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.     

Thermal diffusivity and thermal conductivity of steam in the crossover region

In this paper, we present a comparison of the thermal diffusivity and thermal conductivity data of steam in the temperature range 0.02 K<T-T _c< 140 K with a recent formulation of crossover from singular to regular behavior of the transport properties of fluids. We have used two sets of experimental data previously obtained by the authors. The agreement between experimental and calculated data is good.     

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.     

A noncontact method for measuring thermal conductivity and thermal diffusivity of anisotropic materials

A noncontact method for measuring the thermal conductivity and thermal diffusivity of anisotropic materials is proposed. This method is based on the fact that the surface temperature variation with time depends on the thermal properties of the material when its surface is heated locally. The three-dimensional transient heat conduction equation in the material is solved numerically. The dimensionless average surface temperature variations are obtained along each principal axis: that is, thex andy axes. The relation between the dimensionless temperature and the Fourier number is expressed by a polynomial equation and used as a master plot, which is a basic relation to be compared with measured temperature variation. In the experiments, the material surface is heated with a laser beam and the surface temperature profiles are measured by an infrared thermometer. The measured temperature variations with time are compared with the master plots to yield the thermal conductivity λ _x and thermal diffusivityx _v in thex direction and the thermal conductivity ratioE _xy (=λ _y λ _x ) simultaneously. To confirm the applicability and the accuracy of the present method, measurements were performed on multilayered kent-paper, vinyl chloride, and polyethylene resin film, whose thermal properties are known. From numerical simulations, it is found that the present method can measure the thermophysical properties λ _x , α _x andE _xy within errors of ±6, ±22, and ±5%, respectively, when the measuring errors of the peak heat flux, the heating radius, and the surface temperature rise are assumed to be within ±2, ±3%, and ±0.2 K, respectively. This method could be applied to the measurement of thermophysical properties of biological materials.     

Experimental measurement of thermophysical properties of H2O/KCOOH (potassium formate) desiccant

This paper presents the measurement of the thermal conductivity and the dynamic viscosity of H₂O/KCOOH (potassium formate) desiccant with a salt concentration from 60 to 80% in the temperature range 1–80 °C. The thermal conductivity measurement gives evidence of a great sensitivity to salt concentration and lower sensitivity to temperature: H₂O/KCOOH desiccant shows a thermal conductivity from 23 to 33% lower than water at the same temperature. H₂O/KCOOH desiccant exhibits a Newtonian behaviour in all the investigated ranges of temperature and concentration. The relative viscosity shows a great sensitivity to salt concentration and weak or no sensitivity to temperature up to a solution concentration of salt around 70%. For higher solution concentration of salt (75 and 80%) the relative viscosity shows a great sensitivity also to temperature. H₂O/KCOOH desiccant presents a dynamic viscosity from 4 to 30 times higher than water at the same temperature.     

Thermal Conductivity Measurements of Aqueous SrCl2 and Sr(NO3)2 Solutions in the Temperature Range Between 293 and 473 K at Pressures up to 100 MPa

Accurate high-pressure thermal conductivity measurements have been performed on H₂O+SrCl₂ and H₂O+Sr(NO₃)₂ mixtures at pressures up to 100 MPa over a temperature range between 293 and 473 K using a parallel-plate apparatus. The concentrations studied were 0.025, 0.05, 0.10, 0.15, and 0.20 mass fraction of the salts. The estimated accuracy of the method is about ±1.6%. The pressure, temperature, and concentration dependences of the thermal conductivity have been studied. Measurements were made on six isobars, namely, 0.1, 20, 40, 60, 80, and 100 MPa. The thermal conductivity shows a linear dependence on pressure and concentration for all isotherms. Along each isobar, a given concentration shows the thermal-conductivity maximum at a temperature of about 413 K. The measured values of thermal conductivity at atmospheric pressure are compared with the results of other investigators. Literature data at atmospheric pressure reported by Ridel and by Zaitzev and Aseev agree with our thermal conductivity values within the estimated uncertainty.     

超临界二氧化碳物性参数测量方法综述

对于超临界二氧化碳(S-CO2)物性参数的获取,一般可从美国国家标准与技术研究院(NIST)直接查询,但S-CO2在临界点处物性发生剧烈变化.在实际工业应用中,S-CO2的密度、粘度和热导率这3个方面对与超临界二氧化碳布雷顿循环的研究有着很重要的作用,为了解目前S-CO2物性的测量方法,该文对当前所涉及到的S-CO2基...     

