Thermal Conductivity Measurements Research Articles

Shell Shape Influence on Latent Heat Thermal Energy Storage Performance during Melting and Solidification

Phase-change materials have various applications across industries from thermal energy storage through automotive battery temperature management systems to thermal stabilisation. Many of these applications are shell and tube structures with different shell shapes. However, it is not yet known how the shape of the shell affects the melting, solidification times, and heat transport processes in such structures. To fill this research gap, seventeen shell shapes/orientations were compared using a simulation study. The well-known and validated enthalpy porosity algorithm implemented in the Fluent 2021R2 software was used. The numerical calculations were preceded by the measurement of thermal conductivity, phase change enthalpy, and specific heat during melting and solidification of the phase-change material. The shortest melting time was achieved for a semi-circular shell shape in the downward position, which was 44% shorter than the reference circular case. The shortest solidification times were recorded for an isosceles trapezium in an upward orientation relative to the reference circular case. Therefore, it is possible to significantly reduce the melting time in shell-and-tube systems as a result of the appropriate selection of the shell shape.

Predictions and measurements of thermal conductivity of ceramic materials at high temperature

Predictions and measurements of thermal conductivity of ceramic materials at high temperature

Microstructure evolution of porous calcium hexaluminate compacts during first heating

Porous calcium hexaluminate (CaAl12O19 or CA6) structures show long-term stability at high temperatures, high refractoriness, low thermal conductivity, and excellent chemical resistance, thus being Taylor-made for thermal insulation. Previous works attained porous structures from in situ formation of CA6 combining CaO and Al2O3 sources in solid-state reactive sintering. Although the approach is time-and-energy-saving regarding the production of large parts of complex shapes, it faces considerable difficulties related to the expansive formation of calcium aluminates. To overcome such drawbacks, pre-formed porous CA6 aggregates improve the system's dimensional stability before sintering. Despite the straightforward processing, few studies have investigated such materials systemically. This article addresses the combination of pre-formed porous CA6 aggregates and organic and inorganic binders for the production of porous structures by two shaping processes, namely, uniaxial pressing and direct casting of aqueous suspensions. After drying, the samples' microstructure and physical properties evolution were investigated up to sintering (1100-1500 °C) through total porosity, Young's modulus, compression strength, pore diameter, and thermal conductivity measurements, dilatometric analyses, and scanning electron microscopy. Reference samples of coarse calcined alumina were tested under the same conditions to highlight the impacts of CA6 particles' morphology. Compared to them, the CA6-containing samples showed almost no variation in total porosity and average pore size levels during thermal treatments, although their strength and rigidity increased significantly after sintering. Their microstructure remained practically unchanged after drying, showing clusters of large asymmetrical CA6 crystals bonded to each other by their edges, and surrounding a large fraction of 1.8–2.2 μm pores. According to the results, although water content, processing method, and compacting levels are important parameters, particles' microstructure, ratio of intra-particle pores, and asymmetrical shape are key variables for the development of physical properties. Such characteristics strongly contributed to their densification resistance and lower thermal conductivity after exposure to high temperatures.

Development of CNT-Based Nanocomposites with Ohmic Heating Capability towards Self-Healing Applications in Extrusion-Based 3D Printing Technologies

In the present study, a series of carbon-based nanocomposites based on recycled thermoplastic polyurethane (TPU) matrix and MWCNT fillers synthesized in a laboratory environment were prepared at various loadings and assessed in terms of their functional thermal, dielectric, and rheological properties, as well as their ohmic heating capability, for self-healing applications in extrusion-based 3D printing technologies. The synthesis of nanomaterials focused on the production of two different types of carbon nanotubes (CNTs) via the chemical vapor deposition (CVD) method. A comparative assessment and benchmarking were conducted with nanocomposite filaments obtained from commercial nanomaterials and masterbatches with MWCNTs. For all the polymer nanocomposites, samples were prepared at additive contents up to 15 wt.% and filament feedstock was produced via the melt-extrusion process for 3D printing; these were previously characterized by rheological tests. The measurements of thermal and electrical conductivity resulted in a selected composition with promising ohmic heating capability. As a preliminary assessment of the self-healing ability of the above samples, artificial cracks were introduced on the surface of the samples and SEM analysis took place at the crack location before and after applying voltage as a measure of the effectiveness of the material remelting due to the Joule effect. Results indicate a promising material response with a partial restoration of artificial cracks.

