Measurement Of Effective Thermal Conductivity Research Articles

Serial Laboratory Effective Thermal Conductivity Measurements of Cohesive and Non-cohesive Soils for the Purpose of Shallow Geothermal Potential Mapping and Databases—Methodology and Testing Procedure Recommendations

The article presents the methodology of conducting serial laboratory measurements of thermal conductivity of recompacted samples of cohesive and non-cohesive soils. The presented research procedure has been developed for the purpose of supplementing the Engineering–Geology Database and its part–Physical and Mechanical Properties of Soils and Rocks (abbr. BDGI-WFM) with a new component regarding thermal properties of soils. The data contained in BDGI-WFM are the basis for the development of maps and plans for the assessment of geothermal potential and support for the sustainable development of low enthalpy geothermal energy. Effective thermal conductivity of soils was studied at various levels of water saturation and various degrees of compaction. Cohesive soils were tested in initial moisture content and after drying to a constant mass. Non-cohesive soils were tested in initial moisture, fully saturated with water and after drying to a constant mass. Effective thermal conductivity of non-cohesive soils was determined on samples mechanically compacted to the literature values of bulk density. Basic physical parameters were determined for each of the samples. In total, 120 measurements of thermal conductivity were carried out, for the purposes of developing the guidelines which allowed statistical analysis of the results. The results were cross-checked with different measuring equipment and with the literature data.

Experimental study of porosity and effective thermal conductivity in packed bed of nano-structured FeTi for usage in hydrogen storage tanks

The porosity and effective thermal conductivity, which were measured simultaneously, changed owing to expansion during hydrogen absorption and contraction during hydrogen desorption. The metal hydride used in the experiment was a newly developed nano-structured FeTi (n-FeTi) by mechanical alloying. The porosity was measured by directly observing the changes in the volume of a metal hydride packed bed under free expansion. In the authors' previous study of LaNi5, the volume of the packed bed gradually increased, whereas the total volume of the n-FeTi packed bed decreased with increase in the cycles of expansion and contraction. The effective thermal conductivity measurements of the metal hydride packed bed were carried out using a cylindrical device with heating from a central heater. The results showed that the effective thermal conductivity of n-FeTi changes from 0.4 to 1.1 W/mK with rise in reacted fraction and decrease in porosity.

Repeatable experimental measurements of effective thermal diffusivity and conductivity of pebble bed under vacuum and helium conditions

Effective thermal diffusivity and conductivity are two crucial parameters to determine inherent safety in porous pebble bed of High Temperature Gas-cooled Reactor. A full-radius-scale heat test facility has been developed to measure these thermal properties at high temperature. Two vacuum tests and two atmospheric-pressure helium tests up to 1200 °C have been conducted to determine the temperature-dependent parameters by the inverse method. Repeatable results are validated by both tests with different heating processes. Furthermore, the primary experimental errors from installation positions of thermocouples as well as random packing effect are reduced by retrieving positions in the inverse method. Sensitivity and uncertainty analyses are carried out in respective tests to give the reasonable descriptions of the results of effective thermal diffusivity and conductivity in these tests.

Experimental investigation of heat transfer and fluid flow in octet-truss lattice geometry

This paper undertakes an experimental measurement of effective thermal conductivity, permeability, inertial coefficient, friction factor and wall convective heat transfer from octet truss lattice (OTL) structures as air-cooled compressive load-bearing light-weight heat sinks. Three aluminium alloy (AlSi10Mg) OTL structures of porosities 70.6%, 86.9% and 91.3% were fabricated using an additive manufacturing technique. Experimentally measured effective thermal conductivity, permeability, inertial coefficient, friction factor and wall Nusselt number for OTL structures are reported. Owing to high solid-to-fluid thermal conductivity ratio (∼4250), heat flows through the metal ligament and air independently and establishes steady-state, i.e. peripheral conduction coupling between solid metal and air is negligible. This is conferred from various theoretical and semi-empirical models from the literature. Friction factor and wall Nusselt number are characterised using a single length scale as against different length-scales often used in the literature. Flows through OTL structures seem to follow Forchheimer's flow law with negligible viscous contribution even at low pressure gradients. Further, the results are compared with previous analytical and experimental results for stochastic open-cellular foams. Owing to periodic nature of OTL structures, friction factor is lower than stochastic foams as mechanical dispersion effects are negligible. In conclusion, OTL structures are an excellent candidate for multifunctional applications where heat sinks are expected to bear compressive loads as well as dissipate heat while being lightweight.

