Change In Effective Thermal Conductivity Research Articles

Numerical simulation on the influence of metal radiation shield on the thermal insulation performance of the semi-transparent materials

Foams, flexible felt and thermal barrier coatings are widely used in the thermal insulation fields. This type of material has lower density and thermal conductivity, as well as higher specific surface area and porosity, and is generally referred to as the semi-transparent materials. The radiation heat transfer inside the semi-transparent materials belongs to medium radiation, so the radiation thermal conductivity is larger at high temperature. However, the metal radiation shields (MRSs) play an important role in weakening radiation heat transfer. In order to study the effect of MRSs on the thermal insulation performance of the semi-transparent materials, the numerical simulation of coupling heat transfer of heat conduction and heat radiation is carried out. The effective thermal conductivity (ETC) of the semi-transparent materials with different extinction coefficient is calculated when the different layers of MRSs are evenly inserted into the semi-transparent materials at the specific temperature, so that the influence of MRSs on the thermal insulation performance of the semi-transparent materials can be obtained. The simulation results show that whether it is an optical thin medium or an optical thick medium, when 7 layers of MRSs are inserted into the semi-transparent materials, the thermal insulation performance is greatly improved. After that, the ETC changes little with the increase of the layer of MRSs.

Topology Morphing Insulation: A Review of Technologies and Energy Performance in Dynamic Building Insulation

Topology morphing insulation enables the on-demand switching of thermal properties between insulative and conducting states through shape change. The adaptive nature of these systems allows them to regulate heat transfer by dynamically altering insulation materials or systems in response to changing conditions, including environmental factors, electrical grid dynamics, and occupant requirements. In this article, we highlight the potential of topology morphing insulation for advancing building envelope design, improving energy efficiency, and facilitating on-demand adjustments in effective thermal conductivity. We provide a comprehensive overview of topology morphing insulation, delving into its underlying principles, mechanisms, and potential applications. This review explores cutting-edge research and the potential application of insights from non-building concepts, such as nature, textiles, and origami. Additionally, it examines crucial aspects such as actuation mechanisms, effectiveness, lifecycle considerations, sustainability implications, and manufacturing feasibility. We discuss the potential benefits and challenges associated with implementing topology morphing insulation solutions. Thanks to its transformative capabilities, topology morphing insulation holds tremendous promise for advancing building envelope design, driving energy efficiency improvements, and facilitating responsive changes in effective thermal conductivity.

Molecular Dynamics Study of Energy Transport Mechanism in Nanofluids: Spatial and Component Decompositions of Effective Thermal Conductivity

Nanofluids, which are nanoparticle-suspended fluids, have gathered extensive attention for decades. Yet, the energy transport mechanism of nanofluids has not been comprehensively understood. The present study employs spatial and component decompositions of the effective thermal conductivity (ETC), and numerically investigates the energy transport mechanism of nanofluids, with a focus on influence of liquid layering (i.e., the adsorption layer of the fluid molecules) around nanoparticles. The local ETC of the nearest adsorption layer increases as the nanoparticle wettability improves, but this value has a certain upper limit. Compared with the case of the nearest adsorption layer, the local ETC of the liquid, except for the adsorption layers, gently increases as the nanoparticle wettability improves. However, a main contribution to the ETC component contributed by the liquid is the change in the local ETC of the liquid, except for the adsorption layers, since its volume fraction is relatively large. In conclusion, as a whole, the nanofluid’s ETC is mainly determined by the change in the ETC component contributed by the liquid.

Thermal conduction through a cool well

We study a differentially-heated square cavity with inlet and outlet ports at the top of the side walls, quantifying how the effective thermal conductivity changes with the temperature difference across the system. As the Richardson number, Ri, is increased, we observe the formation of a series of system-spanning vortices through the merging of Moffatt eddies growing from the bottom corners of the cavity. We find that the convection, which enhances the transport of heat, is strongly suppressed by increasing the temperature drop.

