FE simulation of hydrogen diffusion in duplex stainless steel

ABAQUS FE simulations of hydrogen diffusion in duplex stainless steel have been performed. Three models with different ferrite–austenite configurations have been applied and the hydrogen diffusion and the hydrogen coefficient have been evaluated as a function of austenite phase size and shape and the calculated diffusion coefficients compared to literature. Hydrogen concentration due to stress and plastic strain close to an embedded flaw has also been evaluated. An important observation is that the simulations show that when the austenite phases are saturated with hydrogen there is no large difference in the overall diffusion rate between the small and large phased models, i.e. no influence of tortuosity is observed. The work clearly demonstrates that both microstructure and flaws will influence the hydrogen diffusion and the hydrogen concentration and hence, must be taken into account when evaluating the susceptibility of hydrogen stress cracking in duplex stainless steels.     

The DC decay test for determining synchronous machine parameters:measurement and simulation

The DC flux decay test has been proposed as an attractive method of extracting synchronous machine parameters. This test is an attractive alternative to the variable-frequency static impedance test. It is quicker and easier to perform and with carefully chosen short-circuit resistance can give good values for transient and subtransient parameters (although ideally a test is required for each). With the machine stationary, direct current is passed through two phases and the decay of current is observed following the short-circuiting of the supply. Tests on two large turbine-generators performed and simulated by finite-element electromagnetic calculations are reported. The combination of tests, calculations, and analysis has enabled the cause of various difficulties associated with the test to be isolated. Recommendations are made to improve the accuracy of measurements     

燃气机组发电特性及其在电网中运行方式的研究

在全世界对节能减排和环境保护极其重视的情况下,对清洁、高效的燃气发电方式进行深入研究有重要意义。本文基于近几年我国华东地区安装的大容量新型S109FA联合循环机组的运行状况,对发电特性及其在电网中的运行方式进行分析研究,提出应该考虑不同类型燃气机组的特性,进行电厂规划设计和电网运行调度,以保证燃气机组在电网中充分发挥其优势。     

In-situ measurement and numerical simulation of nitriding kinetics of grain-oriented silicon steel

Although nitriding plays a critical role on the low-temperature production process of grain-oriented silicon steel, the quantitative modeling on nitriding kinetics is still in lack. We have in-situ measured the nitriding kinetics of grain-oriented silicon steel at 750 °C in the gas mixture of N₂ + H₂ + NH₃ using thermogravimeter (TG), and found that the kinetics could be divided into three stages. First, the weight of sample decreased until the nitriding period of 40s, indicating that no nitriding could occur but just the oxide layer on the surface be reduced. Next, the weight increased with the increasing rate until 120s, suggesting an accelerated nitriding during this period. Finally, the weight increased with a constant rate during nitriding, i.e. the nitriding with a constant rate. It was found that nitrogen atoms could diffuse into the steel matrix along the grain boundaries with the largest depth of about 50 μm during the nitriding for 590s. Moreover, the lamellar oxide particles on the surface that were formed during decarburization process transformed to the spherical ones when they were reduced by H₂ during nitriding, leading to the formation of spongy oxide layer. With a reasonable assumption that the reduction kinetics of oxide layer should determine its resistance to nitriding, we have established a mathematic modeling with a properly defined boundary condition. The calculated average N content and the N-penetrating maximum depth during nitriding both show excellent agreements with the experimental measurements.     

Experimental measurement and numerical simulation of horizontal-coupled slinky ground source heat exchangers

Results from both experimental measurements and 3D numerical simulations of Ground Source Heat Pump systems (GSHP) at a UK climate are presented. Experimental measurements of a horizontal-coupled slinky GSHP were undertaken in Talbot Cottage at Drayton St Leonard site, Oxfordshire, UK. The measured thermophysical properties of in situ soil were used in the CFD model. The thermal performance of slinky heat exchangers for the horizontal-coupled GSHP system for different coil diameters and slinky interval distances was investigated using a validated 3D model. Results from a two month period of monitoring the performance of the GSHP system showed that the COP decreased with the running time. The average COP of the horizontal-coupled GSHP was 2.5. The numerical prediction showed that there was no significant difference in the specific heat extraction of the slinky heat exchanger at different coil diameters. However, the larger the diameter of coil, the higher the heat extraction per meter length of soil. The specific heat extraction also increased, but the heat extraction per meter length of soil decreased with the increase of coil central interval distance.     

