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Modelling effective thermal properties is crucial for optimizing the thermal performance of materials such as new green insulating fibrous media. In this study, a numerical model is proposed to calculate the effective thermal conductivity of these materials. The fibers are considered to be non-overlapping and randomly oriented in space. The numerical model is based on the finite element method. Particular attention is paid to the accuracy of the results and the influence of the choice of the representative elementary volume (REV) for calculation (cubic or rectangular parallelepiped slice). The calculated effective thermal conductivity of fibrous media under different boundary conditions is also investigated. A set of usual mixed boundary conditions is considered, alongside the uniform temperature gradient conditions. The REV rectangular slice and uniform temperature gradient boundary conditions provide a more accurate estimate of the effective thermal conductivity and are therefore recommended for use in place of the usual cubic representative elementary volume and the usual mixed boundary conditions. This robust model represents a principal novelty of the work. The results are compared with experimental and analytical data previously obtained in the literature for juncus maritimus fibrous media, for different fiber volume fractions, with small relative deviations of 7%. Analytical laws are generally based on simplified assumptions such as infinitely long fibers, and may neglect heat transfer between different phases. Both short and long fiber cases are considered in numerical calculations. [ABSTRACT FROM AUTHOR]

Metalized ceramic substrates are widely used as circuit boards for heat dissipation and electrical insulation in high output power modules and are becoming more important since the output power of the power devices has been increasing year by year. Designing a proper module structure requires not only measurements of constituting materials' properties but also understanding of the heat transfer characteristic in a module structure. An accurate assessment method of thermal conduction properties for such metalized ceramic substrates is currently still missing. Under the circumstances we have developed a method for evaluating in-plane effective thermal conductivities of metalized substrates in a mounted state. The key point of this method is that a cavity is made between a metalized ceramic substrate and a cooling plate by interposing a ring-shaped structure to control a heat flow from the center to the periphery. The in-plane effective thermal conductivities of various kinds of metalized substrates could be evaluated by means of surface temperature measurements at two points on the substrate during power supply. The evaluated values were consistent with those of estimated ones calculated by the rule of mixtures. [ABSTRACT FROM AUTHOR]

• The pores act as a "thermal barrier wall" on the heat transfer path in the MPL. • The fractal characteristics of the MPL inhibit heat transfer. • The structural parameters of the MPL is correlated with the thermal properties. Heat transfer in the fuel cells is generally limited by the effective thermal conductivity of the microporous layer (MPL). Due to the complexity of the microstructure and the difficulty of the test conditions, studies on the key parameters that affect the thermal conductivity of MPL is lacking. In this paper, we develop an analytical fractal model for the effective thermal conductivity of the MPL based on a lumped parametric model of the cube and a fractal particle chain model, and provide the contact ratio of the agglomerates using a 3D FIB-SEM reconstruction. The results indicate that the main factors affecting the thermal conductivity of MPL include the structural parameters, fractal dimension, and the thermal conductivity of the carbon particles and pores. The fractal characteristics of the MPL inhibit heat transfer rate and the pores act as a "thermal barrier wall" on the heat transfer path in the MPL. When the volume fraction of the MPL is 40 percent, the contact area ratio and fractal factor of the agglomerates are 0.351 and 0.84, respectively. In addition, this research will help to develop novel MPL structures with high thermal conductivity and improve simulation results for fuel cell temperatures. [ABSTRACT FROM AUTHOR]

Known as one of the super adiabatic materials today, vacuum insulation panels (VIPs) is wildly applied in building industry. It has the double advantage of saving energy and improving space utilization. However, its expensive cost is the application obstacle. A new type of VIPs with recycled and economical cotton fiber core was prepared and innovated in this study. The micromorphology of the recycled cotton core material, as well as the barrier and thermal conductivity of the prepared VIPs, was experimentally and mathematically investigated. A designed vacuum measurement device was introduced to analyze the variation of thermal conductivity versus gas pressure. Three types of laminated films were prepared to investigate VIPs' adiabatic properties. Three typical barriers, respectively AF, MF-Ⅰ, MF-Ⅱ, were involved to investigate the impact of sustainability. The final results showed that AF as the barrier layer for VIPs with performed better than other VIPs considering thermal conductivity. The effective thermal conductivity (ETC) of the VIPs is sensitive to inner pressure and the optimum gas pressure is less than 0.1Pa. The drying process for porous cotton fibers is a continuous heating at a temperature of 120 °C for 30–40 min. The ETC of the VIPs prepared using AF barrier envelopes was about 3.17 mWm−1K−1. The VIPs with cotton fiber core material has good thermal properties and also has the dual characteristics of low price and environmental protection. As a potential candidate for the VIPs core material, the recycled wasted cotton fiber core material is a new development direction. [ABSTRACT FROM AUTHOR]