Heat transfer analysis of irradiation-induced gas bubble in tungsten from a fractal dimension perspective

The mechanism of heat transfer profoundly depends on the microscopic topological structure. In reality, the intrinsic nature of (un)-irradiated metals possesses the fractal characteristic that repeats a pattern at different scales and sizes regardless of the complexity. Herein we combine the conventional thermal conduction model with the analytical fractal solution in order to understand the effect of fractal parameter on estimating the proportion of each phase in dual- and triple-phase microstructures, the role of gaseous pressure in conducting heat, and the variation of effective thermal conductivity of the bubble-containing tungsten. The fractal dimension obtained from the differential box-counting (DBC) method is quantitatively compared with that from the classical box-counting (BC) method, and the relevance between two approaches in assessing the fractal dimension is analyzed and discussed. Importantly, the basic porosity-fractal dimension formula is qualitatively modified by introducing a regulatory factor to identify the microstructure with striking characteristic. This study provides evidence from the analytical fractal model that the effective thermal conductivity changes with grain size and bubble density. Meanwhile, the predicted results from the proposed model match well with the previous experimental data and the corresponding results predicted from the numerical finite element simulation. Focusing on the heat performance of (un)-irradiated material, the specific procedure allows us to extract the valuable data effectively and efficiently.

A Layer-Dependent Analytical Model for Printability Assessment of Additive Manufacturing Copper/Steel Multi-Material Components by Directed Energy Deposition.

Copper/steel bimetal, one of the most popular and typical multi-material components (MMC), processes excellent comprehensive properties with the high strength of steel and the high thermal conductivity of copper alloy. Additive manufacturing (AM) technology is characterized by layer-wise fabrication, and thus is especially suitable for fabricating MMC. However, considering both the great difference in thermophysical properties between copper and steel and the layer-based fabrication character of the AM process, the optimal processing parameters will vary throughout the deposition process. In this paper, we propose an analytical calculation model to predict the layer-dependent processing parameters when fabricating the 07Cr15Ni5 steel on the CuCr substrate at the fixed layer thickness (0.3 mm) and hatching space (0.3 mm). Specifically, the changes in effective thermal conductivity and specific heat capacity with the layer number, as well as the absorption rate and catchment efficiency with the processing parameters are considered. The parameter maps predicted by the model have good agreement with the experimental results. The proposed analytical model provides new guidance to determine the processing windows for novel multi-material components, especially for the multi-materials whose physical properties are significantly different.

ラティス体積分率分布最適化と金属積層造形による傾斜機能構造体の温度分布設計

In recent years, additive manufacturing (AM) has been used in industrial products for fabricating complex geometry. On the other hand, topological optimization is a suitable method for designing complex geometry. Therefore, there are many studies of combining AM and topological optimization these days. As a variant of topological optimization, density optimization of lattice structure is recently studied. The lattice structure is a characteristic one that includes void inside structure fabricated by AM. The designed model by topological optimization should be modified for a suitable structure for AM process. This research tried to use lattice volume fraction optimization for an industrial AM product. Firstly, we have developed a method of designing the lattice unit cell that its effective thermal conductivity is controlled by design variables. Effective thermal conductivity of lattice is calculated by homogenization method and finite element method (FEM). It was confirmed that the effective thermal conductivity changes depending on the size of the internal pores. Next, we consider the minimization problem of the surface temperature of the target domain. By lattice volume fraction optimization, the uniformity of the surface temperature of the target domain was improved. In addition, we fabricated test pieces of lattice and 3D example model by using selective laser melting additive manufacturing. The results showed that the error between the measurement and the analysis using the test piece was sufficiently small.

Temperature Distribution Design Based on Variable Lattice Density Optimization and Metal Additive Manufacturing

Additive manufacturing (AM) is employed for fabricating industrial products with complex geometries. As topological optimization is suitable for designing complex geometries, studies have combined AM and topological optimization, evaluating the density optimization of lattice structures as a variant of topological optimization. The lattice structures of components fabricated via AM comprise voids. Models designed using topological optimization should be modified to ensure structures suitable for AM. As the lattice unit can be easily fabricated using AM with fewer design modifications, this study uses lattice density optimization for an industrial AM product. We propose a method of optimizing the lattice distribution for controlling the surface temperature uniformity of industrial products, such as molds. The effective thermal conductivity of the lattice is calculated using the homogenization and finite element methods. The effective thermal conductivity changes depending on the internal pore sizes. The proposed methodology is validated using a 3D example; the minimization problem of surface temperature variations in the target domain is considered. The variable density of the embedded lattice in the target domain is optimized, and we experimentally validated the performance of the lattice unit cell and optimal 3D structure using metal powder bed fusion AM.