A Novel Banana Fiber Reinforced Green Composite from Maleated Castor Oil and Linseed Oil

In recent decades, the consumerist society has recognized the consequences of petrochemicals, hence the interest of scientists has been drawn toward renewable products obtained from biological sources. In this study a novel biocomposite with the use of the least chemicals was fabricated by crosslinking maleated castor oil (MACO) and linseed oil (LO) by free radical polymerization, using benzoyl peroxide (BPO) as the free radical initiator. The significance of this study is that the polymer matrix is derived entirely from vegetable oils with slight modifications. Banana fibers are one of the strongest natural fibers that may be taken from the banana pseudo-stem that is discarded after the fruit is harvested. To convert this post-harvest waste into a value-added product, NaOH-treated banana fibers were used as the reinforcement for the developed composite. The characterization of the fibers and the composites was carried out by performing Fourier-transform infrared spectroscopy (FT-IR), X-Ray diffraction analysis (XRD), Dynamic mechanical analysis, tensile and compressive stress analysis, Scanning electron microscopy (SEM), Thermogravimetric (TG)/derivative thermogravimetric (DTG) and thermal conductivity analysis. Composites were also analyzed for their water absorptivity and chemical resistivity. The influence of varying banana fiber weights on the composite’s mechanical strength was investigated using a dual cantilever flexural test. It revealed that the composite’s strength increased with increasing fiber content until 50% (w/w) and then dropped as the fiber content increased further. The maximum tensile strength of the optimum composite was 16.92 ± 4.70 Megapascal (MPa) and the composite was resistant against a compressive force of 4,500 kN without any fractures. The composites’ water absorptivity and chemical resistivity tests revealed that increasing the fiber content enhanced the composites'swelling. The composite was acidic and saline water resistant, but not alkaline solution resistant. Additionally, the thermal conductivity measurements of the composite showed that the composite is a thermal insulator. The composite with 50% (w/w) fiber load, 20% (w/w) BPO, 10% (w/w), LO and 20% (w/w) MACO exhibited the best overall performances.

Multicomponent Si–Zr-based lightweight nanofiber films with a hollow structure exhibiting excellent thermal insulation properties

Hollow-structured ceramic fibers with multiple components are ideal as lightweight insulation materials. In this study, tetraethyl orthosilicate, boric acid, and zirconia octahydrate were used as raw materials for fabricating SiZrBOC hollow ceramic fibers through coaxial electrospinning. These ceramic fibers exhibit a low thermal conductivity and an excellent temperature resistance. The electrospun SiZrBOC hollow ceramic fibers display a clean and compact morphology with internal folds. Fourier-transform infrared spectroscopy and thermogravimetric analysis (TGA) were used to assess the transformation process and thermal behavior of the hollow ceramic fibers. The results suggest that the incorporation of B promotes the formation of Si–O–B bonds within the system, leading to a maximum yield of 75.49 wt% for the production of the SiZrBOC hollow ceramic fibers. The phase composition and microstructure of the SiZrBOC hollow ceramic fibers were analyzed via X-ray diffraction (XRD) measurements. The pyrolysis at 1000 °C resulted in the appearance of diffraction peaks corresponding to SiO2 and SiC in the XRD patterns of the SiZrBOC hollow ceramic fibers. Laser thermal conductivity measurements reveal that the SiZrBOC hollow ceramic fibers exhibit a thermal conductivity of only 0.06 W‧m−1K−1 at 25 °C and 0.124 W‧m−1K−1 at 500 °C, indicating a significantly superior thermal insulation performance compared with conventional solid SiZrBOC ceramic fibers.

Minimum Detection Concentration of Hydrogen in Air Depending on Substrate Type and Design of the 3ω Sensor.

