Transient Hot-wire Technique Research Articles

Thermal Conductivity of Liquid 2-Methoxyethyl Acetate, 2-Ethylhexyl Acetate, and Diethyl Succinate

The thermal conductivity of 2-methoxyethyl acetate, 2-ethylhexyl acetate, and diethyl succinate in liquid phase was reported. The measurements of 2-methoxyethyl acetate and 2-ethylhexyl acetate, covering a temperature range from (233 to 373) K and pressure up to 20 MPa, have been performed by a transient hot-wire technique with two anodized tantalum hot wires. The measurements of diethyl succinate ranged from (263 to 383) K and atmospheric pressure to 20 MPa. The experimental data were represented by polynomial functions of pressure and temperature for the purpose of interpolation. The relative uncertainty of the present thermal conductivity data was estimated to be ± 2.0 % with a coverage factor of k = 2.

Simultaneous Measurement of Thermal Conductivity, Thermal Diffusivity, and Specific Heat of Nanofluids

Effective thermal conductivity, effective thermal diffusivity, and effective specific heat of nanofluids were simultaneously measured by using a transient double hot-wire technique. Several types of nanofluids were prepared by suspending different volume percentages (1 to 5%) of titanium dioxide (TiO2), aluminum oxide (Al2O3), and aluminum (Al) nanoparticles in ethylene glycol and engine oil. While effective specific heats of these nanofluids decrease substantially with nanoparticle volume fraction, the enhanced effective thermal conductivity and effective thermal diffusivity were found to increase significantly with increasing volumetric loading of these nanoparticles. The increments of the effective thermal diffusivity of nanofluids were slightly larger than their effective thermal conductivity values. Predictions of the effective specific heats of nanofluids by the volume fraction mixture rule-based model showed fairly good agreement (within 7%) with the experimental results. Besides particle volume fraction, particle material, particle shape and the type of base fluid were identified to have influence on these properties of nanofluids. Both the calibration results of the base fluids (system accurate to ≤2.7%) and uncertainty analysis (uncertainty ≤2.1%) indicate high accuracy of using the double hot-wire method to simultaneously measure the effective thermal conductivity, effective thermal diffusivity, and specific heat of nanofluids.

Performance of thermal conductivity probes for planetary applications

Abstract. This work aims to contribute to the development of in situ instruments feasible for space application. Commercial as well as custom-made thermal sensors, based on the transient hot wire technique and suitable for direct measurement of the effective thermal conductivity of granular media, were tested for application under airless conditions. In order to check the ability of custom-made sensors to measure the thermal conductivity of planetary surface layers, detailed numerical simulations predicting the response of the different sensors have been performed. These simulations reveal that for investigations under high vacuum conditions (as they prevail, e.g. on the lunar surface), the derived thermal conductivity values can significantly depend on sensor geometry, axial heat flow, and the thermal contact between probe and surrounding material. Therefore, a careful calibration of each particular sensor is necessary in order to obtain reliable thermal conductivity measurements. The custom-made sensors presented in this work can serve as prototypes for payload to be flown on future planetary lander missions, in particular for airless bodies like the Moon, asteroids and comets, but also for Mars.

Enhanced thermal conductivity of ethylene glycol with single-walled carbon nanotube inclusions

In the present work, we report measurements of the effective thermal conductivity of dispersions of single-walled carbon nanotube (SWNT) suspensions in ethylene glycol. The SWNTs were synthesized using the alcohol catalytic chemical vapour deposition method. Resonant Raman spectroscopy was employed to estimate the diameter distribution of the SWNTs based on the frequencies of the radial breathing mode peaks. The nanofluid was prepared by dispersing the nanotubes using a bile salt as the surfactant. Nanotube loading of up to 0.2vol% was used. Thermal conductivity measurements were performed by the transient hot-wire technique. Good agreement, within an uncertainty of 2%, was found for published thermal conductivities of the pure fluids. The enhancement of thermal conductivity was found to increase with respect to nanotube loading. The maximum enhancement in thermal conductivity was found to be 14.8% at 0.2vol% loading. The experimental results were compared with literature results in similar dispersion medium. Experimental results were compared with the Hamilton–Crosser model, the Lu–Lin model, Nan’s effective medium theory and the Hashin–Shtrikman model. Effective medium theory seems to predict the thermal conductivity enhancement reasonably well compared to rest of the models. Networking of nanotubes to form a tri-dimensional structure was considered to be the reason for the thermal conductivity enhancement.