用于排架结构的铅剪切阻尼器试验及数值分析

为进一步促进铅阻尼器在工程中的应用,针对常见的排架结构,研制了一种铅剪切阻尼器,通过低周往复加载试验,逐个分析阻尼材料铅的有效剪切面积、受剪区厚度、剪切截面形式、加载位移幅值、加载频率、加载次数等因素对阻尼器耗能性能的影响。试验结果表明,设计的阻尼器的阻尼力—位移滞回曲线饱满,耗能性能良好,符合双线性力学模型,并对试验中两个典型型号的阻尼器进行了数值模拟分析,通过对比数模值和试验值,证明了数值计算模型的实用性,为阻尼器在结构体系中的减震应用奠定了基础。     

Hydrogen storage in molecular clathrate cages under conditions of moderate pressure and ambient temperature

Hydrogen is an ideal and predominant candidate as a clean energy for global sustainable development, while hydrogen storage is the most difficult hurdle for mobile applications. Here, molecular clathrate cages have been constructed via interfacial reaction between cyclodextrin (CD) and trimesoyl chloride (TMC). Both the inner cavities of CDs and outer cylinders of the CD/TMC clathrate architecture can restrain gas molecules in their cages. Interestingly, the gas uptake capability follows the order of α-CD/TMC > β-CD/TMC > γ-CD/TMC, which is in the opposite trend of their inner cavity sizes. All CD/TMC clathrate cages not only exhibit reversible hydrogen adsorption-desorption isothermals but also great hydrogen adsorption capabilities. The excess hydrogen uptake capability of α-CD/TMC is 2.1 wt% at 35 °C and 10 bar, which is one of the highest capacity reported to date for physisorption nanomaterials under conditions of ambient temperature and safe pressure. Comparing with previously reported materials, α-CD/TMC has the hydrogen storage capability most nearby the target set by U.S. Department of Energy (DOE).     

Numerical simulation of reactive flow in liquid composite molding using flux-corrected transport (FCT) based finite element/control volume (FE/CV) method

The mold-filling simulation of liquid composite molding (LCM) is of great importance in optimizing this cost-effective polymer-composites manufacturing process. The flow in LCM is a convection-dominated reactive, non-isothermal flow involving a moving boundary, so the Galerkin finite element method (FEM) has to be adapted with stabilization techniques to solve such problems. The streamline-upwind Petrov–Galerkin (SUPG) method is one of the most popular stabilized methods. However, the use of SUPG still leads to localized numerical wiggles in the vicinity of sharp solution gradients, which is often encountered in the 3D mold-filling simulation of LCM for thick parts. In this study, we propose to use the flux-corrected transport (FCT) based FEM to solve a set of highly convective transport equations. The numerical examples presented in this paper demonstrate the excellent performance of FCT based FEM in suppressing spurious oscillations in the regions of steep solution-gradients as opposed to the numerical instability of SUPG in such regions. For the first time, the FCT based FEM combined with the control-volume method is employed to simulate the non-isothermal mold-filling process in LCM. We have developed a simulation code PORE-FLOW© based on the scheme proposed in the study. Numerical studies have proven the stability of the FCT based FEM while modeling the mold-filling process of LCM.     

A study on Swirling Flows in a Rectangular Channel （LDV Measurement,Flow Visualization and Large Eddy Simulation）

IntroductionPipelines with non-circular section have beenwidely used in the industrial fields. The inner fiowsbecome more complex compared with those ofcircular section due to production of secondary flows.On the other hand, flows, which pass throughmulhple bends and discharge from fluid machinerysuch as an axial fan and blower and a water turbine,are usually accompanied by a directional swirling.When such swiriing flows discharge into the pipeline,it oscillates and happens to be broken becaus…     

