Search

Modern engineering applications use processes that submit materials to high-temperature gradients. However, the traditional experimentation methods applied to determine thermal properties often do not provide reliable data when working temperatures are up to extreme conditions. Hence, the present work demonstrates the use of an inverse heat conduction problem methodology for estimating the thermal properties of a laser beam welding (LBW) process. The applied technique is the quadrilateral optimization method (QOM), which consists of a multivariable estimation approach developed to calculate the function's parameters. Additionally, the future time regularization scheme was implemented in the objective function to regularize the results. The applied numerical process solved the energy equation using the finite volume method implemented in an in-house CUDA-C language code. The software is a multi-thread application run in a graphics processing unit for enhanced computational time efficiency. A validation study compared the estimated results with LBW simulated data. The QOM estimates the parameters of a function representing the range of thermal conductivity values. The proposed method is expected to lower experimental costs for obtaining thermal properties at high temperatures by eliminating the need for sophisticated technical equipment and skilled labor required by traditional direct measurements. [ABSTRACT FROM AUTHOR]

The dominant techniques for laser-based thermal measurement, time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance, generally require that samples be coated with a metal transducer layer. The properties of the transducer and its interface to the sample are then important to the interpretation of results, and heat conduction in the transducer layer can limit measurements of ultra-thin samples or in-plane conductivity. We demonstrate the feasibility of measuring semiconductor samples using standard TDTR instrumentation, without the use of a metal transducer. A mathematical model accounting for non-zero laser penetration depth as well as both thermal and charge carrier effects is presented, and the ability of the model to describe and fit to experimental data for silicon and germanium samples is demonstrated. Though at this stage measurement uncertainties are larger than for traditional TDTR, we anticipate that the use of this technique will expand the range of samples that can be measured with existing TDTR setups. [ABSTRACT FROM AUTHOR]

During the machining of Inconel 718 using SiAlON ceramics, the tool life can be significantly influenced by diffusion wear. To investigate this crucial wear behavior, this study focuses on the thermochemical interactions in the diffusion couple between SiAlON ceramics and Inconel 718 superalloy. So, we examine the microstructural characteristics and phase formation behavior in the reacted layer between these two materials. The interfaces between these two materials exhibit distinguishable elemental contrasts and phase formations. Particularly, the phases indicate vitrification or devitrification in the diffusion layer, influenced by the chemical composition. This issue in the diffusion couple was analyzed with thermophysical properties using light flash analysis (LFA). After, it was confirmed that the vitrification of the diffusion layer can be induced by the conduction behavior of heat trapped between SiAlON and Inconel 718 during supercooling. These characterizations verify that the devitrification of the welding layer (diffusion layer), resulting from the reaction between SiAlON ceramics and Inconel 718 superalloy, can depend on the thermal conductivity of the prepared ceramics. Furthermore, we discovered a possibility that this could contribute to improving tool performance by suppressing diffusion wear. [ABSTRACT FROM AUTHOR]

Thermal properties of frozen soil can be measured to evaluate the engineering construction and agricultural production. However, it is still challenging to determine the thermal properties of frozen soil using a dual-probe heat pulse sensor (DPHP). The thermal pulse can cause the ice around the heating probe to melt during DPHP measurement. The commonly- used analytical solution can only consider the heat conduction, resulting in the less accurate measurement on of the thermal conductivity (λ) and specific heat (Cv ) at −5 ℃ to 0 ℃. The purpose of this study is to test the feasibility of finite element simulation for the frozen soil DPHP. COMSOL simulation was carried out to consider the latent heat and temperature of phase transition in the measurement process of DPHP. The two-dimensional transient fluid module was used for simulation. The three-dimensional of DPHP experiment was simplified to the heat transfer in two-dimensional cylindrical coordinates. The 150 mm × 150 mm square was constructed as the study area, where was approximated the DPHP probe with a 0.6 mm circle (ignoring internal probe padding and needle body material), and spaced the probe by 6 mm. The boundary of the study area was set as the adiabatic boundary, the boundary of the circular heat source was the heat flux boundary, which was set as 48.51 W/m, and the heating time was set as t0 = 8 s, respectively. The parameter settings were consistent with the experimental process. The nonlinear relationships between thermal conductivity and specific heat capacity with temperature was were calculated using the nonlinear relationship between liquid water content and temperature as studied by He et al. 2015. These relationships were then set as the thermal conductivity and specific heat capacity of the frozen soil model. Four phase change temperatures were at −0.05-0.05, −2- 0, −1.5-−0.5, and −0.5-0 ℃, corresponding to phase change intervals of 0.1, 2, 1, and 0.5 ℃, respectively. Four geometric shapes were constructed around the key solving region (around the DPHP probe), each of which the center was located at the center of the heating probe and side lengths of 5, 10, 15, and 20 mm. The reason was that the uniformly refined grids over the domain were resulted in the high computational costs and large memory usage during simulation. The grid was refined at the boundaries of these geometric shapes, in order to achieve the higher solving accuracy. An infinite line heat source (ILS) and real permafrost measurements were used to evaluate the COMSOL simulation. Three COMSOL simulation models were compared with the ILS model and measured with/without considering phase transition and temperature. The results show that: 1) The COMSOL simulation was in better agreement with the ILS model without considering the phase transition. When the initial soil temperature was ranged from 0 ℃ to −1 ℃, the ILS model was significantly deviated from the experimental measurement (R2 < 0.0013). At the same time, the presence of the solid-liquid moving interface caused it to be invalid in the ILS model using single-phase heat transfer, when the temperature was close to 0℃. 2) The deviation of simulation from experimental data was significantly reduced to consider the phase transition occurrence and temperature. When the phase transition temperature was −1.5-−0.5 ℃, the correlation between the simulated and the measured value (R² > 0.7) was significantly higher than that between the ILS model and the measured value (R² < 0.001 3). Therefore, the heat transfer of transient melt phase transition can be effectively performed on COMSOL simulation. The more accurate measurement of frozen soil thermal properties was achieved, compared with the current mainstream traditional single-phase model. 3) COMSOL simulation can be used to predict the measurement of frozen soil by DPHP, where the liquid water content was obtained in the process of freeze-thaw at the interval of phase transition. The finding can provide the a new idea to measure the thermal properties of frozen soil in the field. [ABSTRACT FROM AUTHOR]

