A three-dimensional heterogeneity analysis of electrochemical energy conversion in SOFC anodes using electron nanotomography and mathematical modeling

In this paper a fully three dimensional, multiphase, micro-scale solid oxide fuel cell anode transport phenomena numerical model is proposed and verified. The Butler-Volmer model was combined with empirical relations for conductivity and diffusivity - notably the Fuller-Shetler-Giddings equation, and the Fickian model for transport of gas reagents. FIB-SEM tomography of a commercial SOFC stack anode was performed and the resulting images were processed to acquire input data. A novel method for estimating local values of Triple Phase Boundary length density for use in a three-phase, three-dimensional numerical mesh was proposed. The model equations are solved using an in-house code and the results were verified by comparison to an analytical solution within the range of its applicability. A limited parametric study was performed to qualitatively assess simulation performance and impact of heterogeneity. Despite the high dependence of the SOFC anode performance on the geometry of its anisotropic, three-phase microstructure there are very few micro-scale numerical models simulating transport phenomena within these electrodes.     

Wind over complex terrain – Microscale modelling with two types of mesoscale winds at Nygårdsfjell

Nygårdsfjell, a complex terrain near Norwegian-Swedish border, is characterized by its significant wind resources. The feasibility of using mesoscale winds as input to microscale model is studied in this work. The main objective is to take into account the actual terrain effects on wind flow over complex terrain. First set of mesoscale winds are modelled with Weather Research and Forecasting (WRF) numerical tool whereas second set of mesoscale winds are taken from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) data system. WindSim, a computational fluid dynamics based numerical solver is used as microscale modelling tool. The results suggest that the performance of microscale model is largely dependent upon the quality of mesoscale winds as input. The proposed coupled models achieve improvements in wind speed modelling, especially during cold weather. WRF-WindSim coupling showed better results than MERRA-WindSim coupling in all three test cases, as root mean square error (RMSE) decreased by 70.9% for the February case, 61.5% for October and 14.4% for June case respectively. Raw mesoscale winds from the WRF model were also more correct than the mesoscale winds from MERRA data set when extracted directly at the wind turbine by decreasing the RMSE by 62.6% for the February case, 62.7% for October and 23.7% for June case respectively. The difference of RMSE values between the mesoscale winds directly at wind turbine versus the coupled meso-microscale model outputs are not conclusive enough to indicate any specific trend.     

Advances and challenges in the development of power-generation systems at small scales

The miniaturization of electro-mechanical devices, and the resulting need for micro-power generation (milliwatts to watts) with low weight, long life devices, has lead to the recent development of the field of micro-scale combustion and power generation. The primary objective of this new field is to leverage the high energy density of fuels, specifically liquid hydrocarbon fuels relative to batteries and all other energy storage devices other than nuclear fission, fusion or decay. As such, a miniaturized device even with a moderately efficient conversion of hydrocarbon fuels to power would result in increased lifetime and/or reduced weight of an electronic or mechanical system that are currently most often powered by electrochemical cells. Furthermore, improvements in this field may make possible novel applications and/or capability. In addition to the interest in miniaturization, the field is also driven by the potential fabrication of the devices using Micro-Electro-Mechanical Systems (MEMS) or rapid prototyping techniques, with their favorable characteristics for mass production and/or low unit cost. The micro-power generation field is very young, and still is in most cases in the feasibility stage. However, considering that it is a new frontier of technological development, and that only a few projects have been funded, it can be said that significant progress has been made to date. Currently there is consensus, at least among those working in the field, that combustion at the micro-scale is possible with proper thermal and chemical management. Several meso-scale and micro-scale combustors have been developed that appear to operate with good combustion efficiency. Some of these combustors have been applied to energize thermoelectric systems to produce power, although with low overall efficiency. Several turbines/engines have also been, or are being, developed, some of them currently producing positive power, albeit with low efficiency. Micro-rockets using solid or liquid fuels have been built and shown to produce thrust. More detailed scaling/modeling efforts are required to improve existing designs. Improvements in diagnostic, control and computational tools are expected to have a significant impact on the development of the field. Some brief scaling arguments are given in this work, and more detailed efforts are referred. A brief introduction to several of the fabrication techniques is presented in this work. Hydrogen-based and some preliminary specialty fuel micro-fuel cells have been successfully developed, and there is a need to develop reliable reformers (or direct conversion fuel cells) for liquid hydrocarbons so that the fuel cells become competitive with the batteries. In this work, the technological issues related to micro-scale combustion and the development of thermochemical devices for power generation will be discussed. Some of the systems currently being developed will be presented, ongoing critical research issues under investigation, and other potential areas of development discussed. Comments regarding the opportunities and limitations of each of the techniques are also presented where applicable.     

