Showing 15 results for Thermal Conductivity
Volume 7, Issue 4 (12-2023)
Abstract
Given the ever-increasing demand for energy and the limited nature of fossil fuel resources, improving energy efficiency and storage has become one of the most significant challenges facing humanity. Phase Change Materials (PCMs), substances capable of absorbing and releasing thermal energy at a constant temperature, have emerged as an innovative solution in the field of energy storage. With their high latent heat capacity, ability to maintain a stable temperature, and environmental friendliness, PCMs have great potential for applications in various industries. However, their low thermal conductivity, especially in organic PCMs, has hindered their widespread use. To address this challenge, researchers have been exploring various methods to enhance the thermal properties of PCMs. One of the most effective approaches involves incorporating high thermal conductivity nanoparticles into the PCM matrix. This research comprehensively reviews recent advancements in the preparation and applications of nanoparticle-enhanced phase change materials. It delves into various types of nanoparticles used, production methods for nanocomposites, the impact of nanoparticles on the thermal and mechanical properties of PCMs, the stabilization of nanocomposites with surfactants and surface modification, and also their potential applications in diverse industries. The results of this study indicate that the use of nanoparticles can significantly improve the thermal conductivity of PCMs, with carbon-based nanofillers showing the highest impact. Additionally, nanoparticles have led to a relative reduction in the phenomenon of supercooling in PCMs. Based on the results of numerous studies, nanoparticle-enhanced phase change materials hold great promise for improving the performance of energy storage systems, reducing energy consumption in various industries, and fostering the development of sustainable technologies. These nanocomposites can be employed in the construction, automotive, electronics, and textile industries to create more comfortable environments, enhance energy efficiency, and reduce greenhouse gas emissions. Continued research in this field is expected to lead to the development of even more efficient PCMs with a broader range of applications.
Volume 9, Issue 37 (12-2012)
Abstract
Determination of thermo-physical properties of pomegranate juice is necessary to control processes such as drying and concentration. These properties have been affected by soluble solid content and temperature. Therefore the properties including; density, specific heat and thermal conductivity were determined at three levels of soluble solid content, 12o, 40o and 65o Brix and temperature range from 25 to 70 oC. Thermal conductivity by co-axial cylinder apparatus, specific heat by differential scanning calorimeter and density by a volumetric pycnometer were measured. Regression analysis indicated that both soluble solid content and temperature affected on these properties but the soluble solid content has more influence. Increase of soluble solid content from 12 to 65 caused a decrease in thermal conductivity from 0.233 to 0.193 W/m.oC, specific heat from 5.19 to 3.34 kJ/kg.oC and an increase in density from 1027 to 1323 kg/m3 respectively. With increase in temperature from 30 to 70 oC, density decrease from 1173 to 1150 kg/m3 but thermal conductivity increase from 0.188 to 0.229 W/m.oC during temperature increasing from 28 to 42 oC and specific heat increase from 3.9 to 4.4 kJ/kg during temperature increasing from 40 to 60 oC. With regression analysis, the suitable empirical models of thermophysical properties of pomegranate juice with operating variables were obtained.
Volume 14, Issue 5 (9-2012)
Abstract
Application of feed pellets in animal and aquatic farming industries has grown because of both the physical and the nutritional benefits it provides. Development of feed pellets manufacturing industry is also considerable. Steam conditioning process, which plays an important role in pelleting production, includes heating feed particles, adding moisture, and mixing the mash. Pellets cooling and drying processes are also involved in heat transfer phenomena. In this study, thermal conductivity of feed pellets was determined at different temperatures ranging from 25 to 85°C and moisture contents of 11.8 to 18.2% wb. It was measured by the transient technique using the line heat source method assembled in a thermal conductivity probe. It turned out that decreasing moisture contents from 18.2 to 11.8% (wb) produced non-linear reduction in thermal conductivity. The average values of thermal conductivity changed from 0.1509 to 0.2143 W m-1 °C-1 at different moisture contents. Tests conducted on two pellet size categories (based on nominal diameter) revealed a significant difference in thermal conductivity between these categories. The thermal conductivities of the first category (minor than nominal dia.) appeared to be 8.5% higher than those of the second category (superior to nominal dia.). Average values of thermal conductivity changed from 0.1538 to 0.2333 W m-1 °C-1 for the first category and from 0.1235 to 0.2456 W m-1 °C-1 for the second category (in 25°C). In addition, some empirical models were developed to express thermal properties as a function of moisture content and temperature.