The 1045 steel is extensively heat treated because its mechanical properties can be easily improved by thermal processes. Reliable knowledge of the relationship between microscopic structure and thermal properties is essential for the thermal design of parts and tools. This paper reports on coupling microstructural aspects with the effects of tempering temperature on the temperature-dependent thermal properties of 1045 steel. Thermal conductivity (k) and specific heat (cp) were simultaneously estimated using inverse heat conduction analysis. The thermal characterization technique handled a simple and low-cost experimental setup, based on transient one-dimensional heat transfer on samples subjected to different heat-treatment conditions. The results, which were discussed in connection with the resulting microstructures, demonstrated that all quenching and tempering (Q&T) conditions led to lower values of both thermal properties throughout the temperature domain. The tempering temperature was found to impact k more substantially. The robustness of the results was ascertained by recovering the applied heat flux based on linear regression equations for k and cp and temperature measurements. Accuracy analysis indicated that the inverse approach provides estimates with an uncertainty close to 4%. [ABSTRACT FROM AUTHOR]

The current work presents a theoretical framework to analyze the nonlocal thermoelastic model with a general kernel function for memory-based derivatives by incorporating two-time delays. The introduced model aims at studying the thermomechanical response of rotating size-dependent nanobeams. The nonlocal elasticity theory and the generalized heat conduction model with phase delays are utilized to formulate the problem. In the proposed model, the thermal conductivity is expected to vary linearly with temperature and the system is excited by a variable harmonic heat source by considering the time-dependent exponential decaying load. To this end, the semi-analytical solutions for the transverse and axial displacements, thermodynamic temperature and bending moment are examined using the Laplace transform method. The effects of the nonlocal parameter and various loading conditions are also investigated and discussed. Finally, it is demonstrated that how the linear variation of thermal conductivity could affect the performance of the considered system. [ABSTRACT FROM AUTHOR]

Abstract Prediction of the coupled conduction-radiation heat transfer in composite materials with periodic structure is important in high-temperature applications of the materials. The temperature dependence of thermal properties complicates the problem. In this work, a multiscale model is proposed for the conduction-radiation heat transfer in periodic composite materials with temperature-dependent thermal properties. Homogenization analysis of the coupled conduction and radiative transfer equations is conducted, in which the temperature dependence of thermal properties is considered. Both the macroscopic homogenized equations and the local unit cell problems are derived. It is proved that the macroscopic average temperature can be used in the unit cell problems for the first-order corrections of the temperature and radiative intensity, and the calculations of effective thermal properties. The temperature dependence of thermal properties only influences the higher-order corrections. A multiscale numerical method is proposed based on the analysis. The Gaussian process (GP) regression is coupled into the multiscale algorithm to build a correlation between thermal properties and temperature for the macroscale iterations and prevent the repetitive solving of unit cell problems. The GP model is updated by additional solutions of unit cell problems during the iteration according to a variance threshold. Numerical simulations of conduction-radiation heat transfer in composite with isotropic and anisotropic periodic structures are used to validate the proposed multiscale model. It is found that the accuracy and efficiency of the multiscale method can be guaranteed by using a proper variance threshold for the GP model. The multiscale model can provide both the average temperature and radiative intensity fields and their detailed fluctuations due to the local structures.

Long hollow cylinders are commonly utilized in various technological applications, including liquid and gas transmission. As a result, its value is growing, becoming increasingly important to many research efforts. Compared with thermal isotropic homogeneous cylinders, thermo-viscoelastic orthotropic cylinders have less relevant data. In this paper, a thermoelastic fractional heat conduction model was developed based on the Moore-Gibson-Thompson equation to examine the axial symmetry problem of a viscoelastic orthotropic hollow cylinder. Atangana and Baleanu derivative operators with nonsingular and nonlocal kernels were used in constructing the fractional model. The thermal properties of the cylinder materials are assumed to be temperature-dependent. The Laplace transform is applied to solve the system of governing equations. The numerical calculations for temperature, displacement, and stress components are performed by the effect of fractional order, rotation, and changing thermal properties of the cylinder. The results showed that due to the presence of fractional derivatives, some properties of the physical fields of the medium change according to the value of the fractional order. [ABSTRACT FROM AUTHOR]

In this research, two alternative approaches to fractional derivatives, namely Caputo–Fabrizio (CF) and Atangana–Baleanu (AB) fractional operators, are used to propose a generalized model of thermoelastic heat transfer of a rigid cylinder with thermal characteristics. The proposed model can be constructed by combining the DPL model with phase delays and the two temperature theories. It will be taken into account that the solid cylinder has variable physical properties. It was also assumed that the surface of the cylinder is penetrated by a continuous magnetic field and is regularly exposed to thermal loading from a continuous heat source. The numerical solutions of the studied physical fields in the AB and CF fractional derivative cases were derived using the Laplace transform method and are compared visually and tabularly and discussed in detail. [ABSTRACT FROM AUTHOR]

An accurate and efficient algorithm is presented, which allows deriving the structure function of a discretized 3-D heat conduction problem. In this approach, the partial thermal conductances and capacitances in the structure function are computed in terms of weighted spatial averages of thermal resistivity and volumetric heat capacity. As a result, the exact influence of all materials and geometric details on each part of the structure function is determined. The approach is validated on a state-of-the-art SiGe heterojunction bipolar transistor (HBT) for high-frequency applications. [ABSTRACT FROM AUTHOR]

Time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance techniques have been widely used to measure thermal properties. However, the existence of the metal sensor brings some limitations to the experimental measurement, such as temperature limits, disability to measure low in-plane thermal conductivity, in situ measurement cannot be achieved, etc. This paper proposes a transducerless time-domain thermoreflectance method to measure in- and cross-plane thermal conductivity of nanofilms, in which the optical absorption depth and thermal conductivity tensor are considered to establish a new differential equation that can describe the heat conduction process in multilayer structures. This thermal model can also calculate the effects of spot ellipticity and spot offset distance. Then, the analytical solution and relative deviation of this new model and the surface heat flow boundary model used in conventional TDTR are compared by calculating the phase signals. In terms of experimental measurement, this model is successfully used to derive cross- and in-plane thermal conductivity of PdSi and IrNiTa amorphous alloy nanofilms without a metal sensor. [ABSTRACT FROM AUTHOR]

