Transient Hot-wire Method Research Articles

A Novel Environmental Route to Ambient Pressure Dried Thermal Insulating Silica Aerogel via Recycled Coal Gangue

Coal gangue, one of the main hazardous emissions of purifying coal from coalmine industry, is rich in silica and alumina. However, the recycling of the waste is normally restricted by less efficient techniques and low attractive output; the utilization of such waste is still staying lower than 15%. In this work, the silica aerogel materials were synthesized by using a precursor extracted from recycled silicon-rich coal gangue, followed by a single-step surface silylation and ambient pressure drying. A low density (~0.19 g/cm3) nanostructured aerogel with a 3D open porous microstructure and high surface area (~690 m2/g) was synthesized, which presents a superior thermal insulation performance (~26.5 mW·m−1·K−1of a plane packed of 4-5 mm granules which was confirmed by transient hot-wire method). This study offers a new facile route to the synthesis of insulating aerogel material by recycling solid waste coal gangue and presents a potential cost reduction of industrial production of silica aerogels.

燃焼生成微粒子を混合した液体の熱物性に関する研究

Measurements of the thermal conductivity and specific heat of nanofluid of carbon black were performed at room temperature. The transient hot-wire method using parylene-coated Pt wires was employed for measurement of thermal conductivity. The thermal insulation method was employed for measurement of specific heat. The present paper shows these properties of base fluid (aqueous solution of 2.5 w% polyvinylpyrrolidone) and carbon black nanofluid (nanoparticle concentration of 5.0 w%). As a result of experiment, thermal conductivity of the nanofluid is higher than that of the base fluid because it is expected that thermal conductivity of carbon black is higher than that of water. However specific heat of the carbon black nanofluid is lower because it is expected that specific heat of carbon black is lower than that of water.

ВЛАСТИВОСТІ ПЕРЕНОСУ НАНОФЛЮЇДІВ (ЕКСПЕРИМЕНТ І МЕТОДИ РОЗРАХУНКУ)

A review and analysis of experimental studies and methods of calculation of thermal conductivity and viscosity of nanofluids with Al2O3 particles are presented. Analysis of published data shows that the results are not consistent. The greatest influence on the thermal conductivity and viscosity of nanofluids make a concentration of the nanoparticles. In addition to concentration, a major factors influencing the transport properties are the following: a method of producing nanofluids and the methods of their preparation for the research; the size and shape of initial nanoparticles; the ability of the nanoparticles to form clusters with different size; temperature and pressure; surface–active substances (surfactants); type and properties of the base fluids; the methodology of the experiment and its adaptation to the specifics of nanofluids. The results of experimental research of thermal conductivity and viscosity for the model system isopropyl alcohol – Al2O3 nanoparticles at various nanoparticle concentrations and temperatures are given. Study of thermal conductivity was carried out by two independent methods – stady-state and transient hot wire methods. The viscosity was measured with the capillary viscometers. The influence of nanoparticles additives on the viscosity of refrigeration compressor oil is also studied. The results of the measurements are presented in the form of simple models.

A Galinstan-Filled Capillary Probe for Thermal Conductivity Measurements and Its Application to Molten Eutectic $$\hbox {KNO}_3$$–$$\hbox {NaNO}_3$$–$$\hbox {NO}_2$$ (HTS) up to 700 K

The successful measurement of the thermal conductivity of molten salts is a challenging undertaking due to the electrically conducting and possibly also aggressive nature of the materials, as well as the elevated temperatures at which these data are required. For accurate and reproducible measurements, it is important to develop a suitable experimental apparatus and methodology. In this study, we explore a modified version of the transient hot-wire method, which employs a molten-metal-filled capillary in order to circumvent some of the issues encountered in previous studies. Specifically, by using a novel flexible U-shaped quartz-capillary, filled with a eutectic mixture of gallium, indium and tin, commercially known as Galinstan, we proceed to measure the thermal conductivity of molten eutectic $$\hbox {KNO}_3$$ – $$\hbox {NaNO}_3$$ – $$\hbox {NaNO}_2$$ . The new probe is demonstrated as being able to measure the thermal conductivity of this molten salt, which is found to range from $$0.48~\hbox {W}{\cdot }\hbox {m}^{-1}{\cdot }\hbox {K}^{-1}$$ at 500 K to $$0.47~\hbox {W}{\cdot }\hbox {m}^{-1}{\cdot }\hbox {K}^{-1}$$ at close to 700 K, with an overall expanded uncertainty (95 % confidence) of 3.1 %. The quartz is found to retain its electrically insulating properties and no current leakage is detected in the sample over the investigated temperature range. The thermal conductivity data reported in the present study are also used to elucidate a partial disagreement found in the literature for this material.

