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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]

In this study, the effective thermal conductivity of materials based on a porous TPMS structure is investigated. The resulting structure is similar to a matrix and consists of identical pore cells that are strictly periodic in all directions. These pores have several characteristic sizes, and by altering them, materials with predictable effective thermal conductivity can be created. This work investigated materials based on Triply Periodic Minimal Surface (TPMS) of Schwarz P, Schoen IWP and Neovius. The calculation of thermophysical properties was performed using the numerical finite element method in the Ansys software package. Heat transfer in the structures was studied both with and without considering air. The empirical coefficient "n" for the Aivazov–Domashnev formula is expressed analytically. This relationship allows for the creation of materials with predictable thermophysical properties, taking into account the thermal conductivity of non-porous materials. The results of numerical modeling are validated by analytical Austin's models and experimental data. The samples for the experimental study were made using additive technologies from PETG, ABS, PLA plastic and photopolymer resin. The study has scientific and applied significance, since a method is proposed for predicting the thermal conductivity of materials with an ordered structure. Several solid-state samples of elementary cells were made publicly available for designing materials with predictable thermal conductivity. The novelty of the work lies in obtaining the dependence of effective thermal conductivity on the geometric parameters of the ordered structure for materials with thermal conductivity in a wide range from 0.12 to 400 W / (m ° C). • The methodology for determining thermal conductivity of TPMS structure is presented. • A study on the thermal conductivity of TPMS structures with air was conducted. • Porous structures of the types Schwarz' P, Schoen IWP, and Neovius were considered. [ABSTRACT FROM AUTHOR]

• Calculation of cross-scale anisotropic thermal transport in carbon fiber composites. • Use of the dual coordinate system to describe thermal conductivity anisotropy. • The mechanism of interface rotation affecting interfacial thermal resistance. • The heat transport properties of fiber tortuosity and orientation within the resin substrate were investigated. The thermal conductivity parameter of materials is a fundamental factor for thermal management and the study of heat transfer processes. In the present work, we constructed the microscopic topology and interfacial structure of the fiber structure based on the homogenization of finite elements and molecular dynamics theory. For the anisotropy of fiber thermal conductivity, we proposed an anisotropic cross-scale heat transport model for carbon fiber/epoxy resin (CF/ER) composites, with consideration of interfacial thermal resistance (ITR). Based on the numerical validation of the reliability of the method, we established a dual coordinate system to analyze the mechanism of the geometric topological parameters on the structural anisotropic heat transport. The fiber volume fraction significantly affected the effective thermal conductivity (ETC) within a range of 10% to 15%, with ETC growth rates of up to 50%. The fiber orientation angle influenced the main heat transfer direction, and when the fiber volume fraction was 40%, increasing the fiber orientation angle to 90° could enhance the ETC in the axial direction up to 130.28%. Meanwhile, the larger orientation angle could reduce the interfacial phonon heat dissipation and achieve the better phonon transmission effect. The change of fiber structure could effectively modulate the ETC, while the fiber length and cross-sectional shape would have less effect on the ETC. With the introduction of fiber tortuosity, the heat transport could be regulated by changing the tortuosity. Within the variation range of tortuosity from 1 to 3, the ETC regulation of the composite could be achieved from 12% to 42% in the axial direction and 14% to 34% in the radial direction. With increasing curvature, the ETC variation trend became gradually nonlinear. The study serves as a theoretical basis for the preparation and thermal properties evaluation of fiber-reinforced composites. [ABSTRACT FROM AUTHOR]

The composition and microstructure of materials play important roles in their thermal properties. In this study, the temperature and composition dependence of the thermal conductivity of MoSi 2 –RSiC composites with a special 3D interpenetrating network structure was investigated experimentally and numerically. A microstructure-based model was established using serial sectioning technology. The model could describe the connectivity and distribution characteristics of the two phases in the composites. Finite element simulation could accurately predict the effective thermal conductivity of the MoSi 2 –RSiC composites, with a deviation of 6.8%–9.6% between numerical and experimental results at different temperatures. Furthermore, a novel simplified model was developed to predict the effective thermal conductivity of the composites with a 3D interpenetrating network structure at different network inclusions. Landauer's effective medium percolation theory (EMPT) was used to verify the numerical values and analytical results. The calculation results of EMPT and the simplified model fit well with the experimental data of the composites with a high volume fraction matrix or measured below 600 K; the former was due to the effect of matrix density on the porosity and interfacial phase content of the composites, and the latter was due to the reduced difference in thermal conductivity coefficient among SiC, MoSi 2 and the interfacial phase, and air with increased temperatures. [ABSTRACT FROM AUTHOR]