Effective Thermal Conductivity Calculation and Measurement of Litz Wire Based on the Porous Metal Materials Structure

Litz wires are employed in high-frequency electrical machines due to their advantages of reducing the ac losses, including minimizing the skin effect and the proximity effect. In order to improve the reliability of such machines, and enable accurate thermal predictions at the design stage, accurate calculation of the thermal conductivity of litz wire is important. In this paper, a calculation method based on the Gasar porous metal materials model is put forward. In this method, a cell model is extracted from the litz wire, and a thermal resistance network is used to calculate the equivalent thermal conductivity (ETC). Following this, two finite-element analysis (FEA) models for the same litz wire are built, one with actual thermal conductivities for the different constituent materials and another with the calculated equivalent thermal conductivity for an equivalent material, with the two models showing similar thermal characteristics. Finally, an experimental setup is built for measuring the steady-state ETC of litz wire. The apparatus structure and characteristics are described in detail, and the experiment uncertainty and measurement errors are analyzed. Three types of litz wire are measured in the experimental, and the results from experiment and calculation are consistent.

Thermal characteristics of cement microparticle-stabilized aqueous foam for sealing high-temperature mining fractures

The fractures formed at high temperatures by the heat damage of coal and rock are the main causes for the evolution and spread of coal fires. Cement microparticle-stabilized aqueous foam is a new type of material for preventing and controlling coal and rock fractures at high temperatures. The thermal characteristics of the foam fluid are the most important technical parameters. The morphological changes of the foam fluid under different hot surface temperatures were characterized, and the mechanism of the effect of heating on the stability of the liquid film was studied based on the kinetic energy of the surfactant molecules and particle hydration. Using a homemade testing system, the thermal insulation properties of foam fluid with different bracket heights (2 cm, 7 cm, 12 cm, 17 cm, and 22 cm) and injection thicknesses (20 mm, 40 mm, 60 mm, and 80 mm) were tested. It was concluded that the cold surface temperature of all the foam fluids from 20 mm to 80 mm increased with increasing temperature; however, the rate of temperature increase within 0–90 s was greater than that within 90–300 s. The thermal insulation temperature was distributed on a Gauss cum surface, and the relationship of the thermal insulation temperature with the hot surface temperature and thickness was fitted. On the foundation of the traditional steady-state method, a new effective thermal conductivity measurement apparatus with a half-globular shape was developed, and its principle of operation and experimental setup were described. The effective thermal conductivity fluctuated slowly between 0.1222 and 0.2974 W/m·K within the temperature range of 323–373 K. The empirical correlation to calculate the thermal conductivity of the cement microparticle-stabilized aqueous foam at different temperatures was obtained.

Thermal or electrical bulk properties of rod-filled composites

The cell model (or self-consistent scheme) has been largely used to estimate the bulk thermal properties for dilute slender rods embedded in a matrix by applying a thermal gradient perpendicular to the main axis of the rod. This approach is then extended by considering a thermal gradient along the particle, or in other words a thermal flux through the end surfaces of the fiber. For a slender-body, this additional contribution is found to be negligible. Then, to circumvent the diluteness assumption, particle-particle contacts are considered when determining the effective thermal conductivity of the composite. This involves adding to the dilute result a new contribution depending on the magnitude of the Biot number (defined later) and on the micro-structures of the particles, and exhibiting a quadratic dependence on the particle concentration. Two assumed contact surface areas are investigating leading to the development of two micro-mechanical models, the difference being in the choice of the contact area. In addition, the model predictions are in good agreement with previously published data, where in-plane measurements of effective thermal conductivity are performed on highly conductive copper fibers impregnated with a poorly conductive PMMA matrix, for a wide range of fiber orientations and concentrations. When an appropriate cell size dimension for the dilute model is adopted, its predictions are also found to give an accurate fitting. Although this paper focuses on the thermal properties, the derivation can be carried over to the electrical properties with very minor modifications. This has been carried out for estimating the effective electrical conductivity of several CNTs (i.e., SWCNT and MWCNT) embedded in an epoxy matrix, leading again to good agreements.