Thermal Conductivity and Density of (NH4)2SO4 + H2O, NH4NO3 + H2O, and (NH4)2SO4 + NH4NO3 + H2O Solutions at T = (278.15 to 333.15) K

The thermal conductivity of ammonium sulfate + water, ammonium nitrate + water, and ammonium sulfate + ammonium nitrate + water is of great significance in the national defense industry and military engineering for manufacturing of various energetic chemicals. The measurements, covering a temperature range from (278.15 to 333.15) K, have been performed by a transient hot-wire technique over a wide range of concentrations. The densities of ternary aqueous solutions of ammonium sulfate + ammonium nitrate are also presented at the same temperature covering different portions of ammonium sulfate and ammonium nitrate. The thermal conductivities of binary aqueous solutions of ammonium salts are correlated by polynomial functions in terms of temperature and weight fraction for the purpose of interpolation. The Aseyev equation in terms of weight fraction is also used to represent the thermal conductivities of ternary mixtures of these compounds. The ideal mixing rule of Young has been successfully applied to correlate the ternary densities of aqueous ammonium sulfate + ammonium nitrate solutions based on relevant binary solution properties. The agreement between the correlation and the experimental data is pretty good.

Concentration and size dependence of nano-silver dispersed water based nanofluids

Nanometric silver dispersed water based nanofluids have been prepared by a single-step chemical process. The crystallite/particle size, morphology and purity of nanoparticles were characterized using standard microscopic, diffraction and spectroscopic techniques. The thermal conductivity enhancement (with respect to the base fluid) has been determined as a function of concentration and size of silver particles using transient hot-wire technique. The accurate fitting of the experimental data of thermal conductivity enhancement with a theoretical model developed by Patel et al. [24] predicts that high specific surface area of the particles, layering at the liquid–solid interface and Brownian motion may be responsible for enhancement.

Precise measurement of thermal conductivity of liquid over a wide temperature range using a transient hot-wire technique by uncertainty analysis

One of the standard methods used to measure the thermal conductivity of liquids is transient hot-wire technique. The principle of this technique is to fit the unbalanced voltage of the hot wire in a bridge circuit using a theoretical model. The uncertainty in the measurement mainly relates to the procedure used and the measuring instruments, which we evaluate for toluene in liquid phase at atmospheric pressure and in the temperature range from −30°C to 90°C using the Guide to the Expression of Uncertainty in Measurement. The uncertainty of the results was estimated to be within ±0.50% below 60°C and ±0.96% at a temperature of 90°C. The maximum departure between recommended values and the results of our experimental runs is 0.31% at the highest temperature, and the thermal conductivity values measured are in good agreement with the recommended values within the level of uncertainty over the whole temperature range.

Simplified Transient Hot-Wire Method for Effective Thermal Conductivity Measurement in Geo Materials: Microstructure and Saturation Effect

