Computational Modeling of GMAW Process for Joining Dissimilar Aluminum Alloys

A three-dimensional transient model is developed to solve for heat transfer, fluid flow, and species distribution during a continuous gas metal arc welding (GMAW) process for joining dissimilar aluminum alloys. The phase-change process during melting and solidification is modeled using a fixed-grid enthalpy-porositytechnique, and Scheil's model is used to determine coupling among composition, temperature, and the liquid fraction. The effect of molten droplet addition to the weld pool is simulated using a “cavity” model, in which the droplet heat and species addition to the molten pool are considered as volumetric heat and species sources, respectively, distributed in an imaginary cylindrical cavity within the molten pool. To establish the model for joining dissimilar alloys, results for joining two pieces of a similar alloy are also presented. The dissimilar welding model is demonstrated using a case study in which a plate of wrought aluminum alloy (with approximately 0.5 wt% Si) is butt-welded to an aluminum cast alloy plate (with approximately 10 wt% Si) of equal thickness using a GMAW process. Macrosegregation, along with the associated heat transfer and fluid flow phenomena and their role in the weld pool development, are discussed. The model is able to capture some of the key features of the process, such as differential heating of the two alloys, asymmetric weld pool development, mixing of the molten alloys, and the final composition after solidification.     

Numerical Modeling of Fin Heat Exchanger in Application to Cold Storage

The fin-and-tube heat exchanger are extensively used in refrigeration systems applied to cold storage. The performance of the heat exchanger affects the efficiency of the refrigeration system. Prediction of temperature, humidity, as well as velocity distribution in a cold storage chamber requires accurate prediction of operation of the finned air cooler. The operation of the air cooler unit is usually taken into account in modeling of operation of the cold storage chambers, but with very simplified geometry and physics. On the other hand numerical investigations of the heat exchangers are focused on thermal performance mostly in order to improve the overall heat transfer coefficient or to optimize its geometry. Results of numerical modeling using the computational fluid dynamics software ANSYS FLUENT of fin-and-tube air cooler applied on a cold storage chamber is presented in the paper. Two different approaches were used: the dual cell model, and porous media conditions. Numerical predictions of air temperature as well as air velocity at the air cooler outlet were also validated on the basis of the own experimental data.     

煤粉燃烧过程中NOx生成的实验和数值研究

本文采用完整的模拟３维气固两相流动，煤粉燃烧和传热的数值程序对实验室卧式炉分级送风以降低ＮＯｘ生成的燃烧过程并进行数值计算，程序中对气相采用欧拉法的通用输运方程，对煤粉颗粒采用拉格朗日的随机轨道法，用Ｄｅ’Ｓｏｅｔｅ的化学反应机理计算燃料ＮＯｘ的生成和用Ｚｅｌｄｏｖｉｃｈ机理计算热力ＮＯｘ的生成，针对分级燃烧降低ＮＯｘ生成的机理，实验中研究了３种配风工况，并结合数值研究具有分析了各工况下温度场和氧     

筒体与法兰环焊缝数值模拟

运用有限元分析方法,采用双椭球体热源模型,通过制定合理的工艺方案,建立筒体与法兰环焊缝焊接模型,并对焊接应力场进行数值分析。结果表明:依据本文方案施焊,产生的最大残余应力为325.6MPa,小于母材的屈服强度,可以满足使用要求。焊接过程中,应力主要集中在焊缝处及其周围,焊后残余应力集中在焊缝及压板附近,其随着焊接过程中温度的逐渐降低而逐渐产生,是一个过程量而不是瞬时量。     

煤粉燃烧过程中NOx生成的数值模拟

本文提出用有限反应速率２阶矩封闭模型来模拟湍流对煤燃烧过程中ＮＯｘ生成的影响。     

往复流多孔介质燃烧器的二维数值模拟与结构改进

对往复式惰性多孔介质燃烧器进行了二维数值模拟,模型的有效性通过实验数据进行验证.在燃烧器中分别填充4孔/cm泡沫陶瓷或小球,研究其内部的燃烧温度和压力损失.结果表明,由相同材料制成但结构不同的多孔介质对燃烧器内的高温区域和压力损失有显著的影响.孔隙率较大的泡沫陶瓷适合于布置在燃烧区,而孔隙率较小的小球适合于布置在热交换区域.改进燃烧器结构,即在燃烧器的中间布置泡沫陶瓷,而在两端布置小球.对于当量比为0.1的甲烷与空气混合气,得到了更为宽广的高温区域和适度的压力降.     

