Effective Thermal Conductivity Coefficient Research Articles

ЧИСЛЕННАЯ МОДЕЛЬ СВОБОДНОЙ КОНВЕКЦИИ В ПРОСТРАНСТВЕ ЭКРАННО-ВАКУУМНОЙ ИЗОЛЯЦИИ ПРИ ТЕПЛОВОМ ВОЗДЕЙСТВИИ ПОЖАРА

The objective of the study is to find the patterns of the conjugate heat transfer process in a closed space of a coaxial cylinder’s pair under the influence of an external source of thermal radiation within the framework of the free convection model. It is assumed that the priority type of heat transfer in the gas filling the coaxial space is turbulent free convection, and thermal conductivity in the external thermal insulation of the pipeline. A model of the free convection process in a coaxial gas cavity in the presence of local heating in the form of a system of differential equations is created. Comparative results of a full-scale and numerical experiment on measuring the effective coefficient of thermal conductivity of screen-vacuum insulation are presented.

Simulation of Frost-Heave Failure of Air-Entrained Concrete Based on Thermal-Hydraulic-Mechanical Coupling Model.

The internal pore structural characteristics and microbubble distribution features of concrete have a significant impact on its frost resistance, but their size is relatively small compared to aggregates, making them difficult to visually represent in the mesoscopic numerical model of concrete. Therefore, based on the ice-crystal phase transition mechanism of pore water and the theory of fine-scale inclusions, this paper establishes an estimation model for effective thermal conductivity and permeability coefficients that can reflect the distribution characteristics of the internal pore size and the content of microbubbles in porous media and explores the evolution mechanism of effective thermal conductivity and permeability coefficients during the freezing process. The segmented Gaussian integration method is adopted for the calculation of integrals involving pore size distribution curves. In addition, based on the concept that the fracture phase represents continuous damage, a switching model for the permeability coefficient is proposed to address the fundamental impact of frost cracking on permeability. Finally, the proposed estimation models for thermal conductivity and permeability are applied to the cement mortar and the interface transition zone (ITZ), and a thermal-hydraulic-mechanical coupling finite element model of concrete specimens at the mesoscale based on the fracture phase-field method is established. After that, the frost-cracking mechanism in ordinary concrete samples during the freezing process is explored, as well as the mechanism of microbubbles in relieving pore pressure and the adverse effect of accelerated cooling on frost cracking. The results show that the cracks first occurred near the aggregate on the concrete sample surface and then extended inward along the interface transition zone, which is consistent with the frost-cracking scenario of concrete structures in cold regions.

Calculation of heat protective parameters of clothing packages by method simulation modeling (Monte Carlo method)

The heat-protective properties of clothing have been studied taking into account the influence of deterministic factors, and the influence of random factors on the heat-protective properties of clothing has not been studied. In this regard, to obtain objective values of indicators of the heat-protective properties of clothing, taking into account the nature of the distribution of factors for a batch of products that are random. Knowing the laws of distribution of random factors effectively, it is possible to carry out calculations of the thermal insulation properties of clothing material packages using the simulation method. The work has developed a method for calculating the thermophysical parameters of clothing packages based on simulations of random parameters distributed according to a normal law. For the calculation, an algorithm and calculation program were compiled in the Python programming language. The thermophysical parameters of the clothing package were calculated, and the influence of wind flow speed on them was analyzed. When calculating the wind flow speed, which have a significant impact on the values of the effective thermal conductivity coefficient of the packages and the values of the total coefficient of the packages, they are taken taking into account the Beaufort table and the recommendations of the Ministry of Emergency Situations of Kazakhstan. The developed program for calculating the thermophysical characteristics of a package of clothing materials allows to change the model parameters during the calculation process: introduce new layers of materials into the package, change the thermophysical and linear characteristics of individual layers of the package, change external conditions.

