Hot-wire Method Research Articles

Investigation on thermal conductivity of decamethylcyclopentasiloxane (D5) and decamethyltetrasiloxane (MD2M) in the liquid phase

In this work, the liquid thermal conductivities of Decamethylcyclopentasiloxane (D5) and Decamethyltetrasiloxane (MD2M) were measured by using the transient hot wire method. The measurement was conducted in the temperature range of 253.15 K to 353.15 K and pressures ranging from 0.1 MPa to 8.0 MPa, with an uncertainty of 3.0 % in the present measurements. Besides, an empirical correlation was used to represent the experimental data. The Average Absolute Deviation (AAD) between the experimental data and calculations for D5 and MD2M are 0.34 % and 0.31 %, respectively. This study provides accurate liquid thermal conductivities of D5 and MD2M. The present experimental thermal conductivity data are useful for the design and optimization of the relevant heat exchange system.

Integrated measurement of thermoelectric properties for filamentary materials using a modified hot wire method.

Thermoelectric materials have been rapidly developed due to the urgent need for the mutual conversion of thermal energy and electrical energy. Accurately measuring the thermoelectric properties of micro/nano thermoelectric materials is very important and highly required. Compared with traditional measurement methods, integrated measurement can avoid multiple sample preparations and reduce measurement errors. Herein, this work designed an improved integrated measurement method for the thermoelectric properties of microscale thermoelectric materials based on the hot wire method. The results demonstrated that the average ZT values of Pt and Ag2S wires are 0.75 × 10-3 and 0.44 × 10-3 with an uncertainty of ∼2.61%. It provides a novel way for the development of accurately measuring the thermoelectric properties of thermoelectric materials.

Compositional dependence of thermophysical properties in binary alkaline earth borate melts: Insights from structure in short-range and intermediate-range order

• Thermal conductivity of alkaline earth borate melts is investigated systematically for the first time using transient hot-wire method. • Thermal diffusivity is extrapolated from thermal conductivity, heat capacity and melt density, providing a new approach to thermal diffusivity measurement. • Short-range and intermediate-range order of alkaline earth borate structure is studied using high-resolution MAS-NMR, Raman spectroscopy and XPS for a precise and comprehensive insight into structure-conductivity relationship. • Compositional dependence of thermal conductivity is revealed with the discussion on “boron anomaly” phenomenon in alkaline earth borate melts. In this work, the thermal conductivity of alkaline earth borate melts was measured using hot-wire method from 1323 to 1623 K, and the thermal diffusivity was extrapolated from the thermal conductivity and heat capacity. The compositional dependence of thermophysical properties was interpreted according to the structure in short-range and intermediate-range order. Based on the Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and 11 B magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra, modifier cation with higher field strength prefers the formation of non-bridging oxygens (NBOs) for the charge compensation at high BO 1.5 / M O ratios. A lower amount of covalent bond and greater isolation of large borate groups lead to a lower thermal conductivity in calcium borate melt compared with strontium and barium borates. But the large size of Ba 2+ encounters difficulty in fitting around B [4] -O-B [4] linkages inside the overcrowded large borate groups when BO 1.5 /BaO = 2.5, promoting the formation of NBOs on the edge of borate groups for the charge compensation of modifier cations and leading to the decline in the thermal conductivity. Thermal conductivity plays a major role in regulating the thermal diffusivity at a given temperature since the compositional dependence of volumetric heat capacity is relatively weak compared with that of thermal conductivity.

Thermal Conductivity of Solid Triphenyl Phosphite

The thermal conductivity, κ, of solid triphenyl phosphite was measured by using the transient hot-wire method, and its temperature and pressure dependencies were analyzed to understand heat transfer processes in the solid polymorphic phases, as well as in the glass and the exotic glacial state. Phase transformations and the structural order of the phases are discussed, and a transitional pressure-temperature diagram of triphenyl phosphite is presented. The thermal conductivity of both the crystalline and disordered states is described within the theory of two-channel heat transfer by phonons and diffusons in dielectric solids. In the glass and glacial states, the weakly temperature-dependent (glass-like) κ is described well by the term associated with heat conduction of diffusons only, and it can be represented by an Arrhenius-type function. In the crystal phases, the strongly temperature-dependent (crystal-like) κ associated with heat transfer by phonons is weakened by significant heat transfer by diffusons, and the extent of the two contributions is reflected in the temperature dependence of κ. We find that the contribution of diffusons in the crystal phases depends on pressure in the same way as that in amorphous states, thus indicating that the same mechanism is responsible for this channel of heat transfer in crystals and amorphous states.