Non-invasive measurement of effective thermal conductivity of human skin with a guard-heated thermistor probe

This paper describes a non-invasive method for measurement of the effective thermal conductivity of human skin with a guard-heated thermistor probe. The guard-heated thermistor probe makes it possible to accurately measure the absolute surface temperature of a material at temperatures higher than the ambient temperature. In this study, the probe was used for a non-invasive measurement of the effective thermal conductivity of human skin by measuring skin surface temperature response for a short time. The effective thermal conductivity of human skin was measured at different body locations, such as cheeks, forearms, and heels, on five healthy volunteers. The measured values reflect the effects of multi-layered skin structure and biological processes, such as blood perfusion and metabolism beneath the skin. The results reveal a correlation between the effective thermal conductivity and the skin structure. A numerical simulation was performed to assess the effects of the epidermis thickness and the thermal conductivities of skin layers on the effective thermal conductivity of human skin. The results show that the thickness of the epidermis has a large impact on the effective thermal conductivity measured with a guard-heated thermistor probe. This is because the measured effective thermal conductivity is a function of the thermal conductivities of both the epidermis and dermis, depending on the thermal penetration depth during measurement. At locations on the body with a thicker epidermis, such as the heel, the epidermal thermal conductivity dominates over the effective thermal conductivity. At locations on the body with a thinner epidermis, such as the cheek and forearm, the dermal thermal conductivity dominates over the effective thermal conductivity. The influences of physiological conditions, such as blood perfusion beneath the skin and metabolism, on the effective thermal conductivity of human skin, were also analyzed numerically. Variations in the blood perfusion rate and metabolic heat generation rate of the dermis layer were found to have a slight impact on the effective thermal conductivity; however, only the blood perfusion rate and the arterial blood temperature were found to influence the skin surface temperature.

Method and Validation for Measurement of Effective Thermal Diffusivity and Conductivity of Pebble Bed in High Temperature Gas-Cooled Reactors

The effective thermal diffusivity and conductivity of pebble bed in the high temperature gas-cooled reactor (HTGR) are two vital parameters to determine the operating temperature and power in varisized reactors with the restriction of inherent safety. A high-temperature heat transfer test facility and its inverse method for processing experimental data are presented in this work. The effective thermal diffusivity as well as conductivity of pebble bed will be measured at temperature up to 1600 °C in the under-construction facility with the full-scale in radius. The inverse method gives a global optimal relationship between thermal diffusivity and temperature through those thermocouple values in the pebble bed facility, and the conductivity is obtained by conversion from diffusivity. Furthermore, the robustness and uncertainty analyses are also set forth here to illustrate the validity of the algorithm and the corresponding experiment. A brief experimental result of preliminary low-temperature test is also presented in this work.

Applying Petroleum the Pressure Buildup Well Test Procedure on Thermal Response Test—A Novel Method for Analyzing Temperature Recovery Period

The theory of Thermal Response Testing (TRT) is a well-known part of the sizing process of the geothermal exchange system. Multiple parameters influence the accuracy of effective ground thermal conductivity measurement; like testing time, variable power, climate interferences, groundwater effect, etc. To improve the accuracy of the TRT, we introduced a procedure to additionally analyze falloff temperature decline after the power test. The method is based on a premise of analogy between TRT and petroleum well testing, since the origin of both procedures lies in the diffusivity equation with solutions for heat conduction or pressure analysis during radial flow. Applying pressure build-up test interpretation techniques to borehole heat exchanger testing, greater accuracy could be achieved since ground conductivity could be obtained from this period. Analysis was conducted on a coaxial exchanger with five different power steps, and with both direct and reverse flow regimes. Each test was set with 96 h of classical TRT, followed by 96 h of temperature decline, making for almost 2000 h of cumulative borehole testing. Results showed that the ground conductivity value could vary by as much as 25%, depending on test time, seasonal period and power fluctuations, while the thermal conductivity obtained from the falloff period provided more stable values, with only a 10% value variation.

A novel method used to study growth of ash deposition and in situ measurement of effective thermal conductivity of ash deposit

AbstractAsh deposition always brings boilers some trouble due to fouling or slagging. In this paper, a completely controlled system was developed to study the growth of ash deposit. A novel sampling probe was designed to online measure the heat flux through ash deposit. Additionally, the thickness of ash deposit can be obtained by an online figure collecting system. The results of this research showed that as the thickness of ash deposit increased, the heat flux decreased. It was also found that at the initial stage of ash deposition when the thickness of ash deposit is approximately 1 mm, the heat flux through ash deposit had a sharp reduction. An effective method was attempted to situ measure the effective thermal conductivity of the ash deposit in the simulated combustion flue gas. It was found that temperature of the ash deposit layer had no obvious effect on its value. It was concluded that the structure of ash deposit had no obvious change in a short deposition time of 30 min with varied surface temperatures of the probe head between 400 °C and 600 °C.

