预测复合材料导热系数的热阻网络法

借助计算机模拟复合材料的空间结构,直接迭代求解热阻网络,得到复合材料的导热系数。分析了在随机分布条件下取样尺度对导热系数的影响,以及二维和三维条件下导热的差别。与文献中实验数据的比较表明,所述方法能够较好地预示颗粒弥散型复合材料的导热系数。     

晶体材料导热性质的经验方程与预测方法

依据现有相关的功能晶体材料导热系数实验数据，寻求了晶体材料导热系数与平均原子密度和平均原子量之间关联的经验方程，并对Ⅲ-Ⅴ族化合物半导体和碳化物晶体材料给出了经验方程中的常数，从而为这些晶体材料导热性质的预测提供了一种可靠的方法。     

A model for the thermal conductivity of plasma-sprayed ceramic coatings

The very low thermal conductivity of plasma-sprayed ceramic coatings can be explained in terms of a model for the coating microstructure involving limited regions of good contact between lamellae. The non-contact regions may be regarded as very thin planar pores which have limited thermal conductance at low temperatures because their width is comparable with the mean free path of the gas molecules within them. At higher temperatures there is a radiative contribution across the pores which results in a smaller temperature coefficient of thermal conductivity than for a sintered ceramic. Heating at high temperatures results in changes in the shape of the planar pores and a large increase in the thermal conductivity.     

A rough hard-sphere model for the thermal conductivity of molten salts

A new model based on the rough hard-sphere theory is proposed for the correlation and prediction of the thermal conductivity of molten salts. The model is capable of predicting the thermal conductivity of all the members of a family of molten salts characterized by a common anion if the behavior of any single member of the family of salts is known. Only the molar volumes of the molten salt and the solid salt at the melting temperature are required in the calculations. In addition, the model is easily extended to mixtures with a simple mixing rule.     

Resistance network-based thermal conductivity model for metal foams

A network model for the estimation of effective thermal conductivity of open-celled metal foams is presented. A nodal network representation of three aluminum foam samples from DUOCEL – 10 ppi, 20 ppi and 40 ppi – is constructed out of X-ray microtomography data obtained by computed tomography (CT) scanning of the samples using a commercial CT scanner. Image processing and 3D skeletonization are performed with commercially available image processing software. The effective thermal conductivity is estimated through a 1D conduction model, representing individual ligaments as an effective thermal resistance using the topological information from the scan data. The effective thermal conductivity data thus obtained are compared with the Lemlich theory and other pore-based models. Further, microstructural characterization of foam features – pore size, ligament thickness, ligament length and pore shapes – is performed. All the three foam samples are observed to have similar pore shapes and volumetric porosity, while the other features scale with the pore size. For a given porosity the computed permeability is found to scale as the square of the pore diameter, as also noted by previous researchers.     

A semi-analytical model for the effective thermal conductivity of a multi-component polydisperse granular bed

A theoretical model to predict the effective thermal conductivity of a multi-component polydisperse granular bed is presented. A simple energy balance analysis is used to arrive at an approximate analytical expression for the effective thermal conductivity. Simulation of heat transfer in a granular bed is carried out using an open source Discrete Element Method (DEM) package called LIGGGHTS. The derived analytical expressions for the effective thermal conductivity compares well with the results obtained from DEM simulations for granular beds comprising of different components with different sizes.     

In-situ monitoring of the time evolution of the effective thermal conductivity of snow

We report on a 3-month long time series of in-situ measurements of the effective thermal conductivity (k_eff) of snow at 6 heights in an Alpine snowpack in the Mont-Blanc mountain range, France, at an altitude of 2400 m. Measurements were carried out automatically every 2 days using heated-needle probes embedded in the snowpack. The experimental procedure used is presented in detail and demonstrates the applicability of single heated-needle probes for the evaluation of k_eff in snow, both for long-term measurements within the snowpack and occasional use in the field. Results based on 139 automatically collected data show k_eff values ranging between 0.04 and 0.35 W m^− 1 K^− 1, and a consistent pattern of effective thermal conductivity increase throughout the measurements campaign. The temporal rate of change of k_eff varies up to 0.01 W m⁻¹ K^− 1day^− 1, with maximum values just after snowfall.     

