Solid Conductivity Research Articles

Thermal conductivity analysis of natural fiber-derived porous thermal insulation materials

Natural fibers, derived from plants such as wood, hemp, straw and cotton, have been explored for the fabrication of porous structures for thermal insulation applications due to their widespread availability, sustainability, and cost-effectiveness. Understanding the fundamental heat transfer mechanisms within natural fiber-derived porous structures is crucial for both optimized geometric design and real-world insulation applications. Herein, we developed a theoretical framework considering geometric parameters, including pore size, fiber diameter and porosity (i.e., density), to examine the contribution of various heat transfer modes (i.e., conduction, convection, and radiation) on the effective thermal conductivity of porous structures derived from natural fibers. Our results indicate that thermal radiation is largely responsible for the rapid increase in effective thermal conductivity of the natural fiber-derived porous insulations in low-density regions (< 50 kg/m3) and that natural convection rarely occurs within these materials. The insulation materials derived from natural fibers with diameters in the micron range (5–50 μm) can achieve their minimum thermal conductivity at an optimal density of 50–90 kg/m³. Effective strategies to lower the effective thermal conductivity of natural fiber-derived porous materials include increasing porosity to curtail solid conduction, incorporating nanoscale pores by using nanosize fibers to diminish gaseous thermal conductivity. This research offers valuable insights into the heat transfer mechanisms in natural fiber-derived materials and should guide the structural design and optimization process toward developing super-thermal insulation materials derived from natural fibers.

A coupled model of finite element method and Mie theory for heat transfer inside expanded perlite vacuum insulation panels (VIPs) at high temperatures

The present study introduces the first coupled model of the finite element method and Mie theory that predicts the heat transfer mechanism inside the expanded perlite vacuum insulation panels at a temperature range of 300–800 K. The finite element method is applied to discretise the governing equation of the steady state heat transfer and the three-dimensional Fourier's law is used. COMSOL Multiphysics is the finite element numerical solver which predicts solid and gaseous thermal conductivity inside the VIPs. Moreover, Mie theory predicts the radiative conductivity. Heat transfer radiation is superimposed on the heat transfer through conduction. The model is capable of predicting total thermal conductivity of VIPs at high temperatures ≥ 500 K, which makes it a unique model with clear advantage over previously developed numerical models. The numerical predictions of the developed coupled model are validated experimentally. The predictions of the coupled model are in good agreement with the experimental measurements, which indicate the capability of the developed model to predict the mechanisms of heat transfer at high temperatures. For instance, the values of total thermal conductivity for numerical prediction and experimental measurement, at 557 K, are 14.6 and 14.7±1.2 mW/m/K, respectively, excellently agreeing within ± 8.9 %.

Layered Inorganic Silicate Aerogel Pillared by Nanoclusters for High Temperature Thermal Insulation.

Layered inorganic material, with large-area interlayer surface and interface, provides an essential material platform for constructing new configuration of functional materials. Herein, a layered material pillared with nanoclusters realizing high temperature thermal insulation performance is demonstrated for the first time. Specifically, systematic synchrotron radiation spectroscopy and finite element calculation analysis show that ZrOx nanoclusters served as "pillars" to effectively produce porous structures with enough boundary defect while maintaining the layered structure, thereby significantly reducing solid state thermal conductivity (≈0.32Wm-1 K-1 , 298-573K). Moreover, the layered inorganic silicate material assembled aerogel also exhibits superior thermal insulation performance from room temperature (0.034Wm-1 K-1 , 298K, air conditions) to high temperature (0.187Wm-1 K-1 , 1073K, air conditions) and largely enhanced compressive strength (42kPa at 80% compression), which is the best layered material-based aerogel that has achieved synergistic improvement in thermal and mechanical performance so far. Layered inorganic silicate aerogel pillared by nanoclusters will pave a new avenue for the design of advanced thermal insulation materials under extreme conditions.

