水合物导热系数和热扩散率实验研究

基于Hot Disk热常数分析系统的单面测试功能,建立了一套新的天然气水合物热物性测试系统,并实验研究了I型水合物（甲烷）、H型水合物（甲烷和甲基环己烷）的导热系数和H型水合物的热扩散率。结果显示甲烷水合物样品导热系数随温度的变化非常小,平均导热系数约为0.53 W/(m•K)。结合文献报道和实验分析发现零孔隙率甲烷水合物的导热系数大约为0.7 W/(m•K),水合物样品在压缩过程中虽然减少了孔隙,但是却引起晶体破碎,导致导热系数与理想值差距较大。水合物的导热系数与水合物的类型及客体分子有关,大体顺序为I型 > II型 > H型 > 半笼型水合物。甲烷−甲基环己烷生成的H型水合物热扩散率为0.205 ~ 0.26 mm2/s,和其他类型的水合物相当;水合物的热扩散率大约为水的两倍,而导热系数和水相近。     

甲烷水合物及其沉积层导热特性的研究现状

总结了近年来国内外甲烷水合物及其沉积层导热特性的研究现状,从实验测试和模拟研究两方面分析了甲烷水合物及其沉积层的导热影响因素、相关规律和导热机理。研究结果表明：纯质水合物的导热性能与外界温度、压力、客体分子数量、多孔介质、笼形结构等因素有关,其值大小主要由主体水分子形成的笼形结构决定;而水合物沉积层导热系数的大小主要依赖于实验样品的组成成分、初始水饱和度、各组分分布,与温度、压力以及垂直有效应力关系不大。最后,指出了现存研究方法中一些值得改进的地方,并对今后的研究工作进行了展望。     

基于变导热系数的蒸汽长距离输送模拟

为了提高长距离输送蒸汽参数的计算精度,建立了蒸汽热力和水力耦合计算方程,并在能量方程中考虑了蒸汽流动过程中的相态变化。对于具有多层保温结构管道的节点热阻,提出了一个导热系数随温度变化的热阻计算方法,在介质流动方向上将长距离管道保温热阻进行分段计算。通过实测数据对模型进行验证,结果表明,与将平均温度作为保温材料定性温度相比,新计算模型模拟出的蒸汽温降和压降值更小且计算精度更高,其误差分别不超过1%和4%。     

碳纤维发热线温升特性研究

碳纤维发热体替代金属发热体是一种必然的趋势。研究了碳纤维发热线在空气中工作时的温度、时间、电压、电流的变化规律,进行了不同规格、长度、电压的升温试验,拟合温升曲线得到了温度变化与时间的数学关系式。研究结果表明:碳纤维发热线在空气中工作时最高温度和长度成反比、和电压成正比。其研究成果将为碳纤维的应用提供工程参考。     

考虑多因素影响的混凝土导热系数试验研究

混凝土是一种多相复合材料,其导热性受温湿度、孔隙率、骨料体积分数、龄期等多个因素的综合影响。通过试验研究了不同温度、饱和度、孔隙率、骨料体积分数、龄期等因素变化条件下水泥砂浆、骨料和混凝土导热系数的变化规律,结合Wiener边界理论,提出一种包含多因素影响的混凝土导热系数预测模型,分析了各因素与混凝土导热系数的相关性。结果表明,混凝土导热性的各影响因素之间具有明显的耦合作用特征,即混凝土含水率越高,温度变化对其导热性的影响越显著;混凝土导热系数随饱和度增大而增加,孔隙率越大,增幅越明显;随着骨料体积分数的增加,导热系数增大较明显。     

同轴地埋管换热器岩土热响应试验研究

岩土热物理性质是影响地源热泵系统设计和运营的关键因素,对位于武汉市洪山区的2口不同深度的同轴地埋管换热孔分别进行48 h的热响应试验,并对同轴地埋管换热器内外管之间环形空间中的平均流体温度进行测试.根据同轴地埋管换热器的几何特性,以简便实用的方式测量同轴地埋管换热器环状空间传热流体的平均温度,结合同轴地埋管换热器钻孔热...     

