Decrease In Thermal Diffusivity Research Articles

Thermal study of clay bricks reinforced by three ecological materials in south of morocco

An experimental measurement of thermal properties of clay mixed with three ecological materials, date palm fibers (DPF) and olive waste (OW) and straw was carried out by using the transient and steady state hot-plate and flash methods. The experimental methods are applied to measure the thermal properties of the composite materials. The composite samples were prepared with different volume fractions of the additive materials in the mixture. The thermal conductivity and diffusivity decrease from 0.65 W.m-1.K-1 and 4.21×10-7 m2.s-1 to 0.26 W.m-1.K-1 and 2.01×10-7m2.s-1, respectively, according to the variation of the volume fraction of additive materials varying from 0 to 30%. In order to valorize these ecological materials in building efficiency energy, the Taourirt kasbah which is a most important historical building in Ouarzazate, constructed using unfired clay bricks was simulated by Design builder software with the climate data for south Morocco location. The improvement of the thermal performances of simulated traditional buildings can contribute to prove the comfort without heating and cooling demands in order to get energy savings. The results showed the decreasing energy consumption without affecting indoor comfort levels according to outside temperatures.

Experimental study on response characteristics of micro–macroscopic performance of red sandstone after high-temperature treatment

The macroscopic responses to the progressive process of thermal damage of sandstone, as well as its micro-damage mechanisms, were obtained by measuring the macroscopic physical–mechanical properties and the microscopic properties of red sandstone that were heated at various temperatures up to 800 °C. The experimental results showed that the red sandstone sample had a critical damage threshold near 400 °C. When the heating temperature exceeded the threshold temperature, new fractures developed gradually, while the original fractures expanded rapidly. Some chemical reactions occurred in the minerals, i.e., decomposition, dehydroxylation, and fusion. These reactions resulted in the gradual damage of the mineral crystals and the microstructure. The response characteristics of the macroscopic physical–mechanical properties to the microscopic damage within this temperature range were intense. The damage mainly manifested as the rapid decrease in compressive strength, elastic modulus, tensile strength, P-wave velocity, and thermal diffusivity, and as the rapid increase in the strain and mass loss rate. The results are of great significance for understanding the mechanisms of rock thermal damage, as well as the study of the physical and mechanical properties of the deep rock mass.

Evaluation of aging process of silicone rubber composite insulators with photothermal radiometry

Silicone rubber composite insulators are increasingly used in high-voltage power transmission and distribution grids. However, the aging of composite insulators is a serious safety issue of power systems. Understanding the aging mechanism of composite insulators is therefore of great importance. In this paper, a 3D photothermal radiometry (PTR) is employed to determine the thermal and absorption properties of silicone rubber composite insulators aged in natural environment. A 3D PTR model was developed to extract precisely the thermal diffusivity and absorption coefficients of the composite insulators. In this PTR model both surface and bulk absorptions were accounted for by taking into consideration the fact that a surface absorption of aged composite insulators would be created and changed by surface crazing, chalking, and contamination during aging process. By multi-parameter fitting the measured modulation frequency dependences of PTR amplitude and phase to this 3D PTR model, the thermal diffusivity, surface absorption, as well as bulk absorption coefficient of the composite insulators were extracted simultaneously. The comparisons between the thermal diffusivities, absorption characteristics of fresh and aged composite insulators indicated that aging caused significant decrease of thermal diffusivity and substantial increase of absorption of composite insulators, and the thermal diffusivity and absorption changes were aging level dependent. These experimental results demonstrated that both thermal and absorption properties may be useful parameters for evaluating the aging level and understanding the aging mechanism of silicone rubber composite insulators.