Revised Centrality Measures Tell a Robust Story of Ion Conduction in Solids.

The three most commonly used centrality measures in network theory have been adapted to consider ion conduction time rather than the number of steps. Flow-IN centrality highlights sites with the largest flow of ions from the nearest neighbor sites. Return-flow centrality highlights sites with a fast rate of first returns for the conducting ion. Flow-through centrality highlights which sites support significant flow of conducting ions and appears more robust to removal of the most central vertices. Exploring these centrality measures with the sample system of proton conduction in yttrium doped barium zirconate shows flow-through centrality to provide a robust picture with high contrast between sites involved in the most probable long-range periodic conduction paths and kinetic Monte Carlo trajectories versus sites rarely visited. The flow-through centrality, including all paths further highlights that when the most central proton site is filled, the remaining highest flow-through centrality sites are nearby, corroborating earlier studies suggesting proton pair motion. Finally, while both return-flow and flow-through centrality measure images deteriorate with noise, image restoration is possible when a detailed balance is used to calculate the smaller rate constant in a forward/backward pair.

BEM for transient nonhomogeneous heat conduction in axisymmetric solids with heat source

Boundary equation approach is revisited and extended to be applicable for axisymmetric transient heat transfer applications with spatially varying field variables and heat sources. For the derivation of axisymmetric formulation, three-dimensional fundamental solutions are integrated with respect to the circumferential direction and then the radial integration procedure using the stabilized temperature values is implemented to convert axisymmetric plane integrals to axisymmetric line integrals. All integrals are evaluated numerically. Four Numerical examples are given in order to assess the reliability of the proposed formulation.

The importance of solid phase longitudinal conduction in flame spread over cylindrical and flat fuels: A comparative scale analysis

This work presents a scale analysis to explore the role played by forward (longitudinal) solid conduction in the mechanism of opposed-flow flame spread over flat and cylindrical fuels. Closed-form expressions in terms of known parameters and properties are derived for a non-dimensional conduction number, which compares solid phase forward conduction with the gas to solid conduction in the preheat zone. The corresponding expressions for cylindrical fuels are related to the flat fuel expressions using a curvature factor that depends on fuel radius and flow velocity. The results of the analysis are compared to existing experimental data for flame spread over thick flat and cylindrical fuels. For thin fuels, a comprehensive numerical model is used to evaluate the conduction number for comparison with the prediction of the analysis. An energy balance at the leading edge, taking into account the longitudinal conduction through solid, produces a spread rate formula that is related to the well-known thermal limit through the conduction number.

Heat Transfer in Solid by Using Linear Differential Equation

Abstract: Differential equations are fundamental importance in engineering mathematics because any physical law s and relations appear mathematically in the form of such equations. . The rate of heat conduc- tion in a specified direction is proportional to the temperature gradient,which isthe rate of change in temperature with distance in that direction. In this paper we discussed about first order linear homogeneous equations, first order linear non homogeneous and the application of first order differential equationto heat transfer analysis particularly in heat conduction in solids.

Investigating the impact of modeling assumptions and closure models on the steady-state prediction of solar-driven methane steam reforming in a porous reactor

