Measurement Of Effective Thermal Conductivity Research Articles

Measurement and theoretical analysis of effective thermal conductivity of lanthanum pentanickel powder beds for hydrogen storage in different particle sizes and bed porosities

Particle diameter and bed porosity change due to the expansion and contraction of metal hydride particles during hydrogen absorption and desorption, which leads to variations in the effective thermal conductivity of metal hydride beds. Therefore, the effective thermal conductivity of the metal hydride beds under different bed porosities and particle diameters must be measured for the design of metal hydride-based hydrogen storage tanks. In this study, the effective thermal conductivities of four kinds of lanthanum pentanickel (LaNi5) powder beds with different bed porosities and particle diameters were measured in various gas atmospheres with gas pressure varying from 0.1 to 4.0 MPa. The volume mean diameters of the four kinds of LaNi5 powders equal 335.2, 196.4, 147.7, and 23.7 μm, respectively. Subsequently, the effects of bed porosity and particle diameter on the effective thermal conductivities were analyzed through the modified Zehner–Schlünder–Damköhler model considering the Smoluchowski effect. Experimental results show that particle diameter and bed porosity influence markedly the effective thermal conductivity of lanthanum pentanickel (LaNi5) powder beds. However, the findings of theoretical analysis based on the modified Zehner–Schlünder–Damköhler model reveal the considerably weaker effect of particle diameter on the effective thermal conductivity under identical bed porosity compared with the measurement results. Such a finding was observed because the variation in particle diameter inevitably changed bed porosity during the effective thermal conductivity measurement, which also affected the effective thermal conductivity of the beds. In addition, decreasing bed porosity and increasing particle diameter can remarkably improve the heat transport via the gas–solid mixture pathway, which improved the effective thermal conductivity of the beds and thus explained the experimental results.

Measurement of effective thermal conductivity of composite powders of 2D materials and metals for additive manufacturing

Laser Powder Bed Fusion (LPBF), a promising additive manufacturing technique for composites of 2D materials and metals, requires knowledge of thermophysical properties, such as the thermal conductivity of powder, for process optimization. In this study, we measured the effective thermal conductivity of the most representative Cu-graphene composite powder in the field of heat transfer applications. To measure thermal conductivity, we propose a differential scanning calorimetry (DSC) method to measure the thermal resistances of a powder bed under steady-state heat flow conditions. We observe that the thermal conductivity of a composite powder is 1,000 times lower than that of the bulk metal without 2D material addition; e.g., exhibiting ~ 0.30 W/mK in Cu-1wt.% graphene powder. Furthermore, we discuss powder size and morphology impact on thermal conductivity influencing the number of contact points and thermal contact resistance. Our findings contribute to understanding the thermal conductivity of composite powders in a powder bed and aid in optimizing LPBF processes.

Thermal Conductivity and Microstructure of Novel Flaxseed-Gum-Filled Epoxy Resin Biocomposite: Analytical Models and X-ray Computed Tomography.

The growing awareness of the environment and sustainable development has prompted the search for solutions involving the development of bio-based composite materials for insulating applications, offering an alternative to traditional synthetic materials such as glass- and carbon-reinforced composites. In this study, we investigate the thermal and microstructural properties of new biocomposite insulating materials derived from flaxseed-gum-filled epoxy, with and without the inclusion of reinforced flax fibers. A theoretical approach is proposed to estimate the thermal conductivity, while the composite's microstructure is characterized using X-ray Computed Tomography and image analysis. The local thermal conductivity of the flax fibers and the flaxseed gum matrix is identified by using effective thermal conductivity measurements and analytical models. This study provides valuable insight into the thermal behavior of these biocomposites with varying compositions of flaxseed gum and epoxy resin. The results obtained could not only contribute to a better understanding the thermal properties of these materials but are also of significant interest for advanced numerical modeling applications.

Effective Thermal Conductivity Measurement of Additively Manufactured Lattice Structures by Application of Modified Temperature Profile Method

