土壤样品有效导热系数的分形计算模型

选择上海地区具有代表性的两种土壤样品,根据其微结构的特点,在粒径分布、剖面颗粒分布自相似规律的基础上建立了土壤结构分形模型;结合并联传热,得到土壤样品有效导热系数的分形表达式.通过与土壤样品有效导热系数的实测数据相比较,确立了符合样品有效导热系数的分形表述的最佳面积度量尺度.     

定形相变材料的有效导热系数

为更有效地预测定形相变材料的有效导热系数,以高密度聚乙烯/石蜡定形相变材料为对象,在分形理论的基础上,考虑空腔的分布和石蜡不同相态下的体积变化,建立一种新的导热系数模型——分形-空腔模型。采用准稳态平板法对此模型进行验证,探讨多个参数对有效导热系数的影响。结果表明:该模型的预测值更加符合实际。提高石蜡含量、体积收缩率、初始空腔率、分形维数等参数均会影响定形相变材料的导热系数,初始空腔率对熔融状态下的热导率的影响强于凝固状态下的影响。     

纳米流体强化导热系数机理初步分析

从添加纳米粒子改变了液体结构和纳米粒子微运动两个方面,分析了纳米流体强化导热系数的机理,研究表明,相对于在液体中添加毫米或微米级固体粒子以增加导热系数而言,纳米流体强化导热系数的原因主要来自于纳米粒子的微运动,通过测量不同温度下纳米流体的导热系数,验证了纳米粒子微运动是纳米流体强化导热系数的主要因素。     

多孔介质导热的分形模型

多孔介质中热量传递与多孔介质内部的几何结构有密切的关系，讨论了多孔介质的分形结构和相关的分形维数，利用能量方程，导出了分形维数为D的有限尺度多孔介质中的广义热传导方程，在此基础上，假定热量在多孔介质中的传导路线也是一种分形结构，提出了一个筒化的多孔介质并联通道分形导热模型，求出了基于分形理论的多孔介质有效导热系数表达式。     

聚氨酯泡沫塑料热分析

针对聚氨酯开展导热性能分析,对聚氨酯导热系数、密度、闭孔率进行了实验研究,考察了聚氨酯密度、闭孔率对其导热系数的影响,并根据发泡过程中出现的孔洞开展模拟计算,建立基于多孔介质分形结构的物理模型,分析了孔洞对聚氨酯导热性能的影响。研究结果表明:随着密度的增大,聚氨酯的导热系数随之升高,聚氨酯的导热系数主要受固体孔壁结构的影响;聚氨酯闭孔率越低,导热系数越高,且过低的闭孔率会降低其密度;建立分形结构物理模型,通过数值模拟分析,闭孔率相同时,具有较大内部孔洞的聚氨酯导热系数较低。     

含湿非饱和多孔介质的完全分形传热研究

为了探究在含湿情况下多孔介质有效导热率的变化,基于分形理论,考虑多孔介质在含湿时加热过程中相变的影响,结合加热过程中的热量守恒方程和傅里叶导热定律推导出计算有效导热率的新公式。将该模型相关数据代入进行计算,分析了孔隙率、含湿率、面积分形维数和迂曲分形维数对有效导热率的影响。研究发现,孔隙率与有效导热率呈负相关,含湿率与有效导热率呈正相关,分形维数与有效导热率呈负相关。该研究能够反映多孔介质内的传热进程,对于探究微孔结构物质的传热具有一定的指导意义。     

变密度多孔介质强化导热模型及实验研究

以等温容器截面为研究对象,基于多孔介质模型,采用变密度填充容器,强化中心向容器壁的导热。首先,基于稳态导热最小化热阻模型,确定了变密度铜丝2层和3层填充方案。其次,按确定的分层填充方案搭建实验台并测定容器有效导热系数,与均匀填充相比,导热系数分别增大了52.9%和77.9%。最后,对变密度填充下的瞬态导热进行了数值模拟研究,在中心温度200℃和中心热功率恒定两种情况下,经过一定时间热传导,变密度填充缩小了容器中心到容器壁的温度差。以上研究结果表明基于最小热阻的分层变密度填充铜丝可以强化由中心向周围的导热。     