稳态平板导热仪最优预热时间的确定

研究了稳态平板法测定保温材料导热系数时的最优预热时间;获得了预热—保持两阶段工作方式下试件内瞬态温度变化的解析解,并从中获得了最优预热时间的数学表达式。计算结果表明,按照最优预热时间预热,可使试样达到稳态的时间显著缩短。     

点加热稳态导热系数测量方法研究

针对稳态导热系数测量方法测量过程时间较长、测量装置复杂、以及样品制备和加工工艺复杂等现状,提出了一种新型的点加热稳态导热系数测量方法,构建相应的三维稳态传热物理模型,使加热面温升只与热流密度、样品导热系数和测温点位置相关。通过聚焦连续激光加热样品,缩短样品达稳态时长至分钟量级;建立对照光路消除表面发射率和激光稳定性对温度测量的影响;红外热像仪测量加热表面稳态温度分布,结合物理模型实现导热系数测量。采用多种已知导热系数的标准材料和线性法对测量方法进行验证,并应用该方法测量硅藻土导热系数为0.49～0.60 W/(m·K),误差为6.06%。该方法的测量迅速及非接触特性使其可应用于工程实地测量。     

碳纳米管堆积床导热及热松弛

运用热线法测量了碳纳米管堆积床在120 K-370 K温度范围内的导热系数和传热松弛时间。测量数据表明:碳纳米管床导热系数极低,其在低温段随温度升高呈线性增加,在高于室温的范围趋于稳定。测量过程中碳纳米管床表现出的传热松弛时间,较已有文献报道的最大的碳纳米管床传热松弛时间大一个数量级。基于此数据并结合经典的(CV双曲型热传导)模型分析单个碳纳米管接触节点上的瞬态导热及热电特性,分析认为:利用纳米多孔材料的传热延迟特性可提高瞬态热电转换效率。     

微通道轴向导热的拉曼测温实验研究及数值模拟

建立了使用共焦显微拉曼光谱图的微尺度水温测量方法,并用于单相水的对流传热实验。将实验与数值模拟相结合,研究了微通道内轴向导热对流体的传热影响。研究发现,通道入口处壁面热通量最大、液温和壁温均呈非线性发展;局部Nu数曲线出现奇异点,且奇异点位置随着雷诺数的增大往出口处移动;努塞尔数随着雷诺数的增大而增大。     

碳纳米管纤维导热性能的实验研究

利用瞬态电热技术对碳纳米管纤维（CNTF）的导热性能进行测量，研究了不同加热电流下CNTF导热性能的变化规律。在测量过程中，发现热扩散系数存在“突变”现象，CNTF的高热扩散系数比低热扩散系数大约提高1.65-3.85倍，随着电流的增加，热扩散系数呈下降趋势。对电流加热过程中的声子散射机制进行了分析总结，并探究了CNTF中微观结构演变。研究表明，热扩散系数下降主要是由于高温加热过程中，倒逆声子散射增强和组成CNTF中的多壁碳纳米管结构变化造成。     

石墨烯/碳纳米管/环氧树脂复合材料的导热性能的实验研究

在微电子领域中,随着元器件的体积微小化,要求导热材料具备体积小、高导热的特点。高分子导热复合材料能很好的解决器件在不同的工作环境中仍能保持正常的散热问题。以环氧树脂(EP)为基体,石墨烯粉末(GP)和多壁碳纳米管(MWCNTs)为导热填料,采用溶剂和超声分散法,制备出石墨烯/碳纳米管/环氧树脂复合材料。实验采用瞬态电热技术测量其导热系数,结果显示,石墨烯与碳纳米管协同作为导热填料时,复合材料导热性优于单独添加导热填料(GP或MWCNTs),且随着GP所占比例的增大复合材料的导热系数越大。当GP和MWCNTs比例分别为0.7%和0.3%时,复合材料导热系数为0.940 W/(m·K),相比于纯EP导热系数提高了286.83%。     

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.     

石墨烯粉末导热性能的研究

采用瞬态电热技术测量了5~6层纯石墨烯粉末中石墨烯的热扩散率,其值为1.15×10-5 m2/s,相应的导热系数为18.000 W/(m·K)。借助导热仪研究了不同密度下石墨烯粉末导热系数的变化情况,发现其导热系数与密度成正比。密度由0.02增加到0.22g/cm3时,导热系数总体提升了8.09%。另通过实验得到了含水率对石墨烯粉末导热性能的影响。实验结果显示,石墨烯粉末含水率由0.0%增加到99.8%的过程中,导热系数先是上升随后下降最终直线降至最低点(约为0.765~1.030 W/(m·K))。其中当含水率达到96.7%时,混合物(石墨烯与水)的导热系数提高了62.80%。该研究为石墨烯热应用及热管理提供了理论支撑。     

Thermal conductivity and its temperature dependence in selected steel samples

The transient plane source (TPS) technique was employed to measure experimentally the thermal conductivity of three selected steel samples. Measured values of thermal conductivities are critically analysed, taking into account factors such as the composition and the nature of the alloying elements. An effort is made to determine the temperature dependence of thermal conductivity of these samples from room temperature to 300°C. Results of the present measurements are also discussed in the light of the values of thermal conductivity on equivalent steels measured by other workers.     