Thermal stability and behaviour of paraffin nano Al2O3, graphene and surfactant sodium oleate composite as phase change material

Organic phase change materials like paraffin possess high latent heat yet incredibly low thermal conductivity. For improving the thermal conductivity, nanomaterials are introduced into the phase change materials. Thermal energy storage applications benefit from the use of phase transition materials with high thermal conductivity and latent heat of fusion. In this work to increase the dispersion of the alumina and graphene nanoparticles, a novel nanocomposite phase change material was produced by utilizing sodium oleate as a surfactant. The surfactant sodium oleate is prepared with oleic acid and methanol, The mixture is prepared using sodium oleate, Alumina Nanoparticle, and Graphene in the mass ratio of 1:3:0.5 is mixed with paraffin in the weight percentage of 7.5 and 10 and thermal stability study was carried out. Alumina nanoparticles were synthesized and prepared by using a microwave-assisted chemical precipitation approach which is more effective and graphene nanoparticles were prepared by using modified hummer’s method. Thermocycling was used for up to 100 cycles to determine the melting point, latent heat, and long-term thermal stability of nanocomposites with phase change material. Differential Scanning Calorimetry (DSC) was used to evaluate the heat storage behaviour of the samples, and the heating rate of nanocomposites containing PCMs was investigated. The transient hot wire method was then utilised to assess the PCM’s actual thermal conductivity. From the obtained results, nanocomposite with 7.5 wt% additives show maximum thermal stability and latent heat (161.09 KJ Kg−1) for 100 cycles with an increase in 42% effective thermal conductivity, Nanocomposite with 10 wt% shows 57% higher thermal conductivity. But shows lower thermal stability and very low latent heat (120.44 KJ Kg−1). It is understood from the results that nanoparticle and surfactant addition gives a positive rise in latent heat.

On the possibility of expanding the studied dynamic ranges in thermal anemometry

The paper presents the results of a study of the possibility of expanding the dynamic ranges of hot-wire methods. The relevance of the work is due to the demand in new methods for measuring the thermal and hydrodynamic parameters of high-speed gas flows for the purposes of scientific instrumentation. The novelty of the presented technical solution lies in the use of two hot-wires anemometers with significantly different thermal inertia for simultaneous measurement of the flow velocity and the heat transfer coefficient in it. The theoretical basis of the proposed method is based on the phenomenon of the inertia of the response of any thermodynamic system under a stepwise thermal effect on it. The method consists in placing in the investigated flow two bodies of the same shape and size which have significantly different thermal inertia. The bodies are subjected to a stepped thermal effect, and the non-stationary temperature delay of the bodies relative to each other is recorded. Using the maximum value of the temperature delay, the value of the heat transfer coefficient of the bodies with the investigated flow can be evaluated by calculation. The flow rate is found from the value of the moment of time corresponding to the maximum temperature delay while using the calibration characteristic previously obtained on the reference flow. The new thermal anemometry method is scientifically substantiated, the measurement equation of the method is obtained, the measurement algorithm and the generalized scheme of the device implementing the method are developed, and the value of the expected uncertainty of the measurement results is given. The simulation results showed that the relative uncertainty provided by the presented method does not exceed 1.5 %. The developed method makes it possible to significantly increase the accuracy and expand the studied dynamic ranges of the required values for a wide range of gas flows. The method can be used in the flow measurement of gas-air flows.