In thermal wave physics the surface temperature of a material depends on the thermal diffusivity and thermal effusivity of the components. Therefore these thermal properties of buried structures are expected to be retrieved from the measurement of the surface temperature using photothermal techniques. Then, from the constitutive equations, thermal conductivity and specific heat of the inclusions can be calculated. In this paper we demonstrate analytically that when the thermal properties of the inclusions are very different from those of the matrix they are degenerate. Three kinds of inclusions have been studied: layers, cylinders, and spheres. If the transport thermal properties of the inclusion are much higher (much lower) than those of the matrix; only its specific heat (thermal effusivity) can be retrieved. On the other hand, for a gas inclusion only its thermal conductivity can be determined. Photothermal measurements performed on three calibrated samples containing buried cylinders confirm the theoretical conclusions. [ABSTRACT FROM AUTHOR]

A simple photothermal methodology for determining simultaneously and in four different ways the thermal diffusivity and thermal effusivity of the illuminated layer in a two-layer system is presented. The method is based on the analysis of the photothermal signal of the whole system normalized by the corresponding signal of the illuminated one-layer system. Our approach uses the modulation frequencies at which the normalized amplitude (phase) passes through unity (zero) and reaches its maximum (minimum). It is shown that these four characteristic modulation frequencies are independent of the thermal effusivities of the layers, and their values can be used to obtain the thermal diffusivity of the illuminated layer. Then, by using the exact complex expression of the normalized photothermal signal with the experimental data for its amplitude and phase, the thermal effusivity of the same layer can be found. In order to show the usefulness of this approach, the method is applied to a glassy carbon sample in thermal contact with three different types of very thick back layers of air, water, and glycerine. It is shown that the highest (lowest) accuracy on the measurement of the thermal properties corresponds to the frequency at which the zero (minimum) of the normalized phase occurs. [ABSTRACT FROM AUTHOR]

High-temperature superconducting (HTS) coils generate local heat during the transmission of alternating current (AC), resulting in a decrease in thermal stability. The influence of relevant factors on the local heating location and temperature of the coil is still unclear. In order to strengthen the protection and operation monitoring of the superconducting coil, it is necessary to research this. Based on the H-formulation, the paper uses the electromagnetic–thermal coupling finite element method (FEM) to establish a two-dimensional (2D) axisymmetric model of the YBCO coil. The AC loss and temperature when the coil transmits alternating currents of power frequency are analyzed. Firstly, the internal temperature distribution of the coil is analyzed, and the influence of the turn number on the location of the highest temperature is discussed. For a 16-turn coil, the effects of the convective heat transfer coefficient and the thickness of the insulating layer between two turns on the magneto-caloric properties of the coil are discussed, respectively. The results show that, below 100 turns, the highest temperature of the coil occurs near the inner side; improving the heat transfer efficiency and appropriately reducing the thickness of the inter-turn insulating layer is beneficial to suppress the temperature rise and reduce the temperature difference inside the coil. The research conclusions provide a reference for the design and protection monitoring of HTS coils.

This study investigates spatially resolved phonon-mediated thermal transport across nano-sized grains and grain boundaries (GBs) in 3C-SiC using molecular dynamics (MD) simulations. This investigation involves controlling the complete range of inter-grain misorientation tilt angles (θ = 0°–90°) and nanoscale grain sizes (d = 2.18–130.77 nm). The grain boundary energy and interfacial thermal transport are found to be highly θ -sensitive and asymmetric with respect to θ = 45° due to the low symmetry associated with two interpenetrating diatomic SiC fcc lattices. When adjacent grains are tilted at θ = 14.25°, the interfacial heat conduction is highly suppressed compared to other θ values, especially for larger grains. The most stable atomic configuration of the GB region associated with the minimal GB energy results in the highest suppression of heat conduction across the GB interface. Spatially resolved thermal anisotropy reveals a strong GB-mediated nanoscale hydrodynamic phonon Poiseuille effect when heat flows parallel to the GB planes, as shown by our perturbed MD study. With the reduction of d , the intra-grain and inter-GB thermal conductivities decrease due to the enhanced phonon scattering from interfaces, but the difference between these conductivities becomes negligible for the heat flow normal to the GB planes. It is envisioned that nanoscale spatially resolved control of thermal energy will provide useful guidance to engineer nanocrystalline ceramics with tunable interfacial thermal properties. [ABSTRACT FROM AUTHOR]

In the paper, generalized boundary conditions were used for the homogenization of coefficients of the Laplace partial differential equation in the context of Darcy flow and heat diffusion phenomena. The mesoscopic boundary value problem was defined and analyzed from the variational perspective and the finite element formulation of the homogenization problem was provided. The matrix equation for the apparent macroscopic properties, resulting from FEM discretization, was derived and utilized in two illustrative examples: homogenization of the filtration coefficient of clay amended with expanded shale and thermal conductivity of the soil with multiple fractions. It is shown, that generalized boundary conditions can provide very good homogenization results without the assumption of the periodicity of the material. For best results, the microscopic length parameter has to be properly estimated. [ABSTRACT FROM AUTHOR]

The present study goal was to determine the thermal properties of apples during their cooling. Fuji apple was cooled in a domestic refrigerator by natural convection. Thermal properties were determined assuming the geometry of an equivalent sphere. An optimizer software (LS Optimizer) was used for the inverse problem. A new heat conduction software was developed to solve the direct problem, using the spherical geometry of the diffusion equation with the boundary condition of the third kind. Thus, thermal properties of apples and their uncertainties were calculated during cooling. The developed software for the direct problem was used to simulate the cooling kinetics for any previously specified point within the sphere. The fitting of the simulated curve to the experimental points showed coefficient of determination (R2) greater than 0.99 and low values of the chi‐square function (χ2). Therefore, it can be concluded that the spherical geometry and the boundary condition of the third kind were adequate to describe the process of heat removal for apple. Determined values of thermal diffusivity and convective heat transfer coefficient were α = (1.38 ± 0.07) × 10−7 m2 s−1 and h = (1.360 ± 0.018) × 10−6 m s−1, respectively, with confidence intervals of 95.4%. Another confirmation of the developed model was the fact that the thermal diffusivity was similar to the estimate by Riedel correlation (α = 1.38 × 10−7 m2 s−1). The position of the thermocouple in the equivalent sphere calculated by numerical solution was r = 6.6 mm. Practical Applications: One of the main contributions of this paper is the calculation of thermal parameters along cooling with their uncertainties. This will result in very accurate simulation of the pasteurization and freezing processes. Furthermore, the values obtained in the study can be used to predict new cooling curves for the same product, but with different dimensions. [ABSTRACT FROM AUTHOR]