Analysis of the results of studies of the thermal regime of natural and man-made kurums of the cryolithozone

The subject of research is natural and man-made kurums (blocky accumulations of rocks). The purpose of the work was to analyze the main literary sources, in which the results of research on the formation of the thermal regime of rock dumps (kurums) were published. The retrospective period of the research extends for more than 50 years. Domestic and foreign sources of information are considered. The most representative ones have been selected for analysis, in which the results of research with theoretical value and practical significance have been published. The analysis showed that on many important aspects of the formation of the thermal regime of natural and man-made kurums, all the authors of studies conducted in different regions and at different time periods, independently of each other, came to the same conclusions. This indicates the reliability and objectivity of the results obtained. An analytical review of scientific articles was used as a research method, in which research materials on the formation of the thermal regime of both the kurums themselves and their soil bases were published. The kurums of the permafrost and island permafrost zones are considered. An analytical review of literary sources containing theoretical and practical results of research on the formation of the thermal regime of natural and man-made kurums is carried out. The results of the analysis allow us to draw the following main conclusions. 1. Any types of natural and man-made kurums lead to progressive cooling of the base on which they are located. 2. When interacting with the rock base, the technogenic kurum works as an active thermal protection, changing its thermal resistance during the annual cycle. 3. When modeling the process of thermal interaction of technogenic kurum, it is quite acceptable to consider the kurum as a heat-protective layer, the thermal resistance of which depends on the effective coefficient of thermal conductivity, which changes its value when the sign of atmospheric air temperature changes. Based on the analysis of the results of the published works, conclusions are drawn about the possibility of a number of simplifications in the construction of computational models for forecasting and controlling the thermal regime of frozen bases of technogenic kurums.

ТЕРМОМЕХАНИКА ДИСПЕРСНО-НАПОЛНЕННЫХ КОМПОЗИТОВ И КОМПЬЮТЕРНЫЙ ДИЗАЙН МАТЕРИАЛОВ С РЕКОРДНО ВЫСОКОЙ ТЕПЛОПРОВОДНОСТЬЮ

On the example of metal-diamond composites (MDC), a number of issues of thermomechanics of disperse-filled materials with high thermal conductivity used for thermal management are formulated and solved. Due to the importance of the thermal conductivity factor of the interfacial layer, a refined method is proposed for calculating the boundary thermal resistance. This method considers two counter heat flows: from the matrix to the filler and back, and also provides the condition of zero thermal resistance at the same values of the thermomechanical characteristics of these components. Based on the micromechanical model of the disperse-filled composite, an analytical method is developed for determining the effective thermal conductivity coefficient of the metal-diamond composites. The method makes it possible to take into account the boundary thermal resistance, the presence of a thin coating on the diamond particle, the anisometry of diamond particles and the porosity of the metal matrix. The results of the performed parametric analysis are compared with known experimental data and estimates obtained within the framework of existing models. The conclusion on the validity of the developed method is made. A simplified finite-element model is developed for a representative volume of the metal-diamond composites in the form of a cube formed by an aluminum matrix and containing 27 spherical diamond particles of the same radius with a modifying tungsten coating. At a given temperature difference on the opposite faces of the cube, the distribution of heat flux density and the effective heat transfer coefficient of the metal-diamond composites are calculated. Comparison of the results of using the finite element model and the analytical method mentioned above shows their good agreement. Modification of the finite element model is carried out in order to better match the real internal structure of the metal-diamond composites studied by high-resolution X-ray microtomography. Numerical analysis of the temperature field, thermal stress state and fracture kinetics of the aluminum-diamond composite during thermal cycling is performed.

