Static thermal conductivity methods for solid materials

Various experimental arrangements for the determination of steady-state thermal conductivities, as adopted for different situations, are reviewed. The case where the sample geometry and dimensions are imposed is treated distinctly from that where they may be chosen by the experimenter. In the latter case, it is shown that the set-up adopted may vary significantly according to the magnitude of the conductivity, the temperature range investigated, the geometry, and the dimensions of the samples available. Some devices which were recently developed in the author's laboratory are taken as illustrative examples and briefly discussed.     

纤维多孔材料导热系数的测试误差研究

对平板测试传感器测试下的纤维多孔材料内部热传导进行了数值模拟。首先针对纤维多孔材料建立了温度场速度场耦合热传递方程,然后利用数值模拟结果对纤维多孔材料导热系数的测试误差进行了考察,并对导热系数测试误差与纤维多孔材料物理以及结构参数之间的关系进行了分析。在查清误差产生根源的基础上对导热系数、厚度、渗透系数与热流量的关系进行了回归拟合,从而建立了更为精确的导热系数计算公式,并通过实验进行了验证。     

Apparent thermal conductivity approach at high-temperature measurements of porous materials

The apparent thermal conductivity approach for high-temperature investigations of porous materials taking into account moisture transport, convective and radiative modes of heat transport, phase-change processes and chemical reactions is presented. Methods for the measurement of apparent thermal conductivity as a material characteristic are analyzed. An example of the practical application of the approach is given, for a hybrid-fiber reinforced cement composite exposed to high temperatures up to 1000 °C. The aspects of the utilization of apparent thermal conductivity as a tool in an assessment of porous materials and their multi-layered systems exposed to high temperatures are discussed.     

The measurement of thermal conductivity variation with temperature for solid materials

An apparatus was designed to routinely measure the thermal conductivity variation with temperature for solid materials. The apparatus was calibrated by measuring the thermal conductivity variations with temperature for aluminum, zinc, tin and indium metals. The variations of thermal conductivity with temperature for the Zn-[x] wt.% Sb alloys (x = 10, 20, 30 and 40) were then measured by using the linear heat flow apparatus designed in present work. From experimental results it can be concluded that the linear heat flow apparatus can be used to measure thermal conductivity variation with temperature for multi component metallic alloys as well as pure metallic materials and for any kind of alloys. Variations of electrical conductivity with temperature for the Zn-[x] wt.% Sb alloys were determined from the Wiedemann–Franz (W–F) equation by using the measured values of thermal conductivity. Dependencies of the thermal and electrical conductivities on composition of Sb in the Zn–Sb alloys were also investigated. According to present experimental results, the thermal conductivity and electrical conductivity for the Zn-[x] wt.% Sb alloys decrease with increasing the temperature and the composition of Sb.     

A unit for measuring the effective thermal conductivity of thin-layer materials

An experimental setup for determination of the absolute values of thermal-conductivity and heattransfer coefficients of thin-layer spacers at room temperature and higher is described. The experimental data processing procedure is given.     

Analysis of a thermal storage unit containing multiple phase change materials dispersed with high conductivity particles

This paper investigates numerically the combined effect of employing multiple Phase change materials (PCMs) and dispersion of high conductivity particles on the thermal performance of an energy storage unit. The key parameters that govern the performance have been identified following a simple reduction of variables. Among the different parameters, melting temperature of the PCMs, latent heat and melting temperature difference between two consecutive PCMs (ΔT_m) are found to have a significant influence on the performance. A case study considering a 3-PCM unit to study the effect of key parameters during melting has been presented. The results show that for the 3-PCM unit, the performance is better when the second and third PCMs placed have their latent heats larger than the first PCM. It has been recommended to have ΔT_m1 > ΔT_m2 for the 3-PCM unit in order achieve more melting.

     

A model for prediction of the effective thermal conductivity of granular materials with liquid binder

A model is developed to predict the effective thermal conductivity of a three-component system containing sand particles bonded with liquid binders. The effective thermal conductivities of the bonded and unbonded sands are measured by the line-heat-source method at room temperature. The parameters of a two-component model are determined from the measurements for unbonded sands. Finally the model for the three-component systems such as bonded sand is developed using the assumption of the coalescence of the liquid binder at the particle contact points. Comparison between the experimental results and the model predictions shows that the model developed in this study predicts well the effective thermal conductivities of fluid-saturated sand or sand bonded with liquid binders. Also, this model can be used to determine the effects of the parameters such as the thermal conductivities and the volume fractions of components and the geometrical characteristics of solid particles on the effective thermal conductivity of granular materials with or without the liquid binder.     

Laser flash method for measuring thermal conductivity of liquids-application to low thermal conductivity liquids

A laser flash method developed for the measurement of thermal conductivity of solids was applied to liquids of low thermal conductivity. The sample liquid was sandwiched in between a small thin metal disk and a sample holder. When the laser beam is absorbed in the front surface of the metal disk, the temperature of the disk quickly rises about 2 K and heat then flows downwards through the sample liquid as one-dimensional heat flow. The thermal conductivity of liquid can be obtained from the temperature fall of the disk without employing any reference materials and also without measuring the thickness of the sample liquid layer. Thermal conductivities of water and toluene near room temperature were measured by this method with a mean deviation of 2.6%. This laser flash method may be applied to the measurement of the thermal conductivity of liquids such as molten salts at elevated temperatures.     