Thermal conductivity measurements using the transient hot-wire method: a review

The accurate determination of the thermal conductivity of any material is a key element in understanding its actual thermal performance, thus assigning its suitability for a particular application. This, of course, includes its efficiency while being used, lifetime, probability of failure or breakdown, and most importantly, user safety. Several methods are used to measure the thermal conductivity of materials. However, the transient hot-wire method has many practical advantages over other methods due to its relative simplicity and suitability for different materials. The hot-wire method can deliver accurate measurements of gases, liquids, and some solids over a relatively-wide thermal conductivity range. Furthermore, with careful design of the hot-wire instrument, it can be used to measure the thermal conductivity at elevated temperature and under high pressure, which is essential for many industrial applications. In turn, this has made the method one of the most frequently used. This review paper explains the theory of the hot-wire method and demonstrates the technical developments of hot-wire instruments. The paper also presents the advances of electric circuits used to measure the resistance of the hot wire, thus its temperature, during the transient experiment. In addition, it shows the calibration of the hot wire together with the calculation of thermal conductivity.

Application of the Hot Wire Method to Measure the Thermal Conductivity Coefficient of a Gypsum Composite

Currently, there is much discussion about modern technologies and solutions in construction. There are new solutions that save electricity or heat, usually in buildings additionally equipped with intelligent management systems. High hopes are placed on building materials. Every investment begins with them. The basic building materials include materials such as cement, bricks, hollow bricks or plasterboard, and their modification and the use of admixtures ensure the greatest changes in the parameters of the building. This article focuses on the preparation and testing of gypsum mortar consisting of gypsum, phase change material and polymer. The idea was to replace the proven method of adding microencapsulated phase change material by direct binding. This article presents the study of thermal conductivity by the hot wire method. Using this method, tests of temperature changes during plaster hardening were also carried out. Compressive strength tests were also carried out on the 14th, 21st, 28th, 35th and 105th day from the date of making the samples. For each of these tests, three types of samples with different polymer content were used. After a series of tests, the best results were obtained by a series of samples with 0.1% polymer.

Measurement and modeling of liquid thermal conductivity of linear siloxanes: Octamethyltrisiloxane (MDM) and Hexamethyldisiloxane (MM)

In this work, the thermal conductivity data of Octamethyltrisiloxane (MDM) and Hexamethyldisiloxane (MM) were measured by the transient hot wire method, in the temperature range of 263.15 K to 353.15 K and the pressures ranging from 0.1 MPa to 7.0 MPa, with an uncertainty of 3.5 % for MDM and 3.8 % for MM. Then a comparison between the present data and previously reported thermal conductivities was made, indicating a good agreement. Additionally, a thermal conductivity model based on the modified Enskog theory was used to represent the experimental data. This study provides a necessary basis for the calculation and analysis of the heat transfer of typical linear siloxanes for the organic Rankine cycle.

Measuring Thermal Diffusivity of Orthotropic Materials by Multidimensional Constant-Temperature Boundary Method

A method for measuring thermal diffusivity of orthotropic materials by using multidimensional heat transfer model combined with parameter estimation is proposed. The unsteady mathematical model of three-dimensional temperature field is established by heat conductivity product method, the thermal diffusivity is calculated by random conjugate gradient method, and the thermophysical property testing system is developed. The constant-temperature boundary is constructed by using a constant-temperature water bath box combined with graphene stickers wrapped on the surface of the sample, the insulation material covers the upper surface of the sample to construct an insulating boundary, and thermocouples are arranged on the upper surface to monitor the temperature rise change. First, isotropic materials, such as acrylic plate and marble, were tested. Compared with a reference value, the maximum relative deviation of thermal conductivity was 3.72%, which verified the feasibility and accuracy of the test method. Furthermore, the orthotropic-material unidirectional carbon fiberboard was tested. Compared with the results measured by improved parallel hot-wire method, the relative deviations of thermal diffusivity, thermal conductivity, and specific heat capacity were 3.66, 1.59, and 2.15%, respectively, which indicate that this test method could meet the thermophysical testing requirements of orthotropic materials.

Thermal and Mechanical Improvement of Filling Mixture for Shallow Geothermal Systems by Recycling of Carbon Fiber Waste

