Thermal Conductivity Coefficient Research Articles

Heat Conduction of Micropolar Solids Sensitive to Mirror Reflections of Three-Dimensional Space

This article considers a variant of the heat conduction theory of thermal conductivity, in which the heat flux pseudovector has a weight of 1. The pseudoinvariants associated to the heat flux pseudovector are sensitive to mirror reflections and inversions of threedimensional space. The primary purpose of the study was to find a heat flux vector that is algebraically equivalent to the microrotation pseudovector and to measure elementary volumes and areas using pseudoinvariants that are sensitive to mirror reflections. To represent spinor displacements, a contravariant microrotation pseudovector with a weight of +1 was selected. Thus, the heat flux and mass density were expressed as odd-weight pseudotensors. The Helmholtz free energy per unit doublet pseudoinvariant volume was employed as the thermodynamic state potential of the following functional arguments: absolute temperature, symmetric parts, and accompanying vectors for the linear asymmetric strain tensor and the wryness pseudotensor. The results obtained show that the thermal conductivity coefficient and heat capacity of elastic micropolar solids are pseudoscalars of odd weight, indicating their sensitivity to mirror reflections.

Hubbard model and its impact on the thermoelectric properties of the penta-graphene structure

The unique characteristics of penta-graphene structures have captivated scientists.Through our tight-binding and Hubbard calculations, we have made a prediction that the penta-graphene monolayer acts as a semiconductor. Our findings demonstrate that by applying influences like a field parallel magnetic field and voltage bias the band gap of this material decreases. This ability to manipulate its properties holds promise for practical applications in electronics. In this study, we delve into the impact of fields parallel magnetic fields and voltage bias, on the thermal conductivity, electrical conductivity, and Seebeck coefficient of penta-graphene structure using the tight-binding, Hubbard model, and Green function approach. Our analysis explains that the penta-graphene structure is a p-semiconductor, which is changed to an n-semiconductor by on-site Coulomb repulsion. This strategy displays that thermal and electrical conductivity decreases under the effect of parallel magnetic fields. The results presented to control the thermoelectric and electronic properties of the penta-graphene structure can promise a great future for this material in the field of thermoelectric and nanoelectric devices application.

Ultrasonic fatigue testing of AISI 304 and 316 stainless steels under environmental and immersion conditions

Ultrasonic fatigue tests were carried out on stainless steel AISI 316 and 304, under two modalities: at room temperature and in immersion (water for 316 and antifreeze for 304 steels); all tests were carried out with a loading ratio R=-1. The results obtained in the tests at room temperature (without immersion), for both materials exhibited a significant increase in temperature, leaving heat marks on the narrow section of the specimens. This phenomenon occurred due to the low coefficient of thermal conductivity of these stainless steels (16.2 W/ (m °K)), and the recorded temperatures were around 200 °C, generating instantaneous failure of material. Analyzes of fracture surfaces on specimens tested at room temperature reveal that crack initiation was related to the high temperature, causing alteration at the granular scale of the material, followed by a typical behavior crack propagation and failure. For specimens tested under immersion conditions, it was possible to reduce the temperature below 100 °C, which solved the problem of failure due to thermal effect. The results for 316 stainless steel immersed in water showed a fatigue life of 1.188×1010 cycles at188 MPa of stress loading in the specimen; while specimens subjected to 263 MPa stress showed a fatigue life of around 7×106 cycles, representing a significant reduction with an approximate factor of 1700. On the other hand, specimens of 304 stainless steel immersed in antifreeze with the lowest loading values of 169 MPa, showing an infinite ultrasonic fatigue life; while tests subjected to 263 MPa loading stress attains 3.62×106 cycles of ultrasonic fatigue life. The scanning electron microscopy visualizations for both cases of immersion tests showed that the initiation and propagation of the crack occurred on the surface of the specimens, exhibiting the typical mechanical fracture behavior without any apparent thermal influence.

