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。     

基于分形理论超轻中空二次粒子纤维壁的有效导热系数计算方法

利用分形理论描述硬硅钙石超轻中空二次粒子纤维壁的微尺度空间结构,建立了分形单元体导热模型,采用等效电阻方法,推导出了超轻中空二次粒子纤维壁的有效导热系数计算公式,同时探讨了温度、孔隙率、分形维数等因素对材料有效导热系数的影响规律。     

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。     

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。     

土壤源热泵地下水平埋管换热性能及其周围土壤温度场的影响研究

通过建立地下水平埋管换热器模型,模拟了土壤导热系数对埋管及其周围土壤温度场分布和埋管换热量的影响.分析了埋管管材及埋管埋深、管径、管壁厚度等对埋管换热的影响.模拟结果显示,当土壤导热系数从1.1W/(m·℃)增大到2.5W/(m·℃)时,埋管单位管长换热量增幅达100.8%,且到埋管距离越近的点,其土壤温度随土壤导热系数的变化相对较快.地下二层埋管外表面温度及其周围土壤温度变化比地下一层换热稳定性好,换热量大.适当的加大管径,减小管壁厚,有利于增强埋管换热.     

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。     

地下土壤导热系数确定中影响因素分析

利用有限元热分析平台,建立了二维瞬态有限元模型,计算确定了竖孔U型管地下换热器周边的土壤导热系数,并着重对主要影响参数进行了比较分析.讨论了测量时间、土壤初始温度、土壤体积比热、回填料物性参数和管间距等因素对土壤导热系数的影响程度和敏感性.     

竖直U型埋管地热换热器热短路现象的影响参数分析

通过引入换热器出口最高流体温度的概念,对地源热泵竖直U型埋管地热换热器的热短路现象进行了量化,基于竖直U型埋管周围的瞬时有限元模型,对影响热短路现象的主要参数(支管间距和回填料导热系数)进行了模拟分析,得出了量化结果。结果表明,增大支管间距可降低换热器出口最高流体温度,减小由热短路现象引起的热损失;回填料的导热系数对热短路现象的影响较大,当回填料导热系数小于周围土壤的导热系数时,增大回填料导热系数对减小热短路损失有较大作用,而当回填料导热系数大于土壤导热系数时则作用不大,推荐使用导热系数与周围土壤导热系数接近的回填材料。     

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。     

Monitoring of transient 3D temperature distribution and thermal stress in pressure elements based on the wall temperature measurement

Abstract

Two methods for monitoring the thermal stresses in pressure components of thermal power plants are presented. In the first method, the transient temperature distribution in the pressure component is determined by measuring the transient wall temperature at several points located on the outer insulated surface of the component. The transient temperature distribution in the pressure component, including the temperature of the inner surface is determined from the solution of the inverse heat conduction problem (IHCP). In the first method, there is no need to know the temperature of the fluid and the heat transfer coefficient. In the second method, thermal stresses in a pressure component with a complicated shape are computed using the finite element method (FEM) based on experimentally estimated fluid temperature and known heat transfer coefficient. A new thermometer with good dynamic properties has been developed and applied in practice, providing a much more accurate measurement of the temperature of the flowing fluid in comparison with standard thermometers. The heat transfer coefficient on the inner surface of a pressure element can be determined from the empirical relationships available in the literature. A numerical-experimental method of determination of the transient heat transfer coefficient based on the solution of the 3D-inverse heat conduction problem has also been proposed. The heat transfer coefficient on the internal surface of a pressure element is determined based on an experimentally determined local transient temperature distribution on the external surface of the element or the basis of wall temperature measurement at six points located near the internal surface if fluid temperature changes are fast. Examples of determining thermal and pressure stresses in the thick-walled horizontal superheater header and the horizontal header of the steam cooler in a power boiler with the use of real measurement data are presented.     

Liquid crystal quantitative temperature measurement technique

IntrOductionThe quantitative teInPatre measurement usingthermochrOndc liquid crystal is an "area" thermalmeasurement technique. It uhlizes the feature that 1iquidcrystal changes its reflex llght color raPidly withvariahon of temperatUre. On the base of certain colortheory, aPplying an image captUring and processingsystem that consists of a color CCD videO camera and acomPutCr, a callbratiOn curve of liquid crystal's color-temPeratUre can be Obtained. theards, the tectriqueuses uns curve…     

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.     

塔式太阳能热发电站圆月夜聚光实验研究

在塔式太阳能热发电系统中，吸热器采光面上的聚光能流密度分布的测量对优化整个系统的光热性能有着重要意义。本文提出一种基于月光聚光信息的塔式电站定日镜场聚光能流密度分布的间接测量方法。主要介绍2018年9月24日晚在延庆塔式电站开展的两种对月聚光实验：一种是通过塔上布置的照度计标定电荷耦合元件（Charge-coupled Device, CCD）相机拍摄的光斑图像，得到定日镜场聚光光斑的照度分布；另一种是使聚光光斑扫描过照度计，得到不同时刻的照度计数值，通过高斯拟合得到聚光光斑的照度分布。将聚光光斑的照度分布与月光测光站测得的月光法向直射照度对比，得到塔上聚光光斑的相对能流密度分布。实验结果表明，通过月光聚光实验，可以得到塔式电站的聚光光斑的相对能流密度分布（即聚光比分布），为后续依据太阳和月亮之间的亮度分布关系，转换为日光聚光能流密度分布提供实验数据支持。     

Infrared temperature measurements on solar trough absorber tubes

The temperature distribution on solar trough absorber tubes determines thermal losses and hotspots can lead to material stress and limit absorber tube lifetime. The concentrated solar radiation, however, makes it difficult to determine the temperature on solar absorbers. Temperature sensors that require contact to the measurement object are not appropriate and even pyrometry fails, when external light sources interfere. Only solar-blind pyrometry offers reliable temperature readings without perturbation through reflected solar radiation. This paper presents two concepts for a pyrometric solar-blind measurement on solar trough absorber tubes. One solar-blind approach is a spectral measurement range in regions, where the solar spectrum shows gaps due to the discrete absorption of the atmosphere. Another possibility for a solar-blind pyrometric temperature measurement results from the optical behavior, i.e. the distinct angle dependence of the directional reflectance and emittance of a typical selective trough absorber coating. First experimental results are shown and the accuracy and performance advantages and disadvantages of the setups are reported and discussed.     