The reuse of waste materials such as carbon fiber (CF) as filling additive for closed-loop vertical geothermal probes in shallow geothermal systems has been evaluated as a new grout mixture for the improvement of geothermal energy systems efficiency and a sustainable supply of raw materials from special waste. The study evaluates the improvement in both thermal exchange characteristics and mechanical properties of the filling grout for geothermal purposes through the addition of 5% of CF to standard (ST) materials currently on the market. Uniaxial and flexural tests investigating the material response after 14 and 28 days from sample preparation on samples of both standard and mixed grout material as well as non-stationary hot wire method were used to define the thermal conductivity for both the standard and innovative mixtures. The experimental analysis provides evidence for increasing the thermal conductivity by about 3.5% with respect to standard materials. Even the mechanical properties are better in the innovative mixture, being the compressive strength 187% higher and flexural strength 81% higher than standard materials. The obtained results become useful for the optimization of low enthalpy geothermal systems and mostly for the design of the vertical heat exchange system in terms of depth/number of installed probes. Principally, thermal conductivity improvements result in a reduction of about 24% of the geothermal exchanger’s length, affecting the economic advantages in the implementation of the entire system. A simple analysis of the reuse of CF waste shows the reduction of industrial waste and the simultaneous elimination of disposal costs, defining new perspectives for industrial waste management. This research provides essential elements for the development of a circular economy and is well integrated with the European challenges about the End of Waste process and reduction of environmental impact, suggesting new perspectives for economic development and sectorial work.

Characterization of the microstructure and mechanical properties of activated tungsten inert gas welded and hot-wire tungsten inert gas welded super austenitic stainless steel AISI 904L

Super austenitic stainless steel AISI 904L is a premium stainless steel variant known for its superior properties and ability to retain the same in highly challenging conditions. This study investigates the feasibility of welding 10-mm thick plates of AISI 904L through activated tungsten inert gas welding and hot-wire tungsten inert gas welding methods. The fabricated joints are then subjected to solidification mode study, ferrite number analysis, microstructural characterization, and mechanical tests. Results of the solidification mode study indicated primary austenitic solidification in both the weld joints. The ferrite number measurement revealed that the hot-wire tungsten inert gas weld joint had a higher δ-ferrite content of 1.58 over 0.99 of the activated tungsten inert gas weld joint. A microstructure study confirmed the presence of refined equiaxed grains in the fusion zone of the hot-wire tungsten inert gas weld joint, while the activated tungsten inert gas weld joint displayed equiaxed grains crammed between the columnar grains and dendrites. Liquid penetrant inspection of bend-tested samples proved the soundness and quality of both weld joints. The hot-wire tungsten inert gas weld joint excelled over the activated tungsten inert gas weld joint with a higher microhardness value. During tensile tests, activated tungsten inert gas and hot-wire tungsten inert gas weld joints displayed average ultimate tensile strength values of 521 and 514 MPa, respectively. The impact toughness of the activated tungsten inert gas weld joint was 248 J, while it was 220 J for the hot-wire tungsten inert gas weld joint. Field emission scanning electron microscope analysis equipped with energy-dispersive spectroscopy and elemental color mapping provided details about weld microstructure and the reasons for the material's failure. The study proved the efficiency and soundness of the 10-mm thick joints of AISI 904L fabricated by activated tungsten inert gas and hot-wire tungsten inert gas welding techniques.

Effective thermal conductivity and thermal cycling stability of solid particles for sCO2 CSP applications

A supercritical CO2 (sCO2) Brayton cycle solar power system based on solid particle thermal energy storage is proposed to enhance the efficiency of concentrated solar power (CSP). The effective thermal conductivity and thermal cycling stability of solid particles are crucial for sCO2 CSP applications. Some natural and artificial solid particles, such as black silicon carbide, green silicon carbide, white corundum, brown corundum, brown ceramsite sand, garnet, river sand and desert sand are considered as potential thermal energy storage medium due to its low cost and easy availability in large quantities. However, there are few studies about the effective thermal conductivity and stability of these solid particles after thermal cycling tests. In this study, the effective thermal conductivity of these solid particles was measured by the hot wire method. Particularly, the effective thermal conductivity of black silicon carbide and green silicon carbide was measured by modified hot wire method. In addition, the changes of mass, real density, bulk density, void fraction and mean diameter of the above-mentioned solid particles after 100 times thermal cycling tests were also recorded to analyze thermal cycling stability. Based on the results of all the above tests, green silicon carbide exhibits high effective thermal conductivity and good thermal cycling stability, which presents a great potential as a thermal energy storage medium.

