Simultaneous Measurements of the Thermal Conductivity and Thermal Diffusivity of Molten Salts with a Transient Short-Hot-Wire Method

A transient short-hot-wire technique has been successfully used to measure the thermal conductivity and thermal diffusivity of molten salts (NaNO₃, Li₂CO₃/K₂CO₃, and Li₂CO₃/Na₂CO₃) which are highly corrosive. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a short wire with the same length-to-diameter ratio and boundary conditions as those used in the actual experiments. In the present study, the wires are coated with a pure Al₂O₃ thin film by using a sputtering apparatus. The length and radius of the hot wire and the resistance ratio of the lead terminals and the entire probe are calibrated using water and toluene with known thermophysical properties. Using such a calibrated probe, the thermal conductivity and thermal diffusivity of molten nitrate are measured within errors of 3 and 20%, respectively. Also, the thermal conductivity of the molten carbonates can be measured within an error of 5%, although the thermal diffusivity can be measured within an error of 50%.     

MgO nanofluids: higher thermal conductivity and lower viscosity among ethylene glycol-based nanofluids containing oxide nanoparticles

Five kinds of oxides, including MgO, TiO₂, ZnO, Al₂O₃ and SiO₂ nanoparticles were selected as additives and ethylene glycol (EG) was used as base fluid to prepare stable nanofluids. Thermal transport property investigation demonstrated substantial increments in the thermal conductivity and viscosity of all these nanofluids with oxide nanoparticle addition in EG. Among all the studied nanofluids, MgO–EG nanofluid was found to have superior features, with the highest thermal conductivity and lowest viscosity. The thermal conductivity enhancement ratio of MgO–EG nanofluid increases nonlinearly with the volume fraction of nanoparticles. In the experimental temperature range of 10–60°C, thermal conductivity enhancement ratio of MgO–EG nanofluids appears to have a weak dependence on the temperature. Viscosity measurements showed that MgO–EG nanofluids demonstrated Newtonian rheological behaviour, and the viscosity significantly decreases with the temperature. The thermal conductivity and viscosity increments of the nanofluids are much higher than the corresponding values predicted by the existing classical models for the solid–liquid mixture.     

New Nusselt number correlations to predict the thermal conductivity of nanofluids

Research in thermal conductivity of nanofluids started only over the past decades. A great number of numerical and theoretical models have been proposed by several authors to predict the anomalous high thermal conductivity at hus, the total heat flux olow nanoparticles concentrations. This paper presents a new correlation of Nusselt number developed from the experimental data of nanofluids. Than a thermal conductivity model of nanofluids has been derived, based on the Monte Carlo simulations combined with the new Nusselt number correlations. The present work predicts the thermal conductivity of several nanofluids Al₂O₃/H₂O, CuO/H₂O, TiO₂/H₂O and CNT/H₂O with a good accuracy. In addition this model takes into account nanofluid temperature variation and different nanoparticles size. Moreover the results of the present model are shown in good agreement with the existing experimental data.     

Thermal Diffusivity Measurements of Liquid Silicate Melts

The effect of structure on the thermal diffusivities/conductivities for liquid silicates have been summarized based on recent experimental work carried out by the Royal Institute of Technology, Stockholm and the Tokyo Institute of Technology using the laser-flash and the hot-wire methods, respectively. In the former case, the effective thermal diffusivity was measured by a three-layer method. The relationship proposed by Mills that the thermal conductivity of silicates increases with a decrease in the ratio of NBO/T (number of non-bridging oxygens per tetrahedrally coordinated atom) has been well supported by the effective thermal diffusivity data for the liquid CaO-Al₂O₃-SiO₂ slags. However, it has been shown that for the slags having a higher CaO/Al₂O₃ ratio, the effective thermal diffusivity is roughly constant independent of the ratios of NBO/T. It has been concluded that when the silicate network is largely broken down, the phonon mean free path is not affected by the structure. It has been found by the hot-wire method that the magnitudes of thermal resistivity are in the hierarchy Li₂O-SiO₂2O-SiO₂2O-SiO₂ despite their similar values of NBO/T. It has been concluded that the ionicity of non-bridging oxygen ions is also a factor controlling the thermal conductivity of silicates as well as the number of broken bridges in the silicate network. The effective thermal diffusivity was measured for the CaO-Al₂O₃-SiO₂-FeO system to elucidate the radiation contribution to the effective thermal diffusivity. It has been found that the effective thermal diffusivity increases with an increase in FeO content. It can be considered that the strong absorption and emission within the liquid slag films caused by the Fe²⁺ ions enhances the photon heat transfer.     