Hydrogen has emerged as a promising carbon-neutral fuel source, spurring research and development efforts to facilitate its widespread adoption. However, the safe handling of hydrogen requires precise leak detection sensors due to its low activation energy and explosive potential. Various detection methods exist, with thermal conductivity measurement being a prominent technique for quantifying hydrogen concentrations. However, challenges remain in achieving high measurement sensitivity at low hydrogen concentrations below 1% for thermal-conductivity-based hydrogen sensors. Recent research explores the 3ω method's application for measuring hydrogen concentrations in ambient air, offering high spatial and temporal resolutions. This study aims to enhance hydrogen leak detection sensitivity using the 3ω method by conducting thermal analyses on sensor design variables. Factors including substrate material, type, and sensor geometry significantly impact the measurement sensitivity. Comparative evaluations consider the minimum detectable hydrogen concentration while accounting for the uncertainty of the 3ω signal. The proposed suspended-type 3ω sensor is capable of detecting hydrogen leaks in ambient air and provides real-time measurements that are ideal for monitoring hydrogen diffusion. This research serves to bridge the gap between precision and real-time monitoring of hydrogen leak detection, promising significant advancements in the related safety applications.

Three-sensor 2ω method with multi-directional layout: A general methodology for measuring thermal conductivity of solid materials

Anisotropic thermal transport plays a key role in both theoretical study and engineering practice of heat transfer, but accurately measuring anisotropic thermal conductivity remains a significant challenge. To address this issue, we propose the three-sensor 2ω method in this study, which is capable of accurately measuring the isotropic or anisotropic thermal conductivity of solid materials. In this method, several three-sensor groups following the design guidelines are fabricated upon the sample along different characteristic directions, and each group consists of three parallel metal sensors with unequal widths and distances optimally designed based on sensitivity analysis. Among the three sensors, the outer two serve as AC heaters and the middle one as a DC detector. The 2ω voltage signals across the detector in each three-sensor group are measured, and then the data are processed by the proposed Intersection Method to derive thermal conductivities along directions of interest. The application of the detector's 2ω instead of the heater's 3ω voltage signals eliminates errors introduced by the uncertainties of thermal resistance in superficial structures (metal layer, insulation layer, interface, etc.). Meanwhile, by replacing the fitting algorithm with the Intersection Method, the local optimum trap of multivariate fitting is avoided. To verify the accuracy and reliability, four typical monocrystalline semiconductors, i.e., Si, GaN, AlN, and β-Ga2O3, are measured, and the results are consistent with the literature. This method will provide a comprehensive and versatile solution for the thermal conductivity measurements of solid materials.

Applied development of sustainable-durable high-performance lightweight concrete: Toward low carbon footprint, durability, and energy saving

Over the past few decades, the utilization of Lightweight Concrete (LWC) has emerged as a promising solution for reducing the dead load of structures. However, a key concern associated with the reduction of concrete density, is the potential decrease in mechanical and durability properties. This paradox necessitates a precise mix design that fulfills the requirements of high-strength concrete. This article investigates the influence of Lightweight Expanded Clay Aggregate (LECA) on the strength and durability, and thermal conductivity of Lightweight High-Performance Concrete (LWHPC). The LWHPC mixes explored in this study include sustainable cement solutions such as Portland limestone cement, varying water/binder ratios of 0.3 and 0.35, Leca contents ranging from 5 % to 20 % by volume, and two different pozzolanic supplements, namely silica fume and fly ash, which can be reducing porosity and enhancing durability and mechanical strength. The durability properties of the concrete were assessed through water absorption, electrical resistivity, and chloride migration profiles. Additionally, thermal conductivity measurements were conducted on all specimens. The results indicate that LWHPC, with a lower water/binder ratio, exhibits higher compressive strength and lower absorption, indicating good concrete quality. Generally, after a curing period of 90 days, mixtures with a density exceeding 1700 kg/m3 demonstrated superior durability characteristics. Furthermore, LWHPC mixes in both water/binder ratio groups displayed significantly low thermal conductivity with increasing lightweight aggregate content.