Thermo-physical properties measurements of hygroscopic and reactive material (zeolite 13X) for open adsorptive heat storage operation

The determination of the thermo-physical properties (density, specific heat capacity, and thermal conductivity) of hygroscopic and reactive solid materials, as those applied in sorption thermochemical heat storage systems, is not trivial. Lack of precision in the measurement and contradictory results make it difficult to enter these thermal parameters in the programs used for simulating the functioning and operating scenarios of thermochemical heat storage devices. For this reason, different techniques have been applied and compared in order to improve the accuracy of the measurement. In particular, there was a difference in thermal conductivity (about 30%) between the hot disc experiments and the others, as well as a higher dispersion of the experimental data (MAD > 19%). The same was found for specific heat capacity (MAD ≈14%). Hence, the transient hot disk method is not suitable for estimating thermal conductivity and the specific heat capacity of this type of material. Nevertheless, this technique enables a good estimation of the thermal effusivity. The thermal conductivity and the specific heat capacity were investigated by the hot wire method and by TG-DSC, respectively. A slight increase in specific heat with temperature was observed, but in the range of 70–95 °C, this value is constant at 913 J kg−1 K−1. The applied methodology was validated by comparing the experimental and calculated thermal effusivity values.

Physical and Thermal Properties Analysis of Hematite for Thermal Heat Storage.

Energy sustainability represents an important research topic for aiding decreasing energy dependence and slowing down climate changes. In this context, solutions using thermal energy storage through rock start to emerge, due to its natural benefits, when compared to more polluting alternatives. To understand whether a rock material can be considered a good thermal energy storage material for such solutions, it is necessary to evaluate the physical, chemical and thermal properties of such materials. Therefore, it becomes essential to understand how heat propagates in the rock and how voids influence the thermal properties. To achieve these goals, hematite ore from Moncorvo, Northeastern Portugal was used, in particular, to study the effect of grain size on thermal properties for three different sized lots. Chemical and physical changes between heated and unheated lots were detected using X-ray diffraction and particle size, as well as X-ray fluorescence analysis. Regarding thermal properties, a hot wire method approach was used with seven thermocouples. Additionally, a thermal inversion model to simulate the heat exchanges was also proposed, allowing changing the properties of the constituents, to fit the theoretical and experimental temperature curve. Furthermore, the model reveals how heat propagates inside the reservoir filled with hematite ore.

Experimental study on effects of particle melting and micro-convection on measurement of thermal conductivity of ice slurry

Ice slurry is a widely used storage medium for latent heat energy. However, as ice slurry is a liquid–solid two-phase fluid, it is challenging to measure its thermo-physical parameters. In this study, circuit data acquisition was performed by using the transient hot-wire method and Wheatstone bridge, the test system of thermal conductivity was built. Subsequently, a uniform suspension test device with a multi-interlayer was innovatively designed, which suppresses the micro-convection heat transfer of solid particles caused by the solid–liquid density difference, the total uncertainty of the thermal conductivity measurement system was <2%, the reliability of the experimental device was verified by testing the thermal conductivity of four standard liquids, at the same time, on the basis of theoretical study, a theoretical model of the thermal conductivity of ice slurry was established considering the influence of latent heat of ice crystal melting, which is more suitable for experimental measurement process. The results show that the thermal conductivity of the device increased gradually with an increase in the IPF, but when two devices were tested with and without an interlayer simultaneously, the thermal conductivity of the suspension device with the interlayer was generally higher than that of the suspension device without the interlayer. By comparing the experimental results, it was found that compared with the conventional models for predicting the thermal conductivity of liquid–solid two-phase fluids, the accuracy of the proposed model for predicting the thermal conductivity of ice slurry based on the influence of latent heat of ice crystal melting was significantly better, the results were in good agreement with the experimental results, the minimum relative error is 0.05%, and the maximum relative error is 8.96%.This study is helpful to obtain the thermal properties of ice slurry accurately.