Comparison of the thermal conductivities of water and heavy water

New measurements of the thermal conductivity of H₂O and D₂O have been performed from critical temperature up to 510°C and from atmospheric pressure up to 100 MPA. As these measurements have been made with the same cell, a precise analysis of the isotopic effect as a function of temperature and density is possible. Our analysis is presented in terms of corresponding states. It is shown that the critical thermal conductivity excesses for H₂O and D₂O reduced by their respective background thermal conductivity terms are represented by single reduced isotherms.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Comparison of thermal conductivity data for partially stabilized zirconia with values derived from thermal diffusivity results

The thermal conductivity of partially stabilized zirconia was measured over the temperature range 320–1273 K using the radial heat flow method. The data have an absolute uncertainty of about ±2% and repeat measurements showed no evidence of changes in the thermal conductivity at high temperatures. This also was true for the thermal diffusivity data, which were obtained in vacuum over the temperature range 300–1473 K. Both sets of thermal conductivity data pass through minima at high temperatures. Quantitative differences were observed in the temperatures and thermal conductivities of the two minima. The results were analyzed by assuming parallel conduction by phonons and photons, and the phonon component was identified by fitting lower-temperature data. Extrapolating this curve allowed identification of the photon contribution to the thermal conductivity at high temperatures. The photon contribution approached a T ³ function and was larger in the thermal conductivity specimens. The difference in the photon contributions correlates with changes in the optical properties of the samples produced during the high temperature measurments.     

Saturation properties of the refrigerant 143A

1,1,1-Trifluoroethane (CH₃---CF₃; R143a) is currently under consideration as a potential candidate as a component in refrigerant mixtures. Among other things, the accurate knowledge of saturation properties of the pure fluids is a pre-requisite for evaluation of mixture properties. There is a considerable amount of scatter in the available data for R143a. This article presents a conciliation of saturation property data for this refrigerant. Vapour pressures, saturated liquid and vapour densities have been correlated so that the ancilliary equations are related through a power β used for the temperature functions. The coefficients of the equations have been determined from a weighted least squares method using all the available data. The surface tension, viscosity and thermal conductivity in the saturated liquid phase have also been correlated.     

The thermal conductivity of the molten NaNO3-KNO3 eutectic between 525 and 590 K

Molten salts are one of the few remaining classes of fluids for which standardquality (±1% accuracy) data on thermal conductivity have not hitherto been available. We have therefore developed a new apparatus based on the transient hot-wire technique to obtain reference-quality measurements of the thermal conductivity of molten salts at high temperatures. Liquid metal-filled quartz capillaries served as insulated hot wires in our method, and in addition, a two-wire technique was used in order to obtain absolute values of the thermal conductivity. New data for the NaNO₃-KNO₃ eutectic between 525 and 590 K are reported in this paper and comparisons with other recent measurements are shown.     

Probing Anisotropic Thermal Conductivity of Transition Metal Dichalcogenides MX2 (M = Mo,W and X = S,Se) using Time‐Domain Thermoreflectance

Transition metal dichalcogenides (TMDs) are a group of layered 2D semiconductors that have shown many intriguing electrical and optical properties. However, the thermal transport properties in TMDs are not well understood due to the challenges in characterizing anisotropic thermal conductivity. Here, a variable‐spot‐size time‐domain thermoreflectance approach is developed to simultaneously measure both the in‐plane and the through‐plane thermal conductivity of four kinds of layered TMDs (MoS₂, WS₂, MoSe₂, and WSe₂) over a wide temperature range, 80–300 K. Interestingly, it is found that both the through‐plane thermal conductivity and the Al/TMD interface conductance depend on the modulation frequency of the pump beam for all these four compounds. The frequency‐dependent thermal properties are attributed to the nonequilibrium thermal resistance between the different groups of phonons in the substrate. A two‐channel thermal model is used to analyze the nonequilibrium phonon transport and to derive the intrinsic thermal conductivity at the thermal equilibrium limit. The measurements of the thermal conductivities of bulk TMDs serve as an important benchmark for understanding the thermal conductivity of single‐ and few‐layer TMDs.     

Measurement of Thermal Conductivity of TiO2 Thin Films Using 3ω Method

TiO₂ film has been used in many industrial components such as laser filters, protection mirrors, chemical sensors, and optical catalysts. Therefore, the thermal properties of TiO₂ thin films are important in, e.g., reducing the thermal conductivity of ceramic coatings in gas turbines and increasing the laser damage threshold of antireflection coatings. The thermal conductivity of four kinds of TiO₂ thin films, prepared by dc magnetron sputtering, was measured using the 3 method in the temperature range from 80 K to room temperature. The results showed that the thermal conductivity of TiO₂ thin films strongly depends on the thickness and the microstructure of the films. The films with smaller grain size and thinner thickness have smaller thermal conductivities.     

Thermo-physical properties of Al2O3-SiO2/PAG composite nanolubricant for refrigeration system

Thermal conductivity and viscosity of the Al₂O₃-SiO₂/PAG composite nanolubricants for 0.02 to 0.1% volume concentrations at a temperature range of 303 to 353 K were investigated. Al₂O₃ and SiO₂ nanoparticles were dispersed in the Polyalkylene Glycol (PAG 46) lubricant using the two-step method of preparation. Thermal conductivity and viscosity were measured using KD2 Pro Thermal Properties Analyzer and LVDV-III Rheometer, respectively. The result shows that the thermal conductivity and viscosity of composite nanolubricants increase with volume concentration and decreases with temperature. Composite nanolubricants behave as Newtonian in the range of the temperatures and volume concentrations studied. The highest thermal conductivity increment is 2.41% at 0.1% concentration and temperature of 303 K. A maximum value of 9.71% in viscosity at 0.1% concentration is observed at temperature of 333 K. A new correlation model to predict the properties of composite nanolubricants has been proposed for applications in refrigeration systems.     