Direct measurement of in-plane thermal conductivity of microscale suspended thin films using a novel 3ω method

A novel 3ω method has been developed for measuring the in-plane thermal conductivity of microscale suspended thin films. In this method, a heating wire makes contact with the test film using Ampere's force with the assistance of an interstitial material. The thermal impedance is determined by comparing the triple-frequency voltages across the heating wire with and without contact with the test film. The thermal conductivity can be directly obtained from the slope of the real part of the thermal impedance plotted against the logarithm of the heating frequency, even when the thermal contact resistance between the heating wire and the test film is unknown. This method has been successfully applied to determine the in-plane thermal conductivity of various thin films, including stainless-steel and Kapton films. The linear relationship is significantly improved for samples with low thermal conductivity, and its accuracy is verified through repeated measurements under different contact conditions.

Thermal conductivity measurement of uranium–plutonium mixed oxide doped with Nd/Sm as simulated fission products

The thermal conductivities of near-stoichiometric (U,Pu,Am)O2 doped with Nd and/or Sm, which are major fission products (FPs) generated during irradiation of uranium–plutonium mixed oxides (MOX) fuels, as simulated fission products are evaluated from 1073 to 1673 K. The thermal conductivities are calculated from the thermal diffusivities that are measured using the laser flash method. To evaluate the thermal conductivity from a homogeneity viewpoint of Nd/Sm cations in MOX, the specimens with different homogeneities are prepared using ball-mill and fusion methods. A homogeneous Nd/Sm distribution decreases the thermal conductivity of MOX with increasing Nd/Sm content, whereas heterogeneous Nd/Sm has no influence. The effect of Nd/Sm on the thermal conductivity is studied using the phonon transport model λ=(A+BT)−1. The dependences of the coefficients A and B on the Nd/Sm content (CNd and CSm, respectively) are evaluated as: A(mK/W)=1.70×10−2+0.93CNd+1.20CSm, B(m/W)=2.39×10−4.

Experimental and Theoretical Studies of the Thermal Contact Conductance for Bundles of Round Steel Bars.

The paper presents investigations devoted to the analysis of the thermal contact conduction in a bundle of round steel bars. The phenomenon can be expressed quantitatively with the use of thermal contact conductance (hct). The starting points for the presented analysis were the results of the experimental measurements of the effective thermal conductivity. The measurements were performed for samples of a medium in the form of flat packed beds of bars with three different arrangements: staggered, in-line, and crossed and four bar diameters: 10, 20, 30, and 40 mm. Next, a mathematical model was developed, thanks to which the values of the hct coefficient were calculated for the analyzed cases. This approach consists in analyzing thermal resistances in the medium model, which is defined with an elementary cell. It was established that the value of the hct coefficient in the temperature range of 50-600 °C changes within the range of 50-175 W/(m2·K), and it decreases with an increase in the bar diameter. The final effect of the present study was to develop generalized approximation equations describing changes in thermal contact conductance in the heated bar bundle simultaneously in the temperature and bar diameter function.

Rapid and nondestructive testing for simultaneous measurement of thermal conductivity and thermal diffusivity of flat materials based on thermography

This study presents a novel experimental method that simultaneously supplies the thermal conductivity and thermal diffusivity of flat materials, which solves the limitations of traditional methods that have strict demands for thermal excitation, boundary, and size of samples. In this method, IR thermal imaging is used to collect data from the material surface temperature field caused by a laser excitation, and the differential terms in the basic heat-conduction differential equation are estimated by curve fitting. The differential equation is so transformed in an algebraic equation, which allows to simultaneously determine the thermal conductivity and thermal diffusivity. The measurement experiment does not require a strict control of the boundary or initial conditions, or thermal excitation modalities. The hypothesis this method is based on is that the spatial derivative of temperature in the Z-direction is linear with the temperature difference between the material surface and environment. Using this constraint the optimal operation conditions can be identified. The influence of operation conditions, including the heat transfer distance, excitation duration, material thickness, and number of sampling points on the measured surface, together with the results, were analysed by numerical simulation. The optimal operation conditions when testing two 304 stainless steel samples were found from this simulation analysis. Results of 304 stainless steel samples 1 and 2 mm thick show a deviation between the reference and measured values of the two thermal properties within 4.2 %, and repeatability within 5.5 %. Therefore, this method can realise rapid, nondestructive, and simultaneous measurements of thermal conductivity and thermal diffusivity.