[Display omitted] • Importance of thermal/geometric features of composite structures to effective thermal conductivity is studied. • Effective thermal conductivity is correlated with the features using machine learning. • Using a machine learning model, composites are classified based on the existence of percolation path. • Volume fraction of particles and interface thermal resistance are most influential parameters on effective thermal conductivity. Data-driven thermal and percolation analyses are conducted to elucidate the effects of various characteristics on the effective thermal conductivity of complex 3D composite structures. These characteristics include the thermal and geometric properties of the composite constituents, the interface resistance, and the existence of percolation paths. A series of voxel-wise microstructure samples with various characteristics are generated. Their effective thermal conductivities are evaluated using a diffuse-interface-based computational homogenization method. A voxel-based algorithm is employed to identify the potential percolation paths in the structures. The homogenization results show particularly significant effects of the percolation path in composite samples with higher aspect ratios and interface resistances. The importance of different thermal and geometric features to the effective thermal conductivity is analyzed using a data-driven sensitivity study. The analysis also demonstrates that the particle volume fraction and interface thermal resistance are the most influential characteristics for determining the effective thermal conductivity. Finally, employing a surrogate-based classification model, microstructures with and without percolation can be distinguished with an accuracy of 93%. [ABSTRACT FROM AUTHOR]

• Complex heat transfer in steel bar bundles is expressed by the effective thermal conductivity k ef. • The paper analyses the k ef coefficient of bundles within the temperature range 25–700 °C. • The k ef coefficient of bar bundles rises linearly with temperature. • Simple models do not determine the k ef coefficient correctly. Transport phenomena of heat in porous media have been focus of many investigations. Most studies deal with low-porosity granular media. A specific example of such a medium are steel bars heated during heat treatment in the form of cylindrically shaped bundles. The intensity of heating is determined by the processes of heat transfer which occur in the radial direction of the bundle, in which the considered charge is characterized by the lack of continuity of the solid phase. Therefore, the ability of the bundles to transfer heat is expressed by effective thermal conductivity k ef. Knowing this coefficient is necessary to optimize heat treatment processes of bar bundles. The article presents the results of calculations of the k ef for packed beds of round and square steel bars obtained for a few selected analytical models. The quality of the results was verified on the basis of experimental measurements performed in a guarded hot plate apparatus. The results indicate that simple models are not suitable for determining thermal properties of bundles. The k ef values obtained with their use significantly vary from the measured values. Complex models are much more valuable in this respect. The best results were obtained for the Zehner-Bauer-Schlünder model. [ABSTRACT FROM AUTHOR]

Nanoparticle aggregation has been found to be crucial for the thermal properties of nanofluids and their performance as heating or cooling agents. Most relevant studies in the literature consider particles of uniform size with point contact only. A number of forces and mechanisms are expected to lead to deviation from this ideal description. In fact, size uniformity is difficult to achieve in practice; also, overlapping of particles within aggregates may occur. In the present study, the effects of polydispersity and sintering on the effective thermal conductivity of particle aggregates are investigated. A simulation method has been developed that is capable of producing aggregates made up of polydispersed particles with tailored morphological properties. Modelling of the sintering process is implemented in a fashion that is dictated by mass conservation and the desired degree of overlapping. A noticeable decrease in the thermal conductivity is observed for elevated polydispersity levels compared to that of aggregates of monodisperse particles with the same morphological properties. Sintered nanoaggregates offer wider conduction paths through the coalescence of neighbouring particles. It was found that there exists a certain sintering degree of monomers that offers the largest improvement in heat performance. [ABSTRACT FROM AUTHOR]

• The computational microstructure modeling including randomly dispersed coated fibers is established. • The graded interphase has significant influence on the effective thermal conductivity. • The linear form of interphase property can cause higher prediction than the nonlinear form. • The high-conductivity fiber causes a plateau value of the prediction. • The magnitude of plateau value dramatically increases with the increase of interphase thickness. To assess the overall thermal properties of lightweight natural fiber-reinforced cementitious composites (NFRCCs) developed for low-carbon constructions, the comprehensive understanding of properties and characteristics of graded interphase between fiber and matrix is vital. However, there is rare study on the graded interphase in the NFRCCs so far. The main objective of this work is to present a comprehensive numerical investigation of the influence of functionally graded interphase on the effective thermal conductivity of such composites. A computational microstructure model including randomly dispersed fibers coated by functionally graded interphase is established and verified by comparison to the experimental results. Then, the effective thermal conductivity of the composites is characterized with respect to the microstructure parameters. It is found that the overall thermal conductivity is weakly dependent of the fiber distribution, whilst it is significantly affected by fiber content, constituent properties, as well as interphase graded form and thickness. Moreover, the study reveals that the linear variation of the interphase property can cause higher predictions than the nonlinear variation. The proposed work will provide fundamental insight about the role of functionally graded interphase in NFRCCs, which is helpful in guiding the designing and processing of NFRCCs. [ABSTRACT FROM AUTHOR]