带有空穴子模型的石墨烯气凝胶复合相变材料导热系数的分形研究

在分形理论的基础上,确定了基于石墨烯气凝胶(graphene aerogel,GA)骨架复合相变材料的分形维数,并基于改进的Sierpinski地毯建立了带有空穴的导热系数预测模型。预测结果表明,对于基于GA骨架的复合相变材料,在所制得的材料孔隙率为0.7的条件下,无论空穴尺寸如何,均可将导热系数从相变材料本身的0.250 W/(m·K)提升至10.900 W/(m·K),增长幅度达40倍以上。结果显示,复合相变材料的导热系数随着孔隙率的减小而增加,且在低孔隙率下,导热系数随空穴尺寸的减小而增加。     

纳米粒子与石蜡乳状液复合多相功能热流体的制备与性能

以石蜡乳状液为分散介质,纳米Cu粒子为导热介质,采用相转化乳化法制备了纳米Cu/石蜡复合相变乳状液,研究了纳米Cu粒子对复合相变乳状液的稳定性、流变性、导热性和热循环稳定性的影响。研究结果表明：纳米Cu/石蜡乳状液的密度、粘度和导热系数均随纳米Cu含量的增大而增大,密度变化不太明显,而导热系数则明显提高,当纳米Cu粒子含量为0.05 wt%时,复合相变乳状液的导热系数比纯石蜡乳状液提高了161.96%。由于纳米Cu粒子的添加显著增加了Cu/石蜡乳状液的导热性能,Cu/石蜡乳状液在固−液相变热循环过程中的温度平台表现不明显,但其热循环稳定性很好。     

基于Lattice-Boltzmann方法的SiO2多孔绝热材料传热分析

采用四参数随机生成法构造了SiO2多孔绝热材料微观结构,引入二维九速度不可压格子多相Lattice-Boltzmann热模型,该模型能够方便的计算具有复杂边界的多孔材料微尺度传热问题,给出了从结构构造模拟到具体的Lattice-Boltzmann传热分析的程序实现流程,进行了完整的二维多孔绝热材料导热过程的数值分析。结果表明Knudsen数小于10-1时,相同孔隙率下孔径越大,有效导热系数越低。随着孔隙率的减小,有效导热系数明显增加,增加气相导热比重是增强该绝热材料绝热性能的有效途径。骨架结构对SiO2多孔绝热材料有效导热系数的影响显著。     

Thermal conductivities study on silica aerogel and its composite insulation materials

This paper presents a theoretical and experimental study on thermal conductivities of silica aerogel, xonotlite-type calcium silicate and xonotlite–aerogel composite insulation material. The transmittance spectra of silica aerogel and xonotlite-type calcium silicate samples are obtained through FTIR measurements. The corresponding extinction coefficient spectra of the three materials are then obtained by applying Beer’s law. The thermal conductivities of aerogel, xonotlite-type calcium silicate, and xonotlite–aerogel composite insulation material are measured from 300 to 970 K and from 0.045 Pa to atmospheric pressure with the transient hot-strip (THS) method. The thermal conductivity models developed for coupled heat transfer of gas and solid based on the unit cell method are compared with the experimental measurement results. It is shown that the effective thermal conductivity models matches well with the experimental data. The specific spectral extinction coefficients of xonotlite-type calcium are larger than 10 m² kg^?1, and the specific spectral extinction coefficients of aerogel are larger than 7 m² kg^?1 over the whole measured spectra. The density of xonotlite-type calcium silicate is the key factor affecting the effective thermal conductivity of xonotlite–aerogel composite insulation material, and the density of aerogel has little influence. The effective thermal conductivity can be lowered greatly by composite of the two materials at an elevated temperature.     

Fractal structure reconstruction for alumina-silicate refractory fiber and simulation of the thermal conductivity

In this paper, it is proved that the intemal porous structure of alumina-silicate refractory fiber has fractal characteristics, which is reconstructed by the computer and the reconstructed structure further proved to have fractal characteristics. Based on the reconstructed structure, the network-thermal-resistance model is established to calculate the thermal conductivity of the fiber. It is shown that the calculated results agree well with the previous experimental ones, proving the correctness of the method.     

