合金凝固传热传质宏微观耦合数值研究及验证

建立了描述二元合金凝固的平面枝晶一维微观偏析数学模型,考虑溶质在固相中有限扩散,在液相中完全扩散。通过数值模拟,分析比较了A l-Cu和F e-C合金的微观偏析特性。同时,进一步将微观数值模型与宏观凝固实验的传热传质数学模型相耦合,实现了凝固宏微观复合尺度的全数值模拟。研究表明,数值计算结果与实验数据吻合良好,证明微观模型能较准确地反映微观质量传输并能可靠地与宏观相变传热传质模型相耦合。此外,从微观到宏观的计算结果都说明F e-C合金的凝固过程几乎接近平衡凝固。     

微界面扩散过程对二元合金凝固过程数值模拟的影响

针对反向凝固工艺实验研究的传热传质现象，进行了数值模拟，讨论分析了微界面质扩散过程对二元合金凝固过程数值模拟的影响。结合实验数据，认为微观偏析模型的选择对新生相生长的影响不可忽略。     

质子交换膜燃料电池的多尺度传热传质

介绍了目前质子交换膜燃料电池(PEMFC)在膜、电极、单电池、电堆或系统等四个结构尺度上的传热传质过程研究;主要讨论了PEMFC内的多组分传输、膜内水管理和多孔电极内的传热、传质过程;认为建立在孔尺度水平的研究方法是深入探讨电池内多孔材料微结构传热传质的有效途径;多维、多尺度模型的建立及其模拟计算能准确反映PEMFC内部的传递过程机理,为进一步优化电池结构和操作条件提供有价值的参考。     

强化凝结与液膜传热过程的板壳式换热器波纹方案

板壳式换热器敝开的板外侧通道可以实现多种板式换热器所不能实现的传热过程，特别是液膜传热伟质过程。针对不同过程的特点设计波纹的波型是板壳式换热器推广应用的关键，文章介绍了适合于凝结过程的同向双尺度波纹板片和适合于吸收，蒸发等液膜传热传质过程的交叉双尺度波纹板片方案。     

固着液滴蒸发研究进展及展望

固着液滴是指附着于壁面上的液滴,其蒸发行为及传热传质特性是喷雾冷却、喷墨打印等相变传热传质领域的基础问题之一。文中重点针对固着液滴蒸发过程所涉及的自身形态演变规律、气液固三相耦合传热/传质/流动特性进行了综述。结合毫微尺度固着液滴基本蒸发模式、热质传递形式、气液两相流动特征和界面输运行为,分析了液滴性质、壁面条件、气相环境条件等关键因素对固着液滴蒸发过程的内在作用机制和影响规律,提出了微纳尺度固着液滴（群）热质传递过程与机理的相关研究展望。     

喷管内传递特性对微尺度扩散火焰的影响

对均匀空气流中微尺度甲烷扩散燃烧进行了数值模拟,重点考察微喷管内的流动和传热传质对微尺度燃烧特性的影响.结果表明,在低流速下,内径为0.3 mm的微喷管内进气速度为1.0 m/s时燃料与空气的混合已经发生,混合气被管外的热量预热,同时火焰的热损失增加.在喷管直径一定时,减小燃料喷出速度,传热传质现象对微尺度甲烷扩散火焰特性的影响增强;当进气速度为0.5 m/s时,甲烷在微喷管内开始燃烧,放出热量.在进行微尺度解析计算时,必须包含一定的喷管区域.     

喷雾技术在湿空冷上的应用

１喷雾技术应用场合。在气一液两相物料直接接触进行传质、传热时都可应用喷雾技术。被喷雾相称为分散相，另一相为连续相。因此气一液两相物料具有４种混合类型：液一气、液一液、气一液、气一气．２喷雾技术提高传质、传热速率的基本原理。在气一液两相物料直接接触时，分散相通过喷雾技术变成细微的颗粒，增大了传质、传热面积，因此大大提高了传质、传热速率，有利于工业生产的进行。３喷雾技术应用的普遍性。在工业生产中普遍存在气一液两相物料的传质、传热过程。应用喷雾技术提高传质、传热速率过去未被人们充分认识，因此采用增大设…     

