. The present study provides a theoretical support for existing reports that a GDL with a gradient porosity improves cell performance.     

Characterization of defects of pulsed thermography inspections by orthogonal polynomial decomposition

Methods in infrared nondestructive testing such as normalized thermal contrast (NTC) and thermal signal reconstruction (TSR) are based on a pixel-by-pixel time domain analysis that ignores spatial correlation. Other techniques in the frequency domain such as pulsed phase thermography (PPT) present the same disadvantage. In this paper, we propose a method to evaluate defects in composite specimens based on image decomposition into a 2D orthogonal space. We compare NTC, TSR, PPT, and principal component thermography (PCT) with the new approach by inspecting three samples of composite anisotropic materials. Without defining a sound area our method estimates the depths of defects up to 1.2 mm in carbon and glass reinforced plastic specimens. An implementation of the proposed method in Matlab is available at https://github.com/charlielito/2DOrthogonal_Polynomial_Decomposition.     

Transient behavior of vascularized walls exposed to sudden heating

Vascular flow architectures are proposed for controlling the temperature of walls that are subjected suddenly to intense heating from one side. After a short delay, single-phase coolant starts flowing from the other side, and fights off the heating effect. The time-dependent behavior of such vascularized composites is studied and optimized based on full numerical simulations of transient conjugate heat transfer. The focus is on the hot-spot temperatures that build up inside the composite – their evolution, spatial migration, and highest levels. Of interest are vasculature designs that keep the hot-spot temperatures below the safe level associated with long-term operation with steady heating from one side and steady coolant flow from the other side. It is shown that when the driving pressure difference is fixed, the approach to the steady-state temperature is the shortest when the dendrites have an optimal (finite) number of bifurcation levels. The allowable delay time is approximately the same as the duration that the hot-spot temperature reaches the steady-state hot-spot temperature in the absence of coolant.     

有噪信号下ADC量化特性的分析与应用

传统的ADC特性参数的测试,通常是将一纯度很高的模拟正弦信号作为激励信号,然后在无噪的条件下,通过对数字输出结果进行分析从而得出ADC的特性参数.然而,在很多实际应用领域中,通常输入的信号叠加有噪声.本文研究了有噪信号下ADC量化特性,提出了有噪信号下ADC量化特性的分析模型,并从理论上做出了相应解释.研究分析表明,有噪信号下ADC量化有其自身规律,利用本文提出的测试方法,输入信号的任何微小变化都可反映在测试结果中,这一方面可以使得在原高分辨率ADC的基础上获得更高的分辨率成为可能,另一方面可为微弱信号采集系统正确选用性能优良的ADC提供足够的技术支持,有助于改进微弱信号检测或低信噪比情况下的系统设计.最后给出了ADC有噪信号下的量化累加输出信噪比曲线,可为实际工程应用提供参考.     

图像的情感特征分析及其和谐感评价

情感信息处理是日本、美国等科学技术发达国家正着手研究的前沿课题,本文以图像的情感信息为研究对象,首先建立了二维波动的数学模型,然后提出了一种图像波动分析的方法,并用SD尺度法对三类典型波动规律的图像进行了心理测试,同时使用方差分析法验证了心理测试结果差异的显著性.证明了符合“1/f波动”规律的图像能够给人以和谐与美的感觉.     

Database sampling with functional dependencies

During the development of information systems, there is a need to prototype the database that the applications will use when in operation. A prototype database can be built by sampling data from an existing database. Including relevant semantic information when extracting a sample from a database is considered invaluable to support the development of data-intensive applications. Functional dependencies are an example of semantic information that could be considered when sampling a database. This paper investigates how a database relation can be sampled so that the resulting sample satisfies precisely a given set of functional dependencies (and its logical consequences), i.e. is an Armstrong relation.     

Aspects of residual thermal stress/strain in particle reinforced metal matrix composites

The influence of inclusion geometry and thermal residual stresses and strains on the mechanical behaviour of a 20 vol% Al₂O₃ particulate reinforced 6061-T0 Al alloy metal matrix composite is investigated through finite element analysis. The introduction of residual thermal stresses/strains prior to external loading leads to a decrease of the proportional limit, 0.2% offset yield stress and the apparent stiffness. The residual stresses/strains are shown to have a greater effect on the composite behaviour under compressive loading than tensile loading. The residual stresses/strains have little effect on the cyclic behaviour of the composite. In only the second cycle, the difference between the cyclic curves, with and without a thermal history, was 2 MPa. Use of a cube shaped particle, with sharp corners and edges, in the unit cell model led to much greater initial hardening behaviour than spherical inclusions, and therefore a greater 0.2% offset yield stress due to stress/strain localisation at the particle corners and edges. This results in regions of constrained plasticity and high stress triaxiality in the matrix around the particle, producing improved load transfer in the composite. It is shown that inclusion aspect ratio, in the range of 0.5–2.0, has an impact on the yield stress. A minimum yield stress occurred at an aspect ratio of approximately 0.9 with significant increases on either side of this point. The influence of residual stress/strain had a similar effect throughout the aspect ratio range except tensile loading, following thermal treatment, on unit cells with inclusion aspect ratios greater than 1.5 resulted in the highest yield stresses.     

Use of composites in civil engineering infrastracture

Composites have successfully been used in the rehabilitation of concrete, cast iron and steel elements. Dr Holger Garden of Taylor Woodrow Construction Ltd, UK, shows how composites have been used to meet programme and site working constraints in restoration work and also looks at some new build applications.