An effective model to calculate thermal conductivity of polymer composites using core-shell fillers is presented, wherein a core material of filler grains is covered by a layer of a high-thermal-conductivity (HTC) material. Such fillers can provide a significant increase of the composite thermal conductivity by an addition of a small amount of the HTC material. The model employs the Lewis-Nielsen formula describing filled systems. The effective thermal conductivity of the core-shell filler grains is calculated using the Russel model for porous materials. Modelling results are compared with recent measurements made on composites filled with cellulose microbeads coated with hexagonal boron nitride (h-BN) platelets and good agreement is demonstrated. Comparison with measurements made on epoxy composites, using silver-coated glass spheres as a filler, is also provided. It is demonstrated how the modelling procedure can improve understanding of properties of materials and structures used and mechanisms of thermal conduction within the composite.

Highlights • An effective thermal conductivity model is derived based on fractal theory. • Predictions of proposed model show good consistent with experimental data. • The proposed theoretical model reveals the physical basis of heat transfer in porous media. Abstract Thermal conduction in porous media has received wide attention in science and engineering in the past decades. Previous models of the effective thermal conductivity of porous media contain empirical parameters typically with ambiguous or even no physical rationales. This study proposes a theoretical model of effective thermal conductivity in porous media with various liquid saturation based on the fractal geometry theory. This theoretical model considers geometrical parameters of porous media, including porosity, liquid saturation, fractal dimensions for both the granular matrix and liquid phases, and tortuosity fractal dimension of the liquid phase. Effects of these geometrical parameters on the effective thermal conductivity of porous media are also evaluated. This proposed fractal model has been validated using published experimental data, compared with previous models, and thus provides a physics-based theoretical model that can provide insight to geoscience and thermophysics studies on thermal conduction in porous media. [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]

A new, alternative approach is proposed for natural convection simulation by means of stimulated thermocapillary currents created by virtual walls. In contrast to the well-known effective thermal conductivity model, in the proposed approach it is the mass motion due to the convective currents which is intended to be simulated and the heat flux is a consequence of such flows. As a result, no a priori knowledge of the Nusselt number is needed and thus the approach is more suitable for complex geometries. Utilizing a simplified physical model and the definition of hydraulic diameter, a generalized expression for enclosed geometries is derived which offers thermal engineers a powerful analysis tool that can use virtual walls with an associated fictitious Marangoni stress for pre-screening and estimation of Nusselt numbers. [ABSTRACT FROM AUTHOR]

Nanoparticle Enhanced Ionic Liquids (NEILs) were synthesized by dispersing aluminum oxide (Al 2 O 3 ) nanoparticles in 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, ([C 4 mim][NTf 2 ]) ionic liquids (ILs). The experimental assessment of NEILs includes investigating the effective thermophysical properties and forced convection heat transfer under laminar and turbulent flow regime. The results show that thermal conductivity and heat capacity enhanced up to ∼11% and ∼49% respectively for 0.9 vol% NEILs. The rheological behavior of NEILs shows non-Newtonian shear thinning behavior with shear viscosity decreasing with increasing shear rate. The viscosity of NEILs shows much higher value compared to the base ILs for a small amount of nanoparticles dispersion and also has a strong temperature dependency. Measured viscosity and thermal conductivity were found to be much higher than predicted by the well-established model for dilute suspensions. The convective heat transfer performance increases with the nanoparticles concentration within the measured nanoparticles vol%; up to ∼27% and ∼40% enhancement in heat transfer coefficient was found in laminar and turbulent flow regime respectively. The possible mechanisms of the enhanced thermal performance are discussed. [ABSTRACT FROM AUTHOR]

In the present study, the accuracy of predicted effective thermal conductivity of composite materials, determined by the finite volume method (FVM) and the thermal lattice Boltzmann method (TLBM) using single relaxation time (SRT), is investigated. In addition, computational efforts required by both solution methods are also compared. Two- and three-dimensional heat conduction problems in two-phase domains with a high thermal conductivity ratio are considered, where oblique and curved interfaces are represented by the staircase approximation. The discretisation is carried out on a uniform mesh. For TLBM, different stencils are employed and a wide range of relaxation frequencies, corresponding to relative magnitudes of thermal conductivities of different phases, is investigated. For comparison, wherever available, the analytical expression for the effective thermal conductivity is considered. Otherwise, the reference effective thermal conductivity is determined using a commercial software employing an appropriate body-fitted mesh. The present results show that the accuracy of TLBM depends on the employed stencil, relaxation frequency and arrangement of two materials in the computational domain. It is also observed that TLBM can be even more accurate than FVM, although the accuracy of FVM is independent of individual values of thermal conductivities. For the same convergence criterion, however, TLBM requires a higher number of iterations than FVM, although TLBM shows a potential for speed-up. [ABSTRACT FROM AUTHOR]

