Conductivity Of Various Materials Research Articles

Design and application of a testing device for solar reflective materials

Using white polyvinyl chloride pipes, gray polyvinyl chloride pipes, natural rubber foam sheets, and nylon 6 as experimental materials, this article designed a sandwich structure solar reflective material testing device. By testing the thermal conductivity of various materials and the thermal conductivity of the composite materials in the device and conducting thermal insulation verification experiments on the solar reflective material testing device, it was found that the overall thermal conductivity of the device was small and the internal temperature of the device was less affected by the external environmental temperature. The results indicated that the solar reflective material testing device has good insulation performance.

Thermal design of composite cold plates by topology optimization

Topology optimization has been extensively utilized to generate cold plates with efficient cooling performance. However, the design of composite cold plates remains challenging. This study presents a two-solid topology optimization method for conjugate heat transfer based on ordered SIMP (solid isotropic material with penalization). The proposed method aims to develop high-performance composite cold plates by leveraging the differences in thermal conductivity of various materials. The optimization results were investigated for four inlet and outlet combinations at various Reynolds numbers. The results indicate that using a staggered distribution of multiple inlets and outlets yields higher heat transfer than a single distribution. Furthermore, composite cold plates derived from the two-dimensional optimized models were numerically compared with rectangular flow channel and non-composite cold plates. The optimized channels exhibited superior hydrothermal performance over the rectangular flow channel, which was primarily attributed to the weakening of the velocity stagnation region. The average temperature (T¯) and root mean square temperature (RMST) of the heat source were reduced by a maximum of 2.53% and 57.67%, respectively. The friction factor decreased by up to 50.42%. Compared to non-composite cold plates, the decrease in RMST reached 16.24% without any additional increase in the pumping power. Composite cold plates further promote temperature uniformity by accelerating the heat transfer rate through their high thermal conductivity, thereby mitigating the occurrence of hotspots. Finally, the performance of the three test samples was experimentally compared to validate the numerical results. The optimized design enables the strategic allocation of highly thermally conductive material on a cold plate while employing more cost-effective alternatives in other areas. The proposed method is anticipated to expand the design possibilities for cold plates and provide insights into addressing concerns related to hotspots in equipment.

Recent advances on the enhanced thermal conductivity of graphene nanoplatelets composites: a short review

Graphene nanoplatelets (GNPs) have garnered significant attention in the field of thermal management materials due to their unique morphology and remarkable thermal conductive properties. Their impressive thermal properties make them an interesting choice of nanofillers with which to produce multifunctional composite materials and a host of other applications whilst their structural and thermal properties significantly improve their target materials or composites. Therefore, this present study reviewed recent advances in the use of GNPs as nanofillers to enhance the thermal conductivity of various materials or composites. The improved thermal conductivity that GNPs impart in composites is also comprehensively compared and discussed. Therefore, this review may reveal hitherto unknown opportunities and pave the way for the production of materials with enhanced thermal applications including electronics, aerospace devices, batteries, and structural reinforcement.

Comparison of Different Empirical Models for Analysing the Thermal Conductivities of Various Materials for Use in Nanofluid Preparation

For over the past ten years, various empirical models have been studied for analysing the thermal conductivity of different materials. In this analysis, different empirical correlations and models of thermal conductivity have been compared. The thermal conductivities of four types of oxide materials (SiO2, TiO2, CuO, Al2O3) and MWCNTs with volume fractions from 0.5 to 5% in a temperature range of 273–373 K and various nanoparticle shapes were compared. The results illustrated that the thermal conductivity of nanofluids based upon various nanoparticles increased with increasing volume fraction. In comparison, the effective thermal conductivity of nanofluids based on MWCNTs was enhanced much more than that of other types of nanofluids. Furthermore, Maxwell’s model was considered the basis for predicting the effective thermal conductivity of nanofluids. According to the basic models, a new correlation was proposed for predicting the effective thermal conductivity as a function of temperature and nanoparticle volume concentration. The nanoparticle shape has a great impact on the thermal conductivity of nanofluids. Regarding the precise heat transfer enhancement, the analysed nanofluids are suggested to be heat carriers in thermal systems, particularly solar thermal collectors. Typically, the use of nanofluids in solar thermal collectors improves the thermal efficiency of collectors in terms of the precise heat transference of nanofluids.