Thermal diffusivity of tungsten irradiated with protons up to 5.8 dpa

Thermal properties of pure tungsten, irradiated in the Swiss neutron spallation source at the Paul Scherrer Institut, have been studied in the temperature range 25–500 °C. Disk-shaped specimens were prepared from a tungsten sheet which was irradiated with high energy protons and spallation neutrons. The specimens tested in this work received total damages of maximum 3.9 and 5.8 dpa at average irradiation temperatures of 115 and 140 °C, respectively. The thermal diffusivity of the irradiated tungsten was measured using the conventional flash method. For both specimens, the results show a significant decrease in thermal diffusivity after irradiation. Relative to unirradiated tungsten, the irradiated samples show thermal diffusivity values which are 28–51% lower, depending on temperature. Annealing of the irradiated specimen of 3.9 dpa at 1000 °C for 1 h resulted in a slight recovery of thermal diffusivity. In addition, thermal conductivity values were calculated from the thermal diffusivity data.

Effect of size and dimension dependent specific heat on thermal conductivity of nanostructured semiconductors

A thermodynamic model is proposed to study the collective effect of size and dimension dependent specific heat capacity along with phonon surface scattering on thermal conductivity as well as on diffusivity of nanostructured semiconductors. Thermal conductivity and diffusivity of pure and compound semiconducting nanomaterials for partially smooth and rough surfaces of different sizes and dimensions have been calculated. Thermal conductivity and diffusivity decrease with size and increases while moving towards the specular surfaces. Incorporation of size and dimension effect of specific heat capacity with phonon-interface scattering gives the better explanation of thermal conductivity, whereas classical approach alone is inadequate. To validate the obtained results, we have compared our results with the experimental data and other model reported in the literature.

Temperature dependence of the thermal diffusivity of sandstone

High temperature causes a significant impact on the thermal diffusivity of rocks. The deterioration of the thermal properties of rocks, would seriously threaten many applications in engineering and geosciences, such as petroleum engineering, geothermal energy. This paper emphasizes on the variations of thermal diffusivity of sandstone after high-temperature heating. The results indicate that there are two main reasons for the decrease of thermal diffusivity: the variation of mineralogical composition, the physical and chemical reactions caused by thermal stress. The thermal diffusivity of sandstone decreases with temperature and it tends to be constant once the temperature exceeds a critical value. From 25°C to 300 °C, the decrease is mainly due to the escaping of the adhered water, bound water and structural water. Between 300°C and 600 °C, the thermal response of minerals in sandstone increases the development of microcracks and weakens the thermal diffusivity of sandstone.

Thermal characterization of a new effective building material based on clay and olive waste

The influence of thermophysical properties of wall materials on energy performance and comfort in traditional building was investigated. The clay is the most commonly used sustainable building material. The study looked at the effects of the addition of pomace olive on the thermophysical properties of clay bricks to improve the energy efficiency of this ecological material. An experimental measurement of thermal properties of clay mixed with pomace olive was carried out by using the transient and steady state hot-plate and flash methods. The experimental methods are applied to measure the thermal properties of the composite material. The estimation of these thermal characteristics is based on a one dimensional model and the experimental errors are found less than 3%. The composite samples were prepared with different granular classes and mass fractions of the pomace olive in the mixture. The results show that the density of the new material was not substantially influenced by the size of the pomace olive. However, the thermal conductivity and diffusivity decrease from 0.65 W.m-1.K-1 and 4.21×10-7 m2.s-1 to 0.29 W.m-1.K-1 and 2.47×10-7m2.s-1, respectively, according to the variation of the volume fraction of pomace olive from 0 (pure clay) to 71% showing that the olive pomace can be used as effective secondary raw materials in the making of clay bricks.

Thermal characterization of a new effective building material based on clay and olive waste

The influence of thermophysical properties of wall materials on energy performance and comfort in traditional building was investigated. The clay is the most commonly used sustainable building material. The study looked at the effects of the addition of pomace olive on the thermophysical properties of clay bricks to improve the energy efficiency of this ecological material. An experimental measurement of thermal properties of clay mixed with pomace olive was carried out by using the transient and steady state hot-plate and flash methods. The experimental methods are applied to measure the thermal properties of the composite material. The estimation of these thermal characteristics is based on a one dimensional model and the experimental errors are found less than 3%. The composite samples were prepared with different granular classes and mass fractions of the pomace olive in the mixture. The results show that the density of the new material was not substantially influenced by the size of the pomace olive. However, the thermal conductivity and diffusivity decrease from 0.65 W.m-1.K-1 and 4.21×10-7 m2.s-1 to 0.29 W.m-1.K-1 and 2.47×10-7m2.s-1, respectively, according to the variation of the volume fraction of pomace olive from 0 (pure clay) to 71% showing that the olive pomace can be used as effective secondary raw materials in the making of clay bricks.

