Thermal Effusivity Research Articles

Classification of computed thermal tomography images with deep learning convolutional neural network

Thermal tomography (TT) is a computational method for the reconstruction of depth profile of the internal material defects from Pulsed Infrared Thermography (PIT) nondestructive evaluation. The PIT method consists of recording material surface temperature transients with a fast frame infrared camera, following thermal pulse deposition on the material surface with a flashlamp and heat diffusion into material bulk. TT algorithm obtains depth reconstructions of thermal effusivity, which has been shown to provide visualization of the subsurface internal defects in metals. In many applications, one needs to determine the defect shape and orientation from reconstructed effusivity images. Interpretation of TT images is non-trivial because of blurring, which increases with depth due to the heat diffusion-based nature of image formation. We have developed a deep learning convolutional neural network (CNN) to classify the size and orientation of subsurface material defects in TT images. CNN was trained with TT images produced with computer simulations of 2D metallic structures (thin plates) containing elliptical subsurface voids. The performance of CNN was investigated using test TT images developed with computer simulations of plates containing elliptical defects, and defects with shapes imported from scanning electron microscopy images. CNN demonstrated the ability to classify radii and angular orientation of elliptical defects in previously unseen test TT images. We have also demonstrated that CNN trained on the TT images of elliptical defects is capable of classifying the shape and orientation of irregular defects.

Analysis of the Heat Transfer Coefficient, Thermal Effusivity and Mathematical Modelling of Drying Kinetics of a Partitioned Single Pass Low-Cost Solar Drying of Cocoyam Chips with Economic Assessments

This study examines the heat and mass transfer coefficient, thermal effusivity, and other thermal properties of solar-dried cocoyam chips, as well as the drying kinetics. The research also assessed the economics of the solar dryer. For these reasons, a solar dryer with a partitioned collector was developed that creates a double airflow travel distance to delay the airflow inside the collector. The partitioning of the collector delays the airflow and helps to create more turbulence for the airflow with increased energy. The solar dryer was locally developed at the Michael Okpara University of Agriculture and tested during the humid crop harvesting period of September for the worst-case scenario. The obtained drying curves and kinetics for cocoyam drying are subjected to the vagaries of weather conditions. The drying rate showed declining sinusoidal characteristics and took about 25 h to attain equilibrium. Analysis of the airflow velocity showed gravitation between laminar and turbulent flow, ranging from 171.69 to 5152.77. Specific heat capacity, thermal conductivity, and effusivity declined with moisture content while the thermal diffusivity increased. However, the values of thermal effusivity ranged from 12.2 to 47.94 W·s1/2·m−2·K−1, which is within the range of values for insulators. The heat and mass transfer coefficient varied as a function of the airflow velocity. Fitting the drying curve into semi-empirical models showed that the two-term model was the best-fitted model for the experimental data from drying cocoyam. Using the solar dryer in Nigeria can save $188.63–$1886.13 in running costs with a payback period of 0.059–0.59 years (21.54–215.35 days) at a rate of 10–100% of usage.

Influence of Different Ambient Temperatures on the Thermal Properties of Fiber-Reinforced Structural Lightweight Aggregate Concrete

This study reports the influence of different climatic ambient temperatures on the thermal properties of fiber-reinforced lightweight aggregate concrete (LWAC). Lightweight expanded clay aggregates (LECA) with steel (ST) and polypropylene fibers were used in the mix proportions. The steady-state thermal test was performed on concrete samples at the oven-dry state with the measurement taken at six different climatic ambient temperatures of 0 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C. The results show a linear dependence of thermal conductivity, specific heat, thermal diffusivity and thermal effusivity of fiber-reinforced LWACs against the different ambient temperatures. These ambient temperature variations are discussed as a function of the thermal properties of fiber-reinforced LWAC. The thermal conductivity and thermal diffusivity decrease linearly between 0 °C and 50 °C, whilst the specific heat and thermal effusivity increase linearly between 0 °C and 50 °C. Equations with strong correlations to predict thermal properties of fiber-reinforced LWAC were proposed based on the results of this study. The significance of this research is to propose the dynamic ambient temperature-dependent thermal properties equations that can be used in the energy analysis of the buildings.