Sample holder design for effective thermal conductivity measurement of pebble-bed using laser flash method

Since the stress due to the thermal load on breeding blanket structure is one of the main design driver, the thermal conductivity is necessary input data for thermal-structural and thermo-hydraulic analyses performed in order to understand the heat transfer phenomena and estimate the thermal stress. Since for the functional materials of solid type breeding blanket a pebble-bed form is mainly adopted instead of a bulk form such as a block or a disk, it should be needed to measure the thermal conductivity of pebble-bed. In this study, the effective thermal conductivity of pebble-bed is measured by laser flash method, which is one of the various thermal conductivity measurement methods, because this method has several advantages such as a wide thermal conductivity range of the measurement and a small amount of pebbles. A sample holder considering the heat transfer mechanism from the laser source to pebble-bed has been specially designed in order to apply the laser flash technique to the pebble-bed sample and it has been validated by the experiments. This paper introduces preliminary results of the effective thermal conductivity on the pebble-bed using this sample holder.

Measurement of effective thermal conductivity of hydrate-bearing sediments and evaluation of existing prediction models

As a potential alternative strategic energy source, and for their potential impact on global climate, natural gas hydrates have garnered worldwide attention. This study explored the measurement of effective thermal conductivity of methane hydrate-bearing sediments and evaluated existing models for the prediction of effective thermal conductivity. A thermistor-based method combined with Micro-CT observations was employed in the determination of the effective thermal conductivity of porous matrix materials with various hydrate and water saturation levels and physical characteristics. The effects of sample component characteristics, including the volume content of hydrate and water, phase conversion, and properties of porous materials, on the effective thermal conductivity of hydrate-bearing sediments were systematically evaluated. The effective thermal conductivity positively correlated with hydrate saturation, water content, and the thermal conductivity of porous media. In addition, the effective thermal conductivity slightly increased with hydrate dissociation, indicating an increasing heat transfer capacity during gas production. Existing prediction models were evaluated using our measured results, and a hybrid model combining the parallel and series models was proposed with expanded applicability to the scope of our research. The feasibility of the proposed model was also verified in a comparison with previous research. The results of this study are important for future investigation of the actual thermal properties of hydrate-bearing sediment and the understanding of the heat transfer mechanism during gas production. Furthermore, the results can provide guidance in the selection of an optimal technique for gas production from hydrate deposits at the field scale.

A Numerical Model to Explain Experimental Results of Effective Thermal Conductivity Measurements on Unsaturated Soils

Effective thermal conductivity measurements on unsaturated soils by means of the probe method (thermal conductivity probe, TCP) often present a nonlinear trend of \(\Delta T\) versus ln (\(\tau \)). Three different slopes are present in the plots, while a homogeneous material should present only one. Being soils composite structures made of different phases (solid earth, liquid water and air), a possible explanation is the presence of phenomena other than pure conduction, such as water evaporation and vapor migration through the soil structure. A numerical model based on finite differences has been developed to simulate these phenomena. The model takes into account several factors including heat conduction, heat storage due to thermal capacity, water evaporation and water diffusion through a porous medium. Results show that two of the three slopes can be successfully simulated by the model, confirming the interpretation of the phenomena. However, the third slope from the experimental data is lower than the model’s slope, likely indicating the presence of other phenomena not yet taken into account, such as capillarity.

Theoretical Analysis of Effective Thermal Conductivity for the Chinese HTR-PM Heat Transfer Test Facility

The Chinese high temperature gas-cooled reactor pebble bed module (HTR-PM) demonstration project has attracted increasing attention. In order to support the project, a large-scale heat transfer test facility has been constructed for pebble bed effective thermal conductivity measurement over the whole temperature range (0~1600 °C). Based on different heat transfer mechanisms in the randomly packed pebble bed, three different types of effective thermal conductivity have been theoretically evaluated. A prediction of the total effective thermal conductivity of the pebble bed over the whole temperature range is provided for the optimization of the test facility and guidance of further experiments.

Measurement of effective thermal conductivity of compacted granular media by the transient plane source technique

To successfully realise industrial applications handling granular media, especially those involving heating and cooling processes, the temperature fields must be properly evaluated according to the accurate thermal properties of the media. The knowledge the effective thermal conductivity is regarded as one of the fundamental aspects. However, due to the complicated relations between the effective thermal conductivity and the heterogeneity and complexity in the structures and composition of the granular media, the quantitative prediction of the conductivity is challenging. Therefore, experimental investigation of the effective thermal conductivity becomes desired and this can provide first-hand data for industrial reference and serve as the benchmark for the theoretical analysis. In this study, the transient plane source technique is employed to investigate the effective thermal conductivity of compacted granular beds by the application of the commercially available Hot Disk system. The granular beds of different particle size ranges are characterised under different mechanical loading conditions by different sensors. Experimental results are discussed and suggestion to achieve reliable experimental designs is provided.

