Decrease In Thermal Diffusivity Research Articles

Thermal Stability and Heat Transfer of Polyurethanes for Joints Applications of Wooden Structures.

Wood characterized by desired mechanical properties and wood joining material is essential for creating wooden structures. The polymer adhesives are suitable for such applications due to the possibility of energy dissipation from stresses generated by wooden structures and the elimination of thermal bridging, which are common problems in metal joining materials. This research focuses on the thermophysical properties of the laboratory-prepared flexible and rigid polyurethanes to select an appropriate polymer adhesive. Our results showed that the highest thermal stability was in the case of the new PSTF-S adhesive, which reached 230 °C, but the lowest mass loss in the air environment was around 54% for the PS material. The mean thermal expansion coefficient for F&R PU adhesives was 124-164∙10-6 K-1. The thermal diffusivity of examined adhesives varied between 0.100 and 0.180 mm2s-1. The thermal conductivity, depending on the type of polyurethane, was in the 0.13-0.29 W∙m-1∙K-1 range. The relative decrease in thermal diffusivity after heating the adhesives to 150 °C was from 2% for materials with the lowest diffusivity to 23% for the PU with the highest value of heat transfer. It was found that such data can be used to simulate wooden construction joints in future research.

Numerical studies on the propagation of iron dust flames in confinement

Iron powder is a promising alternative fuel owing to its high energy density, non-volatile combustion, and recyclability using green hydrogen. For the safe storage, transportation, and use of iron powder as a reactive energy source, the physical mechanisms behind pressurising dust flames in explosions must be understood. Here, we study the propagation of one-dimensional iron dust flames in confinement using an Euler–Lagrange framework with reflective adiabatic boundary conditions. The gas phase is described by the compressible Navier–Stokes equations, and heterogeneously burning iron particles follow the parabolic rate law for solid-phase oxidation, and diffusion-limited combustion during liquid-phase combustion. We demonstrate that the pressurisation in a closed vessel leads to an increase in unburnt gas density, decelerating flame propagation via a decrease in thermal diffusivity. Flames through dust suspensions at concentrations above the thermodynamic limit are quenched, while for concentrations near the quenching limit, flames are quenched before re-ignition. The trajectories of particles and gas parcels demonstrate that particles are strongly entrained in the gas streams, causing fluctuations in local dust concentrations. The transient evolution of flame speeds in confinement is used to inform a pressure-rise model, which predicts the time to peak pressure in a 20-L vessel with good agreement to experimental measurements. Our insights provide mechanistic understandings about dust flame propagation under pressurising conditions, particularly relevant to the informed design of explosion protection measures.

Thermophysical properties of Cordyceps militaris change during the freezing process

This study focuses on determining the thermophysical parameters of Cordyceps militaris in the freezing stage of the freeze-drying process. When the temperature of Cordyceps militaris changed from an ambient temperature of 25°C to a freezing temperature of -45°C, the thermophysical parameters of Cordyceps militaris changed much. The study determined the initial freezing point temperature of Cordyceps militaris to be -0.6°C. At temperatures above the initial freezing point, the density (ρ) and specific heat (c) increase slightly as the temperature decreases. In contrast, thermal conductivity (k), thermal diffusivity (a), and enthalpy (h) decrease. However, at temperatures below the initial freezing point, when the temperature decreases, the density, specific heat, and enthalpy decrease while the coefficient of thermal conductivity and thermal diffusion increase. The results obtained from this study can be applied in calculating the heat and mass transfer during the freezing phase in the freeze-drying process of Cordyceps militaris.

Direct numerical simulation of a zero-pressure-gradient turbulent boundary layer with passive scalars up to Prandtl number Pr = 6

The objective of the present study is to provide a numerical database of thermal boundary layers and to contribute to the understanding of the dynamics of passive scalars at different Prandtl numbers. In this regard, a direct numerical simulation (DNS) of an incompressible zero-pressure-gradient turbulent boundary layer is performed with the Reynolds number based on momentum thickness ${\textit {Re}}_{\theta }$ ranging up to $1080$ . Four passive scalars, characterized by the Prandtl numbers ${\textit {Pr}} = 1,2,4,6$ are simulated using the pseudo-spectral code SIMSON (Chevalier et al., SIMSON : a pseudo-spectral solver for incompressible boundary layer flows. Tech. Rep. TRITA-MEK 2007:07. KTH Mechanics, Stockholm, Sweden, 2007). To the best of our knowledge, the present DNS provides the thermal boundary layer with the highest Prandtl number available in the literature. It corresponds to that of water at ${\sim }24\,^{\circ }{\rm C}$ , when the fluid temperature is considered as a passive scalar. Turbulence statistics for the flow and thermal fields are computed and compared with available numerical simulations at similar Reynolds numbers. The mean flow and scalar profiles, root-mean-squared velocity and scalar fluctuations, turbulent heat flux, turbulent Prandtl number and higher-order statistics agree well with the numerical data reported in the literature. Furthermore, the pre-multiplied two-dimensional spectra of the velocity and of the passive scalars are computed, providing a quantitative description of the energy distribution at the different length scales for various wall-normal locations. The energy distribution of the heat-flux fields at the wall is concentrated on longer temporal structures with increasing Prandtl number. This is due to the thinner thermal boundary layer as thermal diffusivity decreases and, thereby, the longer temporal structures exhibit a different footprint at the wall.