• Prediction of the effective thermal conductivity of the composite material. • Establishing representative numerical simulation models at multiple scales. • Heat transfer under the condition of conduction & radiation. • Effects of the porosity and fiber volume fraction on the effective thermal conductivity of the composite materials. Composite materials are widely used in the fields of aviation and space, where their thermal properties are of importance since they are often challenged by variations of environmental temperatures. However, thermal properties of composite materials under conditions of conduction & radiation (a key mode of heat transfer in space) remain elusive. In order to address this issue, in this study, numerical simulations of the glass/polyimide composite materials were conducted to determine its effective thermal conductivity under the conduction condition with and without radiation. More specifically, unit cell models of polyimide matrix, glass-fiber yarns, and the composite materials were constructed sequentially to study the effects of the porosity and the fiber volume fraction on the effective thermal conductivities of the composite materials. Simulation results revealed that 1) an increase in porosity can reduce the effective thermal conductivity of the polyimide matrix with the maximal decreasing ratio of 39.5%; 2) an increase in fiber volume fraction can increase the transverse effective thermal conductivity of the glass-fiber yarns with the maximal increasing ratio of 49.0%; 3) differences of 26.0% and 27.6% of the in-plane and out-of-plane effective thermal conductivities of the composite materials were located under the conduction condition with and without radiation. This study provided a simulation approach to investigate thermal properties of composite materials, paving ways for their applications in the fields of aviation and space. [Display omitted] [ABSTRACT FROM AUTHOR]

• Adaptive thermal cloaking is proposed by rotating three-layer configuration. • Effective thermal conductivity of the three rotating-layer configuration is deduced. • Different functions of the three rotating layers are identified and explained. Thermal metamaterials have been extensively studied due to their extraordinary properties beyond natural materials and offered great flexibilities to tune heat flow for desired thermal functionalities, like thermal cloaking, concentrating, rotating, etc. Whereas, the thermal properties of thermal metamaterials are usually fixed once the configuration and the constituent materials are designed and fabricated. For instance, the thermal cloaking effect may be deteriorated when background changes, which limits its practical application significantly. By deducing the effective thermal conductivities of rotating objects, we propose an adaptive thermal cloaking metadevice that is composed by three rotating layers with different roles. The joint effect of three rotating layers makes the effective thermal conductivity a real number on the reciprocity line for feasible implementation. When background changes, we only need change the angular velocities rather than change the configuration or the constituent materials to restore the cloaking effect, which is much more convenient and real-time for practical applications. The underlying physics of the rotating thermal cloak is discussed to identify the key parameters and upper and lower limits of the effective thermal conductivity for further improving the cloaking effect. The present study can trigger more rotating metadevices for novel applications beyond thermal cloaking. [ABSTRACT FROM AUTHOR]

A method to obtain a radiation exchange factor FR in the bundle of steel round bars is presented. This parameter is required for determination of the radiative thermal conductivity krd, which is one of the basic thermal properties of the bar bundles. In the presented approach, the analyzed parameter is calculated indirectly. The initial point for calculations is the geometric model of the medium defined as a unit cell. Then, for the elements present in this cell, the thermal resistance of both conduction and radiation is determined. The radiation resistance is calculated from the radiosity balance of the surfaces enclosing the analyzed system. On this basis, the radiation thermal conductivity krd is calculated. Next, taking into account the bar diameter, the value of parameter FR is also determined. The analysis is performed at the process temperature range of 200 to 800 °C for three bar diameters: 10, 20 and 30 mm, and for the three porosities of the bundle. Different emissivity of bars in the range of 0.5 to 0.9 was also taken into account. Finally, a relationship that allows calculating the FR factor correlated with the emissivity of the bars and the bundle porosity was established. The krd obtained from the methodology presented and compared with the values calculated directly do not exceed 9%; however, after averaging over the entire temperature range of the process, the difference does not exceed 0.2%. [ABSTRACT FROM AUTHOR]

• Some factors influencing effective thermal conductivity changes were studied • Different effective thermal conductivity changes at measuring positions were found • Key factors of effective thermal conductivity changes in two settings were analyzed The effect of hydrate distribution on the effective thermal conductivity changes of the actual hydrate-bearing sediments is uncertain. To define this effect, in the hydrate-bearing quartz sands, the effective thermal conductivity was measured by the transient hot-wire method, and hydrate distribution was analyzed by the electrical resistance changes. At first, the effective thermal conductivity changes were investigated from gas saturation, gas pressure, water saturation, water pressure, and temperature. Afterward, in three kinds of hydrate formation processes, compared with hydrate distribution, the changing trends of the effective thermal conductivity at five measuring points were analyzed. The results showed that the effective thermal conductivity was increased with decreasing gas saturation and increasing water saturation or water pressure, and impacted slightly by gas pressure and temperature. Moreover, at five measuring points, the non-uniform hydrate distribution caused different changing trends of the effective thermal conductivity during hydrate formation. In the excess-water setting, an increasing trend was only observed at some local measuring point. The decreasing trends at the most of the measuring points were governed by the reduced water distribution. But in the excess-gas setting, at the most of the measuring points, this increasing trend was observed and controlled by the enlarged hydrate distribution. Besides, for some actual sediments in the excess-water setting, hydrate distribution may be not the key factor of the effective thermal conductivity changes, and further studies were required. This research could offer some valuable reference to the thermal properties in the actual sediments. [ABSTRACT FROM AUTHOR]