Two Effective Thermal Conductivity Models for Porous Media with Hollow Spherical Agglomerates

Based on the microstructure features of xonotlite-type micro-pore calcium silicate, two unit cell models, the point-contact hollow spherical model and the surface-contact hollow cubic model, are developed. As one of several excellent insulation materials, xonotlite is represented as porous media with hollow spherical agglomerates. By one-dimensional heat conduction analysis using theunit cell, the effective thermal conductivity of xonotlite is determined. The results show that both of the models are in agreement with experimental data. The algebraic expressions based on the unit cell models can be used to calculate the effective thermal conductivity of porous media that have similar structure features as xonotlite.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China     

基于分形理论的开孔聚氨酯泡沫等效导热系数研究

开孔聚氨酯泡沫保温材料是一种典型的多孔介质。采用分形理论描述开孔聚氨酯泡沫材料的微尺度空间结构,建立了简化单元体模型,提出了计算其有效导热系数的分形模型,并导出了气相和固相热传导计算公式、热辐射等效导热系数计算公式、材料总有效导热系数计算公式。模型计算值与实验测量值比较具有较好的一致性,同时总结了多孔介质材料绝热性能的主要影响因素。该分析方法对新型绝热材料的研制和绝热性能的提高具有实用价值。     

基于分形理论的碳泡沫有效导热系数研究

以煤焦油基中间相沥青为原料，在一定的温度和压力条件下升温发泡，然后再经碳化、石墨化便可以制得一种高导热系数的多孔材料——碳泡沫。应用分形理论讨论了这种新型多孔材料的导热特性，推导出了碳泡沫的面积分形维数，并在此基础上建立了石墨化碳泡沫材料的导热模型，采用热阻法导出了石墨化碳泡沫材料的等效导热系数的关系式，计算出了碳泡沫的有效导热系数，计算结果与碳泡沫样品的实测值基本一致，这种方法为更好地利用其优良的导热性能提供了理论基础。     

Application of Effective Media Theory for Determination of Thermal Properties of Hollow Bricks as a Function of Moisture Content

The effective thermal conductivity and effective specific heat capacity of hollow brick blocks are investigated as a function of moisture content. While the effective specific heat capacity as a heat storage parameter is calculated using the linear theory of mixtures, the effective thermal conductivity as a heat transport parameter is analyzed using the effective media theory. At first, the effective thermal conductivity of the dry hollow brick block is calculated using a combination of properties of the brick body and air cavities, and verified using experimental data. Then, the effective thermal conductivity of the brick body is analyzed as a function of moisture content both theoretically and experimentally, and appropriate homogenization formulas are identified. Finally, the effective thermal conductivity of the whole hollow bricks is expressed as a function of moisture content from the dry state to water saturation.     

Effective Thermal Conductivity in a Radial-Flow Packed-Bed Reactor

In this work a theoretical and experimental study of the heat transfer process in a radial flow reactor was carried out under steady- and non-steady-state conditions in order to determine the effective thermal conductivity (k _e). One of the mathematical models proposed was a pseudohomogeneous model in which the effective thermal conductivity varies with radial position. The second model studied was a two-phase model with different thermal conductivities for gas and solid. For the pseudohomogeneous model, an analytical solution was obtained using the method of separation of variables and series approximation. In the two-phase model, the gas and solid temperature profiles were obtained by two numerical methods: orthogonal collocation and Runge–Kutta. Several experiments were performed by changing particle diameter, gas flow and temperature input, and reactor size and time-operation condition: steady and nonsteady. Theoretical results were compared with experimental data in order to calculate the effective thermal conductivity. The values of k _e agree in general with the literature data. At low Reynolds numbers there is no appreciable difference between a pseudohomogeneous model and a two-phase equation model. Constant thermal properties can be used at Re;5 with enough accuracy to predict the thermal behavior of a radial-flow reactor.     