Steel bars are manufactured in the rolling process, whereby they are characterized by strain hardening and poor plastic properties. In many application cases such properties are improper, therefore, additional heat treatment is required. Crucial influence on the products quality after heat treatment has an appropriate selection of process parameters. In many modern technologies of heat treatment the charge of porous structure is subjected to the heating process. Proper control of heat treatment parameters of bundles of rods requires knowledge on their thermal properties. However, it also requires accurate identification of complex heat transfer processes occurring in the porous material. Such analysis, with respect to bundles of bars, provide a response of qualitative nature of the heat exchange area of these charges. The article describes the emissivity measurements of samples of the steel charge using a thermal imaging camera. [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]

Abstract: In many industrial processes, solid particles of different sizes are mixed in different volume or mass fractions for various applications, primarily to reduce void volume or to increase the density of the mixture. A few of these processes include; production of high density ceramics, mortar, concrete, graphite, bricks and carbon blocks. Experiments were carried out with alumina and lithium titanate pebbles (size≥1mm) and particles (size<1mm) of different sizes and volume fractions in cylindrical and rectangular vessels to study the variation of void fraction with volume fraction of component pebbles and with large pebble to small pebble or pebble to particle size ratio. It was observed that the variation of void fraction with volume fraction of smaller pebbles or particles is ‘V-shaped’. Thus there exists a minimum void fraction and two different volume fractions of smaller pebbles or particles which can give same void fraction. The effect of void fraction on the effective thermal conductivity of binary bed of lithium titanate and alumina pebbles and particles of different sizes and volume fractions was investigated. From the experimental results it was found that the binary particulate bed has higher effective thermal conductivity that that of a unary particulate bed. Effective thermal conductivity of binary particulate bed is the maximum when its void fraction is the minimum. The binary particulate bed with less volume fraction of small particles has higher conductivity than that of the binary bed of higher volume fraction of small particles and having same void fraction. This is due to the fact that with increase in volume fraction of small pebbles or particles, the number of small-to-small and large-to-small pebbles and or particles contact point increases resulting in higher resistance to heat transfers. The experimental details and results are discussed in this paper. [Copyright &y& Elsevier]

• 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]

• A closed-loop MPHP with a hydraulic diameter of 350 μm is fabricated. • Effective thermal conductivities of the MPHP charged with FC-72 are measured. • Semantic segmentation-based image recognition is applied to recorded flow images. • Heat transfer simulations are performed using the extracted flow patterns. • Latent heat transfer via liquid films proves to be the most significant. We investigate the relationship between the thermal properties of a micro pulsating heat pipe (MPHP) and the internal flow characteristics via measurements of effective thermal conductivities, flow visualization followed by image recognition of the flow patterns, and heat transfer simulations employing the extracted flow patterns. The MPHP consists of an eleven-turn closed-loop of a meandering square microchannel with a hydraulic diameter of 350 μm engraved on a silicon substrate, which is covered with a transparent glass plate to allow internal flow visualization. The MPHP charged with Fluorinert FC-72 tends to exhibit higher effective thermal conductivities for the coolant temperature of T c = 40 °C compared to T c = 20 °C, and provides the highest effective thermal conductivity of about 700 W/(m K) for T c = 40 °C and a filling ratio of 48%. Interestingly, we observe two different self-oscillation modes having different thermal conductivities, even for identical heat input rates. This tendency indicates a hysteresis of the effective thermal conductivity, which originates from the difference in the heat input rates at which the MPHP falls into and recovers from dryout. Subsequently, semantic segmentation-based image recognition is applied to the recorded flow images to identify the flow characteristics, successfully extracting four different flow patterns involving liquid slugs, liquid films, dry walls, and rapid-boiling regions. The image recognition results indicate that high effective thermal conductivities of the MPHP relate to stable self-oscillations with large amplitudes and high frequencies, along with long and thin liquid films beneficial for latent heat transfer. Finally, we perform numerical simulations of latent/sensible heat transfer via vapor plugs and of sensible heat transfer via liquid slugs using the extracted flow patterns and measured channel temperatures as inputs. We find that latent heat transfer via liquid films accounts for a considerable portion of the overall heat transfer, while the sensible heat transfer via liquid slugs is much less significant. Image, graphical abstract [ABSTRACT FROM AUTHOR]