Combustion irreversibilities: Numerical simulation and analysis

An exergy analysis was performed considering the combustion of methane and agro-industrial residues produced in Portugal (forest residues and vines pruning). Regarding that the irreversibilities of a thermodynamic process are path dependent, the combustion process was considering as resulting from different hypothetical paths each one characterized by four main sub-processes: reactant mixing, fuel oxidation, internal thermal energy exchange (heat transfer), and product mixing. The exergetic efficiency was computed using a zero dimensional model developed by using a Visual Basic home code. It was concluded that the exergy losses were mainly due to the internal thermal energy exchange sub-process. The exergy losses from this sub-process are higher when the reactants are preheated up to the ignition temperature without previous fuel oxidation. On the other hand, the global exergy destruction can be minored increasing the pressure, the reactants temperature and the oxygen content on the oxidant stream. This methodology allows the identification of the phenomena and processes that have larger exergy losses, the understanding of why these losses occur and how the exergy changes with the parameters associated to each system which is crucial to implement the syngas combustion from biomass products as a competitive technology.     

Solar radiation measurement: Techniques and instrumentation

A general survey is presented of solar radiation measurement, the techniques and instrumentation. The importance of determining the total and spectral irradiance of the Sun is examined in the context of the energy crisis and utilization of solar energy. The survey includes the extraterrestrial solar fluxes, their possible variations, problems relating to energy received by collecting surfaces on the ground, major types of instrumentation and the radiation scales to which the measurements are referred. The type of insolation data available from the National Weather Service of NOAA and from other sources is reviewed. Alternate techniques of deriving insolation data with high space time resolution are discussed with reference to solar energy conversion requirements. Energy received on the ground can be computed from known values of the extraterrestrial solar spectrum and of the spectral absorption parameters of the atmosphere. Another technique is based on measurements made by meteorological satellites of the cloud-cover and of the solar energy reflected and scattered back to space by the Earth-atmosphere system.     

Solar passive building in Delhi: Numerical simulation and validation

A two floor hostel for married research scholars consisting of 12 apartments was designed and constructed for the composite climate of Delhi; the design incorporates many passive features. Using modified admittance procedure and Fourier analysis of the periodic parameters, the building was numerically simulated to obtain its thermal performance on an hourly basis. Two apartments of the building (one on the ground floor and one on the first floor have been experimentally monitored by a microprocessor aided data acquisition system. The observations validate the numerical model.     

Small-molecule gas sorption and diffusion in coal: Molecular simulation

Injections of carbon dioxide (CO₂) into unmineable coalbeds can both enhance coalbed methane recovery (ECBM), a high-efficiency energy, and realize underground storage of CO₂. In these processes, the diffusion and sorption of methane (CH₄) and carbon dioxide are key dominant processes. In this study, the diffusion and sorption behavior of CH₄ and CO₂ in coal are investigated and compared based on molecular simulation. The calculated diffusion coefficient of CO₂ was in the order of 10⁻⁹ m²/s, which is reasonably close to the experimental result. The sorption isotherms were obtained using the grand canonical Monte Carlo method. Coal tended to adsorb more CO₂ than CH₄ at a given temperature and pressure. The sorption heat of CO₂ was larger than that of CH₄ (7.9 and 5.8 kcal/mol respectively), accounting for the fact that the CH₄ adsorbed in the coal seam could be replaced by CO₂. This presents an alternative method for directly studying the interactions between coal macromolecule and small-molecule gases under various external environments.     

Computer-oriented building design: Advances in daylighting and thermal simulation tools

Advanced computer simulation tools for analyzing daylighting features and thermal performance of buildings are improved at the Fraunhofer Institute for Solar Energy Systems. For an analysis of daylighting elements in lighting simulations, new light-deflecting structures and materials have been measured and modeled. In order to study the interaction between lighting and thermal behaviour of rooms, thermal and lighting simulation techniques are linked together. Thus, integrated energy concepts can be evaluated. Programs for handling parallel image generation in a network of high-performance Unix workstations have been developed.     