Two-sided estimate of effective thermal conductivity coefficients of a textured composite with anisotropic ellipsoidal inclusions

Two-sided estimate of effective thermal conductivity coefficients of a textured composite with anisotropic ellipsoidal inclusions

Investigation of heating time of a bundle of steel bars

The paper deals with the problem of determining the heating time of a bundle of flat steel bars. To describe a mathematical model of transient heat transfer the energy balance method was used. The most important element is to determine the effective thermal conductivity coefficient using the electrical analogy. The simulations of heating were performed for increasing temperature of furnace. The results show that using different sizes and bundle arrangement the heating time can be reduced by 5–15%. The analysed problem is important from the practical point of view because it allows for optimization of heat treatment processes of steel bars.

Level set-based multiscale topology optimization for a thermal cloak design problem using the homogenization method

Artificially designed composite materials consist of microstructures, that exhibit various thermal properties depending on their shapes, such as anisotropic thermal conductivity. One of the representative applications of such composite materials for thermal control is the thermal cloak. This study proposed a topology optimization method based on a level set method for a heat conduction problem to optimally design composite materials that achieve a thermal cloak. The homogenization method was introduced to evaluate its effective thermal conductivity coefficient. Then, we formulated a multiscale topology optimization method for the composite materials in the framework of the homogenization method, where the microstructures were optimized to minimize objective functions defined using the macroscopic temperature field. We presented examples of optimal structures in a two-dimensional problem and discussed the validity of the obtained structures.

Estimation of effective thermal conductivity and heat transfer coefficient of lithium orthosilicate pebble bed in carbon dioxide–air medium

ABSTRACT Lithium orthosilicate has the highest absorption capacity among other materials used to capture carbon dioxide (CO2) at a higher temperature. To design a CO2 adsorption system consisting of lithium orthosilicate pebble bed, effective thermal conductivity and heat transfer coefficient data are necessary, and till date, these are not reported in the literature. In the present study, experiments were conducted to estimate the effective thermal conductivity and heat transfer coefficient of lithium orthosilicate pebble bed in CO2–air medium. Effects of CO2 concentration in air, bed temperature, and gas flowrate on the hydrodynamic as well as heat transfer properties of the pebble bed of lithium orthosilicate pebbles were also studied. The effective thermal conductivity and heat transfer coefficients were found in the range of 0.2–0.75 Wm−1K−1 and 14–63 Wm−2K−1, respectively.

Analysis and modeling of heat and mass transfer and adsorption isotherms of the aqueous extracts vacuum contact drying

Vacuum contact drying is the most merit drying technology to ensure the quality and throughput of traditional Chinese medicine aqueous extracts. To study the heat and mass transfer of colloidal semi-solid porous media during vacuum contact drying, the sharp interface formulation was established to simulate the thin layer (4 mm) Lonicera japonica Thunb. aqueous extracts vacuum drying at high-temperature (110 and 120 ℃) and low-pressure (10, 15 and 20 kPa). And mathematical models were used to investigated drying kinetics and adsorption isotherms. Results show that the sharp interface model can predict final temperature and moisture ratio with the error is less than 20 %. The models of viscosity variation, specific heat capacity, heat capacity, effective thermal conductivity and heat transfer coefficient of boiling under different conditions were investigated to provide reference for subsequent research. When the vacuum drying superheat is between 50 ℃ and 75 ℃, the boiling heat transfer coefficient is among 6 to 9 W/ (m2·K). The Page is the most suitable semi-empirical model to describe the water migration law of the aqueous extracts. The adsorption isotherms of the extract powder were investigated by experiment and the Peleg is the most suitable model.