The aim of the current work is to investigate the effect of SiC particle weight percent and rolling passes on Al/Cu/SiC laminated composites, fabricated by accumulative roll-bonding (ARB) and cross-accumulative roll-bonding (CARB) processes. The optical microscopy (OM) images of composites revealed that despite the good bonding of the layers, they underwent plastic instabilities as a consequence of strain hardening of the layers. However, these instabilities occurred more in ARBed composites than in composites fabricated by the CARB process. This is because in the latter process, the composites are rolled in two directions, which leads to better strain distribution. Furthermore, with an increase in passes, SiC particles were well distributed in the matrix and interfaces. The mechanical findings showed that, by increasing passes, there was a growth in the values of strengths and elongation. This behavior is believed to be related to increased work-hardening of layers, better distribution of reinforcing particles, and an enhanced bonding of interfaces at higher rolling passes. In addition, the results of thermal conductivities showed a downward trend with an increase in passes; in fact, the increased number of Al/Cu interfaces declined the heat conduction of composites. [ABSTRACT FROM AUTHOR]

High-temperature superconducting (HTS) coils generate local heat during the transmission of alternating current (AC), resulting in a decrease in thermal stability. The influence of relevant factors on the local heating location and temperature of the coil is still unclear. In order to strengthen the protection and operation monitoring of the superconducting coil, it is necessary to research this. Based on the H-formulation, the paper uses the electromagnetic–thermal coupling finite element method (FEM) to establish a two-dimensional (2D) axisymmetric model of the YBCO coil. The AC loss and temperature when the coil transmits alternating currents of power frequency are analyzed. Firstly, the internal temperature distribution of the coil is analyzed, and the influence of the turn number on the location of the highest temperature is discussed. For a 16-turn coil, the effects of the convective heat transfer coefficient and the thickness of the insulating layer between two turns on the magneto-caloric properties of the coil are discussed, respectively. The results show that, below 100 turns, the highest temperature of the coil occurs near the inner side; improving the heat transfer efficiency and appropriately reducing the thickness of the inter-turn insulating layer is beneficial to suppress the temperature rise and reduce the temperature difference inside the coil. The research conclusions provide a reference for the design and protection monitoring of HTS coils. [ABSTRACT FROM AUTHOR]

Estimating thermal properties for thick or solid foods at temperatures greater than 100 °C is challenging for two reasons: the long time needed to reach a constant temperature, and the pressure needed to be maintained in the sealed container. An instrument (“Thermal Properties Cell”, or “TPCell”) was developed based on a rapid non-isothermal method to estimate the temperature-dependent thermal properties within a range of commercial food processes (20–140 °C). The instrument design consists of a custom sample holder and special fittings to accommodate the heater within a pressurized environment. The total time of the experiment is less than 1min., compared to existing isothermal instruments requiring 5–6 h to cover a similar temperature range. Thermal conductivities of different food materials were estimated for the temperature range of commercial food processes. The novelty of the instrument lies in its ability to analyze transient temperature data using a nonlinear form of the two-dimensional heat conduction equations. [ABSTRACT FROM AUTHOR]

The realization of single-molecule thermal conductance measurements has driven the need for theoretical tools to describe conduction processes that occur over atomistic length scales. In macroscale systems, the principle that is typically used to understand thermal conductivity is Fourier's law. At molecular length scales, however, deviations from Fourier's law are common in part because microscale thermal transport properties typically depend on the complex interplay between multiple heat conduction mechanisms. Here, the thermal transport properties that arise from electron transfer across a thermal gradient in a molecular conduction junction are examined theoretically. We illustrate how transport in a model junction is affected by varying the electronic structure and length of the molecular bridge in the junction as well as the strength of the coupling between the bridge and its surrounding environment. Three findings are of note: First, the transport properties can vary significantly depending on the characteristics of the molecular bridge and its environment; second, the system's thermal conductance commonly deviates from Fourier's law; and third, in properly engineered systems, the magnitude of electron hopping thermal conductance is similar to what has been measured in single-molecule devices. [ABSTRACT FROM AUTHOR]

This work aims to present a method for simultaneously estimating the thermal conductivity, k, and specific heat, cp, in samples of stainless steel 304 and cemented carbide by accounting the imperfect thermal contact at the heater–sample interface. A transient one-dimensional heat conduction model with constant thermal properties is used. The metallic sample is placed in the middle of a polyimide heater and a thermal insulator. A constant heat flux is applied on the specimen upside surface and a thermal insulation condition is maintained on the opposing surface, where a type T thermocouple measures the temperature response. Contact resistance is determined and accounted for as a reducing factor on heat flux. Instead of considering a lumped model, a microscopic approach of the contacting regions is used to describe both the surface roughness and the fluid gap. With the intention of guaranteeing simultaneous and reliable estimates for both thermal properties, sensitivity analysis is used to establish heat flux intensity, experiment duration, time step for data acquisition, and other characteristics of the experiments. To simultaneously achieve both thermal properties, the optimization method BFGS (Broyden–Fletcher–Goldfarb–Shanno) is employed to minimize a least-squares objective function comparing experientially measured and numerically calculated temperatures. The numerical solution for the transient conduction problem is determined with COMSOL, which solves the transient heat diffusion problem by discretizing it applying the finite element method (FEM). The linear system of equations is solved using the PARDISO solver and backward differentiation formula (BDF) method. Heat flux is estimated employing the sequential function specification method (SFSM) as a means of verifying the robustness of parameter estimation and experimental procedure. Moreover, uncertainty analysis is performed to confirm the quality of the results obtained. Finally, the uncertainty analysis outcomes and thermal properties estimated are compared with literature. [ABSTRACT FROM AUTHOR]

