Properties Thermal Diffusivity Research Articles

Influence of the Thermophysical Model on the CFD Analysis of Oil-Cooled Transformer Windings

A disc-type winding of an oil-immersed power transformer is modeled with Computational Fluid Dynamics. Different approaches are implemented to evaluate the feasibility of the Boussinesq approximation: (i) constant fluid properties, (ii) variable viscosity and thermal diffusivity and (iii) temperature-dependent fluid properties. Temperature and flow distributions are reconstructed and put into relation with physical phenomena and model assumptions. Their comparison suggests that numerical results are fairly sensitive to the thermophysical model as long as the buoyancy force is a relevant component of the flow. Nonetheless, all the cases converge to very close predictions of the hot-spot value and location, with possibly positive implications for the use of reference parameters when deriving flow and heat transfer correlations for this topic.

Development of superior stable two-dimensional montmorillonite nanosheet based working nanofluids for direct solar energy harvesting and utilization

Nanofluids showing enhanced thermal diffusivity and thermal conductivity have potential applications in the fields of thermal science, engineering, and sustainable energy. It is still a challenge, however, to facilely prepare nanofluids with good dispersion stability using an environmentally friendly method. In the present study, natural layered montmorillonite was exfoliated into two-dimensional montmorillonite nanosheets (MtNS) using a physical ultrasonic method in aqueous solutions. The obtained MtNS suspensions can be directly used as nanofluids without any further operation or any addition of dispersants. Owing to electrostatic repulsion and hydration repulsion among the MtNS nanoparticles in aqueous solution, MtNS/water nanofluids show excellent dispersion stability even over a long time. In addition, owing to the good light absorption ability of MtNS and the thermal diffusion properties of nanofluids, MtNS/water nanofluids exhibit outstanding photothermal conversion efficiency and dramatic solar thermal energy collection performance. Experimental results also proved that suspensions with a high MtNS concentration and small MtNS particle size increase the efficiency of solar energy harvesting. In addition, the stable dispersion of MtNS in fluids leads to extremely good suspension sustainability and recycling performance of the nanofluids. The facile and green preparation method, low cost, excellent dispersion stability, and good solar energy collection performance of MtNS/water nanofluids enable them to be promising solar energy collection mediums. The large-scale application of MtNS/water nanofluids can take full advantage of green solar energy, which can be used in green buildings and contribute to building a sustainable society.

Ethylene propylene diene monomer rubber‐based heat shielding materials for solid rocket motor: Impact of Kevlar fiber reinforcement on the thermal and mechanical properties

This work scrutinizes the utilization of ethylene propylene diene monomer rubber matrix (EPDM) with an embodiment of aramid fiber for the heat shielding applications in solid rocket motor (SRM). Aramid fibers are aromatic poly‐paraphenylene terephthalamide, here deployed are Kevlar fibers (KF). However, the literature that encompasses the thermal and mechanical behavior with the fiber loading is reported nowhere else. The effect of fiber addition on the surface morphology and density was thoroughly studied, and it revealed that the EHSMs were of lower density to act as an efficient payload for the SRM. In this regard, the thermal conductivity, heat capacity, thermal diffusivity, fire behavior, and mechanical properties of the EPDM/KF‐based EHSMs were explored. The results revealed that the EHSMs are thermally insulating and thermally stable material with balanced mechanical properties that can engender the thermal and mechanical strains of the rocket motor. Furthermore, other analytical techniques such as scanning electron microscopy and energy dispersive X‐ray spectroscopy have been exploited to monitor the performance of the char residues of the EHSM to delineate its performance in the fire atmosphere.