Uncertainties Quantification of Effective Thermal Conductivity for Ceramic Fiber Blanket

In the present paper, a probabilistic propagation model for assessing the uncertainty of the effective thermal conductivity was developed based on a combined conduction and radiation heat transfer model of a ceramic fiber blanket composite. The Monte Carlo technique was used to cope with the uncertainties in the material density, radiative properties, and boundary temperatures observed in experimental tests. The calculated effective thermal-conductivity distribution for the sample was compared with the experimental measurements performed on multiple samples, and the predicted mean values were in good agreement with the measured data. The result validates the thermal predictive model and demonstrates the suitability of the stochastic model containing statistical distributions in the input variables. Statistical information also indicates that the uncertainty effect can be enlarged at high temperatures. Response sensitivity analysis between the random inputs and the effective thermal conductivity demonstrates that the randomness in the hot-side temperature, the cold-side temperature, and extinction coefficient of the sample has a significant influence on the variability of thermal-conductivity properties. The extinction coefficient becomes more and more important with an increase of temperature due to the dominant radiative heat transfer contribution at high temperature. The analysis provides good insight into the scattering control in the experimental measurement and theoretical prediction of the effective thermal conductivity of a ceramic fiber composite.     

金属基复合材料界面导热性能的扫描热探针测试

利用扫描热探针方法,以微米级的空间分辨率,分析测试了三种馆长在复合材料（ＭＭＣ）界面特征和界面导热性能,应用数学统计方法对扫描热显微镜（ＳＴhM)的形貌和热电压数据进行处理和转换,获得 面宽度和界面导热率数据,结果表明,金属基复合材料宏观导热性能与增强相一基体界面的导热性能密切相关。     

气凝胶/硅酸铝纤维保温包装材料导热性能研究

目的利用气凝胶/硅酸铝纤维制备一种保温包装材料,并研究其导热性能。方法利用真空抽滤成型方法及微波加热的干燥方法制备保温包装材料,利用正交试验得到影响该保温包装材料的主要因素。结果导热系数测定结果表明,正交实验最佳配方的导热系数为0.0164 W/(m·K),符合温控包装要求。随着二氧化硅气凝胶加入量的增大,导热系数变小;在最佳体积密度范围51.29~55.09 mg/cm3内,材料的导热系数最低可达到0.013 W/(m·K)。结论文中方法扩大了气凝胶和硅酸铝纤维材料在包装行业中的应用范围,对于农产品包装具有一定现实意义。     

颗粒填充型复合材料有效导热系数的数值计算方法

基于Monte Carlo随机摆放方法,建立颗粒填充型两相复合材料的随机分布模型.应用有限差分法对导热微分方程进行离散,获得差分格式的热传导方程.将复合材料模型中的微观随机结构单元的数据信息导入计算机,应用循环迭代法对差分格式热传导方程进行求解,实现了对二维稳态导热温度场的数值模拟.采用此方法对可以理论求解的特殊情形进行模拟验证,结果表明其准确可靠.同时模拟了导热增强颗粒含量、颗粒与基体的导热系数比对复合材料有效导热系数的影响,并将模拟结果与Maxwell-Eucken、Bruggeman模型进行了比较,发现该方法的模拟结果在低填充量时与Maxwell-Eucken模型、在高填充量时与Bruggeman模型吻合良好.研究表明所述方法能够较好地计算颗粒填充型复合材料的导热系数.     

聚合物基复合材料的界面结构与导热性能

在整理近年来国内外对聚合物基导热复合材料性能的研究工作的基础上,总结界面结构与界面处理对聚合物基复合材料界面和导热性能的影响,分析目前已有研究中的缺点和不足,提出未来改善界面结合、提高聚合物基复合材料导热性能的具体方法,并指出探究界面结构与热流散射的关系,特别是弱界面结合情况下的热流传导规律,对提高聚合物基复合材料的热导率有很大的指导意义。     

Effective Thermal Conductivity of Moist Porous Sintered Nickel Material

The effective thermal conductivity of capillary structures is an important parameter in the thermal performance analysis of loop heat pipes (LHP). In this paper, the effective thermal conductivity of porous sintered nickel material filled with water, ethanediol, and glycerin were measured by means of the hot disk thermal constant analyzer. The measured data were compared with similar measured data and calculated values from models in the literature. The results indicate that the thermal conductivity of the porous material depends on the thermal conductivity of the fluid, the filled ratio, and the porosity of the material.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China     

Determination of the Thermal Conductivity in Solidifying Ingots

The problem of identification of the effective thermal conductivity in a solidifying ingot is considered on the basis of experimental data. The possibility of its solution with identifiability conditions obtained and implemented in the present work is determined.     

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al₂O₃ thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al₂O₃ film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al₂O₃/water, ZrO₂/water, TiO₂/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al₂O₃, ZrO₂, TiO₂, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.     