Thermal conductivity and viscosity of 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123)

The thermal conductivity and the viscosity data of CFC alternative refrigerant HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane: CHCI₂-CF₃) were critically evaluated and correlated on the basis of a comprehensive literature survey. Using the residual transport-property concept, we have developed the three-dimensional surfaces of the thermal conductivity-temperature-density and the viscosity-temperature-density. A dilute-gas function and an excess function of simple form were established for each property. The critical enhancement contribution was taken no account because reliable crossover equations of state and the thermal conductivity data are still missing in the critical region. The correlation for the thermal conductivity is valid at temperatures from 253 to 373 K, pressures up to 30 MPa, and densities up to 1633 kg m^–3. The correlation for the viscosity is valid at temperatures from 253 to 423 K, pressures up to 20 MPa. and densities up to 1608 kg·m^–3. The uncertainties of the present correlations are estimated to be 50% for both properties, since the experimental data are still scarce and somewhat contradictory in the vapor phase at present.     

Thermal Conductivity and Viscosity of Aqueous K<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>SO<Subscript><Emphasis Type="Bold">4</Emphasis></Subscript> Solutions at Temperatures from 298 to 575 K and at Pressures up to 30 MPa

The thermal conductivity of three (0.239, 0.499, and 0.782 mol·kg⁻¹) and the viscosity of four (0.0658, 0.2055, 0.3050, and 0.4070 mol·kg⁻¹) binary aqueous K₂SO₄ solutions have been measured with coaxial-cylinder (steady-state) and capillary-flow techniques, respectively. Measurements were made at pressures up to 30 MPa, and the range of temperature was 298–575 K. The total uncertainties of the thermal conductivity, viscosity, pressure, temperature, and composition measurements were estimated to be less than 2%, 1.6%, 0.05%, 30 mK, and 0.02%, respectively. The measured values of the thermal conductivity and viscosity of K₂SO₄ (aq) were compared with data and correlations reported in the literature. The reliability and accuracy of the experimental method was confirmed with measurements on pure water with well known (IAPWS standards) thermal conductivity and viscosity values (deviations, AAD, within 0.31 % and 0.52 %, respectively). The values of the viscosity A-, B-, and D-coefficients of the extended Jones–Dole equation for the relative viscosity (η/η₀) of aqueous K₂SO₄ solutions as a function of temperature were studied. The maximum of the B-coefficient near 340 K has been found. The derived values of the viscosity A- and B-coefficients were compared with results predicted by the Falkenhagen–Dole theory of electrolyte solutions and calculated with the ionic B-coefficient data. The behavior of the concentration dependence of the relative viscosity of aqueous K₂SO₄ solutions is discussed in terms of the modern theory of transport phenomena in electrolyte solutions.     

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%.     

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.     

Experimental investigation on the influence of high temperature on viscosity,thermal conductivity and absorbance of ammonia–water nanofluids

To select the optimal ammonia–water nanofluids and apply to ammonia–water absorption refrigeration systems (AARS), this paper investigated the influence of heating on viscosity, thermal conductivity and absorbance of binary nanofluids. The hysteresis phenomenon was observed after heating at high temperature which is rarely reported in the literature. Experimental results show that most of nanofluids' thermal conductivity increased by about 3–12% after heating. However, their viscosities increased by as much as 15% to 25% except the γ-TiO₂ ammonia–water nanofluid, which was reduced by 2% to 7%. This study also shows that the trend of viscosity is consistent with the absorbance. Due to fact that the thermal conductivity of γ-TiO₂/NH₃–H₂O mixture increased after heating, while the viscosity decreased, even if the concentration of the base liquid is 12.5% or 25%, therefore it is the optimal choice for practical research in AARS at present.     

Measurement of Thermal Transport Properties with an Improved Transient Plane Source Technique

The transient plane source (TPS) technique has been revised with the aim of developing a simple and fast system to measure the thermal transport properties of materials at low temperatures, especially high-T _c superconductors. To ensure reliable results, any new system should be tested with known samples. Fused silica, 0.9% carbon steel (215/3), and halide crystals (silver chloride) were studied with the new setup to check its performance. Data were taken from room temperature down to liquid nitrogen temperature. The assembly was designed for cryogenic (79 to 300 K) measurements in an atmosphere free of humidity. Dry nitrogen gas was used as a heat transfer medium around the sample holder assembly. The measured values for thermal conductivity and thermal diffusivity of these samples are in excellent agreement with values reported earlier. The thermal conductivity and thermal diffusivity for silver chloride crystals are extended down to 80 K although recommended data were available only down to 220 K. A Ba-doped, Bi-based, high-T _c superconductor was prepared by a solid-state reaction method. The nominal composition used was Bi_1.6Pb_0.4Sr_1.6Ba_0.4Ca₂Cu₃O_y. Large-sized samples (diameter 28mm and length 11mm) are investigated for thermal transport properties.