This study aims to establish a general-purpose thermal conductivity measurement method that can take into account the effect of heat loss under atmospheric conditions for measuring the effective thermal conductivity of lattice structures, and to clarify the effective thermal conductivity of lattice structures with different wire diameters. In this paper, calculations by finite element method and measurements using steady state comparative-longitudinal heat flow method and modified temperature profile method were performed to clarify the effective thermal conductivity of the five truncated octahedron unit-cell lattice structures with different wire diameters fabricated by additive manufacturing. The modified temperature profile method is developed to take into account the effect of interfacial thermal resistance in the measurement apparatus. The effective thermal conductivity measured using the steady state comparative-longitudinal heat flow method and calculated with finite element method analysis showed good agreement, confirming that the effective thermal conductivity is strongly dependent on the wire diameter. The effective thermal conductivity obtained by the modified temperature profile (MTP) method was 3 % to 24 % smaller than that obtained by the steady state comparative-longitudinal heat flow method, and the measurement was able to take heat loss into account more concretely. Furthermore, measurements using the MTP method enabled us to obtain reasonable values for the ratio of heat loss in each section, the fin efficiency of the sample, the heat transfer coefficient to the surroundings, and the interfacial thermal resistance between the rods and the sample.

Effects of atmospheric gases and compression load on effective thermal conductivity of Li2TiO3 pebble bed

Tritium breeder material in the solid-type of breeding blanket concepts is usually used in a pebble bed form because of the efficiency and convenience of tritium extraction. The pebble bed is continuously heated by the exothermal reaction between neutron and lithium-6 during the operation of a fusion reactor. Thus, a compression pressure is applied on the pebble bed by the thermal expansion of pebbles and container structure. Therefore, the accurate thermal properties of the pebble bed are one of the important design parameters to ensure a function and stability of the breeding blanket. This study aims at the investigation of the change on effective thermal conductivity of Li2TiO3 pebble beds under the operation conditions of breeding blanket.The measurement of effective thermal conductivities of Li2TiO3 pebble beds has been performed by laser flash technique in several atmosphere gases, such as helium, nitrogen, and air, under the statically compression load controlled by specially design pneumatic device incorporated to the laser flash apparatus. Also, in the case of helium atmosphere, the effects of hydrogen concentration from 0.1 wt.% to 1.0 wt.% on the effective thermal conductivity were also investigated. The effective thermal conductivity of the Li2TiO3 pebble bed was strongly dependent on a kind of atmosphere gases, especially its thermal conductivity, in the relatively low temperature. However, the concentration of hydrogen in the helium atmosphere did not significantly affect the effective thermal conductivity of the pebble bed. Applied axial compression load has influenced on the effective thermal conductivity of the Li2TiO3 pebble bed, especially at high temperature. However, the obvious change of effective thermal conductivity with the value of compression load was not observed.

X‐ray Computed Tomography Characterization of Slag Crust and the Effect of Bubbles and Metallic Iron in Slag on Heat Transfer to Copper Stave

Understanding the heat transfer properties of the slag crust is critical to the safe operation of copper staves. The distribution of metallic iron and bubbles in the slag crust is detected by X‐ray computed tomography. The thermal conductivity of the slag crust is modified based on an effective thermal conductivity model and laser thermal conductivity measurements to predict the effect of metallic iron and bubbles on the heat transfer in the slag crust. The results show that the average volume fractions of metallic iron and air bubbles in the slag crust are 3% and 4.5%, respectively. The measured value is consistent with the trend of the effective thermal conductivity calculation model, and the calculation result of the Maxwell–Eucken effective thermal conductivity model is very accurate. Variations in the volume fraction of metallic iron in the slag crust have a greater effect on the thickness of the slag crust, while variations in the volume fraction of bubbles have a smaller effect on the thickness of the slag crust. It is recommended to keep it between 10 and 30 mm in blast furnace production to help improve the operational life of the copper stave.

Modeling and Measurement of Effective Thermal Conductivity of Materials Reinforced with Bars

Modeling and Measurement of Effective Thermal Conductivity of Materials Reinforced with Bars

Modeling and measurement of effective thermal conductivity of core-shell-structured SiO2 MHSPs-silica aerogel composite

To explore and optimize silica aerogel's thermal insulation performance, we model and test the effective thermal conductivity (λ) of SiO2 micro hollow sphere particle (MHSP)-silica aerogel. The models are for the infrared radiative λ of SiO2 MHSP-silica aerogel, with different core/shell ratios (ld), and the hot surface's temperature response is measured by the steady-state method. The results show that the radiative λ of aerogel doping SiO2 MHSP with ld=0.75 is equivalent to that of doping with solid spheres, whose weight is reduced by 42.19%, and the solid λ is reduced by 88.24%. Furthermore, the λ of aerogel with 60 μm SiO2 MHSPs of 0, 6.89, and 16.28 vol% is 0.02504∼0.02613, 0.02847∼0.03100, and 0.03635∼0.03955 W·m–1·K–1, respectively, at small temperature difference (ΔT <20 K). As ΔT increases (> 550 K), the ‘λ’ of aerogel with 16.28% SiO2 MHSP becomes smaller than aerogel with 6.89% SiO2 MHSP, indicating the optimal insulation performance.

