（相变）复合材料瞬态导热性能的简化计算方法

一般情况下复合材料不存在有效导温系,其瞬态热性能需藉三维非稳态传热模型分析。如掺温材料中涉及相变材料,由于其等效比热容与温度强烈相关,使得描述复合材料瞬态传热性能的热扩散方程呈现很强的非线性,更增加了求解的难度。该文探讨了利用等效一维均质上变材料模型分析（相变）复合材料瞬态传热特性的可行性,结合算例,分析了该方法的适用范围。结果表明,当掺混材料的相对尺寸不太大时,（相变）复合材料可当作均质（相变）材料,其瞬态传热性能可藉等效热物性计算。对于许多实际问题,上述方法可简化分析,节省计算量。     

激光脉冲热导仪测试半球向全发射率的探讨

测试半球向全发射率的方法很多。我们对激光脉冲热导仪稍作改进,用来测试样品背面的温度曲线和半球向全发射率ε_H(从室温到高温)。 卡计法通过测量样品加热后的冷却曲线,由下式可计算得到ε_H:     

光学热带法测量固态材料热物性研究

针对接触式瞬态热带法测量导热系数时,加热丝和样品间接触热阻,会影响实验测量结果以及对固体样品形状大小要求较高的现状,根据瞬态热带法原理,本文提出了一种光学瞬态热带法来测量固体材料的导热系数。采用连续激光为加热源,通过透镜将光斑放大并聚焦照射在样品表面,实现样品非接触式测量。构建二维导热模型,采用红外热像仪记录样品表面温升随时间的变化关系,根据导热理论模型求出待测样品的热扩散系数及导热系数。以K9和石英玻璃为样品对本套测量方法进行验证,制备并测量了纯石蜡、0.5%和1%石墨烯-石蜡的固态复合相变材料的导热系数,探讨了影响实验结果的潜在因素。     

三维离散传播辐射模型的理论及数值分析

本文比较详细地阐述了三维离散传播热辐射模型的理论，并且编制了相应的数值模拟程序，运用该程序对三维立方腔体中的两种辐射传热问题进行了数值模拟，所得模拟结果与精确解吻合较好，这表明三维离散传播热辐射模型是目前解决辐射问题中热流量和温度分布的一种较好的方法。     

一种高密度辐射计的设计及性能分析

分析了辐照投射角、测量桥的传热特性、结构和材料等因素对一种高密度辐射计测量性能的影响.结果表明:铜制辐射计能完成10kW/m2～15MW/m2范围内的辐照测量,响应时间不超过5s,输出温差与投射辐射的密度值成正比;较长的测量桥适用于辐照密度低、分辨率要求高的场合,较短的测量桥则适用于辐照值较高的场合;用热扩散系数高的材料制造的辐射计,其测量上限值较高,而用热扩散系数低的材料制造的辐射计,则有较高的分辨率和较低的测量下限值.     

换热孔回填材料含水率特征变化规律及其对热物性影响研究

通过设计“量筒法”代替环刀法和选用热重法,分别测量地下水渗入换热孔内回填材料的饱和含水率和残余含水率,用滤纸法测量基质吸力与含水率数据,然后基于含水率计算出不同深度下回填材料热特性参数。结果表明,高吸力值时材料的含水率更低,假设饱水带最高点为地下水位线0点,在距地下水位线100 m处仅有3.6%含水率;对比软件模拟数据与实测点,发现所选用的设计方法与拟合结果相近;与干实回填材料相比,受地下水渗入的换热孔下段靠近水位线处蓄热能力更强,温度变化相对延迟,但导热能力和热扩散能力都有所减弱,中上段蓄热和导热能力减弱,但热扩散能力增强。     