In this work, the effect of different modeling assumptions and closure models — frequently considered in the literature of solar thermochemical reactors and closely-related areas — is investigated. The application of different modeling assumptions and closure models may strongly affect the results accuracy and compromise a meaningful comparison across literature works. The following is herein investigated: (i) the role of upstream and downstream fluid regions to the two-phase reactor region; (ii) relevance of species and gas-phase thermal diffusion mechanisms; (iii) reaction heat accounted for in gas- or solid-phase energy balance equations; (iv) local thermal equilibrium (LTE) vs. local thermal non-equilibrium (LTNE) models; (v) model dimension (one-dimensional vs. two-dimensional axisymmetric models); (vi) local volumetric convection heat transfer correlations; and (vii) effective solid thermal conductivity correlations. The relevance of this work extends well beyond the current application (methane steam reforming in a volumetric solar reactor), since similar models and assumptions have also been widely applied for predicting the performance of volumetric solar absorbers and dry reforming solar reactors. The results show that an upstream fluid region should be considered while applying inlet first-type boundary conditions and diffusion transport in the corresponding governing equations to ensure full-conservation and simultaneously to account for developing profiles upstream the reactor inlet section. For the operating conditions considered, species diffusion and gas-phase heat conduction are particularly relevant at the reactor centerline but with a negligible integral effect. A significant difference in the reactor performance is observed while accounting for the reaction heat from surface reactions in the solid- or gas-phase energy balances — for the lowest inlet gas velocity herein considered, the thermochemical efficiency (methane conversion) is approximately equal to 70.8% (76.3%) and 77.3% (84.7%) assigning the reaction heat to the gas- and solid-phase energy balances, respectively. LTE model results are strikingly different from the results obtained with the LTNE model considering the reaction heat accounted for in the gas-phase energy balance but not significantly different from the results computed with the LTNE model with the reaction heat assigned to the solid-phase energy balance. One-dimensional reactor modeling provided with an average concentrated solar heat flux value results in a similar average performance as that given by a two-dimensional model but fails to predict the high temperatures observed at reactor centerline — for the lowest inlet gas velocity, the difference between the maximum solid temperatures predicted by one- and two-dimensional models is about 340K.

Simulation of Impedance Changes with Aging in Lithium Titanate-based Cells Using Physics-Based Dimensionless Modeling

Quantifying aging effects in lithium-ion cells with chemistries that have a flat open circuit potential is challenging. We implement a physics-based electrochemical model to track changes in the electrochemical impedance response of lithium titanate-based cells. Frequency domain equations of a pseudo two-dimensional model are made dimensionless, and the corresponding non-dimensional parameters are estimated using a Levenberg-Marquardt routine. The model weighs the relative contributions of changes in diffusion, ionic conduction within the electrolyte phase against solid phase electronic conduction towards cell aging. Solid-phase diffusion, charge transfer resistance and double layer capacitance at the solid-liquid interface are accounted for in the particle impedance. The estimation routine tracks dimensionless parameters using accelerated cycling data from full cells over 1000 cycles. The model can be deployed within a short time for state estimation using physics-based models without requiring prior knowledge of the battery chemistry, format, or capacity.

Fundamentally new approaches to solving thermophysical problems in the field of nanoelectronics

Currently, there is a rapid development of thermophysics of solids associated with the need of creating models with a high degree of predictive reliability. This paper presents new approaches to solving relevant issues related to the study of heat transfer in semiconductors and dielectrics, mainly concerning nano-structures. The first of the considered tasks is the creation of a statistical model of the processes of interaction of heat carriers – phonons – with rough surfaces of solids. For the first time authors proposed a method based on the statistics of the slopes of the profile of a random surface. The calculation results are the mean free paths of phonon between the opposite boundaries of the sample, which are necessary for calculating the effective thermal conductivity in ballistic and diffusion-ballistic regime of heat transfer, depending on the roughness parameters. The second task is to develop methods for calculating the processes of heat transfer through the contact surfaces of solids. We were able to show that, taking into account the phonon dispersion and the corresponding restrictions on the frequency values, the modified acoustic mismatch model for calculating Kapitsa resistances can be extended to temperatures above 300 K. Previously, the limit of applicability of this method was considered to be a temperature of 30 K. Moreover, the proposed method is also generalized to the case of rough interfaces. The third task is a new approach to determining the thermal conductivity of solids. The authors have developed a method of direct Monte Carlo simulation of phonon kinetics with strict consideration of their interaction due to the direct use of the laws of conservation of energy and quasi-momentum. The calculations of the thermal conductivity coefficient for pure silicon in the temperature range from 100 to 300 K showed good agreement with the experiment and ab initio calculations of other authors, and also allowed us to consider in detail the kinetics of phonons.