Integration of Gold Nanoparticles to Modulate the Ignitability of Nanothermite Films

Thermite multilayered films composed of alternating thin layers of metal/oxidizers have various uses in microelectromechanical systems (MEMS), microelectronics and materials bonding applications. Recently, applied research especially on the micro-initiator applications has engendered an urgent need to improve ignitability without changing the layering, reactant spacing that both affect the combustion characteristics. This work describes an innovative nano-engineering solution to reduce the energy barriers for mass transport making it possible to substantially lower ignition energy of CuO/Al reactive multilayers without manipulating the fuel and oxide layers thickness. To that end, gold nanoparticles exhibiting high thermal diffusivity properties are in-situ grown uniformly inside the first CuO layer to produce localized hot-spots

Physical and thermal behaviour of areca and coconut shell powder reinforced epoxy composites

An understanding of the temperature dependent properties such as thermal conductivity, specific heat, and thermal diffusivity of natural fiber reinforced polymer composites are an important to determining their heat conduction/insulation capability. In the present study, epoxy based hybrid composites were fabricated by reinforcing the areca and coconut shell powder (CSP) to improve the thermal insulation. The two reinforcements were added together with different weight ratios as 1:1, 1:3, and 3:1 and the total fiber loading of 10–40 wt%. The effect of weight ratio and fiber loading on the density, water absorption, thermal conductivity, specific heat, and thermal diffusivity properties of areca-CSP reinforced epoxy composites was examined. The results revealed that the density, thermal conductivity, and specific heat of hybrid composites were decreased with increase in fiber loading. On other hand, it was observed that the water absorption of hybrid composite was increases with the fiber loading.

Thermal properties of zirconia-alumina coatings obtained by thermal spray flame

Thermal spraying makes possible to manufacture coatings of a large number of materials, including those with a high melting point, such as ceramics. There are different ways of manufacturing these coatings, two of them are air-plasma spraying and flame spray. In this study the zirconia and alumina coatings were manufactured by flame spray, their morphology was studied and compared with their thermal conductivity and diffusivity properties. The morphology was studied from scanning electron microscope micrographs and the thermal properties measured by the laser flash method. It was discovered that the type of flame used to obtain the coatings influences the percentage porosity and thickness and that this in turn, affects its thermal properties.

Effects of Pore on Thermal Diffusivity and Thermal Radiation Properties of C/SiC Composites at High Temperatures

The effects of pore in C/SiC composites on thermal diffusivity and thermal radiation properties were investigated systematically. Pores were introduced into C/SiC by oxidizing carbon phase at 700 °C and damaged the thermal properties of C/SiC composites. Because of little changes in the pore shape and the pore orientation in C/SiC, thermal diffusivity of samples increased linearly with porosity. The pores within C/SiC absorbed and reflected the radiated heat, decreasing spectral emissivity. However, the temperature dependence of spectral emissivity didn’t change by the pore. With measurement temperature increasing, the pores weakened the thermal radiation property of samples gradually. A linear relation was suggested to quantify the negative effect of pores on the total emissivity.

Metallurgical characterization and corrosion behavior of laser-cladded 310 s + WC alloy coating

Metal parts in alkali recovery boilers such as water-cooled wall tubes and primary air nozzles are easily damaged by high temperature alkali corrosion. In this investigation, 310 s steel base alloy powders mixed with WC was applied on the surface of 1020 steel by laser cladding technology to improve surface quality and extend service life. The content of WC in the alloy powder has a pronounced influence on the microstructure and properties of the obtained cladding layer and it was considered as a variable. The microstructures and elements of the cladding layer was investigated by OM, SEM and XRD. In addition, hardness, friction properties, thermal diffusion properties and alkali corrosion resistance of the cladding layer were further investigated. Finally, research results showed that the microstructure of the laser cladding layer has an organization gradient which is subdivided into planar crystal, cellular crystal, columnar crystal and dendrites orderly from the weld junction. The 310 s + 10%WC cladding layer has the highest hardness of 880 HV, and also has the best friction characteristics, wear resistance, thermal diffusion properties and alkali corrosion resistance.

Porous boron nitride/polyimide composite films with high thermal diffusivity and low dielectric properties via high internal phase Pickering emulsion method

This work offers a simple fabrication of porous boron-nitride-(BN)/polyimide-(PI) composite films with high thermal diffusivities and low dielectric constants by combining high-internal-phase Pickering emulsification (HIPPE) and subsequent hot-pressing. BN nanoparticles in composite foams were well dispersed and three-dimensionally connected following the surface of the PI skeleton used as the polymer matrix. The BN contents in the BN/PI composite foams were adjusted in the range of 20–80wt%. The porosities of the composite films were controlled according to the hot-pressing conditions such as the temperature. The porous BN/PI composite films exhibited high thermal diffusivities of 0.059–1.033mm2/s and low dielectric constants of 2.08–3.48 at 1GHz for BN contents of 20–80wt%. In particular, the BN/PI composite films had extremely low dielectric loss values, close to zero (<0.002) at high frequencies regardless of the BN content and pressing conditions.