激光闪射法对石墨烯薄膜导热性能的研究

石墨烯是目前发现的导热系数最高的材料,其理论导热系数值可达5 300 W/(m·K),成为新一代最具潜力的高导热材料。文中采用激光闪射法研究了石墨烯薄膜横向和法向的热扩散系数,并根据测试的密度和比热计算得出导热系数。研究表明:横向热扩散系数随着薄膜厚度的增加而不断减小,当MCT检测器在电压为260 V,脉冲宽度为100μs,信号高度为1 V,Inplane+各向同性计算模型下,高温烧制薄膜横向热扩散系数高达740.16 mm~2/s,是法向的238倍,导热系数为1 252.28 W·(m·K)~(-1);压片法制得的石墨烯薄膜的横向热扩散系数为7.58 mm2/s,是法向的19倍,导热系数为9.43 W·(m·K)~(-1)。     

TPS法测定泡沫铜/石蜡复合相变材料热物性

运用瞬态平面热源法(Transient Plane Source-TPS)对4种孔隙率的泡沫铜/石蜡复合材料热物性进行了测量。以10μm厚的镍金属按双螺旋线布置作为测量探头。泡沫铜材料孔隙率分别为ε=97.79%、ε=96.17%、ε=94.94%和ε=93.26%,经线切割加工后向内灌入液态石蜡,凝固后作为测试样品。在室温(25±1℃)和常压下对复合材料的等效导热系数、热容及热扩散率进行了测量。测试结果表明:复合材料导热系数和热扩散率因泡沫铜的加入而大幅提高,在孔隙率ε=93.26%时,等效导热系数已达到单纯石蜡的25倍,而复合材料等效热容则由于铜金属加入的绝对量较少相对原石蜡热容变化较小。以比例加成的方法对泡沫铜/石蜡复合材料的等效热容进行了计算,并利用实验数据拟合了其等效导热系数的计算公式,运用这些公式对复合材料物性的计算结果与实验结果非常吻合。     

水平椭圆管内相变材料接触熔化的分析

对水平椭圆管内相变材料接触熔化过程进行了研究。根据椭圆作图法测，将其分段圆弧化后，利用Ｎｕｓｓｅｌｔ液体边界层理论，对紧密接触熔化的液体传热与运动进行了分析。求得管内固体熔化速度、熔化率平均Ｎｕｓｓｅｌｔ数。讨论了影响它们的诸因素，所得结果包含了圆管时的结论。     

微纳米粒子对辛酸-肉豆蔻酸相变储能体系热物性的影响

该文通过添加3组同族(镍、铝、铜)亚微米级金属、纳米金属及纳米金属氧化物改善其传热性能,制备可用于空调蓄冷系统的新型相变储能材料;通过步冷实验、差示扫描量热法(DSC)及瞬态平面热源法(Hotdisk)对比添加剂对OA-MA在过冷度、相变温度、相变潜热、导热系数及热扩散率等方面的影响,研究添加剂种类与颗粒尺寸对上述热物性的影响规律;利用Maxwell及Zimmerman公式对比不同质量比例的纳米氧化铜(CuONP)对材料导热系数的影响,研究纳米流体传热传质机理,数据拟合得出其拟合公式。     

Modeling and prediction of the effective thermal conductivity of random open-cell porous foams

Although highly desirable, accurate prediction of the effective thermal conductivity of high-porosity open-cell porous foam materials has remained to be a challenging problem. Aiming at this thorny obstacle, we have developed a random generation-growth method to reproduce the microstructures of open-cell foam materials via computer modeling, and then solve the energy transport equations through the complex structure by using a high-efficiency lattice Boltzmann method in this contribution. The effective thermal conductivities of open-cell foam materials are thus numerically calculated and the predictions are compared with the existing experimental data. Since the porosity is high, the predicted thermal conductivity caused by thermal conduction is lower than the measured data when the thermal conductivity of either component is very low and the radiation heat transfer is non-negligible. After considering the radiation effect, the numerical predictions agree rather well with the experimental data. The radiation influence is diminishing as the material porosity decreases. In general the effective thermal conductivity of open-cell foam materials is much higher than that of granular materials of the same components due to the enhanced heat transfer by the inner netlike morphology of the foam materials.     