In-situ thermophysical measurement of flowing molten chloride salt using modulated photothermal radiometry

Molten salts are leading candidates for high-temperature heat transfer fluids (HTFs) for thermal energy storage and conversion systems in concentrated solar power (CSP) and nuclear energy power plants. The ability to probe molten salt thermal transport properties in both stationary and flowing status is important for the evaluation of their heat transfer performance under realistic operational conditions, including the temperature range and potential degradation due to corrosion and contamination. However, accurate thermal transport properties are usually challenging to obtain even for stagnant molten salts due to different sources of errors from convection, radiation, and corrosion, let alone at flowing status. To the best of authors’ knowledge, there is no available in-situ technique for measuring flowing molten salt thermal conductivity. Here, we report the first in-situ flowing molten salt thermal conductivity measurement using modulated photothermal radiometry (MPR). We could successfully perform the first in-situ thermal conductivity measurement of flowing molten NaCl-KCl-MgCl2 in the typical operating temperature (520 and 580 °C) with flow velocities ranging from around 0.3 to 1.0 m s−1. The relative change of the molten salt thermal conductivity was measured. Gnielinski’s correlation was also used to estimate the heat transfer coefficient h of the flowing NaCl-KCl-MgCl2 in the given experimental condition. The work showed the potential of the MPR technique serving as an in-situ diagnostics tool to evaluate the heat transfer performance of flowing molten salts and other high-temperature HTFs.

Effects of thermal annealing on thermal conductivity of LPCVD silicon carbide thin films

The thermal conductivity (k) of polycrystalline SiC thin films is relevant for thermal management in emerging SiC applications like microelectromechanical and optoelectronic devices. In such films, k can be substantially reduced by microstructure features including grain boundaries, thin film surfaces, and porosity, although these microstructural effects can also be manipulated through thermal annealing. Here, we investigate these effects by using microfabricated suspended devices to measure the thermal conductivities of nine low-pressure chemical vapor deposition SiC films of varying thicknesses (from 120 to 300 nm) and annealing conditions (as-grown and annealed at 950 and 1100 °C for 2 h, and in one case 17 h). Fourier-transform infrared spectroscopy, x-ray diffraction spectra, and density measurements are also used to characterize the effects of annealing on the microstructure of selected samples. Compared to as-deposited films, annealing at 1100 °C typically increases the estimated grain size from 5.5 to 6.6 nm while decreasing the porosity from around 6.5% to practically fully dense. This corresponds to a 34% increase in the measured thin film thermal conductivity near room temperature from 5.8 to 7.8 W/m K. These thermal conductivity measurements show good agreement of better than 3% with fits using a simple theoretical model based on the kinetic theory combined with a Maxwell–Garnett porosity correction. Grain boundary scattering plays the dominant role in reducing the thermal conductivity of these films compared to bulk single-crystal values, while both grain size increase and porosity decrease play important roles in the partial k recovery of the films upon annealing. This work demonstrates the effects of modifying the microstructure and, thus, the thermal conductivity of SiC thin films by thermal annealing.

Investigation of thermo-rheological properties of Fe3O4/Ethylene glycol nanofluid in a square cavity

Many fluids used in heat transfer and transport phenomena restrict the effectiveness of heat exchange equipment on account of their low thermal conductivity. Using nanofluids, the ef-fectiveness of heat exchange equipment is enhanced by many folds. The use of magnetic nano-fluids for heat transfer generates a prospect of regulating flow and controlling the thermal and transport properties particularly the thermal conductivity and viscosity using an externally applied magnetic field. The present study involves synthesis of oleic acid-coated magnetic nanofluids at varying concentrations of 0 to 0.643% by volume, measurement of thermal conductivity, rheological properties and corresponding numerical simulation of Nanofluid in a heated square cavity. The thermal conductivity measurement have been carried out by transient hot-wire method using KD2-pro at varying concentrations of solid phase. The re-sults show a significant increase in thermal conductivity with increase in particle concentra-tion. Rheological measurements show variation in viscosity with shear rate, temperature and concentration. Moreover, it has been found that at low particle loading magnetic nanofluids exhibited Newtonian behavior unlike non-Newtonian behavior at increased concentration. Numerical simulation of the magnetic nanofluid in the heated square cavity demonstrates the immense potential of augmentation of heat transfer coefficient using such fluids.

