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.     

Statistical Second-order Two-scale Method for Nonstationary Coupled Conduction-Radiation Heat Transfer Problem of Random Porous Materials

This paper develops a novel statistical second-order two-scale (SSOTS) method to predict the heat transfer performances of three-dimensional (3D) porous materials with random distribution. Firstly, the mesoscopic configuration for the structure with random distribution is briefly characterized Secondly, the SSOTS formulas for calculating effective thermal conductivity parameters, temperature field and heat flux densities are derived by means of construction way. Then, the algorithm procedure based on the SSOTS method is described in details. Finally, numerical results for porous materials with varying probability distribution models are calculated by SSOTS algorithm, and compared with the data by finite element method (FEM) in a very fine mesh and theoretical methods. They show that the SSOTS method is not only valid, but also accurate to predict the coupled heat transfer performances of random porous materials and demonstrate its potential applications in thermal engineering.     

多孔铝对流换热系数的反演分析

建立了多孔铝的一维瞬态对流换热模型,采用反演分析方法求得了多孔铝的体积对流换热系数.给出了多孔铝对流换热的无量纲准则关联式,并在较宽的孔结构范围内(孔隙率为60.0%～95.0%、孔径为2.5～6.0 mm)研究了孔结构对多孔铝换热系数的影响.结果表明:多孔铝的换热系数随着风速的增加而提高.在风速相同的条件下,换热系数随着孔径和孔隙率的减小而提高.在风机功率相同的条件下,换热系数随着孔隙率的增大而提高,在孔隙率约为85%时达到峰值.     

Mixed isogeometric analysis of strongly coupled diffusion in porous materials

In this paper, we develop a mixed isogeometric analysis approach based on subdivision stabilization to study strongly coupled diffusion in solids in both small and large deformation ranges. Coupling the fluid pressure and the solid deformation, the mixed formulation suffers from numerical instabilities in the incompressible and the nearly incompressible limit due to the violation of the inf‐sup condition. We investigate this issue using subdivision‐stabilized nonuniform rational B‐spline (NURBS) elements, as well as different families of mixed isogeometric analysis techniques, and assess their stability through a numerical inf‐sup test. Furthermore, the validity of the inf‐sup stability test in poromechanics is supported by a mathematical proof concerning the corresponding stability estimate. Finally, two numerical examples involving a rigid strip foundation on saturated soil and a swelling hydrogel structure are presented to validate the stability and to demonstrate the robustness of the proposed approach.     

高温氧化对超高温陶瓷材料耦合传热的影响

超高温陶瓷材料暴露于极端高温飞行环境中会导致其发生氧化,表面生成的氧化物具有不同的热物性从而对传热过程造成影响。针对预氧化的ZrB₂和ZrB₂-SiC,基于氧化模型预测氧化层(ZrO₂、B₂O₃、SiO₂和SiC耗尽层)厚度,利用有限元建立圆柱形代表性体积单元,并与外部高超声速流场的CFD (Computational Fluid Dynamics)求解器相耦合,研究了高温氧化对超高温陶瓷材料的耦合传热的影响。计算中采用分区求解方法,通过耦合界面处非匹配网格间的插值完成实时数据交换,实现了基于Navier-Stokes方程的流动求解器与有限元求解器的多场耦合计算。ZrB₂、ZrB₂-SiC以及氧化生成物的热物性均为温度相关,通过理论计算给出了B₂O₃挥发及SiC耗尽导致的多孔结构的有效热导率和有效比热容。瞬态耦合传热分析的结果表明:ZrB₂在预氧化后其热阻能力略有提高, ZrB₂-SiC氧化前后的热阻变化很小,并且在相同流动环境条件下,氧化后ZrB₂的热阻能力高于氧化后ZrB₂-SiC的热阻能力。      

New implementation of MLBIE method for heat conduction analysis in functionally graded materials

The meshless local boundary integral equation (MLBIE) method with an efficient technique to deal with the time variable are presented in this article to analyze the transient heat conduction in continuously nonhomogeneous functionally graded materials (FGMs). In space, the method is based on the local boundary integral equations and the moving least squares (MLS) approximation of the temperature and heat flux. In time, again the MLS approximates the equivalent Volterra integral equation derived from the heat conduction problem. It means that, the MLS is used for approximation in both time and space domains, and we avoid using the finite difference discretization or Laplace transform methods to overcome the time variable. Finally the method leads to a single generalized Sylvester equation rather than some (many) linear systems of equations. The method is computationally attractive, which is shown in couple of numerical examples for a finite strip and a hollow cylinder with an exponential spatial variation of material parameters.     

镁基材料储氢器的传质与传热研究

运用由壳层-缩核模型推导出的镁基储氢材料吸氢过程的动力学方程,分析了储氢材料在吸氢过程中的传质与传热规律,并对小型储氢器传质与传热过程进行了计算,其计算结果与试验数据可以较好地吻合,特别是对镁基储氢材料在吸氢过程所形成的‘引燃'过程进行了准确的描述,为储氢器设计提供了必要的理论基础.     

基于传递矩阵法的车辆-轨道垂向耦合系统动力分析方法

为准确高效求解车辆-轨道耦合系统动力响应,利用轨道结构的周期性特征,基于传递矩阵法(transfer matrix method,TMM)提出了一种便捷的轨道子系统建模和求解方法.该方法根据轨道系统结构特点,分别将有砟轨道和CRTSⅡ型无砟轨道系统的周期性重复部分划分为不同轨道元胞结构,在元胞结合面引入刚度方程假定,基...     

A multi-frontal parallel algorithm for coupled thermo–hydro–mechanical analysis of deforming porous media

In this paper, a multi-frontal parallel algorithm is developed to solve fully coupled heat, water and gas flow in deformable porous media. The mathematical model makes use of the modified effective stress concept together with the capillary pressure relationship and takes phase change and latent heat transfer into account. The chosen macroscopic field variables are displacement, capillary pressure, gas pressure and temperature. The parallel program is developed on a cluster of workstations. The PVM (Parallel Virtual Machine) system is used to handle communications among networked workstations. The multi-frontal method has advantages such as numbering of the finite element mesh in an arbitrary manner, simple programming organization, smaller core requirements and shorter computation times. An implementation of this parallel method on workstations is discussed. The speedup and efficiency of this method is demonstrated and compared with a general domain decomposition method based on band matrix methods by numerical examples. © 1998 John Wiley & Sons, Ltd.