多孔材料辐射-传热耦合性能的统计二阶双尺度计算

对多孔材料辐射-传热耦合计算的数学模型,即Rosseland方程,给出了一种统计的二阶双尺度分析方法,并针对典型问题进行了数值模拟.建立了考虑辐射项的统计二阶双尺度计算公式,给出了统计意义下热流密度极值的预测算法,并通过与理论解的比较对算法进行了验证,利用本文中方法研究了孔洞体分比和空间分布状态对陶瓷多孔材料热传导系数、辐射传导系数和热流密度极值的影响.结果表明:孔洞体积分数的增加将导致有效热传导系数下降;热流密度极值随孔洞体积分数的增加而变大,并且在高温时辐射的作用明显增大;数值试验表明,使用统计二阶双尺度方法及其有限元算法预测孔洞随机分布复合材料结构的热性能是有效的.     

多孔泡沫金属中指数规律变热流平板表面的层流对流传热分析

采用Brinkman-Forchheimer-extended Darcy流动模型和局部非热平衡传热模型(双温度模型),对指数规律变热流密度条件下的多孔泡沫金属中平板表面的层流对流传热进行了分析,并得出了平板表面的热边界层的厚度和局部的对流传热系数的表达式。结果发现:平板表面的热边界层的厚度发展沿流动方向逐渐增大,但是增大的趋势由迅速趋向平缓;局部对流传热系数沿流动方向逐渐减小,而后趋于稳定。最后推导出了局部的对流传热Nusselt数的准则方程。     

基于宏观性能的微观多孔材料拓扑优化

基于均匀化理论,建立与微观材料拓扑形状相关的宏观结构材料等效弹性张量。集成宏观结构所得到的位移场,推导出带有宏观结构力学特性的微观敏度。从而实现在给定材料体积分数前提下,以宏观结构最大刚度为目标,对材料微结构进行拓扑优化的目的。相关算例说明该方法可以得到与宏观力学性能相对应的各种微观结构蜂窝材料或复合材料。揭示了材料的微观结构拓扑形状依赖于宏观结构尺寸、载荷及初始边界条件等因素。     

基于阻尼材料拓扑分布的结构-声辐射优化

基于拓扑优化思想,提出一种人工材料假设,建立了阻尼材料铺设的拓扑优化数学模型。在阻尼材料用量一定的情况下实现阻尼材料最优分布,达到减振降噪之目的。采用遗传算法进行求解,进行了阻尼材料和人工材料的转换,实现了阻尼材料的拓扑优化。数值算例表明:所提方法在减振降噪设计中具有可行性和有效性,有较好的工程应用价值,可为阻尼材料的敷设提供一种可行有效的思路。     

基于材料选型优化的金属-复合材料组合结构-声辐射优化

基于金属铺层假设,以结构材料属性和材料铺层数量为设计变量,建立了基于金属-复合材料的材料选型结构-声辐射优化设计模型。以钢-纤维增强复合材料组合结构为例,开展了多设计变量、多约束条件的结构-声辐射优化研究。最后,采用遗传算法进行求解,实现了结构材料的转换。数值算例表明,通过金属-复合材料组合结构的材料选型优化,可以有效降低钢-纤维增强复合材料结构的振动和声辐射。文中的研究方法为结构材料的分布优化提供了一种可行有效的思路。     