Volume 14, Issue 8 (11-2014)
Abstract
During the past few decades, growing global concern about environmental problems, caused by widespread use of fossil fuels, attracts more research attention toward adsorption systems technology. However, one of the main problems of these systems is the poor heat transfer rate in adsorbent bed due to its low thermal conductivity. In the present study, extended surfaces and metal piece additives are applied to the adsorbent bed in order to numerically investigate the effect of heat transfer enhancement on the adsorption system performance. Employing metal pieces increases effective thermal conductivity of the bed by at least 100%. Results indicate that decreasing fin space and fin height and adding metal pieces to the adsorbent bed reduce the cycle time which finally improves the system specific cooling power. However, it is worth mentioning that the effect of metal piece additives on the cycle time reduction and specific cooling power improvement decreases at smaller fin spaces. Moreover, results show that the increase of fin height improves the coefficient of performance while decreases the specific cooling power of the system. On the contrary, the reduction of fin space simultaneously increases the coefficient of performance and the specific cooling power of the adsorption system.
Volume 15, Issue 1 (3-2015)
Abstract
This work presents a model for calculating the effective thermal conductivity of nanofluids. In this method, the effect of non-uniform sizes of nanoparticles and interfacial layer is investigated simultaneously. The developed model for the thermal conductivity of nanofluids takes into account the effects of the thermal conductivity of base fluids, the thermal conductivity, the volume fraction and the size of nanoparticles, the interfacial layer, non-uniform sizes of nanoparticles, Brownian motion and temperature. Hence, this model has the capability of offering both analytical and numerical Predictions. The accuracy of proposed model for the effective thermal conductivity of water-〖 Al〗_2 O_3, ethylene glycol-〖 Al〗_2 O_3, water- CuO, ethylene glycol-CuO, ethylene glycol-Al, water- TiO_2 is investigated. The effect of temperature, size of nanopartcles and volume fraction of nanopartcles is determined. Results show that the interfacial layer at the nanoparticle-liquid interface and non-uniform sizes of nonparticles are the important parameters for calculating the thermal conductivity of nanofluids. The Comparison between the result and available experimental data of several types of nanofluids indicates that the proposed model provides accurate results and the maximum error is 5%.
Volume 15, Issue 5 (7-2015)
Abstract
Awareness of the thermal conductivity of nanofluids regard to a significant development for use in research,it is necessary with regard to the inability of the analytical and experimental models that presented in most cases, it experimentally thermal conductivity can be measured. In this paper, the design and performance of thermal conductivity of fluids and nanofluidics measurement device without using a Wheatstone bridge is tested. Wheatstone bridge short transient hot wire method has previously been used for construction that requiring complex electronic systems and high power consumption. In this paper, a new method is provided so that no current or voltage is kept constant, but the method of measuring the relative resistance of the copper-clad lacquered with a diameter of 40 microns was used probe is easy to is within reach. The difference between the results of the design references, 1.17% is obtained. In this regard, changes in the magnetic fluid thermal conductivity is studied experimentally. Magnetic fluids are a new class of nanofluids are affected by magnetic fields and their properties can be changed. Fe3O4 magnetic water-based tests for different volume percentages.
Volume 16, Issue 1 (3-2016)
Abstract
In this paper, efficiency of defected graphene nano ribbon incorporating with additional nanoparticles on mass detection operations is studied via the Reverse Non Equilibrium Molecular Dynamics (RNEMD) method. Thermal conductivity management of this structure is challenging because of imposed losses in electrical conductivity and any procedure could manage the thermal conductivity of graphene will be useful. In this paper it is observed that on the mass detection operation, due to the porosity generation in the nano ribbon surface or even creation of external nanoparticles, thermal properties of graphene change considerably. This should be noted in calibration of graphene based mass sensors. In summary, results show that the graphene’s thermal conductivity would reduce by increasing the concentration of nanoparticles and thermal conductivity of graphene is higher when porosities and impurities are at the edges. This indicates that the location of vacancies and nanoparticles influences the thermal conductivity. For a better thermal management with the help of nanoparticles, wither respect to the porosities, addition of nanoparticles decrease the thermal conductivity more and more. By increasing the cavity’s diameter from 0.5nm to 4.4nm in a specific single layer graphene, thermal conductivity was reduced from 67 W/mk to 1.43 W/mk.