Effective thermal conductivity measurement of PCM composites during phase transition by using the transient plane source method

The thermal conductivity of phase change materials (PCMs) during phase transitions is of great significance to the evaluation of the thermal performance of energy-saving buildings. Based on the transient plane source method, a method for measuring the effective thermal conductivity of PCM composites was proposed, by which the variations in effective thermal conductivity with liquid fraction and temperature during phase transitions can be obtained. The correctness of the proposed method was verified through matched simulation experiments. Three kinds of PCM samples were prepared for experimental measurement on the basis of the direct incorporation method, and the volume proportions of decanoic acid to gypsum were set to 10%, 12%, and 15%. The phase transition regions and latent heat values of these PCM samples were measured using the differential scanning calorimetry technique. Results showed that the effective thermal conductivity of the PCM first increased and then decreased during the phase transition. As the fraction of PCM content increased, the phase transition regions gradually expanded, and the effective thermal conductivity changed more drastically. The good measurement repeatability demonstrated the feasibility of this method for measuring the effective thermal conductivity of composites during phase transition.

Bayesian optimization for effective thermal conductivity measurement of thermal energy storage: An experimental and numerical approach

The increasing demand for cooling and refrigeration poses an urgent need in designing efficient and low-cost thermal energy storage systems for future energy systems. While multiple effects may affect the heat transfer behaviors during thermal energy storage, these effects can be lumped into one parameter, the effective thermal conductivity. Effective thermal conductivity provides a simple and reliable solution for accurate numerical simulations in designing a thermal energy storage system. In this study, a novel experimental, numerical and Bayesian optimization-based method is developed and validated that allows for fast and accurate measurement of the effective thermal conductivities over a wide temperature range. The method can also be applied to other bulky and heterogeneous structures that cannot be considered as continuous media. An experimental setup and a 3D numerical model were developed for the plate-type thermal energy storage. After a thorough algorithm comparison, Bayesian optimization using Gaussian process was selected to search for the effective thermal conductivities with high accuracy (root mean square error < 2 K and R-squared between 0.975 and 0.992). The effective thermal conductivities measured using deionized water as the phase change material were validated by a COMSOL simulation. With the accurate effective thermal conductivity results, we revealed that neglecting the effective thermal conductivity for the solid phase while still using conduction models will lead to significant errors in the simulation. A duo arch-shaped graphite sheet-based macrofiller is designed and inserted into the plate-type thermal energy storage, which increased the effective thermal conductivities by around 20% and suppressed the subcooling effect.

Effective Thermal Conductivity Measurement of Additively Manufactured Lattice Structures by Application of Modified Temperature Profile Method

Effective Thermal Conductivity Measurement of Additively Manufactured Lattice Structures by Application of Modified Temperature Profile Method

Measurement of effective thermal conductivity of lithium metatitanate pebble beds by steady-state radial heat flow method

As a functional material of the tritium breeding blankets of the future fusion reactor, different candidates of the lithium-based ceramics have been considered to generate and release tritium. These tritium breeding materials in the form of packed pebble beds of nearly spherical-shaped particles have many advantages over the sintered pellets or blocks. For the design and analysis of the breeding blanket, the effective thermal conductivity (keff) of lithium-based ceramic pebble beds are required to be well characterized under the fusion-relevant conditions. In this study, an experimental setup has been fabricated and installed to estimate the keff of pebble beds as a function of bed temperature and filling gas pressure. The working principle of the experiment is based on the steady-state radial heat flow method. The various experiments have been performed on lithium metatitanate (Li2TiO3) pebbles having a diameter of 1 ± 0.15 mm to measure keff in the temperature range from 400 °C to 800 °C and in the environments of stagnant helium gas. The pressure of helium gas was adjusted from 0.105 MPa to 0.4 MPa (abs.) to account the effect of helium gas pressure. The increase in keff has been found with increasing the bed temperature and also increasing the helium gas pressure. The obtained results have been found comparable with the literature results with minimum difference.