Anomalous thermal structures of subduction zones revealed by thermal properties of clinochlore at high temperature and pressure

Clinochlore is a major hydrous mineral in subduction zones, and its thermophysical properties at high temperature and pressure are critical to the thermal structures of subduction zones. Here, we used the pulse heating method to measure thermal diffusivity and thermal conductivity of clinochlore at 0.5−4.0 GPa and 298−1373 K. Our results indicate that upon heating, thermal diffusivity and thermal conductivity decrease from ∼9.6 × 10−7 m2 s−1 to 4.3 × 10−7 m2 s−1 and from ∼3.5 W m−1 K−1 to 1.9 W m−1 K−1, respectively, before dehydration, but this trend is reversed after dehydration. In general, the pressure derivatives for the thermal transport properties also decrease with temperature before dehydration. Lattice heat transfer is the dominant mechanism before dehydration, but fluid is involved after dehydration. Using our experimental data, we simulated the temperature distribution of subducting slabs containing clinochlore at volume fractions of 0%, 10%, 20%, 50%, and 100%. Our simulations showed that the heat insulation effect caused by the presence of clinochlore could result in an increase in temperature by 30−60 K for the upper part of the subducting slab.

Valorization of Moroccan Hemp Waste: Study of the Possibility of its Use in Thermal and Acoustical Insulation of Buildings

This paper aims to study the possibility of valorizing hemp residues in order to develop new local bio-composites from Moroccan hemp shiv and epoxy. The goal is to use them as thermal and acoustical insulation panels since these hemp residues exist in large quantities in landfills and present a national concern due to a lack of waste management technologies. For this purpose, several samples were prepared for different densities and two sizes of hemp shiv; crushed shiv (CS) and fibred shiv (FS). The results revealed that the increase of density resulted in an increase in thermal conductivity and a decrease in thermal diffusivity. However, the thermal conductivity of composites is still lower than 0.1 W/mK for the most studied samples. The samples show values of acoustic absorption coefficients varying between 0.2 and 0.59 for crushed shiv composites (CSC) at the frequency range (578-1396 Hz) and between 0.2 and 0.73 at the frequency range (662-1396 Hz) for Fibred shiv composites (FSC). It has been observed that the density has a significant effect on the sound absorption coefficient. Increasing the density shifts the acoustic absorption curve towards the low frequencies. Also, decreasing the particle size enhances the sound absorption in the medium frequency range (300-600 Hz). The obtained results are satisfactory for manufacturing these new composites that can be used as thermal and acoustic insulators. Moreover, it offered the best solution for hemp waste management.

Monitoring Congo red discoloration using thermal properties in photocatalytic processes: A new approach

Waste water contaminated with azo-derived dyes is the main cause of contamination of aquatic systems. The photocatalytic process is an alternative remediation technology to counter the negative effects of dyes in aqueous solutions. In this work, the photocatalytic potential of β-PbO nanoparticles to degrade Congo red was investigated. For the first time, monitoring photocatalysis by obtaining thermal diffusivity using thermal lens spectroscopy and thermal wave resonator cavity techniques is reported. The reaction was carried out under alkaline conditions with UVC light irradiation and in the presence of lead oxide nanoparticles. It was possible to recorded a decrease of the characteristic peak of Congo red around 497 nm in the UV-Vis spectra. Likewise, a decrease in thermal diffusivity from 15.53±0.026 × 10−4 cm2•s−1 to 13.96±0.045 × 10−4 cm2•s−1 was observed as the irradiation time increased. The azo-derived dye degradation reaction follows pseudo-first-order kinetics. The model used allowed calculating an apparent rate constant of 0.0188 min−1 and half-life of the Congo red of 36.8 min−1 in the presence of orthorhombic lead oxide. The experimental conditions allowed the degradation of the dye in a percentage up to 90%.