Material and Morphological Heat Transfer Properties of Fuel Cell Porous Transport Layers

The complex structure of the porous transport layer (PTLs), commonly referred to as gas diffusion layers (GDL), makes direct measurement of thermal transport properties difficult. Typically, effective thermal conductivity values are obtained experimentally and computational models are developed to replicate the measurements. This approach is sufficient for generalized one-dimensional models of fuel cell performance, but is not sufficient to predict reactant maldistribution due to the presence of liquid water in the PTL. Further, the anisotropy in the PTL structure results in dissimilar properties for through-plane and in-plane transport. In this work an analytical model is developed to extract material properties from effective thermal conductivity measurements. The model accounts for the change in the effective thermal conductivity as a function of compression. Geometric changes due to compression are separated from morphological changes using a compression modulation function; the shape of which is indicative of how fiber-to-fiber contact morphology changes with respect to strain. Material and morphological properties are determined using effective thermal conductivity data for three commercial PTLs. The properties also enable prediction of in-plane heat transfer using through-plane empirical data and also provide insight into morphological changes due to compression.

Measurement and augmentation of effective thermal conductivity of La0.8Ce0.2Ni5 hydride bed

An experimental set up for measuring Effective Thermal Conductivity (ETC) of Metal Hydride (MH) beds was fabricated using one-dimensional steady-state radial heat transfer absolute method. La0.8Ce0.2Ni5 alloy powder and its hydride were considered in the present study. Two types of compacts were fabricated for ETC augmentation of MH bed. First type was made from the mixture of 6 wt% graphite flakes with the La0.8Ce0.2Ni5 alloy powder. A three dimensional structure, having six radial fins, uniformly distributed around the outer periphery of a hollow cylinder, made with copper wire mesh, was integrated with the mixture of La0.8Ce0.2Ni5 powder and 6 wt% graphite flakes to make the second type of compacts. The ETC of the three types of beds namely loose metal powder bed (LMP), bed of compacts of metal alloy powder mixed with graphite flakes (CMPGF) and bed of compacts of metal alloy powder mixed with graphite flakes and embedded with copper wire mesh structure (CMPGFCu) were measured and compared at different argon and hydrogen pressures, hydrogen concentrations and average bed temperatures. The maximum values of effective thermal conductivity for the bed configurations of LMP/CMPGF/CMPGFCu were 1.3/4.7/6.8 W m−1 K−1. These values of effective thermal conductivity were obtained at 70 bar hydrogen gas pressure and 60 °C average bed temperature. The decrease in mass fraction of hydrogen absorbed by the La0.8Ce0.2Ni5 hydride, as a result of implementation of ETC augmentation techniques, was nominal (around 5.7% in case of CMPGF and 4.5% in case of CMPGFCu) when compared to the respective powder bed configurations.

Effective thermal conductivity of inhomogeneous medium containing gas‐filled inclusions

Determination of effective physical parameters of gas‐filled porous media is of practical interest to a wide spectrum of applications from construction industry to petrophysics. The classical equations of thermal conductivity are not applicable for sufficiently small pores, the characteristic size of which is comparable with the mean free path of gas molecules. In this case, for description of the gas behavior, it is necessary to use the methods of physical kinetics and rarefied gas dynamics. In this work, we have considered so‐called slip flow regime, 0 < Kn < 1, where Kn =λ/R is the Knudsen number, λ is the mean free path, and R is the characteristic pore size. In this regime, we can use the classical equations of mechanics of continuum media with modified boundary conditions that take into account the temperature and energy flux jumps on the pore boundaries. We have solved the so‐called one‐particle problem of thermal polarization of gas‐filled inclusion. The obtained solution was then used in some self‐consistent homogenization methods to calculate the effective conductivity coefficient. Our calculations have shown that the effective conductivity depends substantially on the pore size (the Knudsen number). The obtained results demonstrate the feasibility of pore sizes determination by using measurements of effective thermal conductivity. Copyright © 2015 John Wiley & Sons, Ltd.

A Novel Pre-cooling System for a Cryogenic Pulsating Heat Pipe

To reduce the influence of the pipe material on the measurement of effective thermal conductivity, the pipe of a cryogenic pulsating heat pipe is generally made of stainless steel. Because of the low thermal conductivity of stainless steel, the pre-cooling of the evaporator in cryogenic pulsating heat pipe using helium as working fluid at 4.2K is a problem. We designed a mechanical-thermal switch between the cryocooler and the evaporator, which was on during the pre-cooling process and off during the test process. By using the pre-cooling system, the cool down time of the cryogenic pulsating heat pipe was reduced significantly.