微细管壳式换热器传热性能分析

对微细管管壳式换热器的流动与传热性能进行了实验研究。根据实验获得的微细圆管换热器对流传热努谢尔特数准则式与流动阻力系数的准则式,分析了微细管管壳式换热器的传热流动综合性能,并与传统的管壳式换热器进行了分析对比。结果表明:微细管管壳式换热器传热流动综合传热性能是传统管壳式换热器的2到5倍,且在实验范围内随着雷诺数的增加而增加。     

余热制冷装置中氨吸收器热、质耦合数值研究

船用柴油机废热品位较高，基本满足吸收式制冷装置的需要。作为吸收式制冷装置中重要的部件，吸收器的合理设计直接决定了制冷系统的综合性能。在对水平管氨降膜吸收器中传质与传热的相互关系进行详细分析的基础上，建立了完全基于热、质耦合传递的二维模型。数值计算结果表明，在氨降膜吸收过程中，强烈的传质过程是实质，起着主导作用，而传热过程仪是传质过程的外在表现。     

固体吸附式制冷系统中吸附床传热传质研究进展

总结了近２０年来国内外吸附式制冷循环系统中吸附床传热传质研究的发展及现状。将吸附床传热传质数学模型分为３类进行了讨论：（１）均匀温度场模型；（２）均匀压力场模型；（３）非均匀温度场和压力场模型。以具体的吸附器结构为例，详细描述了不同数学模型的前提建模方法和适用范围，指出了吸附床传热传质数值研究的发展趋势。     

Effect of micro mass transfer through phase interface on numerical simulation of solidification process

Existing models for the solute redistribution during solidification have been reviewed. Some typical models are applied for the numerical simulation of heat and mass transfer with phase change under experimental condition of inverse casting. The results show that the effect of micro mass transfer models on the formation of the new phase cannot be omitted. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 393–401, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20024     

Numerical Simulation of Segregation Phenomena Coupled with Phase Change and Fluid Flow: Application to Metal Matrix Composites Processing

The injection of a liquid metal through a fibrous preform, located in an initially preheated mold, is one of the techniques used to manufacture metal matrix composites (MMCs). In order to reduce the chemical reactions between the fibers and the metal matrix, the fibrous reinforcement and the mold are commonly preheated up to initial temperatures much lower than the metal solidification temperature. Therefore, local metal solidification instantaneously occurs on fiber during liquid metal infiltration. When infiltrating metal alloy, unlike what happens when infiltrating a pure metal, both temperature and composition may vary within the matrix; this heterogeneity induces segregation within composites. A fiber scale numerical simulation was developed taking into account coupled physical phenomena which occur during the processing: flow of the liquid metal around the fibers, phase change phenomena, solute redistribution at the liquid/solid interface during alloy solidification, and species diffusion. This model predicts the segregation phenomena associated with fibrous preform infiltration by a binary alloy.     

合金枝晶生长及微细界面热质传递模拟

利用元胞自动机法(CA)耦合对流和热质传递模型模拟了Al-Cu二元合金微观组织在多种影响因素下的二维生长过程,分析了枝晶凝固、微界面热质传递以及微流动等微细现象之间的相互作用,得到了单枝晶以及多个枝晶在微流作用下的生长规律。模拟结果表明:(1)枝晶凝固过程中溶质富集于生长前沿。随着枝晶生长,凝固前沿远离冷源,枝晶尖端温度逐渐增大,而浓度逐渐变小;(2)流动对于枝晶的生长有着重要影响。流动破坏了枝晶生长的对称性,下游溶质浓度大于上游,枝晶在上游方向优先生长,而在下游方向有所抑止;(3)多个枝晶生长时,枝晶彼此间有阻碍生长的作用,二次枝晶臂的形成相对减少,枝晶间几乎不存在微流动。     

强化太阳能相变蓄热技术的研究进展

为解决太阳能的间歇性问题,常将其与相变蓄热技术进行结合。与传统显热蓄热相比,相变蓄热可将蓄热能量提高数倍以上,具有巨大的研究和应用价值。本文总结分析了相变蓄热的传热机制及在强化太阳能相变蓄热技术上的研究手段,如变换蓄热结构、添加肋片、使用相变胶囊、充注多相变材料、蓄热材料中添加高导热物质等。分析结果显示,相变传热机制中,融化过程主要考虑对流换热,凝固过程热传导占主导;使用肋片、相变胶囊等,主要增大相变材料接触面与蓄热体的比值,进而改善传热;蓄热材料添加高导热物质,可以改善相变材料的团聚、结核及使用寿命,从而提高导热性能,其中添加泡沫金属效果最为显著。     