Evaluation of the thermal effusivity of intermetallic compounds using a modulated thermoreflectance method

In this study, we evaluated thermal properties of the matrix phase and intermetallic compounds in solder joints by a frequency-domain thermoreflectance method with high spatial resolution realized by two two laser beams and considering thermal diffusion length. The values of the thermal effusivities of a tin matrix phase (Sn matrix) and the intermetallic compounds (IMCs) in solder joints were determined for each heating frequency using calibration curves given by standard samples with known thermal properties. These measured heat effusivities of these Sn matrix and IMCs were estimated various values affected thermal conductivity of addition elements. Moreover, to convert from thermal effusivity to thermal conductivity, numerical calculations were conducted with assumed various thermal conductivity of molybdenum-reflector film on the sample surface and the volumetric heat capacity of IMCs. Finally, it is found that the apparent thermal conductivity of the Mo film was 0.5 W m−1 K−1 and the thermal conductivity of IMCs were shown 25–37 W m−1 K−1 with composition dependence clearly.

Influence of hot water blanching and saline immersion period on the thermal effusivity and the drying kinetics of hybrid solar drying of sweet potato chips

The purpose of this study was to investigate the effect of adding pre-treatment to solar dried sweet potato chips. The tested pre-treatments were blanching using hot water and saline immersion. Indeed, the potato chips were dried using a convective indirect solar dryer, equipped with a wind generator and glycerine as thermal storage. The system was built locally at the University of Agriculture in Umuahia, Abia State, Nigeria. The obtained experimental drying curves and kinetics were directly affected by the weather conditions, as the drying rate increased with the increase of the radiation and the ambient temperature and decreased with their decrease. Drying of the various pre-treated potato slices took 8–14 h to attain equilibrium moisture level under the variable external drying conditions. The drying curves were fitted to semi-empirical models. The results showed that the two-term exponential model was the best model that fitted the experimental data. Shrinkage was a vital phenomenon observed during the drying process, as the volume of the chips was reduced by 50%. The volume of samples treated by saline immersion was higher by 6% than non-treated samples. Shrinkage phenomenon was then introduced to determine the thermos-physical properties of sweet potato chips, with particular attention to the particle density, the bulk density, shape analysis, specific heat capacity, thermal diffusivity, and thermal effusivity. The particle and bulk density peaked at 1.41–1.44 g/cm3, and 1.52–1.53 g/cm3 respectively for all treatments. The thermal effusivity ranged from 17.6 to 7.93 W.S1/2.m−2.K−1 while the specific heat capacity ranged from 1028 to 2288 J/kg °C. All the thermo-physical properties were presented as a function of the volume or moisture content variation with a high degree of association. The results demonstrated the importance of shrinkage and its direct effect on the determination of the thermos-physical properties of the studied product for both pre-treated and untreated samples.

Mechanical and Thermal Properties of 3D Printed Polycarbonate

This study aims at showing the potential of additive manufacturing as a new processing route for designing future insulators in the building sector. Polycarbonate (PC) is studied as a possible candidate for designing these new insulators. This polymer offers several advantages, among them fire resistance and stability of its physical properties at high temperatures. The 3D printing of PC is attempted using fused deposition modelling technology. The printing temperature and infill rate are varied to achieve optimal mechanical and thermal characteristics. The results show that an optimal printing temperature of 280 °C is needed to achieve high tensile performance. In addition, thermal properties including thermal conductivity and effusivity increase with the increase of the infill rate in opposition to the thermal diffusivity decrease.