Measurement of volumetric thermal expansion coefficient of various nanofluids

Experiments were carried out for studying volumetric thermal expansion behavior of various nanofluids in order to evaluate their potential application in heat removal systems employing natural convection as mode of heat removal. For this purpose, various nanoparticles such as Al₂O₃, CuO, SiO₂ and TiO₂ were used, which were suspended in the base fluid (water) by ultrasonication. All nanofluids had the same concentration of 1 wt %. Each nanofluid was heated from room temperature to a maximum of about 60°C and the increase in volume due to heat addition was recorded. The volumetric thermal expansion due to heating for each nanofluid was compared to that for the base fluid for same increase in the temperature. The volumetric thermal expansion coefficient was evaluated from the measured data. Surprisingly, it was found that the nanofluids have greater volumetric thermal expansion coefficients as compared to that of the base fluid. 1The text was submitted by the authors in English.     

Thermal Conductivity and Viscosity Measurements of Water-Based TiO<Subscript>2</Subscript> Nanofluids

In this study, the thermal conductivity and viscosity of TiO₂ nanoparticles in deionized water were investigated up to a volume fraction of 3% of particles. The nanofluid was prepared by dispersing TiO₂ nanoparticles in deionized water by using ultrasonic equipment. The mean diameter of TiO₂ nanoparticles was 21 nm. While the thermal conductivity of nanofluids has been measured in general using conventional techniques such as the transient hot-wire method, this work presents the application of the 3ω method for measuring the thermal conductivity. The 3ω method was validated by measuring the thermal conductivity of pure fluids (water, methanol, ethanol, and ethylene glycol), yielding accurate values within 2%. Following this validation, the effective thermal conductivity of TiO₂ nanoparticles in deionized water was measured at temperatures of 13 °C, 23 °C, 40 °C, and 55 °C. The experimental results showed that the thermal conductivity increases with an increase of particle volume fraction, and the enhancement was observed to be 7.4% over the base fluid for a nanofluid with 3% volume fraction of TiO₂ nanoparticles at 13 °C. The increase in viscosity with the increase of particle volume fraction was much more than predicted by the Einstein model. From this research, it seems that the increase in the nanofluid viscosity is larger than the enhancement in the thermal conductivity.     

Prediction and optimization of nanoclusters-based thermal conductivity of nanofluids: Application of Box–Behnken design (BBD)

The existing models to predict the thermal conductivity of nanofluids are based on single particle diameter, whereas, in actual solutions, nanoparticles mostly exist in a cluster form. Experiments are carried out to observe the effects of various surfactants on stability, nanocluster formation, and thermal conductivity of Al₂O₃–H₂O nanofluid, which is found to be improved considerably with SDS surfactant. The prolonged sonication was not adequate to break the clusters of Al₂O₃ nanoparticles, into an average size of less than 163 nm, indicating the tendency of Al₂O₃ nanoparticles to remain in the form of clusters instead of individual nanoparticles of primary size of 20 nm. Response surface methodology has been employed to design and optimize the experimental strategy by taking volumetric concentration, temperature, and surfactant amount as the contributing factors. The developed model has been validated against the experimental data and the existing models with an accuracy level of ± 8% in the former case. Analysis reveals about the formation of nanoclusters and enhancement in thermal conductivity. The results confirmed that the model can predict thermal conductivity enhancement with an accuracy level of R square value of the order of 0.9766.     

Thermal-Conductivity and Thermal-Diffusivity Measurements of Nanofluids by 3<Emphasis Type="Italic">ω</Emphasis> Method and Mechanism Analysis of Heat Transport

A 3ω technique is developed for simultaneous determination of the thermal conductivity and thermal diffusivity of nanofluids. The 3ω measuring system is established, in which a conductive wire is used as both heater and sensor. At first, the system is calibrated using water with known thermophysical properties. Then, the thermal conductivity and thermal diffusivity of TiO₂/distilled water nanofluids at different temperatures and volume fractions and the thermal conductivity of SiO₂ nanofluids with different carrier fluids (water, ethanol, and EG) are determined. The results show that the working temperature and the carrier fluid play important roles in the enhancement of thermal transport in nanofluids. These results agree with the predictions for the temperature dependence effect by the Brownian motion model and the micro-convection model. For SiO₂ nanofluids, the thermal-conductance enhancement becomes strong with a decrease in the heat capacity of the carrier fluids. Finally, according to our results and mechanism analysis, a corrected term is introduced to the Brownian motion model for providing better prediction of heat transport performance in nanofluids.     