Convective Velocity Suppression via the Enhancement of the Subadiabatic Layer: Role of the Effective Prandtl Number

Abstract It has recently been recognized that the convective velocities achieved in current solar convection simulations might be overestimated. The newly revealed effects of the prevailing small-scale magnetic field within the convection zone may offer possible solutions to this problem. The small-scale magnetic fields can reduce the convective amplitude of small-scale motions through the Lorentz-force feedback, which concurrently inhibits the turbulent mixing of entropy between upflows and downflows. As a result, the effective Prandtl number may exceed unity inside the solar convection zone. In this paper, we propose and numerically confirm a possible suppression mechanism of convective velocity in the effectively high-Prandtl number regime. If the effective horizontal thermal diffusivity decreases (the Prandtl number accordingly increases), the subadiabatic layer which is formed near the base of the convection zone by continuous depositions of low entropy transported by adiabatically downflowing plumes is enhanced and extended. The global convective amplitude in the high-Prandtl thermal convection is thus reduced, especially in the lower part of the convection zone via the change in the mean entropy profile, which becomes more subadiabatic near the base and less superadiabatic in the bulk.

Experimental study of thermal properties of a new ecological building material based on peanut shells and plaster

The aim of the present work was to investigate the thermal properties of a new building material consisting of a mixture of plaster and peanut shells for use as insulating materials in building. The properties are commonly measured by using the steady state asymmetric hot plate method, the asymmetrical transient hot plate method and the flash method. The experimental study that we have conducted, enabled us to determine the conductivity, the effusivity and the thermal diffusivity of our material. The influence of the size and the mass fraction of the peanut shell shards on thermophysical properties of tested material, was investigated. Our experimental data show a good efficiency and a significant decrease in the thermal conductivity of material with peanut shell shards compared to simple plaster material. The purpose is to obtain ecological composite materials with better thermal performance, which can contribute to improve the thermal comfort in constructions in Morocco. The results show that the density of the new material was not substantially influenced by the size of the peanut shell shards. However, the thermal conductivity and diffusivity decrease from 0.3Wm−1K−1 and 3.75×10−7m2s−1 to 0.14Wm−1K−1 and 2.11×10−7m2s−1, respectively, according to the variation of the mass fraction of peanut from 0 to20%.

On numerical investigation of nonuniformity in cooling characteristic for different materials of target surfaces being exposed to impingement of air jet

Heat transfer using air jet impingement technique is one of the conspicuous tasks in the looming world of electronic packaging system. Here, the material selection of heat sink becomes one of the prior and important assignments to construct a heat sink with desired characteristic cooling rate. In order to study the material effect of heat sink over the cooling characteristic, the present work takes an initiative in plotting the Nusselt magnitude over the radial distance for different material of heat sink. This is done by computing the flow regime and heat transfer characteristic of a 2D axis symmetric geometry in commercial simulating software, ANSYS CFX. The computation of cooling characteristic in form of Nusselt profile is done using SST + Gamma–theta turbulence model. Since the prediction of heat interaction due to the intermediacy and transition in the flow regime is a unique issue of this problem. The results for Nusselt curve signifies a tangible elevation in local Nusselt value (nonuniformity) with decrease in thermal diffusivity of target surface. Also the nonuniformity is observed to vanish above a critical range (66.76[Formula: see text]mm2/s) of thermal diffusivity. This happens due to presences of abnormal turbulence of heat flow which occurs inside the target surface. Since the variation in thermal diffusivity causes some imbalance competition between the heat storage and dissipation capabilities. Above all the target surface carrying thermal diffusivity less than 66.76[Formula: see text]mm2/s possesses a dominating heat storage capability, on behalf of which some heat transfer occurring in near jet and far jet regions are being restricted. These are transferred towards stagnation region in radial direction.