Influence of coolant and material properties on cooling high-temperature steel spheres in subcooled ethanol-water mixtures

The paper presents the new experimental results on cooling high-temperature steel spheres in subcooled ethanol-water mixtures at mass fractions of ethanol in the range of 10–80% under atmospheric pressure. Three test samples of 39 mm in diameter each have been prepared for the experiments: a carbon steel sphere with the oxidized surface and two stainless steel spheres. In the latter samples, one sphere had a polished surface, while the other one had a carbon coating. Usage of binary liquid mixtures as cooling liquids allows changing their thermophysical properties in a wide range. In general, when decreasing ethanol mass fraction boiling heat transfer intensifies, and it leads to an earlier (at higher surface temperature) transition to the intensive heat transfer regime. The experimental results are qualitatively consistent with a model that was proposed by authors earlier. This model describes the conditions of transition from a stable film boiling to an intensive heat transfer regime (so-called “microbubble boiling”). The main idea of the model is that this regime can arise due to the local contacts between the crests of the wavy liquid flow at the liquid/vapor interface and the protrusions of the surface roughness. The model considers the properties of a coolant (latent heat of evaporation, surface tension and kinematic viscosity) and thermophysical properties of a cooled surface (thermal effusivity). The experimentally obtained values of heat transfer coefficients in stable film boiling regimes are in agreement with the calculation results of the semi-empirical model of heat transfer in stable film boiling of subcooled liquids developed by authors earlier.

MTPS Analytical Temperature and Heat Flux Solution With Thermal Contact Resistance

Abstract An exact transient analytical solution of the temperature fields and heat fluxes for the modified transient plane source (MTPS) method is presented. The MTPS sensor is made of a thin disk with a heater/sensor and a heated guard ring on its surface, providing a one-dimensional heat flow to a semi-infinite sample in contact with its surface. The MTPS sensor measures the thermal effusivity of the sample. Two different solutions are derived, one is in the form of an infinite series with no thermal resistance, and the other is in integral form and includes thermal resistance between the MTPS sensor surface and the measured sample. The theoretical model is supplemented by experimental results.

Determination of Thermal Effusivity of Lunar Regolith Simulant Particle Using Thermal Microscopy

This study aimed to measure the thermal effusivity distribution on a lunar regolith simulant (FJS-1) using a thermal microscope and to calculate the average thermal effusivity and thermal conductivity using density and specific heat. Moreover, discussions were conducted based on the results of the microstructural analysis of the sample. The FJS-1 particles were embedded in an epoxy resin and polished to a mirror finish. The samples were analyzed using scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM–EDS). X-ray diffraction (XRD) was performed to identify the mineral phases in FJS-1. The results of SEM–EDS and XRD showed that a single sand particle was composed of several minerals, such as anorthite and olivine. Then, the thermal microscope was used to obtain the distribution of the thermal effusivity of a particle from the mirror-finished sample in a 1 × 1 mm2 area with intervals of 10 μm. The measured thermal effusivity correlates with the SEM image of the sample. Anorthite has a small thermal effusivity of 1.99 ± 0.31 kJ·s−0.5·m−2·K−1, while olivine has a large thermal effusivity of 2.73 ± 0.35 kJ·s−0.5·m−2·K−1. In both cases, the thermal effusivity was found to be of the same order of magnitude as the reported values. The average thermal effusivity and conductivity of a single particle were determined to be 2.4 ± 0.6 kJ·s−0.5·m−2·K−1 and 2.6 ± 1.3 W m−1·K−1, respectively, based on the proportion of existing phases.

Effect of moisture content on hygrothermal properties: Comparison between pith and hemp shiv composites and other construction materials

This paper deals with the experimental assessment of the influence of water content on thermal conductivity, thermal effusivity, heat capacity and vapor permeability of pith and hemp shiv composites. The results, which will be used to enrich modelling databases, show that all four properties increase their value with relative humidity, especially for pith composites, while remaining within adequate margins. They are compared to the results commonly measured for other porous construction materials, namely hemp concrete, wood, and rammed earth. This comparison allows to better understand the influence of the material morphology and nature on hygrothermal properties in presence of moisture. It has been noted that, in hygroscopic conditions, the specific heat cannot always be deduced from the specific heat of water and of the dry material due to the interactions between the adsorbed water and the material. On the other hand, the transport of liquid water has been observed to play a significant role in the hygroscopic transfers.