Photothermal Study of Two Different Nanofluids Containing SiO2 and TiO2 Semiconductor Nanoparticles

Thermal and optical properties of two different nanofluids containing SiO₂ and TiO₂ semiconductor nanoparticles were studied by thermal lens spectrometry (TLS) and spectrophotometry. In the case of SiO₂ nanofluids the transmission electron microscopy technique was used to obtain the SiO₂ nanoparticle sizes to investigate the size effect of these nanoparticles on the sample thermal diffusivity which is important in some medical applications such as photothermal-modulated drug delivery systems. On the other hand for the case of TiO₂ nanofluids, the photopyroelectric technique, TLS, scanning electron microscopy, and X-ray diffraction were employed to investigate the concentration effect on the thermal properties of these nanofluids. Thermal diffusivities and effusivities as functions of the TiO₂ nanoparticle concentrations were obtained. From the experimental results, an incremental increase in the thermal diffusivities and effusivities was observed when the nanoparticle concentration was increased, indicating that the nanoparticle concentration is an important factor to be considered to obtain nanofluids with more thermal efficiency which are required for some applications, such as degradation of residual water.     

Thermal diffusivity and conductivity of Li2ZrO3 using the modulated electron beam technique

Thermal diffusivity and thermal conductivity of three Li₂ZrO₃ specimens with two different density values, 82.6 and 87% theoretical density, have been measured between 350 and 900°C. The modulated electron beam thermal diffusivity method was used. Heating the specimens above some 850°C but below 900°C results in a reversible increase in both the diffusivity and the conductivity. When heating to higher temperatures generally still higher increases are noticed but measurements become rapidly unstable and irreversible.     

An Experimental Study on Thermal and Electrical Properties of Packed Carbon Nanofibers

In the present study, the thermal and electrical properties of packed carbon nanofibers (P-CNFs) have been investigated. A short-hot-wire (SHW) technique was applied to determine simultaneously the thermal conductivity and thermal diffusivity of P-CNFs. In SHW measurements, a platinum wire coated with an alumina layer served as both the heating source and the thermometric sensor. A curve fitting method by matching the experimental data and numerical simulated values was proposed for determining the thermal conductivity and thermal diffusivity of P-CNFs with different packed densities. The electrical conductivities were measured by a four-terminal method where a special vessel with electrodes with circular plates was used. The results indicated that the electrical conductivity increases linearly with an increase in packed density. The thermal conductivity and thermal diffusivity also increase with an increase in packed density. The relation between the thermal conductivity and the electrical conductivity has been shown to be approximately linear. The SHW technique combined with the curve fitting method would be applicable to many kinds of materials.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Effect of sonication time on enhancement of effective thermal conductivity of nano TiO2–water,ethylene glycol,and paraffin oil nanofluids and models comparisons

This paper presents experimental and theoretical determination of the effective thermal conductivity of various base fluids and nano TiO₂ composition. Ultrasonically assisted sol–gel method was used for synthesising anatase TiO₂ nanoparticles and dispersing them into base fluids using sonication for the synthesis of nanofluids. It is observed that thermal conductivity enhancement is significantly higher than that of base fluid. The thermal conductivity shows an increment with the addition of nanoparticles and confirms a 22% enhancement achievable in base fluids. The effect of base fluids is a complex idea and difficult to understand; lower base fluid viscosities were supposed to contribute higher in enhancement of thermal conductivity, but another important factor; i.e. fluid nanoparticles surface interaction, nanoparticles crystal type also contributes in enhancement. In the further study, as the sonication time increases; an improvement in the thermal conductivity of nanofluids is also observed. Except water-based nanofluids, all others show reasonably good agreement with the data predicted by Bruggeman model and the prediction is in the range of 5%. This study is important since it covers base fluids with a wide range of thermal conductivity and viscosity.     

Measurements of the Thermal Conductivity and Thermal Diffusivity of Polymer Melts with the Short-Hot-Wire Method

In this paper, the thermal conductivity and thermal diffusivity of four kinds of polymer melts were measured by using the transient short-hot-wire method. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a wire with the same length-to-diameter ratio and boundary conditions as those in the actual experiments. The present method is particularly suitable for measurements of molten polymers where natural convection effects can be ignored due to their high viscosities. The results have shown that the present method can be used to measure the thermal conductivity and thermal diffusivity of molten polymers within uncertainties of 3 and 6%, respectively. Further, the thermal conductivity and thermal diffusivity of solidified samples were also measured and discussed.     

Free convection of hybrid Al2O3-Cu water nanofluid in a differentially heated porous cavity

Hybrid nanofluids are a new type of enhanced working fluids, engineered with enhanced thermo-physical properties. The hybrid nanofluids profit from the thermo-physical properties of more than one type of nanoparticles. The present study aims to address the free convective heat transfer of the Al₂O₃-Cu water hybrid nanofluid in a cavity filled with a porous medium. Two types of important porous media, glass ball and aluminum metal foam, are considered for the porous matrix. The experimental data show dramatic enhancement in the thermal conductivity and dynamic viscosity of the synthesized hybrid nanofluids, and hence, these thermophysical properties could not be modeled using available models of nanofluids. Thus, the actual available experimental data for the thermal conductivity and the dynamic viscosity of hybrid nanofluids are directly utilized in the present theoretical study. Various comparison with results published previously in the literature are performed and the results are found to be in excellent agreement. In most cases, the average Nusselt number Nu_l is decreasing function of the volume fraction of nanoparticles. The results show the reduction of heat transfer using nanoparticles in porous media. The observed reduction of the heat transfer rate is much higher for hybrid nanofluid compared to the single nanofluid.     