Testing the thermal properties of modern ventilated facade fastening systems

The study reported in this paper investigated a set of building fasteners used in ventilated facades. For the building fasteners actually present in the industrial market the values of the effective thermal conductivity were measured experimentally. These values were used next in numerical simulations run with COMSOL Multiphysics software application. The validation of the simulation model was done in specific additional experimental test. The paper presents a method of determining the effective thermal conductivity coefficient for fasteners with a novel design. Temperature distributions and heat fluxes were determined for different variants of multilayer walls with the fasteners. The calculation of the effective thermal conductivity coefficient for a structural profile is based on the heat balance of the measuring stand. The performed tests show not only an expected reduction in the coefficient value for structures in which stainless steel is used. The results also demonstrate that the fasteners with holes cut out in their structures have significantly lower effective thermal conductivity coefficients than those with solid walls. This effect can be justified by the formation of labyrinth-like narrowings extending the conductive heat flow path in the fastener. As a final result of the experimental tests and the COMSOL simulations the application of the effective thermal conductivity as the new indicator of a thermal effectiveness of building fasteners is proposed in industrial practice. Consequently the design of the building fasteners with various shapes of holes is recommended for improving their insulation features.

Разработка тепловой изоляции с упорядоченной структурой, основанной на ТПМП Неовиуса

Currently, materials with a random pore arrangement are mainly used as thermal insulation. Such materials, as a rule, have low strength. To solve a number of structural tasks, a heat-insulating material with a high specific strength is required.To date, for these purposes sandwich panels with layers of honeycomb cells filled with air are applied. In this regard, the current goal is to develop more efficient materials with an ordered structure, which will have greater thermal resistance and strength. For numerical simulation of thermal conductivity in a porous material with an ordered structure, the Steady-State Thermal module of the ANSYS software package has been used. Polymers that are often used in 3D printing, such as PETG plastic and Phrozen photopolymer, are chosen as materials for a porous medium with the Neovius TFMT structure. As a result of the study of thermal conductivity in an elementary cell of the Neovius surface in the ANSYS software package, the values of the effective thermal conductivity coefficient are obtained for various characteristic of geometric parameters (cell wall thickness, length of the edge of the cube in which the cell of the Neovius surface is inscribed). Based on the results obtained, it has been found that the dependence of the coefficient of effective thermal conductivity on the cell wall thickness (as well as on the relative cell thickness) is linear. It has been also determined that the geometry under study has a quasi-isotropic thermal conductivity, since it has cubic symmetry. Based on these results, the corresponding graphical and analytical dependencies are designed. The developed heat-insulating material with an ordered structure based on Neovius TPMT is proposed to be used as thermal insulation. The obtained graphical and analytical dependences make it possible to determine the coefficient of effective thermal conductivity of a material with a structure based on the Neovius TFMT with known characteristic geometric parameters (cell wall thickness, cube edge length). The results obtained can be used to design thermal insulation for several tasks when, in addition to thermal insulation properties, structural strength is also important. The porous structure can be produced using modern 3D printing methods, such as selective laser sintering (SLS), laser stereolithography (SLA), etc.

Effective Thermal Conductivity of Structured Porous Medium: Numerical Study

The paper presents a numerical study of thermal conductivity of porous structures using the Ansys software package. Unlike the well-known porous materials used in construction and engineering, it is proposed to use porous materials with an ordered law of cavity placement. The porous material proposed is formed by dividing the volume into cubes of equal size with a spherical cavity placed in the center of each cube. The numerical calculation of an effective thermal conductivity coefficient of a porous medium is performed using the Ansys Mechanical computer modeling tool. The values obtained are compared with solutions based on classical methods for determining the effective thermal conductivity of porous materials. A dependency graph of effective thermal conductivity in a porous material based on pores geometric parameters (distance between cavities, diameter of cavities), as well as an analytical dependence to obtain the effective thermal conductivity value is presented. Additive technologies available today provide producing the proposed porous material with an ordered law of cavity placement with any accuracy and any pore geometric parameters. Such materials open up wide opportunities for engineers, especially in the field of thermal power engineering, because it has predictable thermophysical and mechanical properties.