Thermal conductivity of 3D printed concrete with recycled fine aggregate composite phase change materials

In this study, a novel shape-stabilization phase change materials (PCM) for 3D printed concrete have been generated by impregnating paraffin into the recycled fine aggregates. Based on both steady state hot plate method and transient hotwire method, the impact of printing parameters, including printing layers, path, extrusion rate and materials on thermal conductivity of the 3D printed concrete was investigated. The relationship among thermal conductivity, dry density and porosity of the 3D printed concrete was analyzed. The results showed that recycled fine aggregates can be successfully used as a support material for paraffin. The melting temperature and latent heat is 36.52 °C and 40.65 J/g, respectively. The thermal conductivity of 3D printed concrete was significantly influenced by the multi-layer structure and with thermal anisotropy. Under the same size, density class or mix design, the thermal conductivity of the 3D printed concrete can reach up to 31.04% lower than that of mould cast concrete. The results also indicate that impregnating paraffin into the recycled fine aggregate can improve its thermal properties so as to upgrade the overall performance of 3D printed concrete.

Influence of radiation heat transfer on parallel hot-wire thermal conductivity measurements of semi-transparent materials at high temperature

This paper presents a study of the influence of the radiation transfer on the thermal conductivity measurement in a parallel hot-wire method at high temperature. Simulations of the temperature evolution are first carried out with COMSOL Multiphysics® using the coupled conduction–radiation module based on the P1 approximation for the radiation calculation. A wide range of conditions were investigated: a low and a high density insulator were considered with various radiation characteristics (purely scattering, absorbing and scattering, purely absorbing) and with temperature varying up to 1500°C. This study enables the determination of the validity limit of the modeling of the temperature by a purely conductive model with an equivalent conductivity based on the Rosseland approximation. When this assumption is valid, a new estimation process was proposed to improve the estimation accuracy of the thermal properties. A calibration process of the distance between the hot-wire and the thermocouple has also been proposed and validated, enabling a more accurate estimation of the volume heat capacity.An experimental study carried out on three insulating materials with densities ranging from 581kgm−3 to 910kgm−3 and at temperatures ranging from 20°C to 1200°C confirms the results of the theoretical study. Finally, a method enabling the estimation of the extinction coefficient from thermal conductivity measurements at various temperatures is presented and successfully applied to the three tested materials.

The effect of peroxide cross-linking on the thermal conductivity and crystallinity of low-density polyethylene

In this work, the thermal conductivity of a series of low-density polyethylene (LDPE) with different degrees of cross-linking was measured by the transient hot-wire method at different temperatures. It was found that the thermal conductivity of LDPE samples with different degrees of cross-linking decreased smoothly with increasing temperature at temperatures ranging from ‐20 to 80°C. The higher the degree of cross-linking, the smaller the negative temperature dependence of the cross-linked LDPE samples. The decrease of negative temperature dependence of LDPE was explained by the thermal vibration of molecular chains. Meanwhile, the crystallinity and melting point of a series of LDPE samples with different degrees of cross-linking were determined by differential scanning calorimetry (DSC), and the crystallization process of LDPE samples with different degrees of cross-linking was investigated using the Jeziorny method with a modification of the Avrami equation. The results show that the thermal conductivity of cross-linked LDPE is directly proportional to the crystallinity and inversely proportional to the gel content. This work provides a basis for the future design of semi-crystalline polymers with no temperature dependence of thermal conductivity.

Experimental measurement and prediction of thermal conductivity of fatty acid ethyl esters: ethyl butyrate and ethyl caproate

Biodiesel is expected to replace diesel because of its environmental friendliness. Biodiesel is mainly composed of fatty acid methyl esters (FAMEs) and fatty acid ethyl esters (FAEEs). Thermophysical properties of fuels are essential parameters in the industrial applications. As one of the most important thermophysical properties, thermal conductivity of fuels is essential in the design of heat transfer system and the modeling of engine's temperature distribution, so it is of great importance to measure thermal conductivity of FAEEs. In this paper, thermal conductivity of two FAEEs, ethyl butyrate (EeC4:0) and ethyl caproate (EeC6:0) were measured by transient hot-wire method, the temperature and pressure range is from 293.15 to 523.15 K and 0.1 to 15.0 MPa, the experimental data were fitted by a dimensionless polynomial as a function of temperature and pressure. AADs of measured data and polynomial are 0.35 % for EeC4:0 and 0.31 % for EeC6:0, respectively. Furthermore, the experimental data are compared with literature data, the results show that the present data agree well with literature data. In addition, several predictive models (Latini model, Sastri model, Liu model, Liu model with re-fitted parameters) are used to describe the data, the comparison shows that Liu model with re-fitted parameters describes the data best.