The thermal conductivity measurement by a simplified transient hot-wire technique is applied to geomaterials in order to show the relationships which can exist between effective thermal conductivity, texture, and moisture of the materials. After a validation of the used &#x201c;one hot-wire&#x201d; technique in water, toluene, and glass-bead assemblages, the investigations were performed (1) in glass-bead assemblages of different diameters in dried, water, and acetone-saturated states in order to observe the role of grain sizes and saturation on the effective thermal conductivity, (2) in a compacted earth brick at different moisture states, and (3) in a lime-hemp concrete during 110 days following its manufacture. The lime-hemp concrete allows the measurements during the setting, desiccation and carbonation steps. The recorded <svg style="vertical-align:-2.21957pt;width:57.025002px;" id="M1" height="13.5625" version="1.1" viewBox="0 0 57.025002 13.5625" width="57.025002" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(1.25,0,0,-1.25,0,13.5625)"> <g transform="translate(72,-61.15)"> <text transform="matrix(1,0,0,-1,-71.95,63.41)"> <tspan style="font-size: 12.50px; " x="0" y="0">Δ</tspan> <tspan style="font-size: 12.50px; " x="9.0271664" y="0">𝑇</tspan> <tspan style="font-size: 12.50px; " x="15.903816" y="0">/</tspan> <tspan style="font-size: 12.50px; " x="21.455149" y="0">l</tspan> <tspan style="font-size: 12.50px; " x="24.930983" y="0">n</tspan> <tspan style="font-size: 12.50px; " x="33.270481" y="0">(</tspan> <tspan style="font-size: 12.50px; " x="37.433983" y="0">𝑡</tspan> <tspan style="font-size: 12.50px; " x="41.34742" y="0">)</tspan> </text> </g> </g> </svg> diagrams allow the calculation of one effective thermal conductivity in the continuous and homogeneous fluids and two effective thermal conductivities in the heterogeneous solids. The first one measured in the short time acquisitions (&#x3c;1&#x2009;s) mainly depends on the contact between the wire and grains and thus microtexture and hydrated state of the material. The second one, measured for longer time acquisitions, characterizes the mean effective thermal conductivity of the material.

Determination of effective specific heat of nanofluids

The effective specific heat of several types of nanofluids are measured by transient double hot-wire technique. Sample nanofluids are prepared by suspending 1–5 volume percentages of titanium dioxide (TiO2), aluminium oxide (A12O3) and aluminium (Al) nanoparticles in various base fluids, such as deionised water, ethylene glycol and engine oil. The effective specific heats of these nanofluids were found to decrease substantially with increased volume fraction of nanoparticles. Besides particle volume fraction, particle materials and base fluids also have influence on the effective specific heat of nanofluids. Except Al/engine oil-based nanofluid, predictions of the effective specific heat of nanofluids by the volume fraction mixture rule-based model showed reasonably good agreement with the experimental results. Based on the calibration results obtained for the base fluids, the measurement error is estimated to be within 2.77%.

Thermal-Conductivity Characterization of Gas Diffusion Layer in Proton Exchange Membrane Fuel Cells and Electrolyzers Under Mechanical Loading

Accurate information on the temperature field and associated heat transfer rates is particularly important for proton exchange membrane fuel cells (PEMFC) and PEM electrolyzers. An important parameter in fuel cell and electrolyzer performance analysis is the effective thermal conductivity of the gas diffusion layer (GDL) which is a solid porous medium. Usually, this parameter is introduced in modeling and performance analysis without taking into account the dependence of the GDL thermal conductivity λ (in W · m−1 · K−1) on mechanical compression. Nevertheless, mechanical stresses arising in an operating system can change significantly the thermal conductivity and heat exchange. Metrology allowing the characterization of the GDL thermal conductivity as a function of the applied mechanical compression has been developed in this study using the transient hot-wire technique (THW). This method is the best for obtaining standard reference data in fluids, but it is rarely used for thermal-conductivity measurements in solids. The experiments provided with Quintech carbon cloth indicate a strong dependence (up to 300%) of the thermal conductivity λ on the applied mechanical load. The experiments have been provided in the pressure range 0 < p < 8 MPa which corresponds to stresses arising in fuel cells. All obtained experimental results have been fitted by the equation λ = 0.9log(12p + 17)(1 − 0.4e−50p ) with 9% uncertainty. The obtained experimental dependence can be used for correct modeling of coupled thermo/electro-mechanical phenomena in fuel cells and electrolyzers. Special attention has been devoted to justification of the main hypotheses of the THW method and for estimation of the possible influence of the contact resistances. For this purpose, measurements with a different number of carbon cloth layers have been provided. The conducted experiments indicate the independence of the measured thermal conductivity on the number of GDL layers and, thus, justify the robustness of the developed method and apparatus for this type of application.