Effect of hydrate distribution on effective thermal conductivity changes during hydrate formation in hydrate-bearing quartz sands

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

Effective thermal conductivity changes of the hydrate-bearing quartz sands in depressurization and soaking

The effective thermal conductivity changes in the sediments during depressurization are significant for hydrate exploitation. But these changes cannot be measured directly because of the unavailable stable conditions during depressurization. In this work, in order to form the stable conditions for the measurements, soaking was designed to stabilize temperature and pressure in the sample after depressurization. Afterward, the effective thermal conductivity changes were measured by the transient hot-wire method. To define reasons for these changes, gas-water-hydrate distribution was inferred by the electrical resistance changes. The effective thermal conductivity changes were further analyzed from initial hydrate saturation, back pressure, water-gas production ratio, and gas-water-hydrate distribution, respectively. The results showed that the effective thermal conductivity changes were involved with hydrate dissociation closely. The effective thermal conductivity was increased with increasing initial hydrate saturation and back pressure. Moreover, the effective thermal conductivity was increased by the enlarged water distribution at the early period of hydrate dissociation and decreased by the enlarged gas distribution at the late period of hydrate dissociation. Gas slippage effect and gas-water gravity differentiation played important roles in these gas-water redistribution in the sample. Meanwhile, the sandy grain rearrangement was inferred to improve the contact quality among grains and thus also increased the effective thermal conductivity. Additionally, although there was hydrate dissociation rate of 0.36% during the measurements, the trends of the effective thermal conductivity were not supposed to change. This work may offer some reference to understand the mechanisms of heat transfer in the sediments during depressurization.

Thermal conductivity of nanofluids: A comparison of EMD and NEMD calculations

Extensive studies on the effective thermal conductivity (ETC) of nanofluids have been conducted thus far; however, the mechanisms behind the change in ETC remain unclear. In the present study, we investigated the components of the ETC of nanofluids, based on equilibrium molecular dynamics (EMD) and non-EMD (NEMD) calculations, to elucidate more accurately the mechanisms responsible for change in ETC. Until now, the factors affecting ETC have not been revealed clearly, and not compared quantitatively by the EMD and NEMD. This study also aimed to compare the constituents of ETC quantitatively to validate the NEMD calculation as a method for determining the ETC of nanofluids. Our detailed results demonstrate that the primary factors contributing to the change in ETC are thermal transport in liquid–liquid interactions and the solid–solid interactions in nanoparticles. The ETC components related to liquid exhibit consistency between the EMD and NEMD calculations, with less than 5.0% difference. Conversely, although the components related to nanoparticles result in differences of more than 10%, these properties do not appear to have a significant impact on ETC. On the basis of the findings of this study, we are able to calculate consistent results for not only ETC but also its components, via EMD and NEMD, for a liquid system with a spherical nanoparticle.

The impact of aging and environmental conditions on the effective thermal conductivity of several foam materials