• Structural electronic and thermal properties of Hydrogenated silicon carbide (6H-SiC). • The transfer characteristics of SiO2/6H-SiC MOSFET transistors were calculated. • Evaluation of heat transfer conduction using the single-phase-lag model. • The temporal distribution of temperature at different moments of time. In this paper, studies of thestructural electronic and thermal properties of hydrogenated silicon carbide (6H-SiC) have been carried out applying first-principles calculations based on the linearized augmented plane wave method. A two-dimensional numerical model is proposed for 6H-SiC Metal oxide semiconductor field-effect transistor (MOSFET) to investigate the heat transfer on the transistor characteristics using a commercial software application. Structural and electronic results show 6H-SiC lattice parameters (a) and (c) of 3.0817Å and 15.1179Å respectively and an indirect band gap about 2.015 eV. The thermodynamic calculations show that the heat capacity of 6H-SiC is stable at higher temperature. Moreover, the single-phase-lag heat conduction model is used to calculate the temporal temperature distribution in centerline of 6H-SiC MOSFET. The temperature values obtained are 300K-360K at time 7 ps and 12 ps respectively. [ABSTRACT FROM AUTHOR]

• We used models of heat transfer by conduction and convection to estimate soil thermal properties. • We used 1.5 years of experimental data on a natural pasture area in the Pampa biome in southern Brazil. • We consider the effects of soil heterogeneity and thermal convection due to the vertical movement of water in the soil, W. • Soil thermal diffusivity (k) showed an inverse seasonal variation with soil temperature. • Increasing values of W led to an increase in the variability of k with depth. Soil temperature, composition, and structure directly affect the heat transfer mechanisms in soil. Understanding the thermal behavior of soil is fundamental for describing the processes of mass and energy exchange in the soil-plant-atmosphere system. In addition, estimating thermal property values is necessary to determine the amount of heat transferred, stored, or transmitted by conduction and convection between the soil surface and deeper layers. This study used two soil heat transfer models (the conduction and conduction-convection methods) to estimate soil thermal properties. Soil data were collected in a natural pasture area in the Pampa biome in southern Brazil over 1.5 years. The results showed that soil thermal diffusivity (k) has an inverse seasonal variation with soil temperature (T s). The k and soil thermal conductivity (λ) values are higher as the soil depth and soil moisture (θ) increase, confirming the vertical variation of thermal properties. The liquid water flux density (W) represents the vertical movement of water in the soil and decreases in periods of higher T s. Increasing values of W led to an increase in the variability of k with depth. The conduction–convection method provides the best estimates for T s with R2 = 0.99 for deeper layers compared to the conduction method. However, we found that both models gave similar results for near-surface layers since, in this case, the conduction process was responsible for most of the heat transfer from the surface to the ground. The results obtained here allowed us to evaluate heat transfer at different depths and estimate thermal properties as a function of soil moisture in a natural pasture area in the Brazilian Pampa biome. Furthermore, the results can be integrated into global land surface models intended to represent the behavior of the energy exchange between soil, surface, and atmosphere. [ABSTRACT FROM AUTHOR]

This study presents an experimental technique to simultaneously estimate the thermal conductivity (k) and the specific heat (c p) as functions of temperature of AISI 304 stainless steel. The thermal model was based on the transient nonlinear heat conduction across a three-dimensional slab. This enabled one better assess the heat diffusion effects, increasing the sensitivity, and addressing a more realistic case study. Bayesian statistics evaluated the agreement between the numerical model and the measurements to infer constant k and c p for intervals of 10 °C. Temperature dependence was determined via a linear fit to these piecewise estimates. The direct problem was solved in COMSOL Multiphysics, obtaining the temperature field from known initial and boundary conditions. D-optimality-based sensitivity analysis was used to determine the best location to collect the measurements so that data from a single thermocouple were sufficient to identify these thermal properties. This sensitive temperature information was provided by a low-cost experiment at room temperature. The inverse estimation quality was shown by comparing the outcomes with data from literature and by assessing the 95% confidence bounds, which had deviations below 3.6% for both properties. The simultaneous estimation accuracy was further investigated using uncertainty analysis, which indicated a value close to 3.5%. [Display omitted] • High sensitivity coefficients are achieved through use of a 3D thermal model. • 3D experimental data in Bayesian estimation of k and c p with uncertainty below 4%. • Simultaneous estimation using the whole temperature domain divided into intervals. • Temperature dependence is determined from a single experiment at room temperature. • Single sensor data are sensitive enough to be used in the inverse methodology. [ABSTRACT FROM AUTHOR]

The paper presents the results of the laboratory investigation of thermal conductivity of clayey soils subjected to different moisture contents. The conductivities of soil specimens are evaluated at two extreme cases: fully saturated and dried state. The latter state is achieved by drying the soil specimens in the temperature 105 °C. The needle probe test is used for laboratory measurements. Theoretical analyses, with the use of two different homogenization approximation schemes, namely the Mori-Tanaka and the Self-consistent are performed. Using laboratory results and treating them as effective conductivities the conductivity of solid as well as the geometry of pores are found. Calibrated model gives the possibility of calculating the effective conductivity of the soil in two considered cases: fully saturated and dried state. [ABSTRACT FROM AUTHOR]

We have investigated the thermal conductivity of defective fullerene (C by using the nonequilibrium molecular dynamics (MD) method. It is found that the thermal conductivity of C with one defect is lower than the thermal conductivity of perfect C. However, double defects in C have either positive or negative influence on the thermal conductivity, which depends on the positions of the defects. The phonon spectra of perfect and defective C are also provided to give corresponding supports. Our results can be extended to long C chains, which is helpful for the thermal management of C. [ABSTRACT FROM AUTHOR]

Despite its wide range of real-life applicability, the combined effect of the thermal properties and thickness of a heater on the Leidenfrost temperature (T L) of the system has not been explored. Thus, this study investigates the effects of thermal properties and thickness on the T L using four different metal specimens (Inconel 718, AISI 304 stainless steel, brass, and aluminum). Our experimental results show that the T L decreases with an increase in the thickness of the specimens up to a threshold value. Herein, this varying T L , which varies with thickness, is called transient Leidenfrost temperature (T L,TR) in this study. When the thickness of the sample exceeds the asymptotic thickness, T L became constant, which is termed as the asymptotic Leidenfrost temperature (T L,ASY) in this study. The dependency of the Leidenfrost phenomenon on the thermal properties and thickness can be described by simultaneously considering the heat capacity and heat propagation characteristics; however, in case of the thick specimens, the T L,ASY is mainly considered to be resultant of the speed of heat propagation, including the heat-transfer performance. Consequently, two different trends of the Leidenfrost phenomenon can be predicted. • Leidenfrost temperatures decrease as the thickness of specimens increases to an asymptotic thickness. • Transient Leindenfrost temperature is simultaneously affected by both heat capacity and thermal conductivity. • Asymptotic Leindenfrost temperature is mainly affected by thermal conductivity. • Leidenfrost temperatures are predicted from a transient heat conduction analysis based on the droplet dynamics. [ABSTRACT FROM AUTHOR]