Performance of nitrogen pulsating heat pipes as passive thermal switches in a redundant cryocooler application

A novel redundant cryocooler system, featuring two cryocooler cold heads linked via separate nitrogen pulsating heat pipe (PHP) thermal switches to a common cold plate, is developed and performance tested. The setup allows the cold plate to be maintained at the design temperature when one of the two cryocoolers is inactive by leveraging the low thermal conductivity of the static vapor in the associated dry PHP as insulation from the warm inactive cryocooler. The PHP thermal switches operate passively and require no mechanical actuation, inherently offering a highly reliable switching mechanism. PHP heat load, effective thermal conductivity, effective conductance, evaporator temperature, and condenser temperature measurements are presented for a range of fill ratios and applied cold plate heat loads. The tested PHPs can transfer at least 16 W before dryout and have effective thermal conductivities in the ON and OFF switch states ranging from 10,000 W/m-K to 55,000 W/m-K and 8 W/m-K to 50 W/m-K, respectively. Thermal switch performance is characterized by the switching ratio – the ratio of its effective conductance in the ON state to that in the OFF state – and is demonstrated to approach about 2500 for the tested PHPs. The parasitic heat load through the PHP switch in the OFF state (about 0.6 W) is found to be about 4 percent of its maximum tested ON state heat transfer capacity (about 16 W).

Simultaneous measurement of effective thermal conductivity and effective thermal diffusivity of Li2TiO3 pebble bed using transient hot-wire technique

Lithium-based ceramics in the form of pebble beds have been considered for tritium breeder material in the fusion reactor's breeder blanket. It is essential to study the thermal characteristics of these ceramic pebble beds subjected to fusion relevant conditions. Effective thermal conductivity (k), effective thermal diffusivity (α) and effective specific heat (cp) of a packed bed are some of the crucial parameters for the design of breeder blanket module. In the present study, the transient hot-wire technique was used to measurek, α and cp of lithium metatitanate (Li2TiO3) pebble bed fabricated at Institute for Plasma Research, India. Thermal properties of Li2TiO3 pebble bed (1 ± 0.15 mm pebble diameter and ∼63% packing fraction) are measured within the temperature range of 45 °C to 800 °C in stagnant helium gas environment. The filling gas pressure variation effect on the Li2TiO3 pebble bed's thermal properties for the pressure range of 0.105 MPa to 0.4 MPa has also been studied. Equations are suggested for k and α of Li2TiO3 pebble bed as a function of temperature at different pressure in the helium gas environment.

Measurement and analysis of effective thermal conductivity of LaNi5 and its hydride under different gas atmospheres

In this work, an ETC measurement cell based on the steady-state radial heat flow method was fabricated, and its reliability was validated by a commercial instrument. The ETCs of unactuated and activated LaNi5 powder beds were measured under various filling gases in the pressure range from 0.1 to 1.5 MPa. The experimental results show that the ETC of activated LaNi5 powder is much lower than that of the unactuated material due to the pulverization effect. The ETCs of unactuated and activated LaNi5 powder in helium atmosphere lie in the ranges of 1.04–1.63 W m−1 K−1 and 0.49–0.85 W m−1 K−1, respectively. Furthermore, the ETC increases with the increasing pressure and thermal conductivity of filling gas. The effects of hydrogen absorption and desorption on the ETC were investigated by step-by-step reaction, which depend mainly on the hydrogen concentration and expansion/constriction of MH particles.

Synthesis and Effective Thermal Conductivity Measurements of Hollow Mesoporous SiO2 Spheres for Heat‐Insulating Applications

Mesoporous silica hollow spheres are an excellent model system to investigate the thermal conductivity for an efficient heat‐insulating material with respect to its geometry. Four different monodisperse silica hollow spheres are synthesized via a three‐step synthesis consisting of emulsifier‐free emulsion polymerization, modified Stöber condensation process, and subsequent calcination. In this approach, cetyltrimethylammonium bromide (CTAB) is used as a structure directing component to produce a highly porous silica shell. The systematic investigations of the effective thermal conductivity (ETC) allow distinguishing the respective conduction pathways, such as solid conduction and gas conduction. The carried out thermal conductivity measurements reveal for all four samples promising low ETC values of ≈36 mW m−1 K−1 at 1013 mbar and 35 °C. In vacuum (0.03 mbar) all four samples showed, independent of shell thickness and inner diameter, a comparable reduced ETC of about 10 mW m−1 K−1. The comparison with previous studies on unstructured silica hollow spheres indicates that the solid state conductivity within the bulk is more dependent on the contact strengths and the number of contacts than on the thermal conductivity within the silica material, as the scattering probability of phonons is not influenced by an increased density of defects.