Thermal Properties of Silica Aerogel Formula

The thermal conductivity of silica aerogel developed in this research program was measured using the transient hot-wire technique. The thermal conductivity of monolithic samples drops significantly from 9.3 mW/m · K to 3.2 mW/m · K with modest pressure reduction from 1 atm to 0.1 atm. The same aerogel in granular form has a thermal conductivity of 15.0 mW/m · K at ambient gas pressure with a modest compression applied to compact the granules and reduce the air void sizes. Radiation heat transfer in the hot-wire test may not be representative of its contribution in large scale applications. Measurements of the monochromatic extinction coefficient over the wavelengths of interest resulted in a Rosseland mean extinction coefficient of 2400 m−1 at 300 K. The small thermal penetration distance during the hot-wire measurements suggest that in actual use radiation could contribute approximately 2.5 mW/m · K with a possible upper limit of 3.0 mW/m · K to the effective thermal conductivity over that measured using the transient hot-wire method.

Effect of particle shape on suspension stability and thermal conductivities of water-based bohemite alumina nanofluids

The suspension stability and thermal conductivity of water-based bohemite alumina nanofluids created using nanoparticles of various shapes (brick, platelet, and blade) at concentrations from 0.3 vol% to 7.0 vol% were theoretically and experimentally investigated. To quantitatively examine the effect of nanoparticle shape on suspension stability, this study uses the laser-scattering method rather than the zeta-potential measurement or the sedimentation test because both the zeta-potential and sedimentation tests cannot systematically represent the suspension stability of nanofluids. Using the DLVO (Derjaguin and Landau, Verwey and Overbeek) theory, we explain why the suspension stability varies with nanoparticle shape despite similar volume fraction of nanoparticles, pH, and temperature. The thermal conductivities are also measured by the transient hot wire method, which was developed in house. Experimental data are compared with theoretical results predicted by the Hamilton–Crosser model, which considers the effect of nanoparticle shape. It is shown that the model cannot predict nanofluids thermal conductivity relative to nanoparticle shape. Finally it is clearly shown that the thermal conductivity of nanofluids strongly depends on the suspension stability of bohemite alumina with various shapes.

Experimental evaluation of thermal conductivity, viscosity and breakdown voltage AC of nanofluids of carbon nanotubes and diamond in transformer oil

The present work reports new experimental results concerning the thermal conductivity, viscosity and breakdown voltage of solutions of diamond nanoparticles and multiwalled carbon nanotubes dispersed in mineral insulating oil for electrical transformers. The thermal conductivity of the solutions (also named nanofluids) was measured using the transient hot wire method. A controlled stress rheometer operating with coaxial cylinders was used for obtaining the shear stress versus shear rate curves and dynamic viscosity measurements. The dielectric strengths of the nanofluids were evaluated through the measurements of the insulation breakdown voltage AC using a dielectric strength analyzer with brass electrodes. From the experimental results, it was found that thermal conductivity and dynamic viscosity increase with increasing the particle concentration. On the other hand, and similarly to what is observed in the literature, the addition of these nanoparticles to the oil decreases its breakdown voltage.