Abstract: A micromechanics-based thermo-dynamical model is proposed to predict the distribution of dynamic effective thermal properties of functionally graded fibrous composites resulting from thermal waves. Generally speaking, in functionally graded materials there exist two microstructurally distinct zones: fiber-matrix zone and a transition zone. In the fiber-matrix zone, based on the non-Fourier heat conduction equation in materials, the dynamic effective thermal properties for any macroscopic material points are determined by employing the effective medium method in the corresponding microstructural representative volume element (RVE). In transition zone, a transition function is introduced to make the wave fields continuous and differentiable. Numerical examples of the dynamic effective thermal properties in the gradation direction under different parameters are graphically presented. Analysis shows that the material properties of each phase, the incident frequency, and the gradation parameter of materials have great effect on the distribution of dynamic effective properties in the gradation direction. In different material zones, the effect displays great difference. In the region of higher frequencies, the relaxation time of the two phases also has great effect the dynamic thermal properties. At last, the results are discussed in detail. [Copyright &y& Elsevier]

This article deals with estimation of spatially distributed thermal properties of two-dimensional heterogeneous media from the solution of inverse heat conduction problem. The experimental procedure involves sequential heating of sample at four discrete locations on one side and recording transient temperature on the other side non-intrusively. The inverse problem formulation is carried out as parameter estimation problem and the unknown parameters are estimated through minimization of sum of squared error between measured and simulated temperatures using Levenberg–Marquardt algorithm. The huge computational time required to calculate sensitivity matrix is reduced through parallel computation strategy. Numerical estimations are carried out with synthetic temperature and it is found that the resolution and accuracy of estimated spatially distributed thermal conductivity is in better agreement with the exact distribution when compared to volumetric heat capacity even at an error level of ± 0.1 K. Transient temperature response of the fabricated heterogeneous prototype is recorded using infrared radiation camera and the same is used for estimation of unknown parameters. The estimated thermal conductivity closely mimics the actual distribution. Therefore, this simple proposed method can be directly used in thermal tomography applications for identifying the shape and size of inclusions/inhomogeneities from the estimated thermal conductivity distribution alone. [ABSTRACT FROM AUTHOR]

Abstract: In this work heat transport in layered systems is analyzed using a hyperbolic heat conduction equation and considering a modulated heat source for both Dirichlet and Neumann boundary conditions. In the thermally thin case, with Dirichlet boundary condition, the well known effective thermal resistance formula is derived; while for Neumann problem only a heat capacity identity is found, due to the fact that in this case this boundary condition cannot become asymptotically steady when modulation frequency goes to zero. In contrast in the thermally thick regime, heat transport shows a strong enhancement when hyperbolic effects are considered. For this thermal regime, an analytical expression, for both Dirichlet and Neumann conditions, is obtained for the effective thermal diffusivity of the whole system in terms of the thermal properties of the individual layers. It is shown that the magnifying effects on the effective thermal diffusivity are especially remarkable when the thermalization time and the thermal relaxation time are comparable. The limits of applicability of our equation, in the thermally thick regime are shown to provide useful and simple results in the characterization of layered systems. Enhancement in thermal transport and in the effective thermal diffusivity is a direct consequence of having taken into account the fundamental role of the thermal relaxation time in addition to the thermal diffusivity and thermal effusivity of the composing layers. It is shown that our results can be reduced to the ones obtained using Fourier heat diffusion equation, when the thermal relaxation times tend to zero. [Copyright &y& Elsevier]

A set of tunnel fire experiments using a 1:20 scale model tunnel was conducted taking into account the scaling of heat conduction through the tunnel walls. The critical velocity and backlayering distance measured in the tunnel fire experiments were compared with other reference data to examine the effect of the thermal properties of the tunnel walls on these two parameters. In this paper, we present new correlations for estimating the critical velocity and backlayering distance, and discuss the difference between our correlations and the previous ones from the viewpoint of the thermal properties of the tunnel wall materials. [ABSTRACT FROM AUTHOR]

A new solution for the three-dimensional transient heat conduction from a homogeneous medium to a non-homogeneous multi-layered composite material with temperature dependent thermal properties using a mesh-free Monte-Carlo method is proposed. The novel contributions include a new algorithm to account for the impact of thermal diffusivities from source to sink in the calculation of the particles’ step length (particles are represented as bundles of energy emitted from each source), and a derivation of the three-dimensional peripheral integration to account for the influence of material properties around the sink on its temperature. Simulations developed using the proposed method are compared against both experimental measurements and results from a finite element simulation. [ABSTRACT FROM AUTHOR]

In the paper the solutions for stationary heat conduction in a two dimensional composite with functionally graded heat properties were obtained. Numerical solutions for the taken boundary conditions are shown for several types of changes of composite's thermal conductivity. The solutions were obtained with the use of the finite-difference method. [ABSTRACT FROM AUTHOR]

In this paper an investigation on thermal conductivity of mortar, concrete and its major constituent aggregate is presented. Four different types of rocks were tested for their conductivities at dry and fully saturated states, which were further crushed and used as coarse aggregates in concrete. Thermal conductivity of mortar and concrete have been measured at various moisture contents. The applicability of Campbell-Allen and Thorne''s model in assessing the influence of aggregate type has been investigated. Using a few advanced models for thermal conductivity of porous material, it has been demonstrated that there exists a possibility to gain insight into the nature of the pore that is likely to be encountered in the concrete. The relationship between concrete conductivity and aggregate conductivity at various degrees of saturation including at dry and fully saturated conditions have been developed. Further, equivalent conductivity of solid, which is a necessary input parameter for porous material models, has been evaluated for all the types of concrete under consideration. [Copyright &y& Elsevier]