Experimental and theoretical thermal investigation of bio-composite panels based on sawdust particles

This work focuses on the valorization of sawdust waste in order to develop a new bio-sourced insulation material. To assess the dynamic thermal behavior of a multi-layer external wall insulated with these panels, their thermal characteristics, the time lag, and the decrement factor, were investigated. The results obtained show that when the density increases, the thermal conductivity, the thermal diffusivity, and the volumetric heat capacity increase while the thermal diffusivity decreases. For a thickness of 20 cm of the composite material, the time lag increases, and the decrement factor decreases with increasing density. Moreover, the results obtained for an external wall show that the decrement factor decreases and the time lag increases with the thickness of the thermal insulation. It was also found that the position of the insulation layer has a considerable effect on thermal inertia properties. The wall with insulation on the outside is the most desirable. In addition, the internal global heat transfer coefficient affects the external wall's time lag and decrement factor, while the external global heat transfer coefficient does not affect its inertia properties.

Performance Properties of Cement-Glass Composite Bricks (CGCB) with Additively Manufactured (AM) Polymeric Scaffolding.

This study provides an alternative to traditional masonry materials: a cement-glass composite brick (CGCB), with a printed polyethylene terephthalate glycol (PET-G) internal scaffolding (gyroidal structure). This newly designed building material consists of 86% waste (78% glass waste, and 8% recycled PET-G). It can respond to the construction market's needs and provide a cheaper alternative to traditional materials. Performed tests showed an improvement in thermal properties after the use of an internal grate in the brick matrix, i.e., an increase in thermal conductivity (5%), and a decrease in thermal diffusivity (8%) and specific heat (10%). The obtained anisotropy of the CGCB's mechanical properties was much lower than the non-scaffolded parts, indicating a very positive effect of using this type of scaffolding in CGCB bricks.

The thermal conductivity of unlithified granular volcanic materials: The influence of hydrothermal alteration and degree of water saturation

The thermal properties of volcanic materials are required for modelling and for understanding volcanic surface heat fluxes and timescales for cooling magma. However, compared to volcanic rocks, there are relatively few thermal property data for unlithified granular volcanic materials. Here, we measured the thermal properties of a suite of hydrothermally altered powders from La Soufrière de Guadeloupe (Eastern Caribbean) as a function of water saturation degree. Our data show, under dry conditions, that thermal conductivity and thermal diffusivity decrease, and that specific heat capacity does not change systematically, as a function of the degree of alteration of the unlithified granular material. For example, thermal conductivity decreases from ∼0.3 to ∼0.2 W·m−1·K−1 as the quantity of alteration minerals in the samples increases from ∼10 to ∼70 wt%. We interpret the decrease in thermal conductivity with increasing alteration as the result of the lower thermal conductivity of the alteration mineral assemblage relative to the unaltered assemblage. Our data also show that thermal conductivity increases from ∼0.2–0.3 to ∼0.8–1.1 W·m−1·K−1 as saturation degree increases from dry to at, or close to, complete saturation, due to the higher thermal conductivity of water compared to air. We show that an empirical model for variably saturated granular media is in general agreement with our data and provides a framework to predict the thermal conductivity of unlithified granular volcanic materials as a function of saturation degree. The data and modelling provided herein can help improve heat flux calculations designed to inform on volcanic and geothermal processes.

Thermal properties of nano-silica added concrete

As energy costs are rising, the utilization of thermal insulating materials during construction has assumed paramount significance. The response of different building components to changes in external temperature depends on the material’s thermal properties. The investigation connected to concrete material thermal properties is an essential factor when concrete is used for constructing green buildings. The effect of adding nano-silica on the concrete’s thermal properties is explored, and the findings are shown in this paper. The experimental results reveal that compressive strength increases and density reduce for nano-silica added concrete. The investigation indicates that thermal conductivity and thermal diffusivity decrease for concrete, whereas specific heat increases with a rise in nano-silica content. Also, a mathematical model was developed for the calculation of thermal conductivity.