Investigation on the Lubrication Heat Transfer Mechanism of the Multilevel Gearbox by the Lattice Boltzmann Method

In a gear transmission system in a closed space, the heat transfer between gears and fluids presents highly nonlinear characteristics due to the complex physical processes involved in heat exchange and fluid motion, and constructing and solving the thermodynamic model of the gearbox becomes a task that involves considerable difficulty. This paper takes a conical–cylindrical two-stage gearbox as the research object, proposes a fluid–solid coupled dynamics model based on the lattice Boltzmann (LBM) combined with the large eddy simulation (LES) method, and the adopted lattice model is the D3Q27 velocity model, which is used to numerically simulate the distribution of the flow field inside the gearbox and undertake in-depth research on the fluid motion law of the complex gear transmission system in the enclosed space. The model is solved to reveal the laws determining the gear speed and the effects of the lubricant’s dynamic viscosity and thermal conductivity coefficient on the gear heat dissipation efficiency. By adopting the lattice Boltzmann method, we can simulate the fluid flow and heat transfer inside the gearbox more efficiently, which provides a new way to closely understand the thermodynamic behavior of closed complex gear transmission systems. The application of this method is expected to provide strong support for thermal performance optimization and the design of gear transmission systems.

Laser powder bed fusion of pure copper electrodes

This study explores the fabrication and characterization of a pure copper electrode for an EDM equipment using laser powder bed fusion (LPBF) and comparing its performance with an original cast sample. The microstructural, mechanical, electrical, thermal, and corrosion characteristics of the samples were analyzed. The microstructure of the additively manufactured (AM) sample exhibited superior mechanical properties, with an 18 % increase in ultimate tensile strength and more than two times greater elongation in comparison with those of the cast sample. Moreover, the electrical and thermal conductivity coefficients of the AM sample were found to be consistent with industry standards as 5.52 × 107 S/m and 361 W/mK, respectively. Additionally, the potentiodynamic polarization corrosion test results of the AM sample further exhibited superior corrosion resistance in terms of the corrosion potential particularly in a 3.5 % NaCl environment. Advanced electron microscopy techniques were used to investigate the differences in the properties of the AM and cast samples and further shed light on the superior properties of the LPBF counterpart. Despite challenges in manufacturing pure copper components through LPBF, this study demonstrates the feasibility of producing a functional part that competes effectively with cast counterparts and excels in certain aspects.

Shape Anisotropic Chain‐Like CoNi/Polydimethylsiloxane Composite Films with Excellent Low‐Frequency Microwave Absorption and High Thermal Conductivity

AbstractThe demand for low‐frequency microwave absorption materials is becoming more and more urgent. Novel shape anisotropy chain‐like CoNi is fabricated using polyvinylpyrrolidone as a shape‐directing agent via solvothermal method, which is then mixed with polydimethylsiloxane (PDMS) to prepare corresponding multifunctional chain‐like CoNi/PDMS composite films. Shape anisotropy and strong magnetic coupling effect of chain‐like CoNi enhance natural resonance and magnetic loss capability. The minimum reflection loss (RLmin) is −50.5 dB and low‐frequency effective absorption bandwidth (EAB) is 1.04 GHz (2.64–3.68 GHz) at 3.9 mm for chain‐like CoNi. The corresponding 18 vol% chain‐like CoNi/PDMS composite films present optimal low‐frequency microwave absorption performance with RLmin of −56.7 dB and low‐frequency EAB of 1.04 GHz (2.96–4.00 GHz) at 4.1 mm, which is far superior to 18 vol% spherical CoNi/PDMS composite films with RLmin of −9.5 dB. Meantime, the in‐plane and inter‐plane thermal conductivity coefficients of 18 vol% chain‐like CoNi/PDMS composite films are 2.05 and 0.61 W m−1 K−1, about 1.5 times higher than 18 vol% spherical CoNi/PDMS composite films (1.36 and 0.42 W m−1 K−1), also 220% and 190% higher than pure PDMS (0.64 and 0.21 W m−1 K−1). This composite films with low‐frequency microwave absorption and thermal conductivity can broaden applications in 5G communications and flexible electronics.