Absolute measurement of thermal conductivity of poly (acrylic acid) by transient hot wire technique

The transient hot-wire method is considered the most accurate technique to measure the thermal conductivity of fluids. In this study, a transient hot wire instrument which employs 25.4-μm-diameter tantalum wire with an insulating tantalum pentoxide coating has been used. This hot-wire cell with a thin insulating layer is suitable for measurement of the thermal conductivity of electrically conducting and polar liquids. Measurements of the thermal conductivity of 50 wt% solution of PAA [poly (acrylic acid)] in water and PAA–Na in 50 wt% water are reported here. These measurements were obtained in the temperature range of 299–368 K at 1 atmospheric pressure. The measurement of thermal conductivity is estimated to be accurate within ±4%.

Investigation of thermal transport in colloidal silica dispersions (nanofluids)

Thermal conductivity enhancement in colloidal silica dispersions (nanofluids) is investigated experimentally using a novel optical technique. The effects of nanoparticle size, concentration, and state of aggregation are examined. New data on well dispersed systems are compared to published data obtained using the more conventional transient hot-wire technique and good agreement was found. Experimental results are also compared with model predictions for relative thermal conductivity based on effective medium theory. For systems composed of larger diameter nanoparticles (~30 nm), good agreement was found between the measured thermal conductivity enhancement and that predicted by the classical Maxwell-Garnett model. For systems composed of smaller nanoparticles (∼10 and 20 nm), thermal conductivity enhancement was reduced by as much as 10%, presumably because interfacial thermal resistance effects become important. Measurements on two systems that were induced to form gels exhibited an increase in thermal conductivity of approximately 5% relative to the well-dispersed systems. The observed increase in thermal conductivity is larger than that predicted by a recently proposed model for aggregated nanofluids.

Thermo-physical property measurement of nano-gold dispersed water based nanofluids prepared by chemical precipitation technique

Nano-gold dispersed water based nanofluid has been prepared following the chemical reduction method. Crystallite size, particle size/shape/morphology, and purity of the nanoparticles have been characterized using X-ray diffraction, scanning and transmission electron microscopy and energy dispersion spectroscopy, respectively. The degree of thermal conductivity enhancement of the nanofluid (with respect to the base fluid) as a function of concentration and size of gold nanoparticle has been determined using the transient hot-wire technique. The degree of enhancement increases with increase in concentration and decrease in size of nanoparticles. The maximum enhancement recorded is ∼48% at 0.00026 vol.% concentration and 21 nm average particle size.

Historical Evolution of the Transient Hot-Wire Technique

The paper attempts to describe the historical evolution of the transient hot-wire technique, employed today for the measurement of the thermal conductivity of fluids and solids over a wide range of conditions. Starting from the first experiments with heated wires in 1780 during the discussions of whether gases could conduct heat, it guides the reader through typical designs of cells and bridges, software employed and theory developed, to the modern applications. The paper is concluded with a discussion of the areas of application where problems still exist, and a glimpse of the technique’s future.

A simple method to assess the quenching effectiveness of fire suppressants

A screening concept is suggested for evaluating the effectiveness of fluids to thermally suppress fires. It is based on measuring a fluid's ability to inhibit (or quench) the temperature rise of a material that is rapidly heated. The experimental design is similar to the transient hot wire technique, in which the evolution of the average material temperature is recorded for a given input power, and internal temperature gradients in the material are minimized. A gold wire (100 μm long and 5 μm diameter) is used as the surface which heats the fluid. The wire temperature response due to a power pulse provides a measure of the effectiveness of the fluid to suppress thermally the temperature increase. The results indicate that the “quenching effectiveness”, QE = ( T max − T ∞ ) / ( T max , ref − T ∞ ) , correlates with the ratio of the fluid thermal conductivity to that of the wire, k fluid/k solid , using different Nusselt numbers (representing both conduction and natural convection) for the liquids or gases. The concept developed here could be included in a more comprehensive screening protocol, which would assess the thermal potential of candidate fire suppressants.