Unsteady Squeezing Flow and Heat Transfer Analysis of Magnetohydrodynamic Third-grade Nanofluid between Two Disks Embedded in a Porous Medium subjected to Thermal Radiation using Homotopy Perturbation Method

The non-linear behaviours of non-Newtonian fluids under various flow conditions continue to arouse research interests in recent times. In this work, nonlinear analysis of unsteady squeezing flow and heat transfer of a third-grade nanofluid between two parallel disks embedded in a porous medium under the influences of thermal radiation and temperature jump boundary conditions is studied using homotopy perturbation method. The parametric studies from the series solutions show that for a suction parameter greater than zero, the lower disc's radial velocity increases while that of the upper disc decreases as a result of a corresponding increase in the viscosity of the fluid from the lower squeezing disc to the upper disc. An increasing magnetic field parameter and the radial velocity of the lower disc decrease while that of the upper disc increases. There is a recorded decrease in the fluid temperature profile as the Prandtl number increases due to a decrease in the third-grade fluid's thermal diffusivity. The results of this work can be used to advance the analysis and study of third-grade nanofluid flow behavior and heat transfer processes.

Magnetohydrodynamic mixed convection flow of liquid metals in a vertical channel: A stability analysis

The impact of applied magnetic field on liquid metal flows has good prospects for the industry. For a better understanding of flow dynamics, the stability analysis of liquid metal flows through a confined transverse magnetic field is of fundamental importance in designing advanced magnetohydrodynamic (MHD) devices for the next generation. In this paper, we report the linear stability analysis of mixed convection flow of electrically conducting liquid metals with low Prandtl number (Pr) in a vertical channel under a transverse magnetic field. The flow is described by quasi-static MHD approximation. The instability properties are examined numerically through a spectral collocation method. The influence of magnetic field on instability mechanism is examined for a good range of Pr. The results demonstrate that the growth of the infinitesimal disturbance decreases with increasing strength of the applied magnetic field due to the effective Lorentz force exerted on the flow, i.e., stability of the flow increases. In contrast, the stability of the basic flow decreases on increasing the value of Pr, except for a small range of Pr, where it increases. The kinetic energy balance shows that for very small values of Pr, the shear instability dominates the flow, whereas for relatively larger values of Pr, thermal-buoyant instability plays a significant role in the instability mechanism. The dissipation of energy in the kinetic energy balance occurs prominently due to the presence of the applied magnetic field. The secondary flow pattern shows that the applied magnetic field suppresses the disturbance in the vicinity of channel walls. In contrast, a decrease in thermal diffusivity provokes instability by inviting a strong disturbance cell in the middle portion of the channel. The energy dissipation in the kinetic energy balance occurs mainly due to the presence of the magnetic field.

Microstructure evaluation and thermal properties of Ag–Sb alloys

The microstructure and thermal properties of four Ag–Sb alloys prepared using 12.1, 40.0, 43.9, and 81.1 at.% Sb have been experimentally studied in this paper. The microconstituents and compositions of the prepared samples were studied using energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). The phase transition temperatures of the investigated alloys were determined using differential scanning calorimetry (DSC) and compared with the results obtained using thermodynamic calculations. The temperature of the eutectic reaction was determined to be 484.1 °C. Thermal diffusivity measurements in the temperature range from 25 to 400 °C were performed using the xenon-flash method, followed by determination of the thermal conductivity. The temperature and composition dependence of the density, thermal diffusivity, specific heat capacity, and thermal conductivity for the solid Ag–Sb alloys were determined. The obtained results revealed that the thermal diffusivity and thermal conductivity decrease upon increasing the Ag content in composition range studied due to the formation of low heat-conductive intermetallic phases.

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.

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.

Thermo-physical and mechanical characterization of cement-based mortar incorporating spent tea

The use of biomass waste and plant fibers as aggregates for traditional binders has proven to be a green promising solution for reducing the overall energy consumption and carbon dioxide emissions related to the construction sector. Most research on plant-based additives reported an improvement in thermal insulation at the expense of mechanical strength. Thus, balancing the thermo-mechanical properties is a key to produce innovative insulating materials. In this work, the potential use of Spent Tea (ST) in the manufacturing of bio-sourced cement-based mortars was studied. Different weight ratios ranging of 0–10% were used to evaluate the influence of ST content on the cement performance. Thermal, mechanical, and physical properties of the produced bio-composites were characterized. The results showed that increasing ST content leads to a significant decrease in thermal conductivity (67%), thermal diffusivity (57%), density (24%), and compressive strength (97%). At last, it was found that adding up to 2.5 wt% ST meets the structural requirements of lightweight concretes (>3.5 MPa) and could be used for wall structures. However, the composite incorporating 7.5 wt% of ST should exclusively serve for insulation purposes.