Thermal Radiative Properties of Xonotlite Insulation Material

Introduction Xonotlite-type calcium silicate (6CaO?6SiO2?H2O) is synthesized porous insulation material by hydrothermal processing with quartz powder and limestone as the raw material (with CaO/ SiO2≈1:1). Compared with fire- retardant fibre, xonotlite has more excellent insulating performance, such as low thermal conductivity, environment friendly, high intension, and wide applying temperature range, which has been emphasized in recent years by many scholars and widely used in many indu…     

The prediction of effective thermal conductivities perpendicular to the fibres of wood using a fractal model and an improved transient measurement technique

The porous microstructure of wood samples on their sections perpendicular to the fibres were analyzed using the scanning electron microscope images. The fractal dimensions of these images were calculated using the box-counting method, respectively. They are all approximately equal to 1.4, although the distribution and the scale of wood fibres are extremely different. Then, a fractal model for predicting the effective thermal conductivities of wood was established using the thermal resistance method. In addition, we measured the effective thermal conductivity of wood via an improved transient plane source measurement method. The calculated results by the proposed model are in good agreement with the experimental data as well as the literature data. The comparison shows clearly that this fractal model can be used to accurately and effectively predict the effective thermal conductivities perpendicular to the fibres of wood.     

Lattice Boltzmann modeling of the effective thermal conductivity for fibrous materials

This paper presents a full set of numerical methods for predicting the effective thermal conductivity of natural fibrous materials accurately, which includes a random generation-growth method for generating micro morphology of natural fibrous materials based on existing statistical macroscopic geometrical characteristics and a highly efficient lattice Boltzmann algorithm for solving the energy transport equations through the fibrous material with the multiphase conjugate heat transfer effect considered. Using the present method, the effective thermal conductivity of random fibrous materials is analyzed for different parameters. The simulation results indicate that the fiber orientation angle limit will cause the material effective thermal conductivity to be anisotropic and a smaller orientation angle leads to a stronger anisotropy. The effective thermal conductivity of fibrous material increases with the fiber length and approach a stable value when the fiber tends to be infinite long. The effective thermal conductivity increases with the porosity of material at a super-linear rate and differs for different fiber location distribution functions.     

Thermal properties of particulate TIMs in squeeze flow

Direct numerical simulations based on a thermal Lattice–Boltzmann method are utilized to compute the effective thermal conductivity of particulate thermal interface materials (TIMs). By simulating the squeezing process, we obtain the particle distribution in a situation similar to application. Therefore, there is no need to calculate the average thermal characteristics from several pre-defined random distributions. The effects of particle volume fraction, particle size, and particle to matrix thermal conductivity ratio on the thermal performance are investigated. The results for the effective thermal conductivity are in agreement with the existing semi-analytical and experimental results reported in the literature.     

Effect of solid thermal conductivity and particle–particle contact on effective thermodiffusion coefficient in porous media

Transient mass transfer associated to a thermal gradient through a saturated porous medium is studied experimentally and theoretically to determine the effect of solid thermal conductivity and particle–particle contact on thermodiffusion processes. In this study, the theoretical volume averaging model developed in a previous study has been adopted to determine the effective transport coefficients in the case of particle–particle contact configurations. The theoretical results revealed that the effective thermodiffusion coefficient is independent of the thermal conductivity ratio for pure diffusive cases. In all cases, even if the effective thermal conductivity depends on the particle–particle contact, the effective thermodiffusion coefficient remains independent of the solid phase connectivity. We also found that the porosity can change the impact of dispersion effects on the thermodiffusion coefficients. For large values of the thermal conductivity contrast, dispersion effects are negligible and the effective thermal conductivity coefficients are the same as the ones for the pure diffusion case.Experimental results obtained for the purely diffusive case, using a special two-bulb apparatus, confirm the theoretical results. These results also show that, for non-consolidated porous media made of spheres, the thermal conductivity ratio has no significant influence on the thermodiffusion process for pure diffusion. Finally, the particle–particle contact also does not show a considerable influence on the thermodiffusion process.