Heat transfer through woven textiles

A mathematical model based on the principles of heat transfer to predict the thermal resistance of fabrics has been presented in this paper. The woven fabric is considered as a system of porous yarns, interlacements between warp and weft yarns and air pores and all the basic weaves can be depicted by this system. The conduction and radiation heat transfer together, was calculated based on the construction parameters of the fabric. The thermal insulation, which is equivalent to the thermal resistance, was predicted with the help of these parameters. The total heat transfer by conduction through each part was calculated using Fourier’s equation. Radiation heat transfer through the air pore was calculated with the help of net radiation method. Linear anisotropic scattering was used to model the radiation heat transfer through fibrous media. The total thermal resistance obtained was validated with actual values obtained from a standard thermal resistance measuring instrument.     

保温材料热物性测试的实验及数值研究

针对保温材料导热系数低，采用常规的导热系数测试方法难以获得准确结果的问题，根据瞬态法导热系数测试原理，对常功率平面热源法进行了研究。建立传热的二维瞬态数学模型，借助FLUENT有限体积软件对常功率平面热源法中试样的温度分布和热量传递规律进行数值模拟，开发了一套保温材料导热系数测试装置。测试结果与文献数据能较好的吻合，最大误差不超过4％。测试结果可靠，测试精度较高。     

Near-Field Thermal Radiation: Recent Progress and Outlook

Any materials at temperatures higher than absolute zero emit electromagnetic waves due to the thermal fluctuations of free charges or ions. When two or more bodies at different temperatures are brought sufficiently close to each other with vacuum gap spacing smaller than the characteristic thermal wavelength, near-field radiative heat flux can exceed the far-field blackbody limit, governed by the well-known Stefan-Boltzmann law, by orders of magnitude. This article reviews the recent progress on both theoretical and experimental studies of near-field thermal radiation with an emphasis on its potential applications. Recent theoretical developments are presented, such as near-field radiation of general anisotropic materials and 2D materials, application condition of effective medium theory, and exact numerical methods for dealing with arbitrary particles or periodic nanostructures. Recent experimental achievements are also discussed, focusing on tip–plane and plane–plane configurations with various materials. The wide applications of near-field radiation are summarized in terms of thermal imaging, energy harvesting, and contactless thermal management such as modulation, rectification, and amplification. An outlook is provided on the promise of near-field thermal radiation in energy harvesting, novel thermal sources, and sensing, as well as the development of related fields such as reducing Casimir stiction, minimizing noise current, and increasing spontaneous emission rate. Challenges are also discussed, such as developing exact and fast computational techniques for complex metamaterials, understanding near-field radiation at extreme gap spacing, overcoming weak mode coupling in thermophotovoltaic (TPV) systems, measuring large-area plane–plane configuration at small gap spacing, and realizing devices based on near-field radiation.     

Predicting the thermal performance characteristics of staggered vertical pin fin array heat sinks under combined mode radiation and mixed convection with impinging flow

A theoretical and experimental study was carried out investigating the influence of thermal radiation on the thermal performance of a pin fin array heat sink with the purpose of developing accurate predictive capability for such situations, and to determine the particular design parameters and environmental conditions under which thermal radiation might be advantageous to the thermal performance. Several different types of experimental tests were run with the corresponding physical parameter variations including the emissivity of the heat sink, elevated ambient air temperature, the temperature of a visible hot surface, and its radiation configuration factor. A theoretical model, validated by experimental data, which includes the capability of predicting the influence of thermal radiation on the thermal performance of a pin fin array heat sink, was developed by introducing an effective radiation heat transfer coefficient that was added to the convective heat transfer coefficient.     

Measurement of thermal conductivity, thermal diffusivity and heat capacity of highly porous building materials using transient plane source technique

This paper presents both experimental and theoretical works concerning evaluation of the thermal conductivity, thermal diffusivity and heat capacity of wood composites. Moreover, the aim of this study is to show that the transient plane source technique originally used for measuring thermal properties of isotropic materials can be spread worthy of heat capacity, thermal conductivity and thermal diffusivity measurements of highly porous materials. Measurements of the thermal conductivity, thermal diffusivity and heat capacity have been performed at room temperature (20 ± 0.5°C) and normal pressure. An attempt has been made to predict the thermal diffusivity of wood composites from the predicted values of thermal conductivity using a Verma et al's model based on Ohms law and the calculated values of heat capacity using the enthalpy concept. The predicted values by the proposed model are compared with the values of the thermal diffusivity measured using the TPS method. A comparison shows a good agreement.     