Thermal effect on the modal characteristics of the rotor of a typical cryogenic turbine for helium refrigerator

The helium turbine is a critical component of the cryogenic system for nuclear fusion devices. To ensure the turbine's steady functioning and avoid resonance, it is essential to understand and clarify the influence of key factors on its modal characteristics. This paper investigates the influence of thermal effect, fluid-structure interaction and rotational effects on the intrinsic frequency of a helium turbine used in the Comprehensive Research Facility for Fusion Technology (CRAFT) cryogenic system. The effects of coefficient of thermal expansion, thermal conductivity and Young's modulus on the intrinsic frequency of the rotor are explored, considering these thermal factors that vary with temperature. The results indicate that thermal effect has the greatest influence on the intrinsic frequency of the rotor over rotational effectand fluid-structure interaction. Lower-order modes are more sensitive to changes in thermal conductivity, while higher-order modes are more sensitive to changes in Young's modulus at low temperatures. The intrinsic frequency of the rotor is more sensitive to changes in the thermal factors of the shaft rather than the thermal factors of the impellers.

Математичні моделі локального нагрівання елементів електронних пристроїв

Linear and non-linear mathematical models for the determination of the temperature field, and subsequently for the analysis of temperature regimes in isotropic spatial heat-active media sub-jected to internal and external local heat load, have been developed. In the case of nonlinear boundary-value problems, the Kirchhoff transformation was applied, using which the original nonlinear heat conduction equations and nonlinear boundary conditions were linearized, and as a result, linearized second-order differential equations with partial derivatives and a discontinu-ous right-hand side and partially linearized boundary conditions were obtained. For the final linearization of the partially linearized differential equation and boundary conditions, the ap-proximation of the temperature according to one of the spatial coordinates on the boundary sur-faces of the inclusion was performed by piecewise constant functions. To solve linear bounda-ry-value problems, as well as obtained linearized boundary-value problems with respect to the Kirchhoff transformation, the Henkel integral transformation method was used, as a result of which analytical solutions of these problems were obtained. For a heat-sensitive environment, as an example, a linear dependence of the coefficient of thermal conductivity of the structural material of the structure on temperature, which is often used in many practical problems, was chosen. As a result, analytical relations for determining the temperature distribution in this envi-ronment were obtained. Numerical analysis of temperature behavior as a function of spatial co-ordinates for given values of geometric and thermophysical parameters was performed. The in-fluence of the power of internal heat sources and environmental materials on the temperature distribution was studied. To determine the numerical values of the temperature in the given structure, as well as to analyze the heat exchange processes in the middle of these structures, caused by the internal and external heat load, software tools were developed, using which a ge-ometric image of the temperature distribution depending on the spatial coordinates was made.

Experimental investigation of pressure drop and heat transfer in porous media based on 3D printed triple periodic minimum surfaces

ABSTRACT Porous media are widely preferred for heat transfer applications because of complex structure and high porosity. It is relatively easy to produce complex structures with the additive manufacturing method. In this study, Gyroid-based triple periodic minimal surface is produced ABS (Acrylonitrile butadiene styrene) and PLA (polylactic acid) materials using 3D printers. The pressure drops and heat transfer in the porous structure with the gyroid model are investigated experimentally. Permeability and Forchheimer coefficients are determined for each gyroid porous channel. While the permeability and Forchheimer coefficient of the porous medium of PLA were calculated as 2.97 × 10−8 m2 and 0.07, respectively, the permeability and Forchheimer coefficient of the porous medium of ABS were calculated as 5.52 × 10−8 m2 and 0.09. The correlation between the Reynolds number and pressure drop per length is given. The gyroid structure-filled channel is heated by an asymmetric heat flux on the bottom side. The convection heat transfer in polymer-based porous media was investigated. Also, the correlation between the Nusselt number and the Reynolds number based on permeability is presented. While the Nusselt correlation of the PLA porous medium was obtained as N u K = 0.08 R e K 0.57 , the Nusselt correlation of the ABS porous medium was found to be N u K = 0.09 R e K 0.61 . There are improvements in heat transfer due to the flow mixing effect of the porous medium. The study aims to broaden the current knowledge of heat transfer in polymer porous structures. Especially, this study investigates heat transfer and flow in polymer-based porous structures with a low thermal conductivity coefficient produced by 3D printers.