Micro-scale energy dissipation mechanisms during dynamic fracture in natural polyphase ceramic blocks

The dynamic fracture of natural polyphase ceramic (granite) blocks by high-speed impact at 207 m/s, 420 m/s and 537 m/s has been investigated. An electromagnetic railgun was used as the launch system. Results reveal that the number of fragments increases substantially, and the dominant length scale in their probability distributions decreases, as the impact energy is increased. Micro-scale studies of the fracture surfaces reveals evidence of localized temperatures in excess of 2000 K brought on by frictional melting via fracturing and slip along grain boundaries in orthoclase and plagioclase, and via transgranular fracture (micro-cracking) in quartz. The formation of SiO₂- and TiO₂-rich spheroids on fracture surfaces indicates that temperatures in excess of 3500 K are reached during fracture.     

Numerical study of flow anisotropy within a single natural rock joint

The paper investigates the flow anisotropy within a natural joint subjected to mechanical shear. The cubic law is the simplest way to describe fluid flow through rock joints but because of rock wall roughness, deviations from this model have been observed. The Reynolds equation usually gives better results. In this study, micro-scale roughness is taken into account to define a reduced coefficient of permeability. Numerical simulations have been carried out by applying Darcy's law to the rock joint, described as an equivalent porous medium. The numerical simulations are based on experimental data obtained by Hans (PhD, Grenoble, 2002) from a series of hydromechanical shear tests on a rock joint replica. The numerical results have been compared to the experimental ones, and to the results obtained by applying the Reynolds equation, to assess the relevance of the simulations. For the fracture studied, the approach proposed herein can reproduce relatively well the experimental flow anisotropy, and provides consistent values of flow rates, whereas the Reynolds equation tends to give higher flow rates.     

微米和中间尺度机械制造

微米和中间尺度机械制造的概念源于对广泛材料范围内的精密三维微米和中间尺度零件日益增长的需求。微米和中间尺度零件是指尺寸在0.01～10mm范围的微小零件,因而处于基于微电子的MEMS技术与传统的机械加工技术之间的领域。然而,目前的设备性能和尺寸计量方法生产单个微米和中间尺度零件的能力已经显示出局限性。因此,许多研究者正在探索有关新的或传统的机械制造技术在该尺度的应用问题。基于美国国家科学基金会主办的微米和中间尺度机械制造专题讨论会,介绍了日渐显现的微米和中间尺度机械制造技术的背景和现状,讨论了其应用的科学、技术和商业化挑战,以及其有效开展和实现的主要使能技术,给出了需要广泛研究与发展的几个主要方面问题。最后,提出了开展和证明微米和中间尺度机械制造可行性研究比较有希望的方法和策略。以此希望能对我国微米和中间尺度机械制造技术领域的研究有所启迪。     

Surface wetting of the C/SiC brake lining with micro-scale heat dissipation fins to cool off the brake system: Influence of the fibre ending orientation and fin interval