湿-热-机耦合作用下多孔功能梯度梁的振动及屈曲特性

采用一种拓展的n阶广义梁理论（GBT）,研究了轴向机械载荷作用下多孔功能梯度材料（FGM）梁在湿热环境中的振动及屈曲特性。考虑了材料的物性随温度变化,湿-热沿梁厚按三种不同类型分布,采用含孔隙率的修正Voigt混合率模型描述多孔功能梯度梁的材料属性,在宏-细观力学模型框架下应用Hamilton原理统一建立了系统的自由振动及屈曲方程,采用Navier法求解FGM简支梁的静动态响应。通过算例验证并讨论了GBT阶数n的理想取值,可用于丰富梁理论。探讨了湿热效应、湿-热-机耦合、孔隙率、材料梯度指标、跨厚比对FGM梁振动及屈曲特性的影响。结果表明:湿-热加剧降低了FGM梁的频率和临界载荷,且不同类型的湿热分布对其减小程度有显著差异;随着孔隙率增大,梁结构的整体刚度虽有所弱化,但在湿热环境中频率反而增大,稳定性增强;湿-热效应对多孔FGM细长梁频率和稳定性影响十分显著,但对短粗梁的影响比较有限。      

多孔碳添加量对溶胶凝胶-碳热还原法制备磷酸钒锂正极材料的电化学影响

本工作采用一种具有良好导电性能的多孔结构碳材料与磷酸钒锂通过溶胶凝胶-碳热还原法进行复合,研制出一种锂离子电池正极的新型复合材料。新型电极在0.5C倍率下初始比容量为111.0 m A·h·g~(-1),150圈循环容量保持率为99.2%。在10C倍率循环下复合正极仍有79.8m A·h·g~(-1)比容量和71.9%容量保持率,展示出良好的快充/放性能。复合材料的制备工艺简单,其电化学性能优异和较高含量的磷酸钒锂(LVP)含量符合锂离子电池正极材料的产业实用化的要求,该材料的研发为快充电池工业化提供了一种具有实际意义的材料。     

ZIF-67衍生Co-NC多孔碳材料的可控制备及其在锂-硫二次电池中的应用研究

以ZIF-67为模板制备了一系列具有不同金属Co负载量的S/Co-NC复合材料, 并将其应用于锂-硫电池正极中进行电化学性能研究。采用扫描电镜(SEM)和透射电镜(TEM)对Co-NC材料的多面体形貌及多孔结构进行表征; 采用X射线衍射(XRD)分析了Co-NC中金属Co的结晶状态; 采用氮气吸脱附方法分析了Co-NC材料的比表面积及孔结构。研究表明, 当刻蚀时间为48 h, 即Co含量为15.93wt%时, 复合硫正极呈现出最佳的循环性能以及倍率性能, 在0.2C电流密度下从第50圈到200圈循环的容量保持率为94.84%, 5.0C高倍率下的放电比容量为718.8 mAh?g ^-1。     

关于CJJ/T135-2009标准中胶凝材料用量确定方法的对比讨论

文章讨论了《透水水泥混凝土路面技术规程》(CJJ/T135-2009)标准中配合比设计部分关于胶凝材料用量确定方法与其他两种胶凝材料用量确定方法的准确性、便捷性、应变性和可操作性.通过对实验结果的分析发现,CJJ/T135-2009中的胶凝材料用量确定方法误将水胶比理解成每立方米混凝土用水量与所有胶凝材料用量的体积之比,从而导致计算结果偏差.而采用实际测定浆体密度再确定胶凝材料用量的方法更符合工程应用的实际情况,且可操作性更强.     

The statistical second-order two-scale method for thermomechanical properties of statistically inhomogeneous materials

A statistical second-order two-scale (SSOTS) method is established in a constructive way for predicting the thermomechanical properties of statistically inhomogeneous materials. For this kind of composite materials, the complicated micro-characteristics of inclusions, including their shape, size, orientation, spatial distribution, volume fraction and/or material properties and so on, lead to changes of the macroscopic thermomechanical properties, such as stiffness, coefficient of thermal expansion and strength of material. In this paper, a statistical model at an arbitrary position of the composite material is defined to represent the microstructure of the statistically inhomogeneous media at first. And then, the statistical second-order two-scale analysis formulation is derived. Finally, the numerical results for some statistically inhomogeneous composites are calculated by SSOTS algorithm, and compared with the data by experimental and theoretical methods.     