물/부동액-기반Al<sub>2</sub>O<sub>3</sub>나노유체를 이용한 차량용 냉각시스템 성능 향상에 관한 실험적 연구

In this study, the thermal performance of vehicle’s cooling system is experimentally investigated using the water/coolantbased Al2O3 nanofluids as working fluids. For the purpose, the water/coolant-based Al2O3 nanofluids are prepared by twostep method with gum arabic. In order to obtain the well-suspended nanofluids, the agglomerated Al2O3 nanoparticles are precipitated using centrifugal force and the experiments are performed with supernatant of them. The thermal conductivity is measured by transient hot wire method and the thermal conductivity of nanofluids is enhanced up to 4.8% as compared to that of base fluids. Moreover, the cooling performance of water/coolant-based Al2O3 nanofluids is evaluated using vehicle’s engine simulator under the constant RPM condition. The results show that the cooling performance of automobile engine increases up to 5.9% using prepared nanofluids. To investigate the effect of nanofluids on exhaust gas, the NOx emission is measured during the operation with respect to time and 10.3% of NOx emission is decreased. The experimental results imply that the water/coolant-based Al2O3 nanofluids might be used as a next-generation vehicles’ coolant

Quantitative analyses of factors affecting thermal conductivity of nanofluids using an improved transient hot-wire method apparatus

While there are several factors affecting nanofluid thermal conductivity such as particle size, particle and fluid type, temperature and volume concentration, the effect of each factor on the thermal conductivity of the nanofluid is not clarified despite numerous experimental and theoretical efforts. In this work, four nanofluid samples with different particle size and volume fraction were prepared and the impact of the particle size and volume fraction on the effective thermal conductivity of the ethylene glycol/Al2O3 nanofluid was investigated using advanced transient hot wire system which avoids the capacitance influence and natural convection. The analysis result shows that particle size is more influential than volume fraction at the very low volume fraction less than 0.25% and the impact is more obvious with lesser volume fraction. Other than particle size and volume fraction, particle number density was turned out to be statistically very important factor affecting thermal conductivity of nanofluid when the volume fraction reaches to a certain degree of 0.2%. Under this volume fraction, the thermal conductivity of nanofluid decreases with the number density and this result may be attributed to hydrodynamic interactions induced by large fluid bodies traveling with nanoparticles in Brownian motion. This work provides not only the quantitative analyses of factors affecting the thermal conductivity of nanofluids but also the possible mechanisms of enhanced thermal conductivity.

Study of thermal conductivity enhancement of aqueous suspensions containing silver nanoparticles

Nanofluids are prepared by dispersing polyvinylpyrrolidone coated silver nanoparticles in distilled water. The thermal conductivity of nanofluids is measured by KD2 Pro thermal analyzer which is based on transient hot wire method. The influence of size and concentration of nanoparticles, surfactant and temperature of suspensions on the enhancement of the thermal conductivity is analyzed. The experimental results show that the thermal conductivity of nanofluids increases with the decrease in the size and increase in the concentration of the nanoparticles. Even with low volume fraction of 0.1 % and 20 nm size of silver nanoparticles, a high thermal conductivity enhancement of 54 % has been achieved. The surfactant and the temperature have a significant effect on the thermal conductivity enhancement of the nanofluids. The increase in temperature of the nanofluid from 30oC to 60oC increases its thermal conductivity up to 69 % whereas the addition of surfactant lessens the thermal conductivity enhancement to 34.2% with polyvinylpyrrolidone and 31.5 % with sodium dodecyl sulfate. The experimental results are compared with the existing theoretical models.

Thermal conductivity enhancement of lauric acid phase change nanocomposite with graphene nanoplatelets

In this work, we prepared lauric acid based phase change nanocomposites with chemically functionalized graphene nanoplatelets and measured its thermal conductivity using transient hot wire method. We show that inclusion of graphene nanoplatelets increases the thermal conductivity of phase change material by 230% at a loading of 1 vol%. Comparing the experimental results with the model calculations based on the effective medium theory suggests that graphene based nanocomposites outperforms those with carbon nanotubes or metal nanoparticles reported in the literature. High thermal conductivity, high aspect ratio and low thermal interface resistance at the graphene – host matrix interface makes it the most suitable nano filler candidate to enhance the thermal conductivity of low conductive materials. Differential scanning calorimetry study of the nanocomposites show that the phase change enthalpy and the melting temperature remains similar to that of pristine material, which makes graphene a promising candidate for thermal energy storage applications.