高分子聚合材料的导热系数测定

讨论了平板法测量导热系数的原理。介绍了导热仪的结构。给出了对新型高导热性能有机高分子材料的导热系数测试结果。测试表明,此导热仪可对不良导热材料进行低于０℃的测量,填补了国内这方面的空白。     

聚合物基复合材料导热模型的研究现状及应用

综述了描述聚合物基复合材料导热性能的经典模型和新模型,详细列举了各模型的特点,比较了模型之间的区别,具体分析了影响复合体系导热性能的各种因素.结合对碳纳米管复合材料的研究给出了一些常用模型的应用建议,同时提出了当前研究中存在的模型通用性差的问题,指出今后聚合物基导热复合材料的研究趋势应为开发新的高导热纳米填充物和纳米复合技术以使填充物在基体中能形成有效的导热通路,从而改善复合体系的导热性能.     

Correction to: Impact of Size Distribution of Cell Model on the Effective Thermal Conductivity of Saturated Porous Media

International Journal of Thermophysics - The addition of nanoparticles to base fuel may bring about remarkable changes in a thermodynamic process such as evaporation, due to the intensified...     

高效热管理用金属基复合材料研究进展

从高导热复合构型、高导热复合界面及新型高导热纳米碳增强体3个方面,综述了热管理用铜基和铝基功能复合材料的研究进展,并对高导热金属基复合材料的未来发展方向进行了预测与展望.在材料组分相同的情况下,基于金属基体与导热增强体之间复合构型和复合界面的差异,制备的金属基复合材料的导热与热膨胀性能会发生显著变化.此外,由于纳米增强体与基体在形貌、尺寸及表面化学性质等方面的不相容性,在新型高导热金属基纳米复合材料的研发过程中,更需要兼顾高导热复合构型与复合界面的优化设计.可以预言,采用碳纳米管、石墨烯纳米片等新型纳米碳增强体,设计与制备具有微/纳米跨尺度分级复合构型的金属基复合材料将成为未来的研究热点.     

基于分形理论的玻璃纤维真空绝热热工特性研究

基于分形理论,描述了玻璃纤维多孔介质材料微尺度空间结构,建立分形等效单元体模型,分析了影响其真空下有效导热系数关键因素为固体基质导热系数、空隙率、纤维丝空间结构、分形直径、残余气体压力及导热系数、玻璃纤维材料厚度、使用环境等,并导出了气相、固相热传导计算公式和热辐射等效导热系数计算公式及材料总有效导热系数计算公式.研究表明,玻璃纤维有效导热系数随着分形直径、分形维数、残余气体压力的增大而增大,随着空隙率的增大而减小.同时,模型计算值与实验测量值比较,具有较好的一致性.文章的分析方法对新型真空绝热材料的研制和绝热性能的提高具有实用价值.     

Thermal Conductivity Behavior of Carbon Nanotubes Reinforced Copper Matrix Composites

Carbon nanotubes (CNT) exhibit excellent thermal conductivity.Therefore they are potential reinforcements in composites materials for thermal management applications,where high thermal conductivity and low coefficient of thermal expansion (CTE) are required.In the present study,CNT/Cu composites containing CNTs varying from 0 vol.％ to 15 vol.％ were prepared,and their thermal conductivity behavior was studied in detail.The results indicated that the thermal conductivity of the composites shows no enhancement by the incorporation of CNTs.The presence of interfacial thermal resistance and high level of porosity are the main reasons for this low thermal conductivity.The well dispersed 0-10 vol.％ CNTs composites show a very close to the thermal conductivity of Cu.However,the addition of 15 vol.％ CNTs results in a rather low thermal conductivity of CNT/Cu composites due to the presence a high level of porosity induced by the formation of CNT clusters.The present paper also claims that a further substantial enhancement in thermal conductivity is only possible if the nanotubes are randomly oriented in the plane or if they are all aligned in one direction,for which the processing of CNTs-aligning in metal matrix should be developed.     

Determining Effective Thermal Conductivity of Fabrics by Using Fractal Method

In this article, a fractal effective thermal conductivity model for woven fabrics with multiple layers is developed. Structural models of yarn and plain woven fabric are derived based on the fractal characteristics of macro-pores (gap or channel) between the yarns and micro-pores inside the yarns. The fractal effective thermal conductivity model can be expressed as a function of the pore structure (fractal dimension) and architectural parameters of the woven fabric. Good agreement is found between the fractal model and the thermal conductivity measurements in the general porosity ranges. It is expected that the model will be helpful in the evaluation of thermal comfort for woven fabric in the whole range of porosity.