相变材料相变点温度热物性的测试及误差分析

固-液相变过程中移动的相界面曲线与两相热物性如比热、密度、导热系数以及相变潜热是密切相关的。本文提出通过测定相界面的移动速率来确定相变材料固-液相变温度点导热系数等多个热物性参数的方法,并设计了相应的测试装置。对测试系统的测量误差进行了定量分析,发现采用数值计算与实验相结合的方法测试的系统误差不超过3％。利用研制的测试系统对几种材料的导热系数和热扩散系数进行了测定,得到了满意的结果,表明本文所提出的测试方法是可信的。     

平流层电子设备温度数值模拟

分析了平流层电子设备内外部热环境,考虑平流层大气对流、设备内部自然对流、太阳直射辐射、大气辐射、地面反射太阳辐射、地球红外辐射以及设备自身辐射等因素的基础上,建立了计算电子设备温度分布特征的对流、辐射耦合模型,模拟了其在不同功率、不同对流换热、不同环境条件下的温度分布。结果表明:对于平流层电子设备散热,对流换热和辐射换热都会影响电子设备的温度分布,尽管由于平流层大气压力低、对流换热弱,但对流换热量占到散热总量的60%以上,是散热的主要方式。因此,在平流层电子设备热设计时,可以优先考虑采取开孔等强化对流散热方法来控制设备的温度。最后,开展了平流层模拟环境的实验验证,典型工况实验值与计算值吻合较好,验证了计算模型的正确性。对平流层电子设备热设计有重要的指导意义。     

Heat transfer reduction between two finite concentric cylinders using radiation shields; Experimental and numerical studies

Energy consumption and its efficient utilization are two important factors of thermal systems. This work concerns with numerical and experimental studies about the surface radiation and natural convection effects on the heat transfer and flow field between two finite concentric cylinders, using one radiation shield between them. This study reveals material and geometric effects of the radiation shield on heat losses from two concentric cylinders enclosure at different temperatures and enclosure pressures. The enclosure consists of two concentric cylinders with hotter inner cylinder and colder outer one. The radiation shield with three different materials (aluminum, copper and steel) is inserted between the cylinders at two different radial positions. Validations are carried out for the temperature of the radiation shield with experimental data and numerical ones. After validation, forty eight different experiments and numerical simulations are carried out by varying the inner cylinder temperature between 373 K and 673 K at two enclosure pressures of 0.2 and 1.0 atm, corresponding to three different materials as radiation shields. The outer cylinder temperature from experiments is used in numerical simulations. The results show that the enclosure pressure and radiation shield emissivity together are responsible for reduction in the total heat loss from the inner cylinder. It was also found that among the three considered materials as radiation shields, copper is the most effective one to reduce the heat loss. In a specific case, the total heat loss with copper radiation shield was 14.99% and 57.7% lower than steel and aluminum shields, respectively.     

Numerical Study on the Thermal Stress and its Formation Mechanism of a Thermoelectric Device

The strong thermo-mechanical stress is one of the most critical failure mechanisms that affect the durability of thermoelectric devices. In this study, numerical simulations on the formation mechanism of the maximum thermal stress inside the thermoelectric device have been performed by using finite element method. The influences of the material properties and the thermal radiation on the thermal stress have been examined. The results indicate that the maximum thermal stress was located at the contact position between the two materials and occurred due to differential thermal expansions and displacement constraints of the materials. The difference in the calculated thermal stress value between the constant and the variable material properties was between 3% and 4%. At a heat flux of 1 W·cm~(-2) and an emissivity of 0.5, the influence of the radiation heat transfer on the thermal stress was only about 5%; however, when the heat flux was 20 W·cm~(-2) and the emissivity was 0.7, the influence of the radiation heat transfer was more than 30%.