An effective model to calculate thermal conductivity of polymer composites using core-shell fillers is presented, wherein a core material of filler grains is covered by a layer of a high-thermal-conductivity (HTC) material. Such fillers can provide a significant increase of the composite thermal conductivity by an addition of a small amount of the HTC material. The model employs the Lewis-Nielsen formula describing filled systems. The effective thermal conductivity of the core-shell filler grains is calculated using the Russel model for porous materials. Modelling results are compared with recent measurements made on composites filled with cellulose microbeads coated with hexagonal boron nitride (h-BN) platelets and good agreement is demonstrated. Comparison with measurements made on epoxy composites, using silver-coated glass spheres as a filler, is also provided. It is demonstrated how the modelling procedure can improve understanding of properties of materials and structures used and mechanisms of thermal conduction within the composite. [ABSTRACT FROM AUTHOR]

• A DEM-CFD model considering contact conduction was used to study the effective thermal conductivity (ETC) of pebble beds. • The ETC predicted by the current model is consistent with experimental data in a wide range of k s / k f from 1 to 10000. • If k s / k f is larger than 20, the difference of the ETC for the contact model and the non-contact model is greater than 10%. Pebble beds are main schemes for tritium breeding zones in the solid blanket for fusion reactors. The effective thermal conductivity of pebble beds (k e f f) is an important thermal property used in the thermal hydraulic design of solid blankets. In this study, a 3D DEM-CFD one-way coupling method considering contact conduction is applied to analyze the influence of solid contact conduction and solid-fluid-solid conduction on the effective thermal conductivity of pebble beds. The current numerical model gives a k e f f consistent with experimental data in a wide range of solid-to-fluid conductivity ratio k s / k f from 1 to 10,000. It can be concluded that when the k s / k f is more than 20, contact conduction should be taken into account to make a good prediction for k e f f. [ABSTRACT FROM AUTHOR]

Effective thermal conductivity (ETC), as a necessary parameter in the thermal properties of rock, is affected by the pore structure and the thermal conduction conditions. To evaluate the effect of fractures and saturated fluids on sandstone's thermal conductivity, we simulated thermal conduction along three orthogonal (X, Y, and Z) directions under air- and water-saturated conditions on reconstructed digital rocks with different fractures. The results show that the temperature distribution is separated by the fracture. The significant difference between the thermal conductivities of solid and fluid is the primary factor influencing the temperature distribution, and the thermal conduction mainly depends on the solid phase. A nonlinear reduction of ETC is observed with increasing fracture length and angle. Only when the values of the fracture length and angle are large, a negative effect of fracture aperture on the ETC is apparent. Based on the partial least squares (PLS) regression method, the fluid thermal conductivity shows the greatest positive influence on the ETC value. The fracture length and angle are two other factors significantly influencing the ETC, while the impact of fracture aperture may be ignored. We obtained a predictive equation of ETC which considers the related parameters of digital rocks, including the fracture length, fracture aperture, angle between the fracture and the heat flux direction, porosity, and the thermal conductivity of saturated fluid. [ABSTRACT FROM AUTHOR]

In the present paper, the effective thermal conductivities of Li4SiO4-packed beds with both ordered and random packing structures were investigated using thermal resistance network methods based on both an Ohm's law model and a Kirchhoff's law model. The calculation results were also validated and compared with the numerical and experimental results. Firstly, it is proved that the thermal resistance network method based on the Kirchhoff's law model proposed in the present study is reliable and accurate for prediction of effective thermal conductivities in a Li4SiO4-packed bed, while the results calculated with the Ohm's law model underestimate both ordered and random packings. Therefore, when establishing a thermal resistance network, the thermal resistances should be connected along the main heat transfer direction and other heat transfer directions as well in the packing unit. Otherwise, both the total heat flux and effective thermal conductivity in the packing unit will be underestimated. Secondly, it is found that the effect of the packing factor is remarkable. The effective thermal conductivity of a packed bed would increase as the packing factor increases. Compared with random packing at similar packing factor, the effective thermal conductivity of packed bed would be further improved with an ordered packing method. [ABSTRACT FROM AUTHOR]