Experimental validation and tightly coupled numerical simulation of hot air anti-icing system based on an extended mass and heat transfer model

When an aircraft flies through a cloud containing supercooled liquid water droplets, ice accretion may deposit, posing a threat to flight safety. Hot air anti-icing is one of the most widely used anti-icing techniques. An extended mass and heat transfer model for runback water on a hot air anti-icing surface is developed. The model is based on the mass conservation and energy conservation of continuous water film and rivulet water film, considering the conditions under which continuous water film may break up into rivulet water film. Additionally, the effect of water vapor resulting from liquid water evaporation is also taken into account. Then, a three-dimensional tightly coupled numerical method for hot air anti-icing systems is constructed based on the developed model. The method includes modules for the calculation of external cold air flow, solid skin heat conduction and internal hot air flow, the calculation of water droplet impingement, and the calculation of anti-icing thermodynamics. The accuracy of the method is validated against experimental studies on an enhanced heat transfer piccolo tube anti-icing system in the icing wind tunnel of China Aerodynamics Research and Development Center (CARDC). The temperature results obtained from the three-dimensional numerical simulations showed good agreement with the experimental results, indicating that this method can be utilized in preliminary and detailed designs to evaluate the thermal performance of hot air anti-icing systems.

Strong acoustic-optical phonon coupling in high-performance thermoelectric material Bi0.5Sb1.5Te3

Understanding the origin of intrinsic lattice thermal conductivity in crystalline solids is essential for the optimization of thermoelectric properties. Here, we investigate the phonon dynamics of Bi0.5Sb1.5Te3 by means of Raman scattering, specific heat measurement, and first-principles calculations. All the Raman modes display the three-phonon scattering processes below 490 K. The Boson peak of the low-temperature specific heat and the avoided-crossing of the phonon dispersion together reveal the existence of strong coupling between low-lying optical branches and acoustic branches. The vibration of Bi atoms is responsible for this acoustic-optical phonon coupling. Specifically, the lower group velocity, larger scattering space, larger scattering rate, and larger Grüneisen parameter near the avoided-crossing frequency lead to the lower lattice thermal conductivity of Bi0.5Sb1.5Te3. This work provides new insights for understanding the thermal properties of Bi2Te3-based compounds.

Effective thermal conductivity of composite strut-based hollow periodic cellular solids

Effective thermal conductivity of composite strut-based hollow periodic cellular solids

ДИЭЛЕКТРИКТЕРДІҢ ЭЛЕКТРЛІК БЕРІКТІГІ

Abstract. In human life, even in practice, solids play an important role. Metals, dielectrics, electrical engineering, semiconductors, electrons, magnets, conductors lying close to each other are all solids. It can be argued that the basis for the foundation of scientific and technological progress is a solid body. But in their study, not only practical activity mattered. For its subsystems of development, solid state physics has opened a scientific path to understanding the properties of importance. all materials used in engineering are divided into three groups: conductors, semiconductors and dielectrics. These materials differ in the magnitude of electrical resistance, the nature of its changes during heating and the type of conductivity.The electrical resistivity of the conductors is in the range of 10-6 + 10-3 oms • cm and is used as a material with a small intermediate electrical resistance for resistance elements, heating elements, contacts, etc. for conducting constant and rotational currents. Electrical resistivity of semiconductors 10-3+10+10 Om* is within cm and decreases when heated. They are used for rectification, amplification, conversion of various types of energy into electricity. Electrical resistivity of dielectrics 10+10+10+18 Oms* are within cm, they are used as insulators. The conductivity of solids is determined primarily by the electronic structure. In solids, as a result of the interaction of the electromagnetic field of atoms, energy electronic sublevels are classified with the formation of an energy zone [1]. With the transition to a higher energy, the width of the zone of permissible sublevels increases, and the zones intersect. When approaching the distance between atoms Forbidden energy zones even disappear. The density of the zones filled with electrons and their crosslinking determine the electrical conductivity of solids. Keywords: dielectric, composition, automation, electronics, radio engineering, aerospace, rocket, electrical, conductor.