Equivalent thermal diffusivity of natural and recycled aggregate concrete at temperature up to 350 ⁰C

The paper presents the evolution of equivalent thermal diffusivity with temperature of concretes with natural coarse aggregates and recycled concrete aggregates. Thermal diffusivity was evaluated based on the temperature measurements made on cylindrical samples using so-called "inverse technique". Thermal diffusivity parameter was estimated for concretes manufactured with different replacement ratios of natural by recycled coarse aggregate (0, 50 and 100 %) produced by crushing the worn-out concrete pavement that was dismounted after exploitation period. The diffusivity of concrete was tested up to 350 ◦C. The progressive decrease of thermal diffusivity was observed, and the lowest D values were obtained for higher recycled concrete aggregates content. The results were presented against the background of other authors results and referred to the D values proposed in EUROCODE 2.

Experimental study on the thermal imbalance and soil temperature recovery performance of horizontal stainless-steel ground heat exchanger

Soil thermal imbalance has become a common problem for ground source heat pump (GSHP) system, leading to the fluctuation of ground temperature and decline of heat transfer efficiency. However, compared with vertical ground heat exchanger (GHE), there is a lack of research and evaluation on the soil thermal imbalance for horizontal GHE. Therefore, in this paper, to investigate the extent of soil thermal imbalance and its effects on performance of GSHP system, a sandbox experiment embedded with horizontal stainless-steel GHE was implemented. Various operation conditions were designed to explore effects of intermittent time, water content (rainfall) and buried depth of pipes on soil temperature distribution and restoration performance. The experimental data was validated by a numerical simulation. The results indicated that 5-day intermittent operation of GSHP leads to an increase of 6.41 ℃ in soil temperature. After 2-day natural recovery, soil temperature recovery rate can only reach 0.72. Increasing soil moisture content and intermittent control are effective solutions to mitigate thermal accumulation and improve thermal performance. The heat transfer performance of GHE with an intermittent ratio of 6:18 which GSHP system runs for 6 h and stops for 18 h during a day is 33% higher than that with the intermittent ratio of 14:10 and 10:14. The higher the water content, the lower the soil temperature fluctuation and the stronger the soil temperature recovery capacity. This can be attributed to the increase of soil thermal conductivity and capacity, while the decrease of thermal diffusivity.

The thermal properties of hydrothermally altered andesites from La Soufrière de Guadeloupe (Eastern Caribbean)

The heat flux of an active volcano provides crucial information on volcanic unrest. The hydrothermal activity often responsible for volcanic unrest can be accompanied by an increase in the extent and intensity of hydrothermal alteration, which could influence the thermal properties of the volcanic edifice. Therefore, an understanding of the influence of alteration on the thermal properties of rocks is required to better interpret volcano heat flux data. We provide laboratory measurements of thermal conductivity, thermal diffusivity, and specific heat capacity for variably altered (intermediate to advanced argillic alteration) andesites from La Soufrière de Guadeloupe (Eastern Caribbean). We complement these data with previously published data for altered basaltic-andesites from Merapi (Indonesia) and new data for altered rhyodacites from Chaos Crags (USA). Our data show that thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, whereas the specific heat capacity does not change systematically. Thermal conductivity decreases as a function of alteration (the percentage of secondary minerals) for the rocks from La Soufrière and Merapi (from ~1.6 to ~0.6 W·m−1·K−1 as alteration increases from ~1.5 to >75 wt%), but increases for the rocks from Chaos Crags (from ~1.1 to ~1.5 W·m−1·K−1 as alteration increases from ~6 to ~15 wt%). Although the thermal diffusivity of the rocks from Chaos Crags increases from ~0.65 to ~0.75–0.95 mm2·s−1 as alteration increases from ~6 to ~15 wt%, the thermal diffusivity of the rocks from La Soufrière and Merapi does not appear to be greatly influenced by alteration. The specific heat capacity is not significantly affected by alteration, although there is a slight trend of increasing specific heat capacity with alteration for the rocks from La Soufrière. We conclude that the decrease in thermal conductivity as a function of alteration in the rocks from La Soufrière and Merapi is the result of the low conductivity of the secondary mineral assemblage, and that a combination of the high thermal conductivity of cristobalite and the reduction in porosity as a result of the void-filling mineral precipitation can explain the increase in thermal conductivity in the rocks from Chaos Crags. Calculations show that an increase in alteration of a dome or edifice can result in decreases and increases in heat flow density, depending on the type of alteration. Therefore, alteration-induced changes in the thermal properties of dome or edifice rocks should be considered when interpreting volcano heat flux data. We conclude that it is important not only to monitor the extent and evolution of alteration at active volcanoes, but also the spatial distribution of alteration type.