Cooling Curve Analysis of Micro- and Nanographite Particle-Embedded Salt-PCMs for Thermal Energy Storage Applications

In recent years, the focus of phase change materials (PCM) research was on the development of salt mixtures with particle additives to improve their thermal energy storage (TES) functionalities. The effect of addition of microsized (50 μm) and nanosized (400 nm) graphite particles on TES parameters of potassium nitrate was analyzed in this work. A novel technique of computer-aided cooling curve analysis was employed here to study the suitability of large inhomogeneous PCM samples. The addition of graphite micro- and nanoparticles reduced the solidification time of the PCM significantly enhancing the heat removal rates, in the first thermal cycle. The benefits of dispersing nanoparticles diminished in successive 10 thermal cycles, and its performance was comparable to the microparticle-embedded PCM thereafter. The decay of TES functionalities on thermal cycling is attributed to the agglomeration of nanoparticles which was observed in SEM images. The thermal diffusivity property of the PCM decreased with addition of graphite particles. With no considerable change in the cooling rates and a simultaneous decrease in thermal diffusivity, it is concluded that the addition of graphite particles increased the specific heat capacity of the PCM. It is also suggested that the additive concentration should not be greater than 0.1% by weight of the PCM sample.

Interactions between molten salts and ash components during Zhundong coal gasification in eutectic carbonates

Molten salt gasification provides a promising way for the combined usage of solar energy and coal, during which molten salts act as a heat storage and transfer medium. However, the interactions between coal ash components and molten salts might have undesired effects on the molten salt gasification. The present study investigated the distribution of mineral matters in typical Zhundong coals and the fate of coal ashes in the high-temperature Li2CO3-Na2CO3-K2CO3 eutectic system. The results showed that alkali and alkaline earth metals (AAEMs) were widely distributed in Zhundong coals in the forms of NaCl, CaCO3, CaSO4 and organic matter, which were predominantly dissolved in the molten salts. After the dissolution of these AAEMs, the melting temperature and enthalpy of the molten salts were changed, while the viscosity of the mixtures was hardly affected in melting phase. On the other hand, Si/Al-compounds were also of high content in some Zhundong coals, existing as quartz, kaolin and amorphous species. Some of these compounds tended to react with molten salts, by forming Li-silicate or Li/K-aluminosilicates, which were precipitated out of the system. Meanwhile, the suspension of the unreacted Si/Al-compounds increased the viscosity of molten salts. Additionally, interactions between coal ashes and molten salts brought about the decrease in thermal diffusivity and thermal conductivity of the eutectic system as well as the increase in specific heat capacity.

Photopyroelectric Characterization of Magnetic Nanofluids. Influence of Type and Size of Nanoparticles on the Thermal Parameters

The influence of type and size of nanoparticles on the thermal parameters of some magnetic nanofluids is investigated. Two types of carrier liquids (transformer oil and polypropylene glycol) were combined with two types of iron based magnetic nanoparticles (\(\hbox {Fe}_{3}\hbox {O}_{4}\) and \(\hbox {MnFe}_{2}\hbox {O}_{4})\). Different sizes (10 nm–80 nm) and shapes (spherical, octahedral or cubic) of nanoparticles were obtained depending on the oleic acid/oleylamine molar ratio, which drastically influences the nanocrystals growth rate. This influence is due to the different binding ability of the two stabilizers onto crystal facets. The average size of nanoparticles was 10 nm, 35 nm and 50 nm for \(\hbox {Fe}_{3}\hbox {O}_{4}\) and 10 nm, 20 nm and 80 nm for \(\hbox {MnFe}_{2}\hbox {O}_{4}\) at a concentration of 50 mg\({\cdot }\)ml\(^{-1}\) in all cases. The results obtained by PPE technique indicate that, at this concentration, the presence of the nanoparticles reduces the value of the thermal parameters of pure carrier liquids and both thermal diffusivity and effusivity decrease with increasing nanoparticles size, independently on the carrier liquid. The influence of the nanoparticles size is more pronounced for the thermal effusivity (relative change 24 %) compared with thermal diffusivity (relative change 7 %).