Improved measurement of local thermal conductivities of coatings by the steady-state Raman spectroscopy method: A case study in TRISO particles

Tristructural-isotropic particle (TRISO) is a fully ceramic microencapsulated fuel of high-temperature gas-cooled reactors with complex microstructural morphology and heat transfer process, hindering the accurate measurement of its thermal conductivity. Here, a dual test wavelength steady-state Raman spectroscopy method (SSRS) for determining the thermal conductivities is proposed, resolving the issue of the conventional SSRS failing to measure the thermal conductivity of 3C-SiC. By demonstrating prerequisites for using the SSRS method and modeling the conductive-radiative coupled heat transfer, the thermal conductivities of all layers in TRISO have been accurately measured. It is also found that the thermal conductivity of an underlying material can be estimated by measuring the material on its top if the outer material has low absorption under the applied laser wavelength, offering the possibility of measuring the buried layers in localized structures. The work extends the usable range of the SSRS method for obtaining thermal conductivities in wide applications.

Insight into the dynamics of active and passive controls over the measurement of thermal conductivity of nanofluids subject to magnetic field and thermal radiation through the stretching surface

In comparison to conventional heat transfer fluids, nanofluids tend to significantly enhance thermal efficiency. This extensive study is to enhance heat transformation while accounting for the magnetohydrodynamic (MHD) nanofluid flow with the impact of non-linear thermal radiation and an irregular heat source over the stretching sheet. The impact of Arrhenius activation energy on active and passive controls of nanoparticles, thermophoresis, and Brownian diffusion has elaborated. The system of partial differential equations is first represented mathematically and then converted into the system of ordinary differential equations by using similarity transformations. The influence of various parameters is displayed numerically through tables, graphical impact, and surface plots on heat and mass transfer, micro-rotation distribution, and the Nusselt number. It is noticed that the temperature profile is intensified with increasing values of the temperature ratio parameter and thermal radiation (Rd) with active and passive controls. The impact of the magnetic field Biot number and thermophoresis parameter on the heat transfer rate has been scrutinized. .

Governing factors for actively heated fiber optics based thermal response tests

The accurate measurement of ground thermal conductivity is crucial for the evaluation of shallow geothermal energy potential. Actively heated fiber optics-based thermal response test (ATRT) is a recently developed novel method for the measurement of ground thermal conductivity, yet the governing factors for ATRT remain unclear. This study used numerical and physical models to examine the ATRT heat transfer process and the factors that may affect the measurement accuracy. The results showed that ATRT has a typical heat transfer process with three stages of composition, revealing diverse heat transfer in the cable, grout, and surrounding ground. The minimum heating duration of ATRT is approximately 150 min under a typical setup. The heating power is recommended to be within the range of 15–40 W·m−1, considering the temperature measurement accuracy. Using grout with a slightly higher thermal conductivity than the surrounding ground is beneficial. Actively heated fiber optic cable positions have a negligible effect as long as the heat duration is sufficient. A small borehole radius improves test accuracy by minimizing grout thickness effects. Understanding the ATRT heat transfer process and governing factors guarantees the validity of its in-situ applications and enables the standardization of ATRT.

Ultralow-temperature heat transport study of noncentrosymmetric superconductor CaPtAs

The noncentrosymmetric superconductor CaPtAs with time-reversal symmetry breaking in its superconducting state was previously proposed to host nodal superconductivity. Here, by employing ultralow-temperature thermal conductivity measurement on CaPtAs single crystal, we study its superconducting gap structure. A negligible residual linear term of thermal conductivity (κ 0/T) in zero magnetic field and the field dependence of κ 0/T indicate that CaPtAs has multiple superconducting gaps with a dominant s-wave component. This is consistent with recent nuclear quadrupole resonance measurements on CaPtAs. Our work puts a strong constraint on the theories to describe the superconducting pairing symmetry of CaPtAs.