Improvement in Modeling and Testing of Induction Motors

The paper presents a method for identifying the parameters of a new model of induction motor. This model has two rotor loops in order to take into account the deep-bar effect and a non-linear leakage reactance which represents magnetic saturation effect. These phenomena cannot be neglected when induction motor dynamic characteristics have to be evaluated. The test method is simple and non-iterative. Only two locked-rotor tests at rated current and different frequency are required to derive the parameters of the two rotor loops. A further locked-rotor test at a suitable current value is necessary to determine the non-linear leakage reactance of the model.     

焊接工艺因素对带极电渣堆焊焊道成形的影响

焊接工艺因素对带极电渣堆焊焊道成形的影响机理,主要包括焊接电源、焊接工艺参数和外加磁场的作用。     

四角切向燃烧锅炉炉内冷态与热态流场特性的对比研究

通过冷态和热态实验测量研究了四角切圆燃烧锅炉的炉内流场特性。得出如下的结论：①热态的实际切圆直径相比冷态有增大的趋势；②炉内气流的旋转动量流率矩沿高度下降，热态工况旋转动量流率矩衰减速度比冷态工况慢；③一次风反切的配风方式可有效降低炉内旋转动量流率矩的水平、明显削弱炉膛出口的残余旋转和减小水平烟道入口的速度偏差。图8表3参3     

Numerical Modeling of Alternate Melting and Solidification

A finite-volume model is used to analyze alternate melting and solidification, which is the fundamental operational mode of latent thermal energy storage (LTES) systems. The simulated cases include: (1) melting of tin with natural convection, (2) alternate melting and solidification of sodium nitrate, and (3) cyclic phase change of gallium. For each case, temporal evolution of the heat transfer rate and liquid fraction is presented. In addition, snapshots of phase interface, temperature, pressure, and liquid velocity distributions are presented. The implications of the modeling results are discussed.     

铸造铝活塞模具材料的改进及表面处理

本文简要介绍了活塞生产行业当前用模具材料的概况及主要存在的问题；论述了模具材料改进的必要性和可行性，重点介绍几种适合做活塞芯模用的新型热作模具钢，同时，向用户推荐使用辉光离子渗氮技术。     

含铝复合推进剂燃烧过程数值模拟

在固体推进剂AP/HTPB燃烧模型基础上,针对含铝复合推进剂,建立包含金属燃料铝的三明治热流耦合气相稳态燃烧数值模型,研究含铝复合推进剂燃烧火焰结构与燃烧特性。仿真结果表明,含铝复合推进剂燃烧火焰结构基本符合BDP三火焰结构模型,同时随压强升高,沿着AP/HTPB及HTPB/Al物质界面形成高温扩散火焰带,峰前火焰不断靠近燃烧截面。     

冷风稀释器的工艺设计和数值分析

为进一步优化冷风稀释器的结构,提高混合、传热性能,利用有限元分析软件Workbench建立冷风稀释器流场的数值分析模型,研究"突缩-突扩"缩口直径、烟气入口与主管夹角、烟气与空气入口速比等设计参数对设备性能的影响。研究表明:减小缩口直径、增大烟气入口与主管夹角有利于提高混合气体流速、增强流场湍动状态从而改善混合传热效果;保持合适的烟气与空气入口速比,在增强流动的同时、可以控制流场死区,保证良好的混合传热状态;在烟气与空气入口速比约为0.31时,流场内的混合传热效果达到最佳。     