A model of thermal conductivity of nanofluids with interfacial shells

We derive an expression for the effective thermal conductivity of nanofluids with interfacial shells. Comparing with conventional models, the expression is not only depended on the thermal conductivity of the solid and liquid and their relative volume fraction, but also depended on the particle size and interfacial properties. The theoretical results on the effective thermal conductivity of CuO/water and CuO/ethylene glycol nanofluids are in good agreement with the experimental data.     

A two-stage model for predicting crack growth due to repeated thermal shock

The growth of cracks in equipment that is exposed to repeated thermal down shocks presents a complex problem of analysis. The transient, highly non-linear nature of the stress profiles that are developed during the shock in addition to localized plasticity and environmental interactions makes difficult any accurate analytical predictions. The use of current analysis techniques based on linear stress approximations can result in overly conservative results that may lead to unnecessary and costly component replacements.This paper outlines results from an experimental investigation into crack growth in notched, flat plate specimens exposed to repeated one-dimensional thermal shocks. Analysis of the results shows that a simple two-stage growth model may be applicable for describing the crack growth. The model is comprised of a high strain fatigue region where crack growth is in the plastic range and a region where growth is described by linear elastic fracture mechanics. Allowances for the effects of mean loads and environment on the crack growth are also included in the model. The model is currently limited to the consideration of carbon steel components, operating at temperatures below the creep range.     

A nonlinear effective thermal conductivity model for carbon nanotube and nanofiber suspensions

It has been experimentally demonstrated that suspensions of carbon nanotubes (CNTs) and nanofibers (CNFs) significantly increase the thermal conductivity of nanofluids; however, a physically sound theory of the underlying phenomenon is still missing. In this study, the nonlinear nature of the effective thermal conductivity enhancement with the particle concentration of CNT and CNF nanofluids is explained physically using the excluded volume concept. Specifically, the number of contacting CNTs and CNFs could be calculated by using the excluded volume concept, where the distance for heat to travel in a cylinder between the contacting cylinders in the thermal network of percolating CNTs and CNFs increased with the excluded volume. In contrast to the effective thermal conductivity model of Sastry et al (2008 Nanotechnology 19 055704) the present revised model could reproduce the nonlinear increase of the thermal conductivity with particle concentration, as well as the dependence on the diameter and aspect ratio of the CNTs and CNFs. It was found that the alignment of CNTs and CNFs due to the long range repulsion force decreases the excluded volume, leading to both the convex and concave nonlinear as well as linear increase of the thermal conductivity with particle concentration. The difference between various carrier fluids of the suspensions could be explained as the result of the change in the excluded volume in different base fluids.     

A molecular dynamics-stochastic model for thermal conductivity of nanofluids and its experimental validation

A model to predict the enhanced thermal conductivity of water based copper nanofluid on the basis of molecular dynamics simulation coupled with stochastic simulation shows for the first time that the temperature of a copper nanoparticle colliding with a heat source can rise rapidly within the short collision period (e.g., 10-50 ps) estimated by impact dynamics due to phonon transfer. Thereafter the particles undergo Brownian movement in the base fluid and transfer the excess heat in about 2 to 3 ms to the surrounding fluid resulting in an appreciable enhancement of the thermal conductivity of the fluid. Microconvection has minor contribution to the enhanced thermal conductivity of nanofluids. The predicted thermal conductivity of nanofluid and its variation with the volume fraction of the nanoparticles agree well with the present experiments, as well as, with the data reported in the literature.     

An instrument to index snow surface hardness for predicting snow drifting

A motor-driven, spherical probe attached to a load cell was used to measure the external force needed to fracture the uppermost elements of several natural snow surfaces. These initial failures took place with a probe travel distance that averaged less than 3 mm. The force needed to cause failure varied from less than 1 to more than 200 N/m². This type of surface hardness index should be useful in predicting the onset, intensity, and duration of snow drifting.     