Permeability and thermal conductivity of host compressed natural graphite for consolidated activated carbon adsorbent

Permeability and thermal conductivity test units were set up to study the heat and mass transfer performance of the host material, i.e. expanded natural graphite (ENG), for consolidated activated carbon (AC) adsorbent. The permeability was tested with nitrogen as the gas source, and the thermal conductivity was studied using steady-state heat source method. The results showed that the values of permeability and thermal conductivity were 10^-15 to 10^-12 m² and 1.7 to 3.2 W/(m·K), respectively, while the density compressed expanded natural graphite (CENG) varied from 100 to 500 kg/m³. The permeability decreased with the increasing density of CENG, whereas the thermal conductivity increased with the increasing density of CENG. Then the thermal conductivity and permeability of granular AC were researched. It was discovered that the thermal conductivity of samples with different grain size almost kept constant at 0.36 W/(m·K) while the density was approximately 600 kg/m³. This means that the thermal conductivity was not related to the grain size of AC. The thermal conductivity of CENG was improved by 5 to 10 times compared with that of granular AC. Such a result showed that CENG was a promising host material for AC to improve the heat transfer performance, while the mass transfer performance should be considered in different conditions for utilization of adsorbent.     

The Measurement of Thermal Conductivities of Silica and Carbon Black Powders at Different Pressures by Thermal Conductivity Probe

This investigation was done to study the gas filled powder insulation and thermal conductivity probe for themeasurement of thermal conductivity of powders.The mathematical analysis showed that the heat capacity ofthe probe itself and the thermal resistance between the probe and powder must be considered.The authorsdeveloped a slender probe and measured the effective thermal conductivity of silica and carbon black powdersunder a variety of conditions.     

Temperature dependence of thermal conductivity and thermal diffusivity of some composites using the transient plane source technique

The recently developed transient plane source (TPS) technique has been applied for the simultaneous measurement of thermal conductivity and thermal diffusivity of two composite materials namely, marble and magnesium oxychloride cement in the range of temperatures from 30 to 150°C. The experimental results of these samples show that there is very slight variation in thermal conductivity and thermal diffusivity of these materials in this range of temperature. An effort has been made to express this variation of thermal conductivity and diffusivity with temperature by a linear relation, in these materials.     

Measurements of thermal conductivity and thermal diffusivity of alternative refrigerants in liquid phase with a transient short‐hot‐wire method

A transient short‐hot‐wire method has been proposed to simultaneously measure the thermal conductivity and thermal diffusivity of liquids. The method has been applied to the refrigerant HCFC‐22, alternative refrigerants HFC‐32, HFC‐125, HFC‐134a and refrigerant mixtures HFC‐32/125 [35/65, 50/50 (HFC‐410A), 68/32 wt%], HFC‐32/125/134a [23/25/52 (HFC‐407C) wt%]. From the present study, it is shown that the measured results agree well with the literature values on thermal conductivity and those on thermal diffusivity obtained from NIST's thermophysical property database, REFPROP Ver. 6.0. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 540–552, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20032     

Measurement of in-plane thermal conductivity of carbon paper diffusion media in the temperature range of −20 °C to +120 °C

Carbon paper is commonly used as the gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells as it exhibits high chemical and mechanical durability. This diffusion medium is also anisotropic, which directly affects its transport properties and specifically the thermal conductivity. In this study, the in-plane thermal conductivity of the carbon paper GDL was determined using thermal diffusivity measurements for a temperature range from −20 to +120 °C and four Teflon loadings (0, 5, 20 and 50 wt.%). It is important to understand the effect of temperature on the thermal conductivity since PEM fuel cells are designed to operate under various temperatures depending on the application of use. Further, Teflon is used to change the hydrophobic properties of the carbon paper GDL with 20 wt.% as the most widely used percentage. In this study, the Teflon loadings were chosen to gain a comprehensive understanding of the thermal resistance due to Teflon. In this study, a quasi-steady method was used to measure the thermal properties of the carbon paper; hence, the phase transformation in the presence of PTFE was investigated. The thermal conductivity decreases with an increase in temperature for all samples. The addition of as little as 5 wt.% Teflon resulted in high thermal resistance decreasing the overall thermal conductivity of the sample. Further addition of Teflon did not have major effects on the thermal conductivity. For all treated samples, the thermal conductivity lies in the range of 10.1–14.7 W/mK. Finally, empirical relations for the thermal diffusivity and conductivity with temperature were deduced.