Analytical analysis of non-boiling heat transfer during droplet impact on heated wall and the effect of thermophysical properties

Droplet impact process exhibits excellent heat and mass transfer characteristics. Analytical solution with simple explicit form and certain accuracy can clearly reflect the correlation of physical parameters, however the analytical study on droplet impact heat transfer is quite insufficient and limited to flow field analysis. In this paper, the analytical solutions of heat flux and temperature variation of droplet impact on heated wall in non-boiling regime were obtained based on boundary layer asymptotic matching method. Parameter analysis of thermophysical properties was conducted to explore the decisive properties of droplet and wall materials. The results showed that the analytical solutions are appropriate to predict non-boiling heat transfer at low impact velocity and low initial wall temperature. Parameter analysis showed that thermal effusivity is the decisive property rather than the thermal diffusivity. Wall thermal effusivity could independently determine heat flux, but should determine temperature variation together with wall thermal conductivity. Droplet thermal effusivity and Prandtl number were the decisive properties, however the effect of Prandtl number in reality can be ignored. Wall materials and droplet liquids with higher thermal effusivity showed higher heat transfer capacity. The importance of thermal effusivity was demonstrated and high thermal effusivity was preferred in future thermal management.

Electrospray deposition of AlN-PVA composite coating as thermal interface material

In this study, aluminum nitride (AlN)-polyvinyl alcohol (PVA) composite coatings as a novel thermal interface material for electronic applications were fabricated by using the electrospray deposition (ESD) technique at room temperature. The influence of deposition time on the morphology and the thermal behavior of AlN-PVA composite coatings was investigated. The microstructural inspections indicate that homogeneous, highly dense, and paste-like AlN-PVA coatings were formed. Increasing the deposition time from 15 min to 30 min reduced the surface roughness of the AlN-PVA coating. Infrared thermography images revealed that the deposition of AlN-PVA composite coatings on aluminum alloy substrates dramatically improved its thermal effusivity. The thermal diffusivity of 15 min (38.767 mm2/s) and 30 min (37.485 mm2/s) coated samples were approximately 30% higher than the uncoated substrate (26.638 mm2/s).

Machine learning prediction of critical heat flux on downward facing surfaces

Application of external reactor vessel cooling (ERVC) in-vessel retention (IVR) was an important measure to ensure the integrity of the lower head of a reactor pressure vessel (RPV). As a typical boiling heat transfer process, the critical heat flux (CHF) plays a crucial role in the safety margin of a RPV's IVR-ERVC strategy. Although there have been a lot of correlations and experiments about the CHF of pool boiling on downward facing surfaces, their application range was relatively limited. To further expand the usability, machine learning was introduced in this research after collecting most accessible CHF data of pool boiling on downward facing surfaces. Considering the small amount of these experimental data, some pseudo data obtained by fitting the existed experimental data were added. Three machine learning methods, the ε-support vector machine (ε-SVM), back propagation neural network (BPNN) and random forest were used to predict CHF. Among the three methods, ε-SVM provided the best accuracy in the prediction. The effects of orientation, surface dimensions, heating surface material and pressure on the downward facing surface boiling crisis were predicted by ε-SVM, the results showed that in general, the CHF increased with the increase of orientation angle, and increased with the pressure from 1 bar to 10 bar. Under atmospheric pressure, the CHF decreased with increasing the width of the heating surface, but there was a width value that could eliminate the influence of width. In addition, the CHF seemed to increase with the increase of thermal effusivity. However, there are still some inexplicable phenomena that need to be revealed by further research. Overall, this method is expected to be widely used in predicting the CHF of pool boiling on downward facing surfaces.