Electrothermal Coupling Modeling and Electrothermal Dynamic Characteristics of Nickel-Cadmium Battery

Although Nickel-Cadmium batteries are widely used, there is a general problem of thermal failure and even thermal safety, and a lack of thermal characteristics and temperature rise law research. Accordingly, a typical Nickel-Cadmium battery’s equivalent circuit model, thermal model, and electrothermal coupling model were established in this article. The model’s open-circuit voltage, resistance, and capacitance, as well as its entropy heat coefficient, thermal conductivity and heat transfer coefficient were measured, and finally, according to these parameters, the verification analysis of the electrothermal coupling model was carried out in Simulink under the HPPC working condition and constant current working condition. The results show that the simulation results are basically consistent with the experimental data, the absolute value of voltage error is less than 0.02V under HPPC condition, the maximum voltage error is only 0.7% under constant current condition, and the temperature maximum error is only 3%. Consequently, the constructed electrothermal coupling model can well reflect the electrothermal characteristics of the battery, which provides data support and reference for solving the thermal failure problem.

Thermal properties of poly(vinyl butyral)/iron oxide composite nanofibers

AbstractIn the present study, iron(II, III) oxide nanoparticles are added to the electrospinning solution of poly(vinyl butyral) in order to yield electrospun composite poly(vinyl butyral)/iron oxide nanofibers. Scanning electron microscopy, energy dispersive x‐ray spectroscopy, and Fourier transform infrared spectroscopy analyses confirm the successful incorporation of iron(II, III) oxide nanoparticles in the nanofibers. Differential scanning calorimetry results show a decrease of approximately 6 °C in the glass transition temperature of the nanofibers with the addition of 1.0 % iron(II, III) oxide nanoparticle compared to neat poly(vinyl butyral) nanofibers. The infrared thermograms suggest the formation of a thermally conductive network for poly(vinyl butyral)/iron oxide nanofibers with faster and more evenly heat dissipation compared to that of neat poly(vinyl butyral) nanofibers while thermal conductivity coefficient of the poly(vinyl butyral) nanofibers is found to increase by 57 % with 1.0 % of nano iron oxide loading. The increased thermal conductivity and thermal absorptivity values suggest the cooling effect of the poly(vinyl butyral)/iron oxide composite nanofibers, which are promising materials for thermal management applications.

Study of the thermal conductivity of natural carbonates

The thermal conductivity of natural monoliths of calcite, dolomite marble, and limestone from various deposits was measured using the absolute stationary method of longitudinal heat flow in the temperature range of 50–300 K and the dynamic method in the range of 323–573 K. A majority of calcite marbles were inferior in thermal conductivity to dolomite marbles. At room temperature, the thermal conductivity coefficients of all studied samples were lower k = 5 W/(m K). The obtained data were compared with the literature data. The diversity of experimental data from different authors on the thermal conductivity of carbonates is associated with qualitative differences in the samples studied

Preparation of autoclaved-aerated concrete with waste ash of petroleum coke gasification

ABSTRACT In this paper, a novel route for the utilisation of petroleum coke gasification ash in the preparation of autoclaved-aerated concrete (AAC) is presented. The waste ash along with quartz sand, cement, lime, and gypsum are used as the primary raw materials, with aluminium powder paste serving as the pore-forming agent. The effect of incorporating petroleum coke gasification ash on the dry density and thermal conductivity of the resulting AAC was investigated. The results indicate that the thermal insulation performance of AAC is significantly enhanced by reducing the thermal conductivity coefficient upon the addition of treated petroleum coke gasification ash. At a 40% ash content, the AAC exhibits remarkable performance that meet the requirements of A2.5B05 in GB 11,968-2006, with compressive strength higher than 2.5 MPa and dry density lower than 525 kg m−3. Furthermore, the study examined the optimal calcium silicon ratio after the addition of the ash, with the results demonstrating that AAC prepared with a ratio of 0.25–0.26 exhibits the best compressive strength and dry density. The present study offers a novel perspective on waste management, which carries substantial significance for both the environmental and construction industries.