Thermal-Conductivity Measurements of Aviation Kerosene RP-3 from (285 to 513) K at Sub- and Supercritical Pressures

The thermal conductivity of a representative endothermic hydrocarbon aviation kerosene fuel RP-3 was accurately measured using the classical transient hot-wire method at sub- and supercritical pressures. The measured data cover a temperature range of 285 K to 513 K and a pressure range of 0.1 MPa to 5 MPa. The expanded uncertainty of the experiment was less than 3.0 % based on an uncertainty analysis. Furthermore, the measured data were correlated using a polynomial equation to analyze the deviations; 97.6 % of the measured data were within a 2 % error band. The average absolute deviation (AAD) and maximum absolute deviation (MAD) of the fitted thermal-conductivity data were 0.209 % and 2.31 % for all values, respectively.

Influence of various factors on the thermal conductivity of nanofluids

Influence of the main factors on the properties of nanolubricants, including the methods of their preparation, the size and shape of initial nanoparticles, their concentration, temperature, type and properties of the base fluids, the measuring procedure are considered. In this study, the results of experimental research of thermal conductivity of the model system isopropyl alcohol - nanoparticles Al 2 O 3 are presented. All measurements were conducted over a temperature range from 270 to 370 K at different mixture compositions using two independent methods: the steady-state hot-wire method and the transient hot-wire method. The size and concentration of nanoparticles in the lubricant were determined by dynamic light scattering (laser correlation spectroscopy). The analysis of the obtained data show that thermal conductivity become considerably increased due to nanoparticles even at small nanoparticle concentration (at the Al 2 O 3 volume concentration of 2.5 %, the thermal conductivity increases by 15-20 %). Based on the obtained data, the modified Maxwell model for thermal conductivity was developed.

Thermal conductivity of powders used in continuous casting of steelPart 2 – Powders

Measurements of the thermal conductivity of casting powders are needed to determine the magnitude of the vertical heat flux in the powder bed of a continuous casting mould. The thermal conductivities and thermal diffusivities of four mould powders have been determined using the transient plane source (TPS) and the transient hot wire (THW) methods. The values reported in this investigation are in good agreement with the results of earlier studies which used the THW method. However, the results were significantly different from λeff values obtained in thermal insulation tests. This was attributed to the large contribution to λeff from gaseous conduction which arises from the large temperature gradient across the sample. It was found that the thermal conductivity (λpowd) of the powders had a mean value of 0.125±0.025 W m−1 K−1 for the four powders studied; this value is in good agreement with two other studies; it increased as the temperature increased with a mean temperature coefficient of (dλ/dT = 9×10−5 W m−1 K−2 for the range 295–1100 K and increased as the bulk density increased (λ295 = 0.010+1.69×10−4ρbulk W m−1 K−1).

Selection of nanofluid for heat transfer applications from existing models of thermal conductivity

Nanofluids are gaining much importance over the past decade due to their enhanced thermal conductivity, specific heat, cooling capacity, electrical conductivities. Novel properties of nanofluids are yet to be explored to the highest potential applications. One of the prominent applications of nanofluids is in thermal conduction. The presence of nanoparticle in a fluid can enhance the thermal conductivities by several orders of magnitude (100-250).Experimental techniques involved in measuring thermal conductivity are transient hot wire method, steady state technique, and temperature oscillation technique, but suffer from drawbacks arising during measurement. Theoretical models are also proposed by Maxwell, Hamilton Crosser, Yu and Choi, Koo and Kleinstreuer, and Kumar. The present paper deals with identification of the best combination of nanoparticle and fluid by making use of existing models.

Comparative study of characteristics of water-ethanol binary mixture nanofluids for heat pipe efficiency enhancement

A nanofluid is a fluid containing suspended solid particles, with sizes on the order of nanometers. Normally, nanofluids have higher thermal conductivities than their base fluids. Therefore, we manufactured binary nanofluids by adding multi-walled carbon nanotubes to water-ethanol (ethanol concentration is 0.2) binary mixture nanofluids for efficiency improvement of a heat pipe. And we measured the thermal conductivity and viscosity of water-ethanol binary mixture nanofluids. The thermal conductivity and viscosity of binary nanofluids were measured by using transient hot-wire method and rotational digital viscometer, respectively. As a result, at the same volumetric ratio and temperature, the water-ethanol binary mixture nanofluids offer a lower rate of viscosity increase and a higher rate of thermal conductivity increase than distilled-water-based nanofluids. Therefore, nanofluids, formulated based on binary fluids mixed to the proper ratio, offer excellent performance that is clearly superior to that of single-component-based nanofluids.