Imidazolium‐Based Stabilization of Aqueous Multiwall Carbon Nanotube Dispersions

AbstractImidazolium bromide, an ionic liquid surfactant acrylate, as well as its homopolymer and various copolymers are demonstrated to be superior dispersing aids for preparing aqueous multiwall carbon nanotube (MWCNT) dispersions. We demonstrate apparent complete exfoliation in water with extinction coefficients at 500 nm of about 60 cm2 mg−1 of MWCNT dispersed, a new lower bound and the highest reported extinction to date. The efficacy or efficiency of dispersion (activated by ultrasonication) is examined in terms of a quotient of extinction and weight ratio of the active monomer and MWCNT. A rank ordering of the results obtained for seven stabilizers based on the same imidazolium bromide monomer provides insights into roles of π‐overlap adsorption onto MWCNT surfaces and how hydrogel properties of some of these polymers provide stability in water at higher concentrations than previously considered feasible for surfactant‐stabilized and polymer‐stabilized MWCNT dispersions. Simple nanocomposite film formation is demonstrated by casting and thermal diffusivity and electrical conductivity properties are examined.

COMPARATIVE STUDY OF SPAN 40 AND SPAN 60 BASED SOY-GELS FOR TOPICAL DRUG DELIVERY

Objective: Hydrogels or emulgels are recommended for topical application to elicit a local effect. However, they suffer from stability problems. The present study deals with the formulation and comparison of thermally stable soybean oil-based novel topical organogels (soy-gels) using two different gelators (Span 40 and Span 60) for controlled drug delivery.&#x0D; Methods: Soy-gels (8 batches) were developed with Span 40 and Span 60 by solid fiber mechanism and characterized for viscosity, gelation kinetics behavior, gel-sol transition parameters, drug content, in vitro drug release pattern, and changes occurring during accelerated thermal stability studies.&#x0D; Results: Fourier transformed infrared spectroscopic confirmed the compatibility among the organogel components and paracetamol. The formulations exhibited skin and hemocompatibility. The viscosity of Span 60 based soy-gels was found to be approximately 10 times higher than those of Span 40 based formulations. In comparison to Span 40, Span 60 induced faster gelation (3–6 min) of soybean oil at lower concentration (16% w/v) forming less flexible but thermally more stable soy-gels demonstrating higher Tg values. Higher flexibility and lower viscosity accounted for improved drug diffusion (both Fickian and non-Fickian) from Span 40 gels of varying concentrations in pH 5.8. However, zero-order drug release was observed in organogel with 18% w/v Span 40 only and all Span 60-based formulations except the one with 22% w/v Span 60. Non-Fickian drug diffusion occurred from Span 60 based soy-gels. A gradual increase in gelation time was observed until five cycles of freeze-thaw.&#x0D; Conclusion: Therefore, the choice of organogelator governs the rheological, thermal, and drug diffusion properties of soy-gels intended for topical application.

Heat and Mass Transfer in an Unsteady Second Grade Nanofluid with Viscous Heating Dissipation

This study presents the boundary layer flow of a second grade fluid in a rotating reference frame. The model equations are solved numerically using the bivariate spectral quasi-linearization method to obtain the fluid properties. We investigate the effects of measurable quantities such as the rate of rotation, thermal diffusivity, energy dissipation, viscoelastic parameter and other material properties of the fluid on the flow. The results show that slow rotation leads to a quick decline in the thicknesses of the momentum, thermal and concentration boundary layers. There is also a region of reverse flow for fluids with high Prandtl number.

Phase-sensitive lock-in detection for high-contrast mid-infrared photothermal imaging with sub-diffraction limited resolution.