复杂阻尼结构汽轮机末级长叶片寿命评估相关研究进展及展望

目前汽轮机叶片设计和安全性考核主要采用的是以许用应力为基准的静强度准则和以安全倍率为基准的动强度准则,随着汽轮机设计制造水平的提高,尤其是近年来在超临界和超超临界大功率汽轮机中广泛采用具有复杂阻尼结构的长叶片,该方法已经表现出越来越多的局限性,建立一套从寿命损耗角度评估叶片安全性的方法十分必要。总结了近年来国内外在叶片寿命评估相关研究如静态应力分析、动态应力分析、叶片高周和低周疲劳寿命分析以及叶片水蚀疲劳分析等4个方面的研究进展,并对进一步深入研究提出了一些思路和方法。     

拉杆结构非线性接触的刚度融合修正方法

为建立更加准确的航空发动机高压转子的有限元模型,对表征高压转子拉杆结构中盘与盘之间接触的力学模型进行研究,引入包含库伦摩擦力的非线性弹塑性滑动模型对该类接触进行描述。提出弹簧刚度矩阵和有限元模型整体刚度矩阵的融合修正方法,建立了有限元刚度的整体优化模型。运用拉直算子将模型中需要修正的参数分离出来,结合实验模态参数对有限元模型刚度进行了修正。以实际航空发动机高压转子为例,通过与线性模型的修正结果比较,证明该模型准确描述了拉杆结构的复杂接触,且融合算法是可行且有效的。     

Solar drying of foods: Modeling and numerical simulation

A simulation code was developed to predict the batch drying performance of a packed bed of particles, e.g. cylinders or slices of carrot, apples, etc., subjected to time-varying air conditions. This model allows for shrinkage of the particles. The time-dependent inlet drying air conditions permit the simulation of the case of a solar dryer in which the inlet air temperature is necessarily a function of the hour of the day. All the parameters involved in the model were obtained independently from experimental solar dryer data. The results compared well with published experimental data for solar drying of diced carrot. Effects of various key parameters of this process are presented and discussed.     

Integrated methanol reforming and oxidation in wash-coated microreactors: A three-dimensional simulation

A microreactor consisting of two parallel channels is numerically simulated where methanol steam reforming takes place in one channel, and the required heat is supplied by methanol oxidation in the other channel. Effects of different parameters on methanol conversion, hydrogen yield and CO concentration are examined. Results from the parametric study are then used to propose conditions for high methanol conversion and hydrogen yield. A microreactor with enhanced output conditions is thus designed which is capable of producing a gas stream consisting of 74% hydrogen (dry). CO concentration in the generated synthesis gas stream is low enough to require only a PROX reactor for CO clean-up, eliminating the need for a bulky water–gas shift reactor. The produced hydrogen from an assembly of such microreactors can feed a low-power PEM fuel cell. A cluster of these microreactors would take a volume of about 91 cm³ to feed a typical 30-watt PEM fuel cell.     

Microfiltration of thin stillage: Process simulation and economic analyses

In plant scale operations, multistage membrane systems have been adopted for cost minimization. We considered design optimization and operation of a continuous microfiltration (MF) system for the corn dry grind process. The objectives were to develop a model to simulate a multistage MF system, optimize area requirements and stages required for a multistage system and perform economic analysis of a multistage MF system for a 40 million gal/yr ethanol plant. Total area requirement decreased with number of stages but there was tradeoff between higher capital costs involved at higher number of stages. To achieve thin stillage total solids concentration from 7 to 35%, a 5 stage membrane system was found to be optimum with area requirement of 655 m² for minimum cost. Increase in the input stream flow rate from 1.54 × 10⁶ to 2.89 × 10⁶ L/day significantly increased the total capital cost of the system by 47%. Compared to a single stage system, an optimal system had a 50% reduction in operating costs. Optimal system also showed potential to process more than twice the amount of thin stillage compared to a 4 effect evaporator system for given conditions.     

Correspondence among PL measurement, MBIC measurement and defect delineation in polycrystalline cast-Si solar cells

We report the correspondence between the photoluminescence (PL) measurement, monochromatic-light-beam-induced current (MBIC) measurement and defect delineation in polycrystalline cast-Si solar cells. It was found that the peak of the band-edge PL emission in the hydrogenerated and non-hydrogenerated cast-Si shifted from 1.093 eV in the single crystalline CZ-Si to 1.075 eV at room temperature and the band-edge PL mapping corresponded with the MBIC mapping and defect delineation pattern if excluding the surface damages delineated by the MD-1 etchant.