The thermal conductivity, a fundamental property for insulating materials, is often advertised using a single value implied to be constant. However, research shows that the effective thermal conductivity changes as a result of material aging and the environmental parameters, including temperature and moisture content levels. In recent years, linear temperature-dependent laws have been occasionally proposed for inorganic fibrous materials, although there is increasing awareness of the fact that foam insulating materials have less regular temperature-dependent behaviours. This depends on the fact that the equilibrium among the different gasses in the material changes over time. In this paper, several polyurethane and polyisocyanurate foams are analysed in order to determine how the effective thermal conductivity is altered after accelerated aging obtained by exposing them to high temperature, high relative humidity levels, and freeze-thaw cycling. For each material, and after each aging exposure condition, measurements of the thermal conductivity were collected over a large temperature range from −20 °C to +40 °C using a Heat Flux Meter. These experimental results allow to build 3-D plots showing the effective thermal conductivity as a function of temperature and moisture content for both pristine and aged materials. Then, the measured results are used in hygrothermal simulations performed inputting the measured temperature- and moisture-dependent thermal conductivity in order to determine the effective performance of the considered insulating materials in both pristine and aged conditions. Results show that the aging of the foams and the operating temperatures have higher impacts on the insulating performance of polyisocyanurates than on polyurethanes. Additionally, high moisture levels contribute to lower performance in all foam materials, with open cell foams experiencing the greatest thermal resistance reduction. The increase in the energy fluxes across the insulating layer with respect to the constant thermal conductivity assumption was significantly higher once the effective thermal conductivity of aged materials was considered, especially when polyisocyanurate foams were modelled in cold and humid conditions.

Effective thermal conductivity of unsaturated granular geocomposite using lattice element method

Soil thermal conductivity has an important role in geo-energy applications and heat transfer modelling. The performances and efficiencies of such applications are strongly affected by the saturation conditions, especially at the lower saturation, which can negatively affect the soil thermal conductivity. Therefore, it is essential to improve the soil thermal conductivity at lower saturation levels. Here, we investigate the effect of the fillers, soil gradation and the water saturation on the thermal conductivity of sand as an innovative improvement method. The experimental results show a significant improvement in the thermal conductivity at dry and lower saturation degrees and a considerable improvement in the case of full saturation. The improvement in the thermal conductivity is comparatively high in dry state and is decreasing with increase in the saturation. The existing soil thermal conductivity models, which are limited to a specific boundary conditions and soil types, fail to consider the effect of soil filler composite behaviour. Therefore, a mesoscale numerical method is developed to model the change in effective thermal conductivity. The developed model shows a good agreement with the measured thermal conductivity values.

Thermal characterization of three-dimensional printed components for light-emitting diode lighting system applications

This study investigated the thermal properties of three-dimensional (3-D) printed components with the potential to be used for thermal management in light-emitting diode (LED) applications. Commercially available filament materials with and without a metal filler were characterized with changes to the print orientation. 3-D printed components with an in-plane orientation had >30 % better effective thermal conductivity compared with components printed with a cross-plane orientation. A finite-element analysis was modeled to understand the effective thermal conductivity changes in the 3-D printed components. A simple thermal resistance model was used to estimate the required effective thermal conductivity of the 3-D printed components to be a viable alternative in LED thermal management applications.

Changes in Effective Thermal Conductivity During the Carbothermic Reduction of Magnetite Using Graphite

Knowledge of the effective thermal diffusivity changes of systems undergoing reactions where heat transfer plays an important role in the reaction kinetics is essential for process understanding and control. Carbothermic reduction process of magnetite containing composites is a typical example of such systems. The reduction process in this case is highly endothermic and hence, the overall rate of the reaction is greatly influenced by the heat transfer through composite compact. Using Laser-Flash method, the change of effective thermal diffusivity of magnetite-graphite composite pellet was monitored in the dynamic mode over a pre-defined thermal cycle (heating at the rate of 7 K/min to 1423 K (1150 °C), holding the sample for 270 minutes at this temperature and then cooling it down to the room temperature at the same rate as heating). These measurements were supplemented by Thermogravimetric Analysis under comparable experimental conditions as well as quenching tests of the samples in order to combine the impact of various factors such as sample dilatations and changes in apparent density on the progress of the reaction. The present results show that monitoring thermal diffusivity changes during the course of reduction would be a very useful tool in a total understanding of the underlying physicochemical phenomena. At the end, effort is made to estimate the apparent thermal conductivity values based on the measured thermal diffusivity and dilatations.

Particle-Level Engineering of Thermal Conductivity in Matrix-Embedded Semiconductor Nanocrystals

Known manipulations of semiconductor thermal transport properties rely upon higher-order material organization. Here, using time-resolved optical signatures of phonon transport, we demonstrate a "bottom-up" means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. Growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap.