Measurement of the transient heat transfer to liquid helium from a thin metal film

A method to measure transient heat transfer to liquid helium from a thin metal film heater under the condition of pulsed heating during τ ≤ 400 ns is proposed. The experimental equipment used for the measurements is described. The method is based on the comparison of heat pulses transfered from the heater into a monocrystal substrate which is surrounded either by vacuum or by liquid helium. The method can also be used to investigate the heat flux density transmitted into liquid helium over a wide region of thermal loads. Experimental results showing the heat flux density radiated from a Cu heater into liquid helium at 3.8 K as a function of the electric power fed into the heater by pulses of 200–400 ns duration are demonstrated.     

Peak and recovery heat flux densities in bath of subcooled superfluid helium

This paper describes the measurement of the peak and recovery heat flux densities (q_P, q_R) in a bath of subcooled superfluid helium. A RhFe wire (d = 51 μm, L = 34.8 mm) was horizontally immersed in a He Ilp cryostat, which served as the sample, the heater and the temperature sensor. For the steady state measurement a programmed voltage method with a triangular waveform of 20 s was applied. The measured q_P and q_R values cover the temperature range 1.81–2.1K at a pressure of 0.1 MPa. On the basis of the heat and mass transfer through phase interfaces, theoretical relations (without geometrical parameters) of q_P and q_R are derived, which have very wide application range due to the relative critical heat flux density method used for the calculations. The theoretical results fit the experimental results well for both q_P and q_R.     

金属多孔介质泡沫自然对流换热实验研究

本文对饱和水-泡沫铜多孔介质在方腔内的自然对流换热进行了实验研究。讨论了孔隙率、孔隙密度及倾斜角度对多孔介质自然对流换热的影响。结果表明:自然对流换热随着孔隙率增大而减小。当孔隙率一定时,随着孔隙密度的增大,自然对流换热减小。Nu数随着加热功率的增大而增大,随着腔体倾斜角度的增大而减小。根据相似理论建立了以孔隙率、孔隙密度、倾斜角度为影响变量的换热准则关联式,且误差小于±7.5%。     

混合工质物性计算及在微通道中换热特性研究

微尺度高效换热器具有结构紧凑、热阻小、换热系数大等特点。以A/B混合制冷剂为工质,对其密度、饱和蒸气压、表面张力和黏度进行了计算并拟合出标准差在1%内的相应的物性温度曲线,并对其在-45、20、60℃入口温度下在1.1/1.2 mm尺寸的矩形微通道内的换热特性进行了实验研究,在给定400 W外热流下,不同入口温度有效换热热流密度都在50 000 W/m²左右,入口温度越高,平均换热系数越大。     

基于二阶敏度的共轭梯度法求解多宗量稳态传热反问题

建立多宗量稳态热传导反问题的求解模式，导出了一阶和二阶的敏度计算公式，应用共轭梯度技术进行求解，数值验证给出了令人满意的结果，并探讨了测点数目、测量误差和初值对反演结果的影响。     

Computer Simulations that Illustrate the Heat Balance of Landfills

The characterization of landfills with respect to geometry, temperature, gas flow, water content, and heat generation yields important information about landfill activity and possible risks; insufficient or defective surface sealing, for example, can lead to increased biogas release or to local overheating. The aim of this work was to develop a model in order to simulate the thermal behavior of landfills. The most important variables of interest have been identified, in order to make recommendations for the completion of existing landfills. Furthermore, sealed landfills requiring action regarding the temperature development can be studied. Simulations thus offer the possibility to identify causes and make proposals for solutions. Heat transport within the landfill can be described by the model developed in this work. The maximum temperatures predicted in the center of the landfill are in the range of values measured on different landfills up to 353 K. The interaction with gas flow gives good results, too. A temperature rise of some kelvin between the leak surface site and surrounding surface parts was determined, in agreement with measurements.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1-4, 2002, London, United Kingdom