Imaging of the phase output of a lock-in amplifier in mid-infrared photothermal vibrational microscopy is demonstrated for the first time in combination with nonlinear demodulation. In general, thermal blurring and heat transport phenomena contribute to the resolution and sensitivity of mid-infrared photothermal imaging. For heterogeneous samples with multiple absorbing features, if imaged in a spectral regime of comparable absorption with their embedding medium, it is demonstrated that differentiation with high contrast is achieved in complementary imaging of the phase signal obtained from a lock-in amplifier compared to standard imaging of the photothermal amplitude signal. Specifically, by investigating the relative contribution of the out-of-phase lock-in signal, information based on changes in the rate of heat transport can be extracted, and inhomogeneities in the thermal diffusion properties across the sample plane can be mapped with high sensitivity and sub-diffraction limited resolution. Under these imaging conditions, wavenumber regimes can be identified in which the thermal diffusion contributions are minimized and an enhancement of the spatial resolution beyond the diffraction limited spot size of the probe beam in the corresponding phase images is achieved. By combining relative diffusive phase imaging with nonlinear demodulation at the second harmonic, it is demonstrated that 1-μm-size melamine beads embedded in a thin layer of 4-octyl-4'-cyanobiphenyl (8CB) liquid crystal can be detected with a 1.3-μm spatial full-width at half-maximum (FWHM) resolution. Thus, imaging with a resolving power that exceeds the probe diffraction limited spot size by a factor of 2.5 is presented, which paves the route towards super-resolution, label-free imaging in the mid-infrared.

Real-space and pulse-by-pulse analysis of domain wall creep induced by spin-Hall torque

This study aims to investigate the creep motion of a magnetic domain wall (DW) induced by spin-Hall torque (SHT) in a Ta/Co–Ni/Pt hetero-structured thin wire using photoemission electron microscopy. The displacement of the DW was recorded every time after the duration of an electric current pulse. Results revealed that the existence of pinning centers and the inhomogeneity of the thermal diffusion property of the wire may result in an unstable DW motion. On the contrary, this data also suggested that the SHT induced from both the Ta and Pt layers can significantly improve the DW velocity, which was found to be three orders of magnitude larger than that previously reported for the MgO/Co–Ni/Pt system (Taniguchi et al., Appl. Phys. Exp. 7, 053005 (2014)).

Thermal diffusion and flow property of CO2/CH4 in organic nanopores with fractal rough surface

The kerogen is rich in complex pore networks with a random rough surface, which is a factor controlling the thermal diffusion and flow property of gases. In this work, we construct organic-rich nanopore with fractal surfaces by inserting and deleting carbon atoms. The adsorption ability, thermal diffusion property, and flow velocity of CO2 /CH4 in the nanopore are analyzed using with molecular simulations. The results showed that the adsorption capacity of CO2 is nearly twice that of CH4, which is decided by adsorption enthalpy, whereas the maximum thermal diffusion ability of CO2 is only 23.7% that of CH4. With external pressure gradients imposed on the system, the flow speed of CO2 was lower than that of CH4 for nanopores with different roughness. These findings provide a theoretical basis for the feasibility of CO2 exploitation of shale gas.

Systematic microstructural development with thermal diffusivity behaviour from nanometric to micronic grains of strontium titanate

Strontium titanate is a promising candidate for applications in thermoelectric, thermal management applications, and modern electronic devices because of its desirable thermal stability, chemical stability, and semiconducting behaviour. However, the absence of its important systematic development, having grain size from several nanometric up to micronic size with evolving thermal diffusivity behaviour, triggers the need for filling up the vacuum. Two different heat treatments have been carried out onto the samples which were with presintering and without presintering. Nanometer-sized compacted powder samples were sintered from 500 to 1400 °C using 100 °C increments. The parallel characterizations of structural, microstructural and thermal diffusivity properties were systematically carried out. Interestingly, three significant value-differentiated groups: weak, moderate, and strong thermal diffusivity were observed, resulting from the influence of different phonon-scattering mechanisms through a systematic development of microstructural properties for both heat treatments.