Hot-Wire Method for Kinematic Viscosity Estimation

This paper explores the characterization of thermal and momentum diffusion properties of condensed phase biological fluids. The widely used transient hot-wire technique for determination of thermal diffusion properties is proposed here to investigate also the apparent kinematic viscosity of fluids with the apparatus commonly adopted for thermal conductivity and/or thermal diffusivity determination. The undesired onset of convection in the determination of thermal diffusion properties is in this case the useful effect measured at the wire–fluid interface. From a theoretical point of view, the onset of convection time at a given vertical position along the wire has been related to the Prandtl number, and the reliability of the kinematic viscosity has been studied and preliminarily tested in the case of water.

Thermal conductivity measurement in clay dominant consolidated material by Transient Hot-Wire method.

The transient hot-wire (THW) technique is widely us ed for measurements of the thermal-conductivity of most fluids and some attempts have also been carried out for simultaneous measurements of the thermal-diffus ivity with the same hot wire. This technique was also tried to determine thermal prope rties of soils by the mean of probes which can be considered as wire with some assumptions. The purpose of this paper is to validate the thermal conductivity measurement by th e THW technique in geomaterials, composed of compacted sand + clay mineral that can be used for earth construction (Compacted Earth Brick). The thermal transfer behaviors are mainly governed by the texture and moisture of the geomaterials. Thus the investigations were performed (1) in media made of glass beads of different diameters in dry and saturated state in order to observe the role of grain sizes and saturation stat e on the wire temperature (Δt) measurements and (2) in the compacted clay-geomaterial at different moisture states. The Δt / ln(t) diagrams allow the calculation of two the rmal conductivities. The first one, measured in the short time acquisition (< 1s), char acterizes the microtexture of the material and its hydrated state. The second one, me asured for longer time acquisitions, characterizes the mean thermal conductivity of the material.

Thermal Conductivity of Liquid Diethyl Ether, Diisopropyl Ether, and Di-n-butyl Ether from (233 to 373) K at Pressures up to 30 MPa

The thermal conductivity of diethyl ether, diisopropyl ether, and di-n-butyl ether in the liquid phase was reported. The measurements, covering a temperature range from (233 to 373) K and pressure up to 30 MPa, have been performed by a transient hot-wire technique with two anodized tantalum hot wires. The experimental data have been represented by polynomial functions of pressure and temperature for the purpose of interpolation. The relative uncertainty of the present thermal conductivity data was estimated to be ± 2.0 % with a coverage factor of k = 2.

Thermal Conductivity of Liquid Dimethyl Ether from (233 to 373) K at Pressures up to 30 MPa

The thermal conductivity of liquid dimethyl ether (DME) was measured from (233 to 373) K at pressures ranging from (0.6 to 30) MPa by the transient hot-wire technique with two anodized tantalum hot wires. The experimental data were correlated as a function of pressure and temperature. The average absolute deviation of experimental data from those calculated by the equation was 0.46 %, and the maximum absolute deviation was 1.6 %. The uncertainty of the thermal conductivity was ± 2.0 % with a coverage factor of k = 2.

Thermal conductivity and thermal diffusivity of poly(acrylic acid) by transient hot wire technique

The paper describes a new transient hot wire instrument which employs 25.4 μm diameter tantalum wire with an insulating tantalum pentoxide coating. This hot-wire cell with a thin insulating layer is suitable for measurement of the thermal conductivity and the thermal diffusivity of electrically conducting and polar liquids. This instrument has been used for experimental measurement of the thermal conductivity and the thermal diffusivity of poly(acrylic acid) solution (50 mass%) in the temperature range of 299 to 368 K at atmospheric pressure. The thermal conductivity data is estimated to be accurate within ±4%. Thermal diffusivity measurements have a much higher uncertainty (±30%) and need further refinement.