Measurement of effective thermal conductivity of LaNi[formula omitted] powder packed bed

Effective thermal conductivity of LaNi5 powder packed bed was analyzed with a miniaturized guarded hot-plate (GHP) apparatus. Here, GHP was designed for precise measurement of effective thermal conductivity of metal-hydride powders even with small sample amounts (2.12×104 mm3). Dimensions of sample container and apparatus were determined through two-dimensional (2-D) steady-state heat conduction analysis. Calibration experiment and uncertainty analysis were conducted to validate the accuracy of the GHP. Based on the measurements of the residual thermal conductivity of the LaNi5 packed bed, effect of particle size on contact factor of LaNi5 packed bed was estimated. By applying the Yagi and Kunii (YK) model to the effective thermal conductivity of LaNi5 packed bed, effect of contact factor and gas thermal conductivity on characteristic length of gas film were newly analyzed. Factors of YK model were modified in present work and validated through comparison with experimental data from previous literature.

Heat Transfer in Unfrozen and Frozen Porous Media: Experimental Measurement and Pore‐Scale Modeling

AbstractIn this work, we conducted a mechanistic study on pore‐scale mechanisms controlling heat transfer in partially saturated porous media at unfrozen and frozen conditions. Experimental measurement of effective thermal conductivity (ETC) of simulated partially saturated sediments was carried out to explore the effect of different parameters including pore and overburden pressures, temperature, and water/ice saturation on the pore‐scale mechanisms governing heat transfer in multiphase porous media. The experimental measurements show that the heat transfer is a complex phenomenon affected by several important pore‐scale mechanisms such as particle‐particle conduction and particle‐fluid‐particle conduction, which are governed by water content and distribution, packing structure, wettability characteristics of grains, coordination number, and physical contact among sediment particle. A numerical model was also developed for prediction of ETC using free‐energy lattice Boltzmann model and a space renormalization method. The model predictions were in good agreement with the experimental data, showing that the model is able to reliably estimate ETC with average relative deviations of less than 10%, as it appropriately incorporates the pore‐scale mechanisms influencing ETC. The numerical model predictions were also compared with those of six predictive models available in the literature, and root‐mean‐square errors were calculated to assess its accuracy against the existing models.

Unsaturated cell model of effective thermal conductivity of soils

In this study, a cell model based on new physical conceptualizations is developed to calculate the effective thermal conductivity of unsaturated soils. The evolution of water phase saturation is treated using a cell model consisting of solid particle being enveloped by a mixture of water and air phases. The thermal conductivity of the mixed fluid phase is determined by the volume-weighted average of water and air thermal conductivities. The temperature distribution in the cell of soil and fluid mixture is solved along with boundary condition of linearly varying temperature field. The heat transfer through the cell is calculated by integration of the temperature gradient along with thermal conductivities of soil and fluid. The effective thermal conductivity of the unsaturated soils is determined by conceptualizing the soils as having the same heat transfer as the cell of solid particle and fluid mixture. The new model is compared with 268 data entries of unsaturated soil effective thermal conductivity measurements from the literature. The developed effective thermal conductivity model compares favorably with the experimental data. When the porosity is small, the increase in the effective thermal conductivity with water phase saturation is more significant and non-linear. When the porosity increases, the increases of effective thermal conductivity with water phase saturation approaches a linear relationship.

Measurement of effective thermal conductivity of lithium metatitanate pebble bed by transient hot-wire technique

Abstract In future fusion reactor lithium metatitanate (Li2TiO3) as a functional material in the form of packed pebble beds have been selected in breeding blanket concepts to generate and release tritium. The effective thermal conductivity (keff) of Li2TiO3 pebble beds are needed to be well characterized for the design and analysis of breeding blankets under fusion relevant conditions. The transient hot-wire technique based experimental system has been fabricated, tested and installed for keff measurement of Li2TiO3 pebble beds. Finite element method (FEM) simulations have been performed to optimize the dimension of an experimental setup with the intention of minimizing the uncertainty in keff measurements. The experiments have been performed on 1 ± 0.15 mm diameter Li2TiO3 pebble bed with a packing fraction of 63 %. The influences of temperature (35–800 °C), gas environment (helium and air) and gas pressure (0.105−0.4 MPa (abs.)) have been investigated on keff of Li2TiO3 pebble beds. Reduction in keff was found with a decrease in helium gas pressure while the reduction in keff was not significant with a decrease in air pressure. The measured keff results of Li2TiO3 pebble beds have been compared and agreed well with the literature experimental results. The empirical correlations are also proposed for keff of Li2TiO3 pebble beds in helium and in air environment at 0.105−0.4 MPa and as a function of temperature.