Research progress on heat transfer enhancement technology of phase change energy storage

相变储能是通过相变材料吸/放热过程来实现能量储存的技术,它能够解决热量供需时间、空间和强度上的不匹配,并以其高储能密度成为储能领域的研究热点,但由于相变材料的热导率较低,使其应用受到限制。针对相变储能材料熔化/凝固过程中热导率低引起的传热速率慢的问题,从优化储能设备结构、添加剂提高相变材料热导率以及联合强化传热技术三方面综述国内外相变材料储能强化传热技术的最新进展。通过比较各种强化传热方式的优劣,实验和模拟均显示复合强化传热即可解决相变材料热导率低,又增大传热面积,从而提高相变材料的传热性能;多孔金属作为导热添加剂增强导热效果更好;并提出了相变储能强化传热技术未来需要解决的相关技术难题。     

Analytical and experimental investigations of nanoparticles embedded phase change materials for cooling application in modern buildings

This paper presents the analytical and experimental investigations of the phase change heat transfer characteristics and thermodynamic behavior of spherically enclosed phase change material (PCM) with dispersion of nanoparticles for latent thermal energy storage (LTES) system in buildings. In this study, the heat transfer characteristics in terms of the transient temperature variations, moving interface positions, complete rate of solidification and melting were analyzed for the six different PCMs considered in pure form and with dispersed nanoparticles as well. The heat transfer characteristics of the PCMs considered were analytically modeled and experimentally evaluated for the steady state and transient conditions for various heat generation parameters during freezing and melting cycles of the LTES system. The experimental results infer that for the same thermal load conditions the rate of solidification for the PCMs decreased with the increased mass fractions of nanoparticles while compared to the pure PCMs. For the same operating conditions of the LTES system, similar heat transfer characteristics were observed for the six PCMs considered. In this paper, the analytical model solutions and experimental results for the 60% n-tetradecane: 40% n-hexadecane PCM are presented. The solidification time for the 60% n-tetradecane: 40% n-hexadecane PCM embedded with the aluminium and alumina nanoparticles were expected to reduce by 12.97% and 4.97% than at its pure form respectively. Besides, the test results indicate that by increasing the mass fraction of the nanoparticles beyond the limiting value of 0.07 the rate of solidification was not significant further. Furthermore, the rate of melting was improved significantly for the PCMs embedded with the dispersed nanoparticles than the pure PCMs. The analytical solutions obtained for the pure and dispersed nanoparticles based PCMs were validated using the experimental results. The deviations observed between the analytical solutions and the experimental results were in the range of 10%-13%. Based on the analytical and experimental results the present nanoencapsulated LTES system can be regarded as a potential substitute for the conventional LTES system in buildings for achieving enhanced heat transfer characteristics and energy efficiency.     

Concentration fields in the solidification processing of metal matrix composites

A numerical investigation of the solidification of a binary alloy (Al-1.0 wt.% Cu) around cylindrical fibers with different fiber layouts and thermophysical properties was undertaken to gain insight into the processing of fiber-reinforced metal matrix composites. The focus of this study was on solute transport and redistribution during the solidification process, and the resulting concentration fields in the solidified alloy matrix. Change of phase in the alloy was formulated using a modified version of the temperature-transforming method for the energy equation. A source term that accounts for the solute rejection at the interface was incorporated into the solute concentration equation to model solute redistribution at the interface. Detailed results were obtained from the numerical simulations of low-(alumina) and high-(copper) conductivity fibers in inline and staggered configurations. Effects of the fiber pitch (longitudinal spacing) and transverse spacing were investigated. Higher concentrations of solute were seen to accumulate around copper fibers than for alumina fibers. With an initial, uniform concentration of 1.0 wt.% Cu in the melt, the maximum-recorded solute concentration in the domain for alumina fibers was 1.26% while that for copper fibers was 3.11%. For inline fibers, increasing the fiber pitch beyond a critical value did not change the overall shape of the local solute distribution around the fibers: the critical pitch for alumina fibers was found to be roughly 2.5 fiber diameters while that for copper was 2 fiber diameters.