燃烧过程中NOx形成的数学模拟

本文建立了各种燃料燃烧过程中ＮＯｘ形成的数学模型，并进行了大量的对比计算及计算误差的估计，说明该程序可以比较准确地预报预混及快速混合火焰中ＮＯｘ的形成过程。     

铝合金硬质阳极氧化工艺改进

本文叙述了铝合金普通硬质阳极氧化工艺改为快速硬质阳极氧化的工艺条件，说明了通过改变设备及工艺参数可实现快速硬质阳极氧化生产，从而提高了生产效率，并可获得高质量的硬质氧化膜。     

P91钢焊缝蠕变过程中薄弱带状区域的生成机理

对焊态和蠕变断裂后的P91钢焊缝材料进行了微观组织、硬度和化学成分分析,采用Thermo-Calc软件对蠕变断裂后白色带状区域的碳活度进行了模拟.结果表明:焊道底部边界在焊态时存在合金元素的贫化,具有硬度低、碳化物溶解、碳质量分数低、再结晶的特点,且易于形成蠕变裂纹;焊道底部边界区域的碳活度远高于周围基体材料,当该区域的合金元素贫化为实测平均值时,长期时效后C元素质量分数最终贫化为小于0.01%,碳化物完全溶解;在蠕变过程中,焊道底部边界区域的马氏体组织再结晶为硬度较低的铁素体,其蠕变性能大大降低.     

Numerical study of macrosegregation in Aluminum alloys solidifying on uneven surfaces

Solidification of Aluminum alloys is modeled on uneven surfaces characterized by sinusoidal curves. Wavelengths and amplitudes of these surfaces are varied to study the effect of changing surface topography on fluid flow, macrosegregation and inverse segregation in the solidifying alloy. Solidification is initiated by convective heat removal from the uneven surfaces and simulations are carried out in both vertical and horizontal configurations. Stabilized finite element methods, recently used for modeling solidification in the presence of shrinkage and buoyancy driven flows, are used to discretize and solve the governing transport equations derived by volume averaging. The effect of varying amplitudes and wavelengths is observed in heat transfer, fluid-flow, macrosegregation and inverse segregation processes. In vertical solidification, inverse segregation, that usually occurs at the bottom of the cavities, is studied for different sinusoidal topographies quantified by a particular wavelength and amplitude. The fluid flow here is driven by a combination of shrinkage and thermosolutal buoyancy. Shrinkage driven flow arises due to different densities of solid and liquid phases. During horizontal solidification of an Aluminum alloy from uneven surfaces, thermosolutal buoyancy plays a dominant role in fluid flow and the effect of shrinkage is neglected by assuming the individual phase densities to be equal. Convection in this case is much stronger than that in the vertical case and large scale redistribution of the solute element occurs. To measure variation in macrosegregation with changing surface topography, global extent of segregation and difference between maximum and minimum solute concentrations are calculated for different amplitudes and wavelengths. In both the cases, the main aim is to quantify changes in macrosegregation due to changing surface topography accomplished by varying amplitudes or wavelengths or both.     

UL7005锅炉焊缝裂纹产生的原因及其处理

分析了德国UL7005型蒸汽锅炉运行过程中焊缝产生裂纹的原因,制订出返修方案,并提出了防止裂纹产生的对策。     

稠密气固流动中颗粒聚集现象的数值模拟

基于确定性轨道和真实碰撞，对二维稠密气固两相流动进行了数值模拟，真实展现了颗粒聚集现象从无到有的发展过程。在此基础上，对颗粒聚集现象影响因素的作用规律进行了研究。结果表明，颗粒间的相互碰撞是造成颗粒聚集的重要原因，即使在初始颗粒体积分数低至0.24％的情况下，仍然有显著的颗粒碰撞发生和聚集出现。颗粒弹性恢复系数越小、颗粒初始体积分数越大，则颗粒聚集现象越明显。这与已有的一些实验和数值计算结果是一致的。