The micro-scale heat dissipation fins significantly contribute to cool off a brake system. However, micro-scale heat dissipation fins will change the surface wetting and then change the humidity of components in the brake system. A higher humidity is helpful for improving the thermal conductivity coefficient of the C/SiC component, which aids in further improving the cooling performance. To the best knowledge of the authors, little attention has been devoted to improving the humidity of the C/SiC brake lining by micro-scale fins. The aim of this study is mainly to discuss the surface wetting of the porous C/SiC brake lining with micro-scale heat dissipation fins to facilitate heat dissipation in the brake system by increasing the humidity. In this study, micro-scale heat dissipation fins with various intervals were fabricated by a laser on three typical C/SiC surfaces. Surface wetting was characterized by the spreading time of the water droplets. The theoretical model of the water droplet spreading time was established by a Washburn-type equation. Both the experimental and theoretical results indicated that: (1) a hydrophilic C/SiC surface could be achieved by fabricating micro-scale heat dissipation fins on a circular fibre-ending surface compared with a pillar fibre-ending surface; (2) wetting control of the C/SiC surface is not obvious by changing of the micro-fin interval on the order of micrometers; and (3) the surface wetting of the pillar fibre-ending C/SiC surface was more sensitive to increased repetitions of laser scanning. In the current stage, the experimental results presented a stochastic surface wetting. In this regard, more details on the irregularity of micro-scale heat dissipation fins resulting from a laser process are discussed. The conclusions can be extended to optimize the heat dissipation fin arrangement of the C/SiC brake lining and optimize the overall cooling performance of the brake system.     

微尺度悬臂管颤振的研究

本文基于修正的偶应力理论并考虑Lagrange应变张量所给出的几何非线性,运用Hamilton原理建立了微悬臂管的平面振动微分方程.通过对线性方程的特征值分析,得到了微管的前四阶复频率及临界流速—质量比曲线(临界流速曲线)对材料长度尺寸参数的依赖关系;并且发现宏观管和微尺度管(或者具有不同材料长度尺寸参数的微管)的临界流速曲线可能会相交.运用基于中心流形—范式理论的投影法,计算了临界流速处系统的第一李雅普诺夫系数和退化特征值关于流速的变化率,以此为基础论证了分岔的超临界性质;并对临界流速曲线上的滞后部分及不同尺度管的该曲线的交点处的动力学性质作了探讨,发现了不同的分岔方向.     

Investigation of micro-scale abrasion–corrosion of WC-based sintered hardmetal and sprayed coating using in situ electrochemical current-noise measurements

The micro-scale wear–corrosion interactions of WC-based sintered hardmetals and sprayed coatings are typically investigated by comparing the wear-rates in corrosive environments with neutral (pH 7) conditions and inferring electrochemical activity. However, for a greater understanding of the wear–corrosion interactions, there is a need to examine the repassivation kinetics during micro-abrasion tests under different pH conditions. This paper details in situ electrochemical current-noise measurements performed using a modified micro-abrasion tester to elucidate these wear–corrosion interactions for pH 7–13 conditions for sintered WC–5.7Co–0.3Cr and sprayed WC–10Co–4Cr specimens. Electrochemical measurements and SEM micrographs of worn surfaces are used to detail the degradation process. Discussion will focus on the wear–corrosion interactions present under neutral and alkaline conditions for sintered and sprayed specimens and the influence of microstructure on the electrochemical activity will be detailed.     

A biomass-fired micro-scale CHP system with organic Rankine cycle (ORC) – Thermodynamic modelling studies

This paper presents the results of thermodynamics modelling studies of a 2 kW (e) biomass-fired CHP system with organic Rankine cycle (ORC). Three environmentally friendly refrigerants, namely HFE7000, HFE7100 and n-pentane, have been selected as the ORC fluids. The thermodynamic properties of the selected ORC fluids which have been predicted by commercial software (EES) are used to predict the thermal efficiency of ORC. The results of modelling show that under the simulated conditions (1) the ORC thermal efficiency with any selected ORC fluid is well below (roughly about 60% of) the Carnot cycle efficiency; the ORC efficiency depends on not only the modelling conditions but also the ORC fluid – the highest predicted ORC efficiency is 16.6%; the predicted ORC efficiency follows the following order: n-pentane > HFE7000 > HFE7100 (2) both superheating and sub-cooling are detrimental to the ORC efficiency (3) the electrical efficiency of the CHP system with the selected ORC fluids is predicted to be within the range of 7.5%–13.5%, mainly depending on the hot water temperature of the biomass boiler and the ORC condenser cooling water temperature as well as the ORC fluid, and corresponding to about 1.5 kW and 2.71 kW electricity output (4) the overall CHP efficiency of the CHP system is in the order of 80% for all three ORC fluids although the amount and quality of heating supplied by the CHP system depend on the ORC fluid selected and the modelling conditions.