Instruments to Measure Thermal Conductivity of Engineering Materials - A Brief Review

Insulation materials are used in various engineering applications for improving energy efficiency and performance. Efficient thermal conductivity is one of the most important factors pertaining to the determination of the efficiency of the system, so it is important to know how the material performs under various conditions in the sense of thermal conductivity and how it can be configured for better results. The aim of this paper is to paper is to identify efficacious apparatus and techniques for determining the thermal conductivity values through steady?state methods and transient methods. The review work presented herein discusses about various apparatus/methods considered and utilised by various researchers like the guarded hot plate, hot wire, modified hot wire, laser flash diffusivity, and many more, which are based on the principles of heat transfer for the measurement of thermal conductivity of engineering materials is done. These instruments have been found effective for the measurement of thermal conductivities of various materials. Such apparatuses are examined in the sense of their suitability for specific materials, such that it is possible to determine which instrument to be selected in context to the type of material for which the thermal conductivity is to be determined. Laser flash and Guarded Hot Plate apparatus are very frequently used and their use has increased for quite some time. How to cite this article: Singh S, Sharma V, Narad S. Instruments to Measure Thermal Conductivity of Engineering Materials - A Brief Review. J Adv Res Mech Engi Tech 2020; 7(1&2): 16-25. DOI: https://doi.org/10.24321/2454.8650.202001

A numerical study of the measurement error introduced by TCi theoretical assumptions

Thermal conductivity is the key thermal property of materials that are used in thermal protection of aircraft and energy conservation field, and its accurate test is of great significance in such fields. TCi method is a transient method of measuring thermal conductivity and effusivity, which was developed from the transient plane source method in the last decade. TCi technique has been widely used to measure the thermal conductivity of various materials due to its advantages of simple, rapid and single-side in-situ measurement. But the nominal accuracy is lack of sufficient verification within the full test ranges. Studies have found that the results of thermal conductivity measured by different methods for the same material may be different. Even if the same method and instrument are used to measure the same material, the results by using different test modes may vary significantly. The uncertainty of a dynamic test mainly comes from two aspects, theoretical assumptions of test theory and errors of determining input/output parameters. The latter one can be improved by adopting test facilities with higher precision and resolution, and the error usually can be estimated using the error transfer function. However, the uncertainty introduced by theoretical assumptions is difficult to determine and may lead to an incorrect result when the practical measurement deviates from the theoretical assumptions. For TCi method, except for the errors of determining input/output parameters, the theoretical assumptions in deducing the theory, the calibration curves and the contact resistance will affect the test accuracy. In this study, the influence of the above factors on the accuracy of thermal conductivity and effusivity determined by TCi technique are revealed quantitatively using numerical simulation. Firstly, the practical transient test process of TCi method is reproduced in the simulation with all thermophysical properties given. Secondly, the heat transfer processes of measuring foams, ceramics and metals by TCi method are simulated, and the calibration curves of thermal conductivity and effusivity of each type materials are obtained according to the corresponding temperature increase curve and the theoretical basis of TCi method. The influence of calibration curves on the test uncertainty is analyzed by comparing the calculated values of different calibration curves with the given values in the simulation. Finally, the influence of contact resistance on the test accuracy of different materials is analyzed by setting a thin layer of air between the sensor and the sample. The results show that TCi method could measure the effusivity of different materials accurately, and the different calibration curves have little influence on it. However, the results obtained by different thermal conductivity calibration curves are quite different, especially for the foam materials. The contact resistance between the sensor and the sample will decrease the test results of the high thermal conductivity material, while the effect can be ignored for foam materials. And the influence of contact resistance can be significantly reduced by adding contact agent. Due to the calibration curve and contact resistance in theoretical assumptions of TCi method will affect the test accuracy, the errors introduced by theoretical assumptions should be analyzed when adopting TCi method.