复合材料热传导性能的变分渐近均匀化细观力学模型

为准确预测非均质复合材料的有效热导率和局部温度场分布,采用单胞变分渐近均匀化方法构建了一种新的细观力学模型。首先从非均质连续体热传导变分问题入手,使用变分渐近法将其细观力学模型转换为约束条件下泛函的最小化——取驻值问题;使用有限元法(FEM)推导了离散形式能量泛函的最小化求解过程;根据宏观性能(如全局温度及相应的梯度和波动函数)重构单胞的局部温度场和热通量。采用多个二元复合材料算例验证了所构建理论和程序的有效性和准确性。     

周期性复合材料等效传热系数预测的渐进均匀化新算法及其实现

渐进均匀化方法具有严格的数学基础,预测周期性复合材料的等效热传导系数具有较高的计算精度。本文提出了基于渐进均匀化方法预测周期性复合材料等效热传导系数的新算法。相比原有的算法具有两个优点：它的实现与代表体元方法一样简单,新的算法以现有的有限元商业软件为黑箱,通过简单的几个分析步骤,即可以获得复合材料的等效热传导系数;可以利用商业软件提供的多种单元类型去离散同一个单胞,在处理复杂几何单胞结构时,可以节约大量的计算费用。通过几个典型的算例,验证了方法的有效性。该工作对于推广渐进均匀化方法在预测复合材料等效热传导系数的广泛应用具有积极作用。     

A correlation of thermal conductivity data for helium

A correlation for the thermal conductivity of helium has been developed which covers the temperature range from temperatures just above the lambda line to 830 K, and densities up to about 160 kg m⁻³. The data used incorporate some recent experimental results which cover the temperature range from 4 K to 20 K including the critical region. The correlation gives an equation which generally fits the experimental data within ± 5%. However, at low temperature, the experimental data deviate up to about 10% from predictions based upon viscosity measurements or molecular dynamics calculations.     

霜层生长过程中的导热模型

在逾渗理论的基础上,对霜层的导热系数进行分析.现有研究表明由于冰晶体的不断生长,会形成不同的集团,当在霜层中形成大的连通集团的时候,霜层导热系数的上升速度会突然加快.在此基础上提出了基于逾渗理论的导热模型,并采用数值模拟的方法验证了上述过程.以临界点为分界线,分别导出了霜层生长过程中,不同孔隙率下的导热系数的通用数学表达式.与其他导热模型的结果比较表明,本模型更加符合实际,并且有较大的使用范围.     

A theoretical model for effective thermal conductivity (ETC) of particulate beds under compression

Thermal conductivity of particulate beds is an important property for many industrial handling processes as well as for storage of particulate materials. This paper presents a new theoretical model that is based on heat transfer between particles in three modes: heat conduction through contact area, heat conduction through voids and radiation through voids. The model is further adjusted in order to obtain effective thermal conductivity of a particulate bed by using empirical augmentation factors for the heat transfer coefficient of each one of the heat transfer mechanisms. Comparison of the results predicted by the semi-empirical model to our experimental results show good agreement. The theoretical model was investigated to examine the effect of various parameters (such as: particle elasticity and surface roughness, particle and gas thermal conductivity and particle diameter), on the effective thermal conductivity of various particulate beds. Our results show the significant effect of the contact area (that is a clear function of the compression load) between particles on the effective thermal conductivity.     

A new Peltier sensor for measuring the thermal conductivity offluids

This paper describes a new sensor and a new method to measure the thermal conductivity of many fluids. The principal advantage of the device is self compensation against temperature brought about by an appropriate choice of the materials. Moreover, because the sensor uses both the Peltier and Seebeck effects, measurements can be carried out with accuracy according to an average temperature increase of the device lower than 5 K. Operation of the device brings about a very low Joule power (5 mW). A coherent design rationale is formulated and the various stages in the technical development of the sensor are delineated. Several cases are discussed with a view to increasing the applicability of the method. Notable applications include thermal conductivity gauges for measuring pressures in high-vacuum systems, tank gauging for liquids featuring fire hazards, and low velocity measurements occurring in natural convection mechanisms. It is expected that the versatility of the device will result in a wide number of industrial applications     

A new Peltier sensor for measuring the thermal conductivity offluids

This paper describes a new sensor and method to measure the thermal conductivity of many fluids. The principal advantage of the sensor is self-compensation against temperatures brought about by an appropriate choice of materials. Moreover, because the sensor uses both the Peltier and Seebeck effects, measurements can be carried out with accuracy according to an average temperature increase of the device lower than 5 K. Operation of the device brings about a very low Joule power (5 mW). A coherent design rationale is formulated and the various stages in the technical development of the sensor are delineated. Several cases are discussed with a view toward increasing the applicability of the method. Notable applications include thermal conductivity gauges for measuring pressures in high-vacuum systems, tank gauging for liquids featuring fire hazards, and low-velocity measurements occurring in natural convection mechanisms. It is expected that the versatility of the device will result in a wide variety of industrial applications