Prediction of thermal conductivity of steel

A model of thermal conductivity as a function of temperature and steel composition has been produced using a neural network technique based upon a Bayesian statistics framework. The model allows the estimation of conductivity for heat transfer problems, along with the appropriate uncertainty. The performance of the model is demonstrated by making predictions of previous experimental results which were not included in the process which leads to the creation of the model.     

Permeability and effective thermal conductivity of bisized porous media

In the region of minimum porosity of particulate binary mixtures, heat exchange and permeability were found to be higher than the ones obtained with a mono-size packing built with the same small size particles used in the binary packing. This effect was noticed in the range of the particles size ratio 0.1–1.0.The obtained improvement on thermal performance is related to the increase of effective thermal conductivity (ETC) in the binary packing and to the increase in transversal thermal dispersion due to the porosity decrease and tortuosity increase.Permeability can increase by a factor of two, if the size ratio between small and large spheres of a loose packing stays in the range 0.3–0.5.     

Effective thermal conductivity of highly porous two-phase systems

A theoretical expression for predicting effective thermal conductivity (ETC) of highly porous two-phase systems has been developed. The porous system has been assumed to contain particles of irregular shape, being dispersed randomly within the continuous medium. The concept of averaging the temperature field within different phases has been used. Resistor model has been applied to determine ETC of two-phase porous systems, as a porous medium is neither composed of slabs parallel nor perpendicular to the heat flux. It is proposed to use slabs inclined at an angle θ with the heat flux lines. An effort is made to correlate angle of inclination θ in terms of the ratio of thermal conductivity of the constituent phases and the physical porosity. Best-fitted expression so obtained for θ is used in the derived model and found that the predicted values of ETC are quite close to the experimental results.     

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.     

On the thermal conductivity of sintered metallic fibre structures

The present paper investigates the anisotropic thermal conductivity of a novel sintered metallic fibre structure with different porosities (i.e. 51.9%, 74.9%, 81.4%). Different methodologies are applied: numerical calculations (i.e. Finite Element and Lattice Monte Carlo methods) based on micro-computed tomography images, experimental tests (i.e. steady-state plate method) and analytical modelling. Good agreement between the numerical methods and experimental measurements is obtained for high porosity models (i.e. porosity > 72.5%). Furthermore, the thermal conductivity decreases with increasing porosity. A distinct thermal anisotropy is found where maximum values are in the parallel direction and minimum values in the transverse direction to the fibres.     

Thermal conductivity bounds for isotropic, porous materials

To avoid potential misapplication of effective thermal conductivity models, materials that may be described as ‘porous’ should be divided into two classes; ‘internal porosity’ materials which have bubbles/pores suspended within a continuous condensed phase (e.g. sponges, foams, honeycombs), and ‘external porosity’ materials which include granular/particulate materials. It is proposed that the effective thermal conductivity region bounded by the Hashin-Shtrikman bounds may be divided into internal porosity and external porosity regions by the Effective Medium Theory (EMT) equation. The use of the Hashin-Shtrikman and EMT equations as porosity bounds was supported by experimental data from the literature.     

建筑绝热材料导热系数测试方法探讨

以GB/T 10294—2008为例,对使用防护热板法测试建筑绝热材料导热系数过程中易忽略及不明确的地方进行阐述,主要包括:试件的加工及干燥的处理方法 ;试件厚度的测量方式;环境温、湿度的控制范围;修正系数的标定及参数设置;测量不确定度等方面,并给出了指导性意见和建议。     

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.     

Anisotropic thermal conductivity measurement of carbon-fiber/epoxy composite materials

This project originated from a need for understanding heat transfer in carbon-fiber/epoxy natural-gas tanks undergoing rapid heating during refilling. This paper presents both experimental and theoretical work concerning evaluation of the thermal properties of carbon-fiber/epoxy composite materials. An effective measurement system was implemented based on steady-periodic heating: a platinum film sputtered on the surface of a sample was periodically heated by an AC current at frequency ω. The phase lag and amplitude data of a voltage signal at frequency 3ω related to the temperature response information in the platinum film were collected from the experiment. An impedance analysis model was employed to convert the phase and amplitude of the voltage to those of the temperature response. The anisotropic thermal properties were deduced from an inverse parameter estimation model, which was a least-square systematic comparison between experimental data and the theoretical model. The anisotropic theoretical solution was derived based on the Green’s function solution. Poly(methyl methacrylate) (PMMA) samples were used as reference samples to verify the measurement system. Both in-plane and through-thickness thermal conductivity of carbon-fiber composite samples were obtained and presented simultaneously.     