Abstract: A new method is proposed for estimating temperature-dependent thermal properties using solutions to transient inverse heat conduction problems. The prior information on the functional form of the thermal properties is not necessary for the proposed approach. The unknown thermal property is treated as the optimization variable, and the errors to be minimized are the differences between the calculated temperatures and the measured ones. The least-squares method is employed for the solution of the ill-posed inverse problem. The main contribution of this work is to introduce the complex-variable-differentiation method into transient inverse heat conduction problems, for the calculation of sensitivity coefficient. The inverse estimations of the heat capacity, the thermal conductivity and the thermal diffusivity are reported. Examples are given to demonstrate the effectiveness, efficiency and accuracy of the inverse approach as well as the potential to engineering applications. The effects of measurement errors on the inverse results are also investigated. [Copyright &y& Elsevier]

In this article, the nonlinear flexural analysis of perforated functionally graded composite panel is performed under heat conduction and uniform/sinusoidal pressure. Here, functionally graded composite panel is modeled with single (1 × 1) and multiple (2 × 2, 3 × 3) perforations having identical surface area. A geometrically nonlinear mathematical model is adopted using the Green-Lagrange strain field via the higher-order shear-deformation mid-plane kinematics. The overall temperature-dependent elastic and thermal properties of the functionally graded (metal/ceramic) material are computed using the power-law function via Voigt's homogenization scheme. The equilibrium equations are derived through the minimum total potential energy principle via isoparametric finite element approximations and solved further by adopting Picard successive technique. The convergence and validation of the model are carried out by performing mesh refinement and comparison tests, respectively. In adding, new numerical results for single/multiple perforated FGM panels are illustrated at various sets of parametric combinations, and compared in detail. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of mix design factors on the thermal properties of Superpave asphalt concrete. The thermal properties were measured using a guarded testing device (k-alpha device) developed recently at the University of New Brunswick. Two aggregate types, three gradations (determined by the material retained on a 4.75 mm sieve), and four compaction levels (at 35, 70, 100, and 160 gyrations) were investigated. The aggregate was 12.5 mm maximum nominal size with PG58-34 asphalt binder used for all mixes. The findings show that the thermal conductivity of the asphalt concrete ranged from 1.7 to 2.1 W/m·K. The specific heat capacity varied with the aggregate source and ranged from 940 to 2000 J/kg·K. The thermal diffusivity was found to vary with aggregate source. The aggregate type was found to have the most significant effect on the thermal properties. The compaction level had an effect but not a statistically significant one. The range of thermal properties determined in this paper is comparable to that found in the literature. [ABSTRACT FROM AUTHOR]

An analytical model to predict the extent of the heat-affected zone in laser-welded dissimilar materials with strongly differing thermal properties is presented. The model applies to the joining of two materials, where the heat-affected zone occurs in the joining partner with low thermal responsivity. Heat is supplied to the joining partner with high thermal responsivity and heat conduction is the only mechanism of heat transfer. The model is applied to the laser welding of solid nickel sheets to sheets of nickel foam. With a deviation of less than 1.5%, the extent of the heat-affected zone predicted by the model is in excellent agreement with the ones determined experimentally. [ABSTRACT FROM AUTHOR]

This comprehensive study provides an in-depth examination of the convective drying process and drying kinetics of ginger rhizomes (Zingiber officinale), precisely honing in on the influence of moisture content variation on the thermal property of thermal conductivity. Our research reveals a direct correlation between decreasing moisture content and thermal conductivity during drying, conducted under meticulously controlled conditions with a drying temperature range of 10 °C to 60 °C and an optimum drying temperature identified at 60 °C with a relative humidity of 35%. We scrutinise the thermal properties, namely, the thermal conductivity, in relation to moisture content, shedding light on the intricate dynamics involved. The study uncovers the distinct advantage of convective drying over traditional methods, shortening the drying time to just 24 h, compared to the nine and eight days required for open sun and solar tunnel drying, respectively. We identified optimal moisture levels for various ginger types: unblanched (6.63%, thermal conductivity 0.0553 W / m · K) , blanched (9.04%, thermal conductivity 0.0516 W / m · K ), peeled (8.56%, thermal conductivity 0.0483 W / m · K ), and unpeeled ginger (5.98%, thermal conductivity 0.0460 W / m · K ). As drying progressed, the moisture content fell from 81% to approximately 6–9%, concomitantly lowering the thermal conductivity from roughly 0.0553 W / m · K to around 0.0460–0.0516 W / m · K . These findings offer significant implications for the food industry, proposing improvements in drying processes and strategies for energy conservation when drying ginger rhizomes and similar agricultural produce. Moreover, this study sets a solid foundation for future investigations into potential applications of these insights to other agricultural products and various drying techniques. [ABSTRACT FROM AUTHOR]

Thermoelectric power generation is an important technology in the field of energy conversion, which contributes to the goal of carbon neutralization. The prediction accuracy of thermoelectric generator's (TEG) performance is critical to optimize the design and carbon emission evolution. In this paper, a novel one-dimensional model is developed with comprehensive consideration of the Joule heat, conduction heat, Thomson heat, temperature-dependent properties and side leg dissipation. The numerical solution of the proposed model is compared with the available experimental data and theoretical results. The comparison between the proposed model and experimental data for output power and efficiency of TEG are 3.0–12.0% and 0.9–17.5% respectively, when the temperature difference is within 68–149 K. This implies that the developed model exhibits an improved accuracy, as the published models commonly possess a deviation greater than 20% for the temperature difference higher than 103 K. It is also found that the temperature-dependent properties and side leg heat dissipation have a pronounced effect on TEG performance, when the temperature difference exceeds 68 K. The most influential factor leading to the uneven distribution of temperature and heat flux is the temperature-dependent properties of thermoelectric material, and followed by Thomson heat. • A novel model was proposed for performance calculation of thermoelectric leg. • The model takes the temperature-dependent material properties into account. • It takes the side leg heat dissipation into account. • The developed model exhibits an improved accuracy for prediction. [ABSTRACT FROM AUTHOR]