Volume 16, Issue 12 (2-2017)
Abstract
In this study, a Lattice Boltzmann Method (LBM) has been developed to calculate the distribution of a scalar quantity, like temperature, in a natural convection flow field under the condition of varying fluid thermal conductivity. The standard form of an LBM usually considers the fluid properties to be constant without any source term in conservation equations. The model developed is to account for variation of thermal conductivity with temperature in the presence of an external heat source. The proposed model has been examined against various case studies. It is shown that it is capable of modeling the extremely nonlinear problems. To magnify the nonlinear term in the natural convection case of under study, the radiation and other thermal sources have been used. The multiple relaxation time scheme has been applied to assure the solution stability. Using Chapman-Enskog analysis, the error associated with the proposed model has been estimated. The part of error which was not due to variations in the fluid properties, may be eliminated by introducing a correction term in higher order terms in Chapman-Enskog analysis. In addition, it has been shown that the correction term associated with the fluid conductivity variations, create an error of second order in terms of Knudsen number and is negligible. The present LBM model has an error of the second order of magnitude with respect to time.
Volume 16, Issue 12 (2-2017)
Abstract
This Paper, with the help of the device was made in this university as "rapid prototyping device base on direct metal laser melting", study interaction of metal powder apparent density and heat transfer experimentally. Selective Laser Melting (SLM) is a direct fabrication of part through layer by layer powder deposition and successive laser beam irradiation. One of the important properties of the SLM is thermal conductivity and thermal diffusivity of the metal powder. In this paper, thermal conductivity and diffusivity of metal powder with various apparent densities were studied. According to the method of measuring (the difference between two temperatures), The tests showed the dependence of thermal properties to metal powder apparent density. Changes in apparent density was established through the pressure applied to the raw powder bed. Because achieve to desirable apparent density through proper distribution is much expensive. This study was done in range of apparent density 44.75% to 56.4% compared to the density of pure iron. Comparing the samples produced in different densities it was understood that the pressure applied to the raw powder bed with the optimum point of arrest. In fact, the best quality of the manufactured parts, in density of about 46% was obtained.
Volume 19, Issue 3 (3-2019)
Abstract
In this study, the thermo-physical properties effects of the heat exchanger body on the adsorption chillers performance have been investigated. For this purpose, an adsorbent bed with a rectangular finned flat-tube heat exchanger is simulated by employing a three-dimensional control volume scheme. Furthermore, silica gel SWS-1L-water has been used as a working pair. In order to investigate the effects of thermo-physical properties of the heat exchanger body material, two main parameters including the thermal conductivity coefficient and the volumetric thermal capacity are examined. Also, the effects of these parameters along with variations of the fin height and fin pitch on the specific cooling power (SCP) and the system coefficient of performance (COP) are investigated. The results indicated that the SCP increases with the increase in thermal conductivity coefficient up to a certain value, which increases and decreases with the increase in fin height and fin pitch, respectively. The results also showed that the effects of the volumetric thermal capacity on the SCP are negligible such that it can be considered independent of the heat exchanger body material volumetric thermal capacity. Unlike the SCP, the COP is strongly influenced by the volumetric thermal capacity. The increase in volumetric thermal capacity results in decreasing the COP. The slope of the decrease in the COP decreases with increasing the fin height and pitch. Also, by increasing the thermal conductivity coefficient, the COP slightly decreases.
Volume 19, Issue 9 (9-2019)
Abstract
Heat transfer of polymeric foams is consisting of three different mechanisms including heat transfer through a solid phase, gas phase, and thermal radiation. Thermal insulation properties of polymeric foams are affected by different structural properties. Also, these structural properties have a different influence on the different heat transfer’s mechanisms. Therefore, it is necessary to use theoretical models. Several theoretical models have been presented so far, meanwhile, providing theoretical models that can estimate the thermal conductivity using the easiest measurable properties along with sufficient accuracy and reliability can be very helpful. In this regard in the present study, a theoretical model based on cell size and foam density is developed in order to predict the thermal properties of polymeric foams. It was concluded that the error of the developed theoretical model is lower than 8% in comparison to the experimental results. In the following, the effect of most important structural parameters i.e. foam density and cell size on the thermal conductivity is investigated. Based on the results, determining the optimum density is necessary to achieve the lowest thermal conductivity. Also, the gas thermal conduction has the most contribution to the overall thermal conductivity and achieving the nanometer cell sizes can be useful in order to decrease it.
Volume 19, Issue 9 (9-2019)
Abstract
Polymeric foams are one of the best candidates for thermal insulation. Accordingly, to investigate the thermal insulation properties of polymeric foams has attracted the attention of scientific communities in recent years. In this study, optimization of thermal insulation properties of polymeric foams is performed from solid and radiation thermal conductivities points of view. In this regard, a theoretical model based on cell size and foam density is developed. The results of the developed theoretical model are verified in comparison to various experimental results. Based on the results, the error of the theoretical model is lesser than 5%. Decreasing the foam density increases and decreases the solid and radiation thermal conductivity, respectively. Also, the radiation thermal conductivity is decreased by reducing the cell size. Response surface method (RSM) is applied in order to optimize the solid and radiation thermal conductivities. The results illuminate that the foam density of 23.5 kg.m-3 and cell size of 53 μm are the optimum conditions. At the optimum conditions, both of the solid and radiation thermal conductivities are lesser than 3 mW/mK. According to the results, the data obtained from developed theoretical model and RSM are in a good agreement. The total thermal conductivity is 30 mW/mK at optimum conditions which is a desirable value at aforementioned cell size range.