A synthesis of tortuosity and dispersion in effective thermal conductivity of porous media

Effects of tortuosity and dispersion on the effective thermal conductivity of fluid-saturated porous media are investigated analytically with help of a volume averaging theory. Firstly, a general expression for the effective stagnant thermal conductivity has been derived using a unit cell model, which consists of rectangular solids with connecting arms in an in-line arrangement. The validity of the expression for the stagnant thermal conductivity has been confirmed comparing the present results with available experimental and theoretical data for packed beds, porous foams and wire screens. Secondly, an general expression for the thermal dispersion conductivity has been sought with help of the two energy equations for solid and fluid phases, derived on the basis of a volume averaging theory. It has been shown that the interstitial heat transfer between the solid and fluid phases is closely associated with the thermal dispersion. The resulting expressions for the longitudinal and transverse thermal dispersion conductivities agree well with available experimental data and empirical correlations.     

Mathematical modelling of thermal conductivity process in salt-contaminated wall materials

The results of computational investigations dealing with the influence of moisture and salt on the effective thermal conductivity of the masonry wall were presented. The investigation was carried out on the basis of mathematical modelling of heat transfer in capillary-porous materials. The dependencies between the thermal conductivity of the wall and the content of moisture and salt, as well as the physical state of salt were described. The assessment of the degree of salt influence on the thermal conductivity of masonry wall for the admissible exploitive moisture content was performed. Practical formulae were obtained and presented for realistic estimation of the effective thermal conductivity of ceramic brick and masonry wall.     

Influence of moisture content on measurement accuracy of porous media thermal conductivity

The thermal conductivity measurement accuracy of sand was experimentally studied with a hot disk thermal constant analyzer and water morphologies, distribution, and evolution at the pore scale were observed with a charge coupled device (CCD) combined with a microscope. It was found that thermal conductivities of samples with low moisture content (<25%) could not be accurately measured. For samples with low moisture content, the analysis showed that the water in the region adjacent to the analyzer sensor mainly existed as isolated liquid bridges between/among sand particles and would evaporate and diffuse to relatively far regions because of being heated by the sensor during measurement. Water evaporation and diffusion caused the sample constitution in the region adjacent to the sensor to vary throughout the whole measurement process, and accordingly induced low accuracy of the obtained thermal conductivities. Due to high water connectivity in pores, the rate of water evaporation and diffusion in porous media of high moisture content was relatively slow when compared with that of low moisture content. Meanwhile, water in the relatively far regions flowed back to the region adjacent to the sensor by capillary force. Therefore, samples consisting of the region adjacent to the sensor maintained the constant and thermal conductivities of porous media with relatively high moisture content and could be measured with high accuracy. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20272     

Assessment of structure effects on the thermal conductivity of two-phase porous geomaterials

The effect of structure on the thermal conductivity of geomaterials is studied for solid–fluid combinations representing a wide variety of two-phase porous geomaterials. Nearly 200 thermal conductivity data sets from the literature were analyzed for geomaterials made of natural soil particles, crushed rock particles and sedimentary rock. Two analog models are studied to quantify the effect of structure. It appears that the effect of structure increases with decreasing fluid/solid thermal conductivity ratio and structure effects are negligible from a ratio of approximately 1/15 and higher. A new simplified model is proposed to compute the effective thermal conductivity as a function of the fluid/solid thermal conductivity ratio and the structure of geomaterials. The model applies well to independent data of homogeneous and heterogeneous materials including industrial cement concrete.     

Interstitial materials for controlling thermal conductances across pressed metallic contacts

Previous investigations into the use of interstitial materials as a means of controlling the thermal conductances across metallic contacts are reviewed. The effectiveness of such interstitial materials, used in conjunction with various metallic joint base materials, is assessed. Factors, other than the thermal performance, which influence the choice of a suitable interstitial material are also discussed.     