Directional enhancement and potential reduction of thermal conductivity induced by one-dimensional nanoparticle addition in pure PCMs

To increase the thermal conductivity of organic phase change material (PCM), One-dimensional materials are often used as an additive due to their excellent thermal performance. Many experiments have shown that micro effects have a great influence on the thermal conductivity of composite materials. To clarify the micro effect, numerous molecular dynamics simulations (MD) have been carried out. Previous research has entirely overlooked the thermal conductivity changes of pure PCM in composite systems. Neglecting the thermal conductivity changes of pure PCM is unreasonable, as the mass fraction of nanoparticles is often small. We took paraffin wax (PW) with carbon nanotube (CNT) as an example, and successfully reproduced the phase transition of composite PCM. Then we optimized the details of simulating thermal conductivity, and two processes of replication and movement were taken to get a stable thermal conductivity. A disassembly of thermal conductivity based on heat flux was actualized to investigate the thermal conductivity of pure PCM in composite systems. We investigated the difference in thermal conductivities between solids and liquids and analyzed the underlying mechanism through vibrational changes. The change of thermal conductivity in solids is contributed by CNT itself and the structure of PW; the change of thermal conductivity in liquids is contributed by CNT itself and the interaction of CNT. Among these factors, structure change wouldn't always be beneficial. CNT always induces an axial enhancement of thermal conductivity in pure PCMs. At last, verification was carried out to prove the importance of structure. Our study brought a new view to understanding the behavior of thermal conductivity in composite PCM.

Conjugate local thermal nonequilibrium and non‐Darcy flow inside porous enclosure: Analysis of localized heating and cooling arrangements

AbstractThis study covers a simulation on conjugate free convective in a porous enclosure containing a side wall thickness and partially heated and cooled from sides under the considerations of local thermal nonequilibrium (LTNE) and non‐Darcy flow. Interest has been focused on how the side wall thickness and the locations of cooled and heated parts affect the effectiveness of the Nusselt number (Nu). Three different cases of localized heating and cooling locations have been implemented for the following ranges: scaled heat transfer coefficient (), wall to fluid thermal conductivity ratio (), modified Rayleigh number (), wall width (), inertial parameter (), and thermal conductivity ratio (). Outcomes show that and the locations of cooled and heated parts have remarkable impacts on all the Nusselt numbers. The intensity of LTNE region considerably relies on , and . The total average NuT is highly dependent on , , , , and as compared to H. The increase in leads to change of the convective mechanism to conductive mode. The rise in guides to increase Nu, where can control the flow strength. The actions of on Nuf is more evident than Nus. For low H and Kr, the size of LTNE zone is considerably affected by H as compared to Kr although Kr has a high influence on Nu. For high Kr and H, the LTNE zone has closely vanished. Findings display that the Case 2 provided the highest Nu for all tested parameters except the case of . Finally, it is evident that for the problems that employed solid conduction wall with localized heating and cooling sections, Case 2 is recommended for future use in the applications that implement a porous medium and depend on free convection.

Heat energy transport characteristics of microchannel reactors for hydrogen production by steam-methanol reforming on copper-based catalysts