Volume 19, Issue 9 (9-2019)
Abstract
In this paper, the thermal conductivity coefficient of multi-walled boron nitride nanotubes has been investigated, using molecular dynamics simulation based on the Tersoff and Lenard Jones potential functions. The effects of diameter, length, and temperature on the thermal conductivity of double-walled boron nitride nanotubes have been studied. Also, by considering the 2, 3, 4, and 5-wall nanotubes, the effect of number of walls on the thermal conductivity of boron nitride nanotubes were studied. Finally, by considering of zigzag and armchair nanotubes, the effect of chirality has been investigated. The results showed that the thermal conductivity coefficient of double-walled boron nitride nanotubes increases by increasing the diameter of nanotubes and decreases by increasing temperature. It had been demonstrated that with 73% and 82% increase in the outer diameter of nanotubes, the thermal conductivity increases 93% and 98%, respectively. Furthermore, regarding to the chirality, the armchair nanotubes have a higher thermal conductivity than the zigzag ones. Also, the simulation results showed that thermal conductivity coefficient increases by increasing the length of boron nitride nanotubes and 50% increase of effective nanotube length increases the thermal conductivity by 25% approximately. Finally, by studying the effect of the number of walls, it is concluded that in the same length and temperature, nanotubes with higher number of walls have higher thermal conductivity coefficient in comparison.
Volume 20, Issue 10 (10-2020)
Abstract
Heat exchangers facilitate the transfer of heat between fluids with different temperatures. Compared with solids, most fluids have lower heat transfer coefficients and as a result, the use of high heat transfer coefficient solid particles as additives can increase the convective heat transfer coefficient of the fluid. In this study, the effect of the addition of nanoparticles to the base fluid (deionized water), application of triangular-cut twisted tapes as well as corrugation of shell and tube type heat exchangers pipes, is investigated on heat transfer values, friction coefficient variations as well as variations in performance evaluation criterion. The effects of addition of 0.7 and 1% magnesium-oxide nanoparticles on heat transfer coefficient improvements is investigated and the results of simultaneous application of magnesium-oxide water nanoparticles, corrugated pipes, and twisted tapes are compared. Comparisons against the basic conditions (deionized water without nanofluid, corrugated pipes or triangular-cut twisted tapes) indicate a 48% increase in thermal performance, a minuscule increase of 6.3% in friction coefficient and a 46% increase in the performance evaluation criterion as a result of the application of %0.7 magnesium-oxide water nanoparticles, use of corrugated pipes and triangular cut twisted tapes on the inner surface of shell and tube heat exchanger piping. Also, the application of 1% magnesium-oxide water nanofluid, and simultaneous use of corrugated pipes and triangular-cut twisted tapes on shell and tube heat exchanger piping inner surface results in a 72% increase in thermal performance, a minuscule increase of 6.9% in friction coefficient and a 70% increase in the performance evaluation criterion.
Hadi Bahmani, Davood Mostofinejad,
Volume 24, Issue 6 (11-2024)
Abstract
Prior research has not explored the creation of concrete with superior thermal insulation properties using calcium oxide-activated materials. Furthermore, the impact of substituting large proportions of sand with worn rubber powder and PET on the mechanical and thermal characteristics of high-performance geopolymer concrete remains uninvestigated. This study addresses these gaps by examining the development of geopolymeric concrete with enhanced thermal insulation properties using calcium oxide-activated materials. A novel mixing method has been devised to improve the compaction of thermally insulating concrete, which includes calcium oxide-activated slag. For the purposes of this research, worn rubber powder and PET powder have replaced 10%, 20%, 30%, 40%, and 50% of the aggregates. The mechanical properties of the concrete were determined through compressive strength, four-point bending, and tensile strength tests. Lastly, the thermal conductivity coefficient was tested to ascertain the thermal properties of the developed concrete.
The findings revealed that in the developed concrete, substituting 10% of the aggregates with worn rubber powder or PET powder increased the energy absorption capacity of the concrete by 143% and 107%, respectively, while its mechanical properties decreased by 10% and 7%, respectively. Moreover, using 50% worn rubber powder and PET as aggregate substitutes reduced the samples’ thermal conductivity by 70% and 60%, respectively.