Experimental Study of the Thermophysical Properties of the Red Earth Composite Stabilized with Cement Containing Waste Glass Powder

The aim of this study was to measure the thermophysical properties (thermal conductivity, volumetric thermal capacity, thermal diffusivity, and thermal effusivity) of red earth stabilized with cement and substituted with waste glass powder. Several samples (red earth) were stabilized with 6% and 12% cement and incorporated with different percentages of waste glass powder, which varied from 10% to 30%. The bulk density of the 12 samples was measured in the dry state and at room temperature. All samples were analyzed by a scanning electron microscope (SEM). The thermal conductivity and specific heat of the composite materials were measured experimentally with a thermal conductivity device (CT meter) in the dry state and at ambient temperature. The experimental results showed a decrease in the thermophysical parameters of stabilized red earth containing 12% cement and substituted by 30% glass powder. The following results were obtained: 53.97% for thermal conductivity, 45.42% for volumetric specific heat, 15.66% for thermal diffusivity, and 49.88% for thermal effusivity. The bulk density of the red earth also decreased by 13.66% in the dry state at ambient temperature. Stabilization with 6% and 12% cement played an important role in the compactness of the material and, consequently, improved its thermophysical performance. The composition of this new ternary material significantly affected the thermophysical properties of the red earth.

A simple model of thermal conduction in human skin: temperature perception and thermal effusivity

In this work we address the well-known fact that upon touching different materials, even if at the same environment temperature, one perceives warmer or cooler sensations depending on properties other than temperature itself. A simple, one-dimensional, non-stationary model for thermal behavior of the human skin layer is devised mainly with educational purposes and the associated Fourier heat equation is solved and discussed. In particular, we emphasize the fact that thermal conductivity alone is not adequate to properly characterize different thermal sensations and the so-called effusivity is instead needed to support a quantitative description. We also obtain some information, within the framework of this model, about the specific response of the skin thermal receptors. We furthermore suggest a possible, direct experimental estimate of thermal effusivity based on the use of infrared imaging in a demonstrative and educational perspective.

Integrating Thermal Sensors in a Microplate Format: Simultaneous Real-Time Quantification of Cell Number and Metabolic Activity.

Microplates have become a standard tool in the pharmaceutical industry and academia for a broad range of screening assays. One of the most commonly performed assays is the cell proliferation assay, which is often used for the purpose of drug discovery. Microplate readers play a crucial role in this field, as they enable high-throughput testing of large sample numbers. Common drawbacks of the most popular plate reader technologies are that they are end-point-based and most often require the use of detection reagents. As a solution, with this work, we aim to expand the possibilities of real-time and label-free monitoring of cell proliferation inside a microplate format by introducing a novel thermal-based sensing approach. For this purpose, we have developed thin-film sensors that can easily be integrated into the bottom of standard 96-well plates. First, the accuracy and precision of the sensors for measuring temperature and thermal effusivity are assessed via characterization experiments. These experiments highlight the fast response of the sensors to changes in temperature and thermal effusivity, as well as the excellent reproducibility between different sensors. Later, proof-of-principle measurements were performed on the proliferation of Saccharomyces cerevisiae. The proliferation measurements show that the thermal sensors were able to simultaneously detect relative changes in cell number as well as changes in metabolic activity. This dual functionality makes the presented sensor technology a promising candidate for monitoring microplate assays.

Study of the thermal and mechanical properties of local clay materials activated with quicklime, Sefrou (Morocco)