Fractal Analysis on the Mapping Relationship of Conductivity Properties in Porous Material

The mapping relationships between the conductivity properties are not only of great importance for understanding the transport phenomenon in porous material, but also benefit the prediction of transport parameters. Therefore, a fractal pore-scale model with capillary bundle is applied to study the fluid flow and heat conduction as well as gas diffusion through saturated porous material, and calculate the conductivity properties including effective permeability, thermal conductivity and diffusion coefficient. The results clearly show that the correlations between the conductivity properties of saturated porous material are prominent and depend on the way the pore structure changes. By comparing with available experimental results and 2D numerical simulation on Sierpinski carpet models, the proposed mapping relationships among transport properties are validated. The present mapping method provides a new window for understanding the transport processes through porous material, and sheds light on oil and gas resources, energy storage, carbon dioxide sequestration and storage as well as fuel cell etc.

Third-Order Effective Properties for Random-Packing Systems Using Statistical Micromechanics Based on a GPU Parallel Algorithm in Fast Computing n-Point Correlation Functions.

Estimating the effective properties of a particulate system is the most direct way to understand its macroscopic performance. In this work, we accurately evaluate the third-order approximations involving the three-point microstructural parameter , which can be calculated from a triple integral involving 1-, 2-, and 3-point correlation functions. A GPU-based parallel algorithm was developed for quickly computing the n-point correlation functions, and the results agree well with analytical solutions. The effective thermal conductivity and diffusion coefficient are calculated by the third-order approximates for the random-packing systems of a super-ellipsoid. By changing the parameters of the super-ellipsoid, the particle-shape effect can be predicted for both the thermal conductivity and diffusion coefficient.

Effective heat conductivity of a composite with hexagonal lattice of perfectly conducting circular inclusions: An analytical solution

AbstractPaper is devoted to the effective stationary heat conductivity for the fibre composite materials. We are aimed on getting on analytical expression for effective thermal conductivity coefficient. Asymptotic homogenization approach, based on the multiple scale perturbation method, is used. This allows to reduce the original boundary value problem in multiply connected domain to the sequence of boundary value problems in simply connected domains. These problems include: the local problem for the periodically repeated cell and homogenized problem with effective coefficient. It is shown that for densely packed and high contrast fibre composites, the cell problem can be solved analytically. For this aim, lubrication approach (asymptotics of thin layer) has been employed. We also generalise obtained solution to the case of medium‐sized inclusions in the framework of the Padé approximants.

Overall Heat Transfer Coefficient Evaluation Method for Uncracked Hydrocarbon Fuel in a Regeneratively-Cooled Heat Exchanger of a Scramjet

Regenerative cooling is critical for the thermal protection of hypersonic propulsion systems, wherein the flow and heat transfer characteristics of the hydrocarbon fuel in the cooling channel are crucial. Restricted by measurement, the distribution of the fuel temperature inside the cooling channel cannot be directly detected. As a result, the measurement of the convective heat transfer coefficient is more an overall heat transfer coefficient. Furthermore, the existing overall heat transfer coefficient calculation method is derived without considering the flow state of the hydrocarbon fuel in the cooling channel, which should be taken into consideration. Therefore, a modified calculation method of the overall heat transfer coefficient is proposed in this study. The results of the characteristics of the turbulence-related parameters show that the overall heat transfer coefficient is mainly determined by the effective thermal conductivity and surface convective heat transfer coefficient, which are related to the fuel temperature and flow state. Furthermore, the existing method might overestimate the overall heat transfer coefficient. The modified method is suitable for the calculation of the overall heat transfer coefficient in the heat transfer enhancement process in a cooling channel.

Effect of the Metal–Insulator Transition on the Thermoelectric Properties of Composites Based on $${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_{3}$$ with $${\mathrm{VO}}_{2}$$ Nanoparticles