Senftleben-Beenakker thermal conductivity effect apparatus

An apparatus for measuring the influence of magnetic fields on the thermal conductivity of gases, the Senftleben-Beenakker effect, is described. Conventional 'cold finger type' cooling with appropriate cryogenic fluids is utilized with a helium exchange gas chamber to establish the general temperature level of the cell holder. Automatic temperature control of the trim heater maintains the desired temperature to +/-10(-3) degrees C in the 77-300 K range. A concentric cylinder cell adaptable for use with either conventional electromagnet or superconducting solenoid is described. The use of thin Mylar polyester film end seals minimizes cell 'end effect' corrections and provides a resolution of 5x10(-6) in gas thermal conductivity.     

便携式导热系数测量仪

本文介绍了一种用于固体导热系数测量的新仪器。测量原理采用导热系数λ的新算式；测量控制选用可编程控制器；测量范围0．023 4～2．328 W/(m·K)。仪器具有快捷、轻便、测量精度高等特点。实验结果表明,该仪器测量值与文献参照值相差约＜5%。本文研究的结果对研制开发热物性测量具有一定的参考价值。     

Equivalent thermal conductivity of heat pipes

In precision machining, the machining error from thermal distortion carries a high proportion of the total errors. If a precision machining tool can transfer heat fast, the thermal distortion will be reduced and the machining precision will be improved. A heat pipe working based on phase transitions of the inner working liquid transfers heat with high efficiency and is widely applied in spaceflight and chemical industries. In mechanics, applications of heat pipes are correspondingly less. When a heat pipe is applied to a hydrostatic motorized spindle, the thermal distortion cannot be solved during the heat transfer process because thermal conductivity or equivalent thermal conductivity should be provided first for special application in mechanics. An equivalent thermal conductivity model based on equivalent thermal resistances is established. Performance tests for a screen wick pipe, gravity pipe, and rotation heat pipe are done to validate the efficiency of the equivalent thermal conductivity model. The proposed model provides a calculation method for the thermal distortion analysis of heat pipes applied in the motorized spindle.     

A noncontact thermal microprobe for local thermal conductivity measurement

We demonstrate a noncontact thermal microprobe technique for measuring the thermal conductivity κ with ～3 μm lateral spatial resolution by exploiting quasiballistic air conduction across a 10-100 nm air gap between a joule-heated microprobe and the sample. The thermal conductivity is extracted from the measured effective thermal resistance of the microprobe and the tip-sample thermal contact conductance and radius in the quasiballistic regime determined by calibration on reference samples using a heat transfer model. Our κ values are within 5%-10% of that measured by standard steady-state methods and theoretical predictions for nanostructured bulk and thin film assemblies of pnictogen chalcogenides. Noncontact thermal microprobing demonstrated here mitigates the strong dependence of tip-sample heat transfer on sample surface chemistry and topography inherent in contact methods, and allows the thermal characterization of a wide range of nanomaterials.     

A method for determining the thermal EMF and thermal conductivity

A method for measuring the thermal emf and thermal conductivity of conducting samples is described. After the temperature and potential differences are measured between the sample’s ends, on which metal contacts are deposited, at a specified direct current, the current is turned off, and instantaneous values of the integral thermal emf are measured directly at the metal contacts. Using the results from measuring the temperature and potential differences during passage of the direct current, the electrical and thermal conductivities of the sample are calculated. A specific feature of this technique is the use of additional thick metal layers being in contact with the surfaces of the metal contacts at the sample’s ends, which are aimed at stabilizing the temperature gradient along the sample when the current source is switched off.     

New method for measuring the thermal conductivity

A new experimental method is presented for measuring the thermal conductivity as a function of temperature. The basic innovation lies in extracting from the measured temperature profile of a sample in vacuo, the thermal conductivity of each individual cross-sectional sample element. The estimated experimental error is +/-1%. Not only is high accuracy achieved, but also a self-checking procedure offers the possibility of avoiding systematic errors. Measurements on two samples of type 304 stainless steel are presented. Three independent sets of measurements give consistent values for the thermal conductivity to well within the estimated error of +/-1%.     

Determination of the thermal conductivity of composted material

The objective of this study was to develop a reliable method for the determination of the thermal conductivity of composted material using the TP08 probe. Study was set out to determine whether the selection of a signal fragment used to establish thermal conductivity (λ), has a significant influence on the results. Also minimum number of measurements was determined for every phase of the composting process. No significant differences were reported between results, but certain changes in the value of λ were noted. In successive stages of the process, thermal conductivity of composted material were: 0.31 ± 0.09, 0.45 ± 0.14, 0.27 ± 0.03 and 0.37 ± 0.17 W m⁻¹ K⁻¹.     

Prediction of the thermal conductivity of petroleum products

One of the main functions of a lubricant is to cool the mating surfaces. Detailed studies of thermal behaviour of bearings and coolers requires a knowledge of the thermal conductivity of the lubricant. Thermal conductivity varies from one lubricant to another, and since it is a difficult property to measure, a prediction method has been developed. Over a wide range of temperature, thermal conductivity is described by the equation λ_t = λ₆₀ + α(t?60) and it is shown that λ₆₀ and α can be found from more readily available data. Thus the method allows thermal conductivity of lubricants and a wide range of other petroleum products to be calculated from the equation to within ± 5%.     

松散物料导热系数便携式测试系统

本文设计了一套测定松散物料导热系数的便携式测试装置。利用平行热线法,采用新型便携式装置,方便程度大大增加,成本却大幅降低。测定时对松散物料恒功率加热,将其实时温度送入单片机,处理结果经串口通讯传入笔记本电脑。通过特定的算法,将所测松散物料的导热率计算出来。实测结果与相关文献参照值相符。     

Apparatus for thermal conductivity measurements on thin fibres

A sample holder for the measurement of the low-temperature thermal conductivity of very thin samples is described. The method is based on a potentiometric principle. Some constructional details and typical results are also given.