3D flow of variable properties viscoelastic nanofluids with impacts of Arrhenius energy and Cattaneo–Christov double-diffusion

In several situations, particularly, flow of viscoelastic nanofluids, the dynamic viscosity and thermal conductivity should be considered as variables due to their roles in various practical applications. Therefore, this article aims applying the Cattaneo–Christov double-diffusion theory to investigate the three-dimensional (3D) forced flow of second-grade viscoelastic nanofluids over a permeable stretching surface. The properties of the mixture are considered to be variables where the dynamic viscosity is exponentially varied with temperature and the thermal conductivity and Brownian motion are linearly dependent on temperature and concentration, respectively. The included medium is isotropic porous, and both heat generation and thermal radiation impacts are assumed. At the surface, the nanoparticles are either active or passively controlled, and the convective boundary conditions are considered for the surface temperature. The solution methodology is based on similarity transformations, and the numerical technique is based on 4th-order Runge-Kutta method. The major outcomes revealed that the growth in thermal relaxation time δ e and viscoelastic parameter K causes gradually decreases in temperature and nanoparticles features. Also, constant dynamic viscosity gives higher velocity features compared to the variable case. Furthermore, the considered range of thermal conductivity coefficient ε 1 gives an enhancement in rate of the heat transfer by 5.26%.

Reassessing the transport properties of fluids: A symbolic regression approach.

The viscosity and thermal conductivity coefficients of the Lennard-Jones fluid are extracted through symbolic regression (SR) techniques from data derived from simulations at the atomic scale. This data-oriented approach provides closed form relations that achieve fine accuracy when compared to well-established theoretical, empirical, or approximate equations, fully transparent, with small complexity and high interpretability. The novelty is further outlined by suggesting analytical expressions for estimating fluid transport properties across the whole phase space, from a dilute gas to a dense liquid, by considering only two macroscopic properties (density and temperature). In such expressions, the underlying physical mechanisms are reflected, while, at the same time, it can be a computationally efficient alternative to costly in time and size first principle and/or molecular dynamics simulations.

Effect of temperature and Humidity on the Thermal Conductivity λ of Insulation Materials

The aim of this article is to address the influence of air humidity and testing temperature on the thermal conductivity coefficient (λ) of various thermal insulation materials. This group includes wood-based materials, rock wools, heat-insulating renders, climate boards, and lightweight cellular concretes. These materials are used both indoors and outdoors in buildings. Over the course of several years, data were collected from laboratory tests to determine the thermal conductivity coefficient (λ) in relation to increases in temperature and humidity. The obtained results were compared with values provided by the manufacturers of the insulation materials. The aforementioned research was carried out due to the rather high sorption of most materials and thus the possibility of them becoming humid at high air humidity. Because of the very large difference in the thermal conductivity coefficient of water and air, a relatively small increase in the mass moisture content of the materials results in a loss of insulation.

The Effect of Mineral Wool Fiber Additive on Several Mechanical Properties and Thermal Conductivity in Geopolymer Binder.

The article discusses the effect of additives of waste mineral wool fibers on geopolymer binder. This is an important study in terms of the possibility of recycling mineral wool waste. The paper describes an effective method for pulverizing the wool and the methodology for forming geopolymer samples, labeled G1 for glass-wool-based geopolymer and G2 for stone-wool-based geopolymer. The compressive and flexural strengths and thermal conductivity coefficient of the geopolymer with the addition of mineral fibers were determined. The key element of the article is to verify whether the addition of mineral wool fibers positively affects the properties of the geopolymer. The results obtained prove that the addition of fibers significantly improves the flexural strength. For the G1 formulation, the ratio of compressive strength to flexural strength is 18.7%. However, for G2 samples, an even better ratio of compressive strength to flexural strength values of 26.3% was obtained. The average thermal conductivity coefficient obtained was 1.053 W/(m·K) for the G1 series samples and 0.953 W/(m·K) for the G2 series samples. The conclusions obtained show a correlation between the porosity and compressive strength and thermal conductivity coefficient. The higher the porosity, the better the thermal insulation of the material and the weaker the compressive strength.