Thermal Diffusivity of Metallic Thin Films: Au, Sn, Mo, and Al/Ti Alloy

The thermal diffusivity of Au, Sn, Mo, and Al_0.97Ti_0.03 alloy thin films, which are commonly used in microelectromechanical (MEMs) system applications, is measured by two independent methods — the ac calorimetric and photothermal mirage methods. Both methods yield similar results of the thin-film thermal conductivity, but the uncertainty of the mirage technique is found to be relatively large because of the large temperature increase during the measurement. The measured thermal diffusivities of the thin films are generally lower than those of the same bulk material. Especially, the Al_0.97Ti_0.03 thin film shows a pronounced thermal conductivity drop compared with bulk Al, which is believed to be mainly due to impurity scattering. Comparison of the thermal conductivity with the electrical conductivity measured by the standard four-probe technique indicates that the relation of thermal and electrical conductivities follows the Wiedemann–Franz law for the case of Au and Sn thin films. However, the Lorentz number is significantly larger than the theoretical prediction for the case of Al_0.97Ti_0.03 and Mo thin films.     

无机纳米流体的热物性及其测试新方法的研究

研究了一种能同时测量纳米流体导热系数λ、导温系数α和比热C_p的非稳态多功能测试新方法,它结合了单丝法准确测量导热系数和双丝法准确测量导温系数的优点.对几种液体的导热系数和导温系数的实测值与TPRC推荐值进行比较,最大偏差分别为-0.4％和-2.7％.导热系数和导温系数测试均方根误差分别<±0.5％和±3％.并用此法测量了纳米Al₂O₃流体和纳米TiO₂流体的热物性参数,结果表明在流体中加入无机纳米粉体后其入和α较分散介质均有明显提高,加和原理不完全适用于纳米流体比热计算.     

Fracture energy measurements of Ceramic Thermal Barrier Coatings

The fracture energy of plasma-sprayed thermal barrier coatings of ZrO₂ · MgO, ZrO₂ · Y₂O₃, ZrO₂ · CaO · Al₂O₃, and Al₂O₃ · TiO₂ on metal substrates is obtained for both adhesive and cohesive failure modes of the coatings using notched layer-bonded bend test specimens. The generalized energy release rate was employed as a measure of the fracture energy. It takes account of the nonlinearity found in the load-deflection curves of the specimens. The failure of the specimens within the coating or at the interface is accompanied by the formation and linking of secondary cracks in the coating. This causes the non-linearity in the deformation behaviour and a rate-dependent failure morphology.     

Thermal conductivity of heavy-oxygen water H2O18

The ratio of thermal conductivity coefficients of heavy-oxygen water H₂O¹⁸ with different percentages of enrichment to the thermal conductivity coefficient of ordinary water is measured in the range of 0–40°C. Differences in the thermal conductivities and the temperature coefficients of the thermal conductivity of heavy-hydrogen water D₂O and and heavy-oxygen water H₂O¹⁸ are indicated.     

金属氧化物纳米流体的导热性能研究

采用瞬态热线法测量了4种不同种类、不同体积份额配比的纳米流体的导热系数,分析了纳米颗粒属性、体积分数、悬浮稳定性及温度等因素对纳米流体导热系数的影响.实验结果表明,在流体中加入纳米颗粒将显著提高流体的导热系数.     

Effect of Using CuO–Oil Nanofluid on Surface Roughness and Thermal Performance during Superfinishing Process

This research aims to investigate the effect of adding copper oxide nanoparticles to the oil Gr-6004 base fluid and its concentration changes from 0.1% to 0.4% on the surface roughness of gudgeon pin and the thermal conductivity of the nanofluids during the superfinishing process. The main novelty of this investigation is analyzing the impact of utilizing CuO/oil Gr-6004 nanofluid on thermal conductivity of oil and surface quality of gudgeon pin during superfinishing process. Based on the results, adding nanoparticles to the oil Gr-6004 has significantly reduced the surface roughness. In addition, by increasing the concentration of nanoparticles to 0.4%, the surface roughness has decreased by 57% compared to oil Gr-6004. Also, by adding nanoparticles, the thermal conductivity of the nanofluids has increased to 19.5%. In addition, dispersing CuO nanoparticles into base fluid reduces oil temperature by 17.44%.