Thermal conductivity of simple and tubular nanowire composites in the longitudinal direction

This work establishes a generic model to study phonon transport and the thermal conductivity of periodic two-dimensional nanocomposites in the longitudinal direction (along the wire axis direction). More specifically, the generic model is applied to study the thermal conductivity of silicon-germanium composites with simple silicon nanowire and tubular silicon nanowire inclusions in a germanium matrix, and cylindrical nanoporous silicon materials. The results show that the effective thermal conductivity changes not only with the volumetric fraction of the constituents but also with the radius of the nanowires and cylindrical pores due to the nature of the ballistic phonon transport. The smaller the wire/pore diameter, the smaller is the thermal conductivity of the periodic two-dimensional nanocomposites for a given volumetric fraction. Composites with tubular nanowire inclusions have a lower effective thermal conductivity than simple nanowire composites due to the introduction of additional surface scattering through the pores associated with tubular nanowires. Results of this study can be used to direct the development of both high-efficiency thermoelectric materials and thermal interface material containing high-thermal-conductivity particle or wire inclusions.

Thermal Conductivity Modeling of Core−Shell and Tubular Nanowires

The heteroepitaxial growth of crystalline core-shell nanostructures of a variety of materials has become possible in recent years, allowing the realization of various novel nanoscale electronic and optoelectronic devices. The increased surface or interface area will decrease the thermal conductivity of such nanostructures and impose challenges for the thermal management of such devices. In the meantime, the decreased thermal conductivity might benefit the thermoelectric conversion efficiency. In this paper, we present modeling results on the lattice thermal conductivity of core-shell and tubular nanowires along the wire axis direction using the phonon Boltzmann equation. We report the dependence of the thermal conductivity on the surface conditions and the core-shell geometry for silicon core-germanium shell and tubular silicon nanowires at room temperature. The results show that the effective thermal conductivity changes not only with the composition of the constituents but also with the radius of the nanowires and nanopores due to the nature of the ballistic phonon transport. The results in this work have implications for the design and operation of a variety of nanoelectronic devices, optoelectronic devices, and thermoelectric materials and devices.

OPTIMIZATION OF PIN-FIN HEAT SINKS USING ANISOTROPIC LOCAL THERMAL NONEQUILIBRIUM POROUS MODEL IN A JET IMPINGING CHANNEL

A numerical study has been carried out to optimize the thermal performance of a pin-fin heat sink. A pin-fin heat sink, which is placed horizontally in a channel, is modeled as a hydraulically and thermally anisotropic porous medium. A uniform heat flux is prescribed at the bottom of the heat sink. Cool air is supplied from the top opening of the channel and exhausts to the channel outlet. Comprehensive numerical solutions are derived from the governing Navier-Stokes and energy equations using the Brinkman-Forchheimer extended Darcy model and the local thermal nonequilibrium (LTNE) porous model for the region occupied by the heat sink. Results from this study indicate that the anisotropy in permeability and solid-phase effective thermal conductivity changes substantially with the variation of porosity. Optimum porosity for maximum heat dissipation depends on the pin-fin thickness, the pin-fin height, and the Reynolds number. A correlation for predicting the optimum porosity for a pin-fin heat sink is proposed. Generally, in the case of thin pin-fins the heat sink should be designed to have a high porosity, while in the case of thick pin-fins the heat sink should be designed to have a relatively low porosity.

Kinetics of the approach to sorptional equilibrium by a foodstuff

AbstractThe kinetics of equilibration of a vegetable foodstuff with moist air are studied on the basis of its cellular structure. The problem is modelled as a coupled heat and mass transfer phenomenon; since there is a significant shrinkage as moisture decreases, the model takes the effect into account by changing the basis for the mass and energy balances. The resulting nonlinear second‐order partial differencial equations are solved numerically; predicted values of moisture decrease and temperature gain with time are compared with experimental data. A sensitivity analysis of the solution to values of the transport properties shows that the value of effective diffusivity and heat and mass transfer coefficient may alter significantly the predicted equilibration kinetics and center temperature. The adopted value for heat of sorption has less influence. Changes in effective thermal conductivity and porosity bear little influence on predictions.