Design and Implementation of a Laboratory Equipment for Studying Heat Transfer by Conduction

Abstract Thermal conduction is a heat transfer mechanism. It is present in our everyday lives. Studying thermal conductivity helps us better understand the phenomenon of heat conduction. The goal of this paper is to measure the thermal conductivity of various materials and compare results with the values provided by the manufacturers. To achieve this we assembled a measuring instrument and performed measurements on heat insulating materials.

The effect of external temperature gradients on thermal conductivity in non-equilibrium molecular dynamics simulations: From nanowires to bulk Si

Nonequilibrium molecular dynamics is widely used to calculate the thermal conductivity of various materials, but the influence of temperature gradient to thermal conductivity has received limited attention within current research studies. The purpose of this article is to explore the discrepancy between intrinsic and extrinsic thermal conductivities under different temperature gradients, which can be considered as external fields. The analyses of phonon density of states have shown that the temperature gradient plays a role in the external field, and a larger temperature gradient activates more low-frequency vibrational modes, which leads to larger thermal conductivities. Specially, the thermal conductivity increases linearly with the temperature gradient when using Stillinger-Weber (SW) potential. Moreover, a new formula was derived to satisfactorily fit the thermal conductivities of bulk Si and silicon nanowires (SiNWs) for various cell sizes, and the physical meaning of the formula was explained. It is shown that the SW potential and Tersoff potential of Si produce different thermal conductivities. By comparing the results of first principles simulations, the Tersoff potential gives rise to better description of vibrational modes.

Evaluation of heat transporting losses due to changes of insulation properties during operation

The need for measuring the thermal conductivity of various materials exists in different fields of science and industry. First of all, it relate to constructing and power engineering. Thermal control allows evaluating the thermal insulation properties of the structure, identifying problem areas and assessing the energy efficiency of the pipeline. The paper assesses the transporting heat losses taking into account the actual operating conditions of the pipelines and the heterogeneity of the thermal insulation properties. A number of experiments were carried out. Experiments have shown that the destruction of insulation properties depends on such factors as the surface temperature of the pipe, the humidity of the environment, the service life.

Effects of surface materials on polarimetric-thermal measurements: applications to face recognition.

Materials, such as cosmetics, applied to the face can severely inhibit biometric face-recognition systems operating in the visible spectrum. These products are typically made up of materials having different spectral properties and color pigmentation that distorts the perceived shape of the face. The surface of the face emits thermal radiation, due to the living tissue beneath the surface of the skin. The emissivity of skin is approximately 0.99; in comparison, oil- and plastic-based materials, commonly found in cosmetics and face paints, have an emissivity range of 0.9-0.95 in the long-wavelength infrared part of the spectrum. Due to these properties, all three are good thermal emitters and have little impact on the heat transferred from the face. Polarimetric-thermal imaging provides additional details of the face and is also dependent upon the thermal radiation from the face. In this paper, we provide a theoretical analysis on the thermal conductivity of various materials commonly applied to the face using a metallic sphere. Additionally, we observe the impact of environmental conditions on the strength of the polarimetric signature and the ability to recover geometric details. Finally, we show how these materials degrade the performance of traditional face-recognition methods and provide an approach to mitigating this effect using polarimetric-thermal imaging.

ACHIEVING PAKIS’S THERMAL CONDUCTIVITY AS ARCHITECTURAL BUILDING THERMAL RESISTANT BY GUARDED STEADY-STATE HOTBOX METHOD:PART 1

Thermal conductivity of various materials which are mostly listed available as building or industrial materials in reference books and websites; but one will hardly find out for every new material, and has to be observed and try out itself if we want to know the new thermal conductivity value (k). Nonetheless with new substanct likes pakis-stem blocks that come from natural tree that could be found in the tropical woodland of Indonesia. Steady-state homogeneous temperature applied with hotbox method in an uninfluent environment likes guarded laboratory environment is the right method to obtain the thermal conductivity and resistance of porousness and semi-solidness of the pakis-stem blocks. After investigating almost 24 hours with controller TRSYS01 applying with ASTM C1155, physical semi-solid pakis blocks tend to be more easy to obtain the R-value, k-value, and surface temperatures than the porous pakis. The porous pakis blocks were tend to unstable during the test due to its physical permeable condition. The resistant values (R-value) and thermal conductivity (k) values will be further published on the following discussion of pakis thermal conductivity part 2.