In this paper, a theoretical model of temperature variations is derived for a two-layer optically absorbing structure, including thermal memory effects. It is considered that the two-layer structure is surrounded by gas and illuminated on the front side by a harmonically modulated laser beam. This model is based on the hyperbolic theory of heat conduction and Beer–Lambert's law of absorption neglecting multiple optical reflections in each layer. The derived model represents the generalization of the current models in two aspects. First, the influence of thermal memory properties of both layers is accounted for, and second, both layers are regarded as volume absorbers of the incident light. Based on the derived model, the expressions for surface temperature variations are given and discussed for the special type of two-layer structures, irradiated on the coated side, which is a prominent configuration in photoacoustic and photothermal experiments. It is shown that there exists a frequency range in which the influence of the thermal properties of the coating cannot be neglected, especially at the high-frequency range in which thermal memory of coating becomes significant. It is an important result in terms of understanding experimentally measured photothermal and photoacoustic response and, consequently, accurate characterizations of various high optically reflected and/or optically transparent samples by using these experimental techniques. [ABSTRACT FROM AUTHOR]

Abstract: The continuous increase in the utilization of polymeric materials in many useful short-life applications has demanded the attention of researchers worldwide concerning the solid waste generation and, as an immediate consequence, the generalized environmental pollution. The main alternative for controlling this environmental impact is the polymer recycling process. However, one of the biggest challenges of this process is to keep the polymer performance within levels, which permit their reuse in the same or in other specific important technological applications. In this work, the laser flash technique is employed to study the effect of recycling on the thermal properties of some selected polymers. Since the thermal diffusivity expresses how fast heat propagates across a bulk material and the thermal conductivity determines the working temperature levels of a material, it is possible to assert that those properties are important when a polymer is used as an insulator and also when it is used in applications where heat transfer is desirable. Five sets of virgin and recycled commercial polymers widely used in many applications, including food wrapping, were selected for this study. Disc-shaped samples 10mm diameter and 0.3–1mm thickness were prepared by hot pressing the polymer pellets or powder, or by cutting discs from long cylindrical bars. Measurements were carried out from room temperature up to approximately 50°C above the polymer crystalline melting point. Experimental results show different behavior for the thermal diffusivity of recycled polymers when compared with the corresponding virgin material. [Copyright &y& Elsevier]

In Atlantic bottlenose dolphins Tursiops truncatus, both the thickness and lipid content of blubber vary across 0ntogeny and across individuals of differing reproductive and nutritional status. This study investigates how these changes in blubber morphology and composition influence its thermal properties. Thermal conductivity (W m-1 deg.-1, where deg. is °C) and thermal insulation (m² deg. W-1) of dolphin blubber were measured in individuals across an ontogenetic series (fetus through adult, N=36), pregnant females (N=4) and emaciated animals (N=5). These thermal properties were determined by the simultaneous use of two common experimental approaches, the heat flux disc method and the standard material method. Thickness, lipid and water content were measured for each blubber sample. Thermal conductivity and insulation varied significantly across ontogeny. Blubber from fetuses through sub-adults was less conductive (range=0.11–0.13±0.02 W m-1 deg.-1) than that of adults (mean=0.18 W m-1 deg.-1). The conductivity of blubber from pregnant females was similar to non-adult categories, while that of emaciated animals was significantly higher (0.24 ± 0.04 W m deg.-1) than all other categories. Blubber from sub-adults and pregnant females had the highest insulation values while fetuses and emaciated animals had the lowest. In nutritionally dependant life history categories, changes in blubber's thermal insulation were characterized by stable blubber quality (i.e. conductivity) and increased blubber quantity (i.e. thickness). In nutritionally independent animals, blubber quantity remained stable while blubber quality varied. A final, unexpected observation was that heat flux measurements at the deep blubber surface were significantly higher than that at the superficial surface, a pattern not observed in control materials. This apparent ability to absorb heat, coupled with blubber's fatty acid composition, suggest that dolphin integument may function as a phase change material. [ABSTRACT FROM AUTHOR]

A double base propellant is a polymer which is used as an energetic material to propel rockets. Advances in computers are making it possible to simulate a rocket as a whole, i.e. structurally and thermally. Among the rocket elements, little literature exists regarding the thermal and mechanical properties of propellants at a specified environment. In this study, a double base propellant is produced and then characterised. Accordingly, tensile specimens of the propellant are prepared according to JANNAF specifications to determine its mechanical properties. The tests are carried out at a possible 74°C and also at varying crosshead speeds of 1·3, 6 and 30 mm min-1. As of the thermal properties, the heat conduction coefficient, the specific heat and the density have been determined. All these properties complete the basic data for a thermomechanical simulation of a rocket on computer. [ABSTRACT FROM AUTHOR]

In this paper, in order to improve the electrical and thermal properties of SiC/EP composites, the methods of compounding different crystalline SiC and micro-nano SiC particles are used to optimize them. Under different compound ratios, the thermal conductivity and breakdown voltage parameters of the composite material were investigated. It was found that for the SiC/EP composite materials of different crystal types of SiC, when the ratio of α and β silicon carbide is 1:1, the electrical performance of the composite material is the best, and the breakdown strength can be increased by more than 10% compared with the composite material filled with single crystal particles. For micro-nano compound SiC/EP composites, different total filling amounts of SiC correspond to different optimal ratios of micro/nano particles. At the optimal ratio, the introduction of nanoparticles can increase the breakdown strength of the composite material by more than 10%. Compared with the compound of different crystalline SiC, the advantage is that the introduction of a small amount of nanoparticles can play a strong role in enhancing the break-down field strength. For the filled composite materials, the thermal conductivity mainly depends on whether an effective heat conduction channel can be constructed. Through experiments and finite element simulation calculations, it is found that the filler shape and particle size have a greater impact on the thermal conductivity of the composite material, when the filler shape is rounder, the composite material can more effectively construct the heat conduction channel. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic, transport and thermal properties of single crystal Co2FeGa have been investigated. The small coercivity 20Oe and saturation field 4000Oe of Co2FeGa sample at temperature 5K indicates that the single crystal is magnetically soft. The resistivity (ρ) behaves according to ρ ∼ T 1/2 power law below temperature T =40K, which is due to electron–electron interaction effects in the presence of disorder. The thermal conductivity of Co2FeGa single exhibits anomalous temperature dependence above 50K, i.e., the conductivity increases with the temperature, or dκ/dT >0. We conclude that this anomalous thermal conductivity is due to the strong atomic disorder between the Fe and Co atoms. [Copyright &y& Elsevier]