Bismuth telluride-based materials have been widely investigated due to their applications for the development of high-performance thermoelectric devices. Here, we numerically determine the effective electrical conductivity ( $${\sigma }_{eff}$$ ), thermal conductivity ( $${k}_{eff}$$ ), and Seebeck coefficient ( $${S}_{eff}$$ ) of composite materials made up of $${\mathrm{VO}}_{2}$$ nanoparticles embedded in a $${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_{3}$$ (BST) matrix. The temperature evolution of these three properties along with the thermoelectric figure of merit ( $$ZT={\sigma }_{eff}{S}_{eff}^{2}T/{k}_{eff}$$ ) is analyzed across the metal–insulator transition of $${\mathrm{VO}}_{2}$$ and for temperatures up to 550 K. For temperatures higher than 350 K, it is shown that $${\mathrm{VO}}_{2}$$ nanoparticles with a concentration of 34 % enhance the electrical conductivity and ZT of the matrix by about 16 % and 10 %, respectively, while the Seebeck coefficient remains pretty much constant. This indicates that $${\mathrm{VO}}_{2}$$ nanoparticles provide an effective way to enhance the thermoelectric efficiency of $${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_{3}$$ materials. The calculated ZT values for $${\mathrm{VO}}_{2}$$ are in good agreement with the experimental data reported in the literature for temperatures higher than 350 K. The thermal conductivity values obtained for $${\mathrm{VO}}_{2}$$ in the insulating phase are in good agreement with the experimental data reported in the literature, which are used to calculate the interface thermal resistance between $${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_{3}$$ and $${\mathrm{VO}}_{2}$$ . Furthermore, the ratio $${k}_{eff}/T{\sigma }_{eff}$$ is found to be higher than the Lorenz number for pure metals. Above the transition temperature of $${\mathrm{VO}}_{2}$$ (342.5 K), this ratio increases with temperature and concentration, allowing to evaluate the role of electrons as energy carriers in these systems.

Methodology to determine coefficients of effective thermal conductivity when heating porous bodies using fractal-like structures

Thermal insulation, porous composite, ceramic, charge materials are frequently used in the energy industry. The materials are heat treated to improve the structure and give them the desired properties required for specific production conditions. The important task is to assess the influence of geometric parameters of the fractal-like structure and radiation heat transfer on the thermophysical properties of porous bodies. For numerical description of porous bodies, the technique of replacing geometry of porous body with the bodies with a fractal-like structure having self-similarity properties is significant. The object under study is an array of blanks arranged chaotically, the structures that are called bulk cages. The porous body is replaced by a fractal cube structure of the 2nd rank of partitioning. The simulation has been performed in the COMSOL Multiphysics software based on a three-dimensional model of the Sierpinski carpet. Since the distribution of heat can be non-uniformed over the structure of the object, three variants of the fractal-like cubic structure cross section are considered. A method to determine the effective thermal conductivity coefficients based on the use of fractal-like structures has been developed. Depending on the cross sections, one-dimensional computational models with sufficient accuracy for engineering analysis are obtained. The effective thermal conductivity coefficients are determined. The results of data analysis have shown that the geometric parameters of the structure and radiation heat transfer significantly affect the effective coefficient of thermal conductivity at high temperatures. In comparison to the currently available approaches, the developed method allows solving the problem of determining thermophysical properties without physical experiments. The technique used in the study may be used for mathematical modeling of heat exchange processes of heat-power facilities when calculating temperature fields and determining heating modes.

Pores integrated fractal (PIF) analysis on transportation in porous media considering spatial distribution of pores and genuine tortuosity

A pores integrated fractal model is established to represent the microscopic structures of porous media in consideration of the spatial distribution of pores and the genuine tortuosity in various transportation processes. Transport equations are obtained based on the PIF model, providing high precision for predicting the effective thermal conductivity, the permeability and diffusion coefficient in porous materials. Spatial distributions of pores in realistic samples are acquired, based on which the influence of the equivalent pore distributions is specifically analyzed in the study. In the premise of the same porosity, broader range of pore size, dominant pores in dozens of micron or bimodal distribution of pores are crucial factors that influence the heat conduction and permeation in porous media. It is noted that the PIF model has significant values in the design of porous structure with improved transportation characteristics and in the accurate prediction of vital parameters in complex porous media.