Investigation of dilution effect with N2/CO2 on laminar burning velocity of premixed methane/oxygen mixtures using freely expanding spherical flames

The main objective of this work was to measure the dilution effect of nitrogen and carbon dioxide on the unstretched laminar burning velocities and differential diffusional stability of premixed methane-oxygen mixtures at ambient conditions and different equivalence ratios. Freely expanding spherical flame method was used. The combustible mixture was prepared by following (100-Z)%CH4+2O2ϕ+Z%(N2/CO2) where Z corresponds to the percentage of diluent in the mixture by volume and ϕ refers to equivalence ratio. The percentage of N2/CO2 in the mixture was varied between 50–70%/30–50%. The equivalence ratio was varied from 0.6 to 1.4. Necessary stretch correction to the measured data was done following weak stretch theory. PREMIX with GRIMech3.0 was used to simulate all the experimental operating conditions. The addition of diluents has reduced the burning velocity of the mixture considerably due to the decrease in energy content of the mixture, increase in specific heat capacity and the corresponding reduction in the flame temperature and decrease in thermal diffusivity. At 50%N2/CO2 in the premixed methane-oxygen mixtures, the burning velocity measured with CO2 was 3.6 (average value estimated for all studied equivalence ratios) times smaller than the mixtures diluted with N2. Burning velocity was reduced by 88% due to the addition of 50%CO2 as compared to 0%CO2 in the combustible mixture and out of which reduction due to thermal effect was 81.5% and chemical effect (participation of added CO2 in the chemical reactions) was 6.5% (average value estimated for all studied equivalence ratios). For all the studied dilution cases, the burned gas Markstein length remains positive indicating that the mixtures were stable towards the differential diffusion effects. Agreement of burning velocities obtained between PREMIX with GRIMech3.0 and measurements was not good at lower levels of diluents. Based on the literature, a new species, and few elementary reactions were added to original GRIMech3.0. By doing sensitivity analysis, kinetic parameters of some elementary reactions were also modified. Simulations with the modified GRIMech3.0 showed good agreement with most of the experimental data.

Structure and thermophysical properties of aluminum-matrix composites

The microstructure and thermophysical properties of aluminum-matrix composites have been studied, in which a granulated Al–Zn–Mg–Cu alloy has been used as the matrix, and SiC particles taken in the amounts of 10, 20, and 30 vol % have bee used as the filler. It has been shown that, with an increase in the amount of the filler, the temperatures of the solidus and liquidus of the composites and the values of the thermal expansion coefficient and density increase, whereas the heat capacity, thermal conductivity, and thermal diffusivity decrease. The heat capacity of the composite depends on the amount of the filler: upon heating from 25 to 500°С, the heat capacity of the composite with 10 vol % SiC increases by only 16%, while that of the composite with 20 vol % SiC increases by 19%; and, at 39 vol % SiC, it increases by 36%.

Consolidation Features of Aluminum-Alumina Compositions by Powder Metallurgy Methods

In this paper was followed the processing flow of aluminum-alumina compositions (10÷20% alumina) in powder state, aiming to obtain aluminum matrix composites reinforced with alumina particles, starting from selecting and mixing the grading fraction of both components reaching up to sintering; it was analyzed the way in which reflects the variation of grading fraction ratio (expressed through average particle diameter in the analyzed fractions limits) on the level of technological interest features: apparent density, tapped density, flowability, presability and on densification after sintering (in various environments). By transmission electron microscopy was observed that aluminum particles showed on the surface a nanoscale oxide film, so the sintering occurs between congeneric areas – by solid phase sintering mechanisms [1, 2, 3]. The analysis of thermophysical properties revealed a decrease of thermal diffusivity at an increase of alumina, simultaneous with the decrease of the densification level.