Thermo-mechanical stress analyses of solid oxide fuel cell anode based on three-dimensional microstructure reconstruction

During high-temperature operation of a solid oxide fuel cell, the stresses caused by mismatch of thermal expansion coefficients between different materials and external mechanical loads may cause the rising of damage risk of nickel-yttria-stabilized zirconia anode. It is quite difficult to quantify the mechanical characteristics of a composite anode without investigating on the stress distribution in its real microstructure. However, the high operating temperature and extremely complex microstructure in micro-scale determine the high difficulty in in-situ measurement of thermo-mechanical stress distribution. In this work, the microstructures of six different anode samples, fabricated by using identical nickel oxide-yttria-stabilized zirconia powder mixture, are reconstructed in three-dimension based on the dual-beam focused-ion-beam-scanning-electron-microscopy. The three-dimensional thermo-mechanical stress distributions of different microstructures are conducted at operating temperature based on the finite element method. The effects of both thermal expansion coefficients mismatch between nickel and yttria-stabilized zirconia and external mechanical loads are analyzed. The mechanical failure probabilities of yttria-stabilized zirconia phase in different reconstructions are estimated based on the obtained stress distributions to investigate the influence of microstructure characterizations on nickel-yttria-stabilized zirconia anode strength.     

Stability Analysis of Distributed Parameter Systems on Temperature Measurement of Large-scale Objects

In this paper,the mechanical construction,thermal regulator design and temperature measurement system of alocal area were set up for large-scale linear measurement.Numerical analysis based on temperature correlatedcharacteristic is discussed to obtain optimal observation points for the measurements.The horizontal temperaturedistribution of the measured area is compared with the measurement of the variation of temperature at 15℃ and20~C over time,and characteristic of dynamic response is also discussed.In addition,the long-time stability ofmeasured temperature is analyzed by means of using the standard deviation.It characterizes the temperature dis-tribution performance of a large area and how it may impact the measurement of a large-scale object.     

Simultaneous measurement of thermal properties by thermal probe using stochastic approximation method

A novel thermal probe method is proposed for the simultaneous measurement of the thermal properties by the Monte Carlo stochastic approximation method. In this method, thermal capacity of probe and thermal contact resistance between probe and sample are considered. An experimental system is set up with the method to validate the measurement accuracy of the method. The thermal properties of several liquid samples as well as solid samples are measured. The results show that: (1) the thermal conductivity and the volumetric heat capacity can be measured with an error of less than 1.2% and 3% respectively, therefore, the measurement accuracy by the method is much higher than the conventional method and (2) the thermal contact resistance has a great effect on thermal conductivity for solid sample, while little influence on thermal conductivity for liquid sample and volumetric heat capacity.     

Study of Thermal Conductivity and Geometry Wall Contact Resistance Effect of Granular Active Carbon for Refrigeration and Heat Pumping Systems

The commercial success of sorption refrigeration and heat pump systems depends on a good heat and mass transfer in the adsorbent bed, which allows higher coefficients of performance and greater specific heating or cooling power that reduce capital costs. In this study the thermal conductivity and thermal contact resistance of vibrated and compressed granular active carbon and binary mixtures of active carbon are investigated using two types of conductivity measurements: a steady-state measurement between flat plates and a transient hot tube measurement. With these results is possible to draw conclusions on how the wall geometry, particle size distribution, and bulk density affect the overall thermal performance. Results show that using binary mixtures of grains and powder gives results superior to those of either grains or powder alone. The conductivity of the binary mixtures increases roughly linearly with bulk density and the 2/3 grain mixture achieves the highest densities. The method used to achieve compaction (vibration or compression) did not seem to affect the result. Thermal contact resistances reduce with increasing density but do vary with the mixture ratio, also appearing to be best with a 2/3 grain–1/3 powder mixture.     

Application of a thermally conductive pyrolytic graphite sheet to thermal management of a PEM fuel cell

This work experimentally investigates the thermal performance of a pyrolytic graphite sheet (PGS) in a single proton exchange membrane fuel cell (PEMFC). This PGS with high thermal conductivity serves as a heat spreader, reduces the volume and weight of cooling systems, and reduces and homogenizes the temperature in the reaction area of the fuel cells. A transparent PEMFC is constructed with PGS of thickness 0.1 mm cut into the shape of a flow channel and bound with the cathode gas channel plate. Eleven thermocouples are embedded at different positions on the cathode gas channel plate to measure the temperature distribution. The water and water flooding inside the cathode gas channels, with and without PGS, were successfully visualized. The locations of liquid water are correlated with the temperature measurement. PGS reduces the maximum cell temperature and improves cell performance at high cathode flow rates. The temperature distribution is also more uniform in the cell with PGS than in the one without PGS. Results of this study demonstrate the promising application of PGS to the thermal management of a fuel cell system.