How to improve the accuracy of equilibrium molecular dynamics for computation of thermal conductivity?

Equilibrium molecular dynamics (EMD) simulations through Green–Kubo formula (GKF) have been widely used in the study of thermal conductivity of various materials. However, there exist controversial simulation results which have huge discrepancies with experimental ones in literatures. In this Letter, we demonstrate that the fluctuation in calculated thermal conductivity is due to the uncertainty in determination of the truncation time, which is related to the ensemble and size dependent phonon relaxation time. We thus propose a new scheme in the direct integration of heat current autocorrelation function (HCACF) and a nonzero correction in the double-exponential-fitting of HCACF to describe correctly the contribution to thermal conductivity from low frequency phonons. By using crystalline Silicon (Si) and Germanium (Ge) as examples, we demonstrate that our method can give rise to the values of thermal conductivity in an excellent agreement with experimental ones.

Modeling of Influence of Gas Atmosphere and Pore-Size Distribution on the Effective Thermal Conductivity of Knudsen and Non-Knudsen Porous Materials

Application of porous insulation materials in various gas atmospheres changes their effective thermal conductivity due to many structural and thermal influences. One of those is the pore-size distribution of the material. In this paper a study of the influence and modeling of the modification of the effective thermal conductivity of various materials is presented by exchanging the gas atmosphere. For this purpose 11 different materials with various pore-size distributions are investigated. The experimental work included thermal conductivity measurements along with an analysis of the porous structure. Two additional important effects of thermal accommodation and radiation will be clearly identified as caused by the exchange of the filling gas. Modeling the gas thermal conductivity for different polydisperse systems is carried out based on kinetic theory and the Knudsen effect of rarefaction of the gas in a wide range of the Knudsen number. Development of the single pore model of the gas thermal conductivity to a new predictive model with an exponential parameter gives good agreement with the experimental results.

Effect of cold plastic deformation on electrical conductivity of various materials

This study is focused on the investigation of the effect of cold plastic deformation on the electrical conductivity properties of different materials. The plastic deformation that occurs at the upsetting of the specimen and the varying of the conductivity properties were the main subject of this study. Aluminium, copper and brass were used as the test materials in the experiments. Measurement results of the upset (deformed) specimens were obtained with a conductivity meter instrument. Result which was obtained from the experiments was presented graphically. Finally, it is concluded that in general electrical conductivity decreases slightly due to plastic deformation of the deformed materials.

Development of a unified heat transfer model with special emphasis on thermal conduction

It is postulated that energy fields in space and matter are quantized by an entity having coincident properties of wave and virtual matter. A unified link between thermal radiation and conduction was established as confirmed by Planck's quantum theory, and a unified heat transfer equation was developed. One-dimensional, steady-state thermal conduction was solved by using approximate scatter and mobility models in this equation. Coefficient of thermal conductivity of various materials were calculated and compared with the literature. Even with approximate scatter and mobility models, a good agreement was observed.

Internal Temperature Distribution in a Rolling Tire

Abstract A computer program is developed to calculate the internal temperature distribution in a rolling tire. The geometry of the tire cross section is transformed to simple computational domains by using a boundary-fitted coordinate transformation method. A heat transfer equation in the tire is also transformed to be calculated in the computational domain. Heat generation rates are estimated from load, speed, contained air pressure, and a static stress and strain analysis. An unsteady two-dimensional heat conduction equation is solved with heat convection and radiation boundary conditions. External surface temperature distributions are measured by an infrared camera system and the temperature of the contained air is measured by a thermocouple. The conductivities of various materials in the tire and various heat transfer coefficients along the tire boundary are given in an input file. Some internal temperatures measured by buried thermocouples in the actual tire are compared with the calculated temperatures.