This study aims at testing experimentally the stabilization of unexploited clay from the Bensmim region (Middle Atlas, Morocco) using traditional quick lime from Sefrou located nearly to Bensmim for potential use in building applications. The main purpose of the present work is to evaluate the thermophysical and mechanical behavior of lime-stabilized clay blocks. Many different samples of clay specimens reinforced with lime at six different contents (0%, 10%, 20%, 30%, 40%, 50%, and 70%) were prepared and tested. The thermal characterization was made by the Asymmetrical Hot Plate method in study state and transient regime to identify, respectively, the thermal conductivity and the thermal effusivity. The Flash method was used to evaluate thermal diffusivity. The mechanical performance was measured in terms of compressive and flexural strengths. Besides, a Chemical identification of Bensmim clay was investigated via the X-Ray Fluorescence method. Particle size distribution of used clay was determined following a wet sieving analysis method. The results of clay identification showed that Bensmim clay meets the minimum requirements for manufacturing compressed earth and adobe bricks. The thermophysical tests results showed that the lime addition positively influences the lightness and the thermophysical properties of lime-stabilized clay specimens. For the findings of the mechanical tests, it is observed that the flexural and compressive strengths of lime-stabilized clay blocks continue to increase until a lime content of 30%. After this content, the mechanical performances decrease with lime addition, and no amelioration is detected. Therefore, a lime content of 30% represents the optimal lime content for stabilizing Bensmim clay. At this content, the stabilized clay blocks show optimal mechanical performance and improved thermal properties compared to the reference samples. Stabilized clay blocks at 30% lime content present a 14% improvement in bulk density, 13% in thermal conductivity, 12% in thermal effusivity, 20% in thermal diffusivity, 41% in compressive strength, and 10% in flexural strength, compared to unstabilized specimens.

Comparative Study of Thermophysical Parameters of Different Types of Upholstery Wood and the Influence of Density on Combustion Parameters at Microscale

The fire behaviour of five types of wood was studied on a microscale. Some thermophysical parameters such as thermal conductivity, effusivity and diffusivity were also evaluated. The microscale analysis is based on the analysis of several parameters such as the assessment of heat released rates (HRR), peak heat released rates (pHRR), total heat released (THR), enthalpy variation (Δh) and residue rates with the combustion microcalorimeter (PCFC). The PF2C device was used to measure the conductivity, effusivity and diffusivity of wood, while the PCFC for the microscale study of wood behaviour in combustive situations. In terms of thermal conductivity, wood conducts heat with difficulty, while the other types, namely frake, bete, tek and red wood, more easily conduct heat and therefore cannot be considered as thermal insulators. For results, in terms of measuring effusivity, white wood has the smallest value, which means that it exchanges less thermal energy with its environment. Red and tek woods exchange much more energy with the environment compared to other types of wood. We observe that tek wood has the highest diffusivity, which means that it is the wood that reacts as quickly as possible to the change in temperature. White and frake woods have the same value of thermal diffusivity as red and bete wood which have the lowest thermal diffusivities. The fire behaviour of these materials is a very little variable on the microscale and we can conclude that at this level there is no major difference for our different types of wood. Hence, there is no influence of density on the microscale. We can say that for the white and tek wood the complete combustion begins at 700°C. By this same method of analysis, we come to the conclusion that the frake, bete and red woods tend towards a complete combustion at 675°C. At 675°C the bete and frake woods have the best combustive yields, to a lesser extent than the red and tek woods. The bete wood has the highest activation energy and the white wood the lowest.

Thermosensitive textiles made from silver nanoparticle-filled brown cotton fibers.

Filling fibers with nanomaterials can create new functions or modify the existing properties. However, as nanocomposite formation for natural cellulosic fibers has been challenging, little information is available on how the embedded nanomaterials alter the properties of cellulosic fibers. Here we filled brown cotton fibers with silver nanoparticles (Ag NPs) to examine their thermosensitive properties. Using naturally present tannins in brown cotton fibers as a reducing agent, Ag NP-filled brown cotton fibers (nanoparticle diameter of about 28 nm, weight fraction of 12 500 mg kg−1) were produced through a one-step process without using any external agents. The in situ formation of Ag NPs was uniform across the nonwoven cotton fabric and was concentrated in the lumen of the fibers. The insertion of Ag NPs into the fibers shifted the thermal decomposition of cellulose to lower temperatures with increased activation energy and promoted heat release during combustion. Ag NPs lowered the thermal effusivity of the fabric, causing the fabric to feel warmer than the control brown cotton. Ag NP-filled brown cotton was more effectively heated to higher temperatures than control brown cotton under the same heating treatments.

The Role of Thermal Effusivity on the Early Surface Temperature Raise Caused by Surface Heat Flux During Heat Treatment

The Role of Thermal Effusivity on the Early Surface Temperature Raise Caused by Surface Heat Flux During Heat Treatment