Heat pipes and nanofluids utilization for cooling photovoltaic panels: an application of hybrid machine learning and optimization models

Abstract This study explores the synergies between advanced cooling technologies and photovoltaic systems, seeking to improve their overall efficiency and contribute to the broader goal of mitigating greenhouse gas emissions. To cool photovoltaic panels in more efficiently maner, understanding heat pipes, nanofluids, and panels interaction play key roles. For analysis and optimization, hybrid models of convolutional neural network (CNN) and firefly optimization algorithm are employed. The firefly optimization algorithm is used to optimize the thermosiphon heat pipe’s operational conditions, taking into account inputs such as the filling ratio, nanofluid concentration and panel angle. The study compared the predicted outcomes of a classic CNN model to laboratory experiments. While the CNN model was consistent with experimental findings, it struggled to predict high power values with precision. The proposed model improved high power value predictions by 4.05 W root mean square error (RMSE). The proposed model outperformed the classic CNN model for values greater than 50 W, with an RMSE of 3.95 W. The optimal values for the filling ratio, nanofluid concentration and panel angle were determined after optimization with the firefly algorithm. The research contributes to the advancement of renewable energy technologies and the optimization of photovoltaic panel cooling and energy production. Nanofluid with 1% mass concentration improves photovoltaic collector thermal efficiency due to its higher thermal conductivity coefficient. The photovoltaic collector’s electrical efficiency peaks in the morning, drops at noon due to temperature and radiation and recovers by morning. Electrical efficiency is best with nanofluid at 0.86%. Exergy efficiency closely matches electrical efficiency, with nanofluid at the optimal percentage achieving the highest efficiency and water cooling the lowest.

Fabrication and characterization of lignocellulosic coconut and energy reed straw-reinforced methylene diphenyl diisocyanate-bonded sustainable insulation panels

The use of renewable environmental resources and plastics that can be returned to the biological cycle shows a new direction in the application of composite building materials. This study investigated the potential and strengthening effects of developing composite panels from coconut materials (CM) and reed straw (RS) particles in the presence of methylene diphenyl diisocyanate (MDI). Five composite panels with 100:0, 60:40, 50:50, 40:60, and 0:100% proportions were produced. The nominal density was 600 kg/m3 and the dimensions were 400 × 400 × 8 mm3. The panels were produced by hot-pressing for a total of 10 min at a maximum pressure of 7.2 MPa and at 135 degrees, and then slowly cooled at room temperature. The following experiments were performed: mechanical, physical and morphological properties, thermal conductivity, FTIR (Fourier transform infrared spectroscopy), and EDX (Energy disruptive X-ray). The study gave the following results: the thermal conductivity of the coir and reed panels is competitive with the thermal insulation capacity of similar natural-based insulation materials, and is close to the values of other mineral and plastic-based insulation materials. The values of thermal conductivity coefficient ranged from 0.08 to 0.10 W/(mK). Only reeds (MDI_1) boards have better results than coconut materials. Among the mechanical properties, flexural strength and internal bonding strength increased proportionally with the increase in the amount of coconut. The Young's modulus reached its maximum in the case of a 50–50% coir-reed mixture. The best values of the panels are: 6.33 MPa for flexural strength, 1.83 GPa for Young's modulus and 0.36 MPa for internal bonding strength. The morphological recordings confirmed the proper mixing and binding of the lignocellulosic materials in the composite. FTIR tests proved the successful chemical interactions. The findings support the possibility of creating a promising, environmentally friendly building material.

Investigation of the effect of scandium concentration in the oxide composition Y<sub>2</sub>O<sub>3</sub>–Sc<sub>2</sub>O<sub>3</sub>–Al<sub>2</sub>O<sub>3</sub>–Er<sub>2</sub>O<sub>3</sub> on the thermophysical properties of optical ceramics

The influence of the concentration and position of scandium cations on the thermophysical characteristics of optical ceramics based on a solid solution of Y2O3–Er2O3–Sc2O3–Al2O3 oxides with a garnet structure has been studied. It is established that an increase in the total concentration of scandium leads to a decrease in the thermal conductivity coefficient in ceramics. The dependences of the thermal conductivity coefficient on the concentration and position of scandium cations in the garnet lattice are obtained. As a result the decrease in thermal conductivity of ceramics based on yttrium-erbium-scandium-aluminum garnet is due to structural disordering associated with partial substitution of cations in six- and eight-coordinate positions in the garnet lattice.