Thermal and mechanical properties of mechanically alloyed 304LSS-CNT metal matrix composites

A methodology for the creation of 304LSS-CNT metal matrix composites using the mechanical alloying approach is presented. Planetary ball milled powders were both melted and hot pressed and achieved up to 96% theoretical density. High resolution scanning electron microscopy, Scanning Transmission Electron Microscopy, X-ray diffraction, energy dispersive spectroscopy, thermal diffusivity measurements, and Vickers microhardness measurements are used to characterize as processed and heat treated composites. Melted and solidified samples show highly anisotropic austenite/martensite microstructures with the presence of large dendritic carbon agglomerations, while hot-pressed samples show equiaxed austenite/martensite grains with a large number density of carbide precipitates. Grain size and thermal diffusivity decrease while microhardness increases up to 36% with up to 2% carbon nanotube addition for hot-pressed samples. Thus, mechanical alloying has been shown to be a potential option for the production of homogeneous 304LSS-CNT metal matrix composites for applications requiring increased strength.

Thermal and mechanical properties of polypropylene-iron-diamond composites

Abstract Polypropylene-iron and polypropylene-iron-diamond composites with various amounts of iron and natural or synthetic diamond particles were prepared in a co-rotating laboratory kneader. Test samples were made by injection molding. Heat flow measurements with a differential scanning calorimeter and dynamic mechanical properties in the forced vibration mode were measured from 225 K to 475 K , and thermal diffusivity was measured from 225 K up to 425 K by the laser flash method. Specific heat capacity of the polypropylene-iron-diamond composite can be significantly reduced by the addition of iron particles, while the addition of diamond particles show only a minor influence. Conventional, reversal and non-reversal heat flow measurements are showing two different monoclinic crystal structures ( α 1 , α 2 ) and crystallinities around 50% in the polypropylene. In a heating cycle a conversion between both crystalline structures can appear. Due to different crystallinities and crystal structures the activation energies for the main relaxations at glass transition temperature vary between 275−400 kJ /( mol ⋅ K ) in the amorphous and between 65−190 kJ /( mol ⋅ K ) in the crystalline part of the polypropylene. The storage modulus of polypropylene is increased by the iron and diamond particles from 4000 MPa up to 12500 MPa . The thermal diffusivity can be increased from 0.13 mm 2 / s up to 1.05 mm 2 / s with iron and up to 1.45 mm 2 / s with the secondary addition of diamond powder. With increasing temperature the thermal diffusivity decreases. The particle network shows a dominant contribution of the polymer matrix for mechanical properties whereas the particles show a dominant contribution to thermal properties. The mean free path of phonons in polypropylene decreases with increasing temperature, while it decreases up to 300 K and increases then up to 15 times the value of neat polypropylene again for temperatures above 375 K in the composites.

Fast response of electron-scale turbulence to auxiliary heating cessation in National Spherical Torus Experiment

In this letter, we report the first observation of the fast response of electron-scale turbulence to auxiliary heating cessation in National Spherical Torus eXperiment [Ono et al., Nucl. Fusion 40, 557 (2000)]. The observation was made in a set of RF-heated L-mode plasmas with toroidal magnetic field of 0.55 T and plasma current of 300 kA. It is observed that electron-scale turbulence spectral power (measured with a high-k collective microwave scattering system) decreases significantly following fast cessation of RF heating that occurs in less than 200 μs. The large drop in the turbulence spectral power has a short time delay of about 1–2 ms relative to the RF cessation and happens on a time scale of 0.5–1 ms, much smaller than the energy confinement time of about 10 ms. Power balance analysis shows a factor of about 2 decrease in electron thermal diffusivity after the sudden drop of turbulence spectral power. Measured small changes in equilibrium profiles across the RF cessation are unlikely able to explain this sudden reduction in the measured turbulence and decrease in electron thermal transport, supported by local linear stability analysis and both local and global nonlinear gyrokinetic simulations. The observations imply that nonlocal flux-driven mechanism may be important for the observed turbulence and electron thermal transport.