Local thermal non-equilibrium in porous media with heat conduction

In classical theory of porous media, a local thermal equilibrium between fluid phase and solid phase is assumed. According to this theory, the fluid and solid temperatures reach an equilibrium temperature value rapidly. However, in porous media, the rate of heat transfer between the fluid and solid may not be fast enough to achieve a local thermal equilibrium (LTE) due to their thermal diffusion properties. Therefore, this paper examines the importance of local thermal non-equilibrium (LTNE) in porous media with conductive heat transfer. The LTNE is constituted by energy equations and defines distinctive temperature profiles for both the solid and fluid phases. The Laplace transform and Stehfest algorithm methods have been employed in formulating an exact solution. Using the weighted average method, a temperature for the porous media is defined. Subsequently, the model is applied to a cylindrical hole subjected to a uniform temperature in an infinite porous medium (rock formation). The exact solution and transient temperature profile are the advantages of this model compared to existing LTNE models.

Semi-Analytical Source Method for Reaction–Diffusion Problems

Estimation of thermal properties, diffusion properties, or chemical–reaction rates from transient data requires that a model is available that is physically meaningful and suitably precise. The model must also produce numerical values rapidly enough to accommodate iterative regression, inverse methods, or other estimation procedures during which the model is evaluated again and again. Applications that motivate the present work include process control of microreactors, measurement of diffusion properties in microfuel cells, and measurement of reaction kinetics in biological systems. This study introduces a solution method for nonisothermal reaction–diffusion (RD) problems that provides numerical results at high precision and low computation time, especially for calculations of a repetitive nature. Here, the coupled heat and mass balance equations are solved by treating the coupling terms as source terms, so that the solution for concentration and temperature may be cast as integral equations using Green's functions (GF). This new method requires far fewer discretization elements in space and time than fully numeric methods at comparable accuracy. The method is validated by comparison with a benchmark heat transfer solution and a commercial code. Results are presented for a first-order chemical reaction that represents synthesis of vinyl chloride.

Effect of Equal Channel and Dynamic Channel Angular Pressing of Ni and Further Heat Treatment on its Structure and Grain Boundary Diffusion

Formation of microstructure in Ni under equal-channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), its thermal stability and diffusion properties of grain boundaries are investigated. Grain boundary diffusion in the ultrafine-grained Ni is found to be significantly faster than in the coarse-grained Ni, which indicates a 'non-equilibrium' (deformation-modified) state of grain boundaries in the former. The effect of non-equilibrium state of grain boundaries on the level of internal stresses is analyzed.

Adsorption characterization and CO2 breakthrough of MWCNT/Mg-MOF-74 and MWCNT/MIL-100(Fe) composites

Carbon capture using adsorption processes can significantly mitigate global warming. Mg-MOF-74 is a distinct reticular material amongst other adsorbents owing to its distinguished carbon dioxide adsorption capacity and selectivity under low-pressure applications, while MIL-100(Fe) has lower CO2 adsorption capacity, but extraordinary thermal and hydrostability in comparison to many classes of MOFs. In this paper, we present CO2 adsorption characteristics of new compounds formed by the incorporation of multi-walled carbon nanotubes (MWCNTs) into Mg-MOF-74 and MIL-100(Fe). This was done to improve the thermal diffusion properties of the base MOFs to enhance their adsorption capacities. The new composites have been characterized for degree of crystallinity, and the CO2 and N2 equilibrium uptake. The real adsorption separation has been investigated by dynamic breakthrough tests at 297 K and 101.325. The equilibrium isotherm results showed that Mg-MOF-74 and 0.25 wt% MWCNT/MIL-100(Fe) (MMC2) have the highest CO2 uptake in comparison to the other investigated composites. However, the interesting results obtained from breakthrough tests demonstrate that good improvements in the CO2 adsorption uptake and breakthrough breakpoint over pristine Mg-MOF-74 have been accomplished by adding 1.5 wt% MWCNT to Mg-MOF-74. The improvements of CO2 adsorption capacity and breakpoint were about 7.35 and 8.03%, respectively. Similarly, the CO2 adsorption uptake and breakthrough breakpoint over pristine MIL-100(Fe) are obtained by 0.1 wt% MWCNT/MIL-100(Fe) (MMC1) with improvements of 12.02 and 9.21%, respectively.