Numerical simulations are carried out to understand the heat energy transport characteristics of microchannel reactors for hydrogen production by steam-methanol reforming on copper-based catalysts. Enthalpy analysis is performed and the evolution of energy in the oxidation and reforming processes is discussed in terms of reaction heat flux. The effects of solid thermal conductivity, gas velocity, and flow arrangement on the thermal behavior of the reactor is evaluated in order to fully describe the thermal energy change in the reactor. The results indicate that the thermal behavior of the reactor depends upon the thermal properties of the walls. The change in enthalpy is of particular importance in exothermic and endothermic reactions. The net enthalpy change for oxidation and reforming is negative and positive, but the net sensible enthalpy change is always positive in the reactor. The wall heat conduction effect accompanying temperature changes is important to the autothermal design and self-sustaining operation of the reactor. The solid thermal conductivity is of great importance in determining the operation and efficiency of the reactor. The reaction proceeds rapidly and efficiently only at high solid thermal conductivity. The reaction heat flux for oxidation and reforming is positive and negative. The change in flow arrangement significantly affects the reaction heat flux in the reactor. The parallel flow design is advantageous for purposes of enhancing heat transfer and avoiding localized hot spots.

Recent Progress in Testing and Characterization of Hardenability of Aluminum Alloys: A Review.

In this paper, the progress of the test methods and characterization approaches of aluminum alloys hardenability was reviewed in detail. The test method mainly included the traditional end-quenching method and the modified method. While the characterization approaches of alloy hardenability consist mainly of ageing hardness curves, solid solution conductivity curves, ageing tensile curves, time temperature transformation (TTT) curves, time temperature properties (TTP) curves, continuous cooling transformation (CCT) curves, and advanced theoretical derivation method have appeared in recent years. The hardenability testing equipment for different tested samples with different material natures, engineering applications properties, and measurement sizes was introduced. Meanwhile, the improvement programmed proposed for shortcomings in the traditional hardenability testing process and the current deficiencies during the overall hardenability testing process were also presented. In addition, the influence factors from the view of composition design applied to the hardenability behaviors of Aluminum alloys were summarized. Among them, the combined addition of micro-alloying elements is considered to be a better method for improving the hardenability of high-strength aluminum alloys.

A 3D simulation model of thermal runaway in Li-ion batteries coupled particles ejection and jet flow

A coupled simulation model of the 18650 lithium-ion batteries (LIB) thermal runaway (TR) is presented in this study, which includes TR decomposition reaction, gas generation and combustion processes, solid particles ejection and particles heat transfer process. The model considers a combination of solid heat conduction, gas convection, flame and particles radiation, which is validated to accurately capture the temperature evolution and two typical jet processes during TR. Model validation conducts with experimental measurements for the temperature of the battery surface. The simulation results show that the “ignition” time of the flammable gas mixture is delayed and the rate of flame temperature increase is slowed down when the effect of solid particles is considered. The radiation heat transfer rate and convection heat transfer rate on the adjacent battery surfaces are 80.3W and 12.76W, and are approximately 4.29 times and 1.76 times larger than the results calculated by the model without solid particles, respectively. The difference between these two can be further magnified in confined space. The model developed in this study combines the effect of the solid particles’ radiation into the TR simulation innovatively. It can provide a more accurate calculation method for the prediction of TR propagation.

Solute segregation affected by transport properties during a bubble entrapment in upward solidification

Abstract The effects of transport properties on segregation of a dissolvable gas, for example, carbon dioxide in water in the presence of an entrapping bubble during upward unidirectional solidification is numerically investigated. Microporosity and inter-related solute segregation are two common defects detrimental to mechanical properties of components, especially to fatigue performance, strength and ductility. In this work, transport processes between the bubble, liquid and solid affected by mass, momentum, energy, and concentration Eqs. are solved by finite element schemes provided from the commercial COMSOL code. The results find that solute concentration in solid and liquid surrounding the pore increases with decreasing Henry's law constant and gravitational acceleration, and increasing surface tension and solid thermal conductivity. Solute gas concentration on the top free surface is insensitive to that in the pore rather than that in liquid and solid away from the pore. Apex cap becomes non-spherical affecting solidification rate and shape of the solidification front as ambient pressure at the top free surface and gravitational acceleration increase. Contact angle versus location of the solidification front predicted from this work and Abel's Eq. agrees well. Solute segregation encountered by pore formation is controllable.