Solid Thermal Conductivity Research Articles

Charge-balanced codoping enables exceeding doping limit and ultralow thermal conductivity

Materials with low thermal conductivity are applied extensively in energy management, and breaking the amorphous limits of thermal conductivity to solids has attracted widespread attention from scientists. Doping is a common strategy for achieving low thermal conductivity that can offer abundant scattering centers in which heavier dopants always result in lower phonon group velocities and lower thermal conductivities. However, the amount of equivalent heavy-atom single dopant available is limited. Unfortunately, nonequivalent heavy dopants have finite solubility because of charge imbalance. Here, we propose a charge balance strategy for SnS by substituting Sn<sup>2+</sup> with Ag<sup>+</sup> and heavy Bi<sup>3+</sup>, improving the doping limit of Ag from 2% to 3%. Ag and Bi codoping increases the point defect concentration and introduces abundant boundaries simultaneously, scattering the phonons at both the atomic scale and nanoscale. The thermal conductivity of Ag<sub>0.03</sub>Bi<sub>0.03</sub>Sn<sub>0.94</sub>S decreased to 0.535 W·m<sup>−1</sup>·K<sup>−1</sup> at room temperature and 0.388 W·m<sup>−1</sup>·K<sup>−1</sup> at 275 ℃, which is below the amorphous limit of 0.450 W·m<sup>−1</sup>·K<sup>−1</sup> for SnS. This strategy offers a simple way to enhance the doping limit and achieve ultralow thermal conductivity in solids below the amorphous limit without precise structural modification.

Thermal protection characteristics of non-enclosed thermal cloak

The aerodynamic heat of hypersonic vehicle nose cone can reach tens of MW/m<sup>2</sup> during flight, which could be transferred to the interior of hypersonic vehicle in the form of conduction and radiation. High efficient thermal insulation technology is of significance in keeping internal electronic components working safely. Thermal metamaterials can regulate the macroscopic heat flow path, and they are developing rapidly and have a wide application prospect in the field of thermal protection. In this work, a non-enclosed point transformation thermal cloak is designed to guide heat flow around hypersonic vehicle nose cone by using the transformation multithermotics, which can control thermal conduction and radiation simultaneously. A multi-layer structure is designed as cloak’s simplified approximation due to the anisotropic parameters. Based on the software COMSOL, the thermal protection characteristics and heat transfer mechanism of the point transformation cloak and multi-layer structure are studied numerically. The results show that heat can flow around the object in the form of conduction and radiation in both point transformation thermal cloak and multi-layer structure, so the heat transferred to the inner area decreases. Comparing with the thermal insulation material, the heating rate of the protected area slows down, and the temperature in the front of the hypersonic vehicle nose cone is significantly reduced. However, the improvement of the thermal protection performance of point transformation cloak and multi-layer structures requires that the solid thermal conductivity and radiative thermal conductivity of the material are lower than those of the original thermal insulation material. To solve this problem, a non-enclosed region transformation thermal cloak is further proposed. The solid thermal conductivity and radiative thermal conductivity of region transformation thermal cloak are non-singular, which could be higher than those of the original thermal insulation material. Numerical simulation results show that the region transformation thermal cloak can guide heat flow around object, so the thermal protection capability is improved significantly. Comparing with the thermal insulation materials, the temperature of the front of the hypersonic vehicle nose cone is reduced by 100 K, and the temperature of the inner central zone of the hypersonic vehicle nose cone is reduced by 10 K. The non-enclosed region transformation thermal cloak provides a new approach to realizing thermal protection and is suitable for complex target areas, showing great application potential in thermal protection.

Fluorinated Graphene Thermally Conductive Hydrogel with a Solid-Liquid Interpenetrating Heat Conduction Network.

Hydrogels with excellent mechanical flexibility are widely used in flexible electronic devices. However, it is difficult to meet further applications of high-power integrated flexible electronics as a result of their low thermal conductivity. Herein, highly thermally conductive composite hydrogels with a solid-liquid interpenetrating thermal conductivity network are constructed by aromatic polyamide nanofibers (ANF) and fluorinated graphene (FG) reinforced poly(vinyl alcohol) (PVA) and cross-linked by tannic acid (TA) solution immersion to obtain a hydrogel with a double cross-linked network. The PVA-ANF-FG3T-11.1% composite hydrogel exhibits good mechanical properties compared to PVA-ANFT, with a tensile modulus of up to 0.89 MPa, a tensile strength of up to 1.23 MPa, and an energy of rupture of up to 3.45 MJ cm-3, which is mainly attributed to the multihydrogen bonding interactions in the composite hydrogel. In addition, the friction coefficient of the PVA-ANF-FG3T-11.1% composite hydrogel is 0.178, making it suitable for use in high-friction coefficient applications. The thermal conductivity of the PVA-ANF-FG3T-11.1% composite hydrogel is 1.42 W m-1 K-1, which is attributed to the synergistic effect of the solid thermal conductivity network and the liquid convection network, resulting in a high thermal conductivity of the composite hydrogel. The high thermal conductivity of the PVA-ANF-FG3T-11.1% composite hydrogel shows great potential for flexible wearable electronics and cooling paste applications.

Using infrared opacifiers to reduce the thermal conductivity of micro and nanocellular polymethylmethacrylate

Nanometric cells in cellular polymers leads to a reduced gaseous thermal conductivity but a relatively high heat transfer by radiation, which counteracts the previous reduction. In this work, the impact of the inclusion of graphene nanoplatelets (GnPs), in the thermal conductivity of micro and nanocellular polymethylmethacrylate is evaluated.Production parameters are optimized to produce materials with the desired cellular structure and good macroscopic quality. Nanocellular and microcellular materials containing a 2 wt% of GnPs with similar cellular structures to those obtained with the original PMMA resin are produced.Minimum thermal conductivity of 36 mW/(m·K) is obtained with the microcellular composite containing Nano 307 GnP. The inclusion of GnP block infrared radiation reducing the