Determination of effective thermal conductivity for real porous media using fractal theory

INTRODUCTIONPorousmediaisacompositemediathatincludessolidframeandfluidandexistedwidelyintheeajrthbiosphere.Heatandmasstransferinporousmediaisbothanaturalphenomenoninearthbiosphereandaphysicalchemistryprocessinindustries,agricultureandhumanlife.Thusthestudyonheatandmasstransferinporousmediahasbecomeanimportanttasktoscielltistsandengineers.Heatandmasstransferinporousmediaisaverycomplexobject.Therearestillmanydifficultiestodescribethecoupledheatandmasstransferphenomena.Amongthesedifficultie…     

Hydrogen storage and thermal conductivity properties of Mg-based materials with different structures

Mg-based hydrogen storage materials can be very promising candidates for stationary energy storage application due to the high energy density and low cost of Mg. Hydrogen storage kinetics and thermal conductivity are two important factors for the material development for this kind of application. Here we studied several types of Mg-based materials with different structure-micrometer scale Mg powders, Mg nanoparticles, single crystal Mg, nanocrystalline Mg₅₀Co₅₀ BCC alloy and Mg thin film samples. It seems the Mg materials with good kinetics usually are the ones with nanostructure and tend to show poor thermal conductivity due to electron/phonon scattering resulting from more interfaces and boundaries in nanomaterials. Based on this work, good crystallinity Mg phase incorporated in carbon nano framework could be one promising option for energy storage.     

Numerical simulation of the effective thermal conductivity of wet clothing materials

Clothing materials may be considered composite materials composed of fiber, air, and moisture. For this paper, effective thermal conductivities of wet clothing materials were analyzed numerically using a proposed heat transfer model. The following simplifications were introduced. The clothing material fiber is woven with a single yarn, there is no air movement between fibers, and mass transfer is neglected. Numerical calculations were made using finite difference equations for steady three-dimensional heat conduction for several composite materials representing wet clothing materials. The main results obtained were as follows. The effective thermal conductivity of wet clothing material increases as the thermal conductivity of the yarn and the moisture content increase. We found that our numerical results agree qualitatively with those previously measured. The effective thermal conductivity of a wet layered material depends on the distribution of moisture and attains a maximum in the wet layer. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 243–254, 1998     

Effective thermal conductivity of various filling materials for vacuum insulation panels

Three thermal transport mechanisms of various filling materials for Vacuum Insulation Panels (VIPs) are theoretically investigated with special emphasis on the solid conduction. As the first, the solid conductivities of porous materials such as powder, foam, fiber and staggered beam subject to external atmospheric compression are derived using simplified elementary cell models. The results show that the solid conductivities of the fiber and staggered beam insulation are lower than those of the powder and foam due to the relatively long thermal path. The second mechanism, i.e., gaseous conductivity shows the lowest for the fine powder among the considered materials due to its smallest pore size. The radiative conductivity as the last is calculated using the diffusion approximation. If radiation shields are installed for the staggered beam, the radiation effect can be lowered to a negligible order of magnitude. The predicted total effective conductivities suggest that the fiber and staggered beam structures are promisingly proper filling materials for VIPs.     

Lattice Boltzmann modeling of the effective thermal conductivity for fibrous materials

This paper presents a full set of numerical methods for predicting the effective thermal conductivity of natural fibrous materials accurately, which includes a random generation-growth method for generating micro morphology of natural fibrous materials based on existing statistical macroscopic geometrical characteristics and a highly efficient lattice Boltzmann algorithm for solving the energy transport equations through the fibrous material with the multiphase conjugate heat transfer effect considered. Using the present method, the effective thermal conductivity of random fibrous materials is analyzed for different parameters. The simulation results indicate that the fiber orientation angle limit will cause the material effective thermal conductivity to be anisotropic and a smaller orientation angle leads to a stronger anisotropy. The effective thermal conductivity of fibrous material increases with the fiber length and approach a stable value when the fiber tends to be infinite long. The effective thermal conductivity increases with the porosity of material at a super-linear rate and differs for different fiber location distribution functions.