Cement Reaction and Resultant Physical Properties of Cement Paste

The body of data about the reaction kinetics of cement has greatly grown in recent years, and in terms of techniques for the prediction of the resulting hydrate formation phases, databases contributing to thermodynamic equilibrium calculations have been developed and are growing increasingly sophisticated. On the other hand, though many studies on cement aim to contribute to concrete engineering and numerical model researchers have pointed out the necessity of shedding light on the relationship between the microstructures generated by hydration and physical properties, such data remains scarce. This study investigated Portland cements using two commonly used water-cement ratios and three types of mineral compositions in the material age range of up to one year. The phase composition in relation to cement hydration was determined by X-ray powder diffraction and Rietveld analysis. In terms of physical properties, compressive strength and Young's modulus were measured through loading tests, and Young's modulus and Poisson's ratio were also obtained from the ultrasonic pulse propagation velocity of longitudinal and transverse waves. Further, water vapor adsorption tests to shed light on moisture behavior were conducted, yielding the BET specific surface area. Thermal conductivity, an im- portant determinant of thermal behavior, was measured using the transient hot wire method. On the premise of the ap- plication of these physical properties to numerical calculations, their correlation with the information obtained from the phase composition was considered, leading to the proposal of simple and relatively high-accuracy empirical formulas. Further, background information about raising the correlation of these empirical formulas was also discussed.

Improved thermal properties of paraffin wax by the addition of TiO2 nanoparticles

TiO2 nanoparticles about 20 nm in diameter in the form of anatase were prepared and characterized. The nanoparticles were successfully dispersed into a paraffin wax (PW) matrix without any surfactant. The differential scanning calorimetric instrument and the transient short hot-wire method were used to measure the thermal properties of the TiO2/PW composites. It is found that the phase-change temperature and latent heat capacity vary with TiO2 nanoparticles loading levels. When the loading is not over 1 wt%, the phase-change temperature drops, and the latent heat capacity increases. When the loading is over 2 wt%, the phase-change temperature increases, and the latent heat capacity drops. A significant increase in latent heat capacity is found around 0.7 wt% loading. The thermal conductivity of the composites increases monotonically with increasing TiO2 loading. But such a tendency tends to decrease when the loading is over 3 wt%.

Thermal conductivity of ethyl fluoride (HFC161)

The thermal conductivity of ethyl fluoride (HFC161) was measured in both the saturated liquid and the gaseous phase by the transient hot wire method. Measurements of the thermal conductivity of the saturated liquid HFC161 were reported over the temperature range from 233K to 373K and in gaseous phase from 233K to 373K along 15 quasi-isotherms and at pressure up to 4470kPa. The total uncertainty of the gaseous experiment data was less than ±3% and that of liquid experimental results would be better than ±2%. The results of liquid phase were correlated as a function of temperature and the gaseous phase results were correlated as a function of temperature and pressure for the purpose of interpolation. The absolute mean deviation and maximum deviation of the liquid results from those calculated by polynomial were 0.53% and −1.84%, respectively. The maximum deviation and the absolute mean deviation of the gaseous results from the correlation were 2.94% and 1.18%, respectively.

Algorithm to optimize transient hot-wire thermal property measurement.

The transient hot-wire method has been widely used to measure the thermal conductivity of fluids. The ideal working equation is based on the solution of the transient heat conduction equation for an infinite linear heat source assuming no natural convection or thermal end effects. In practice, the assumptions inherent in the model are only valid for a portion of the measurement time. In this study, an algorithm was developed to automatically select the proper data range from a transient hot-wire experiment. Numerical simulations of the experiment were used in order to validate the algorithm. The experimental results show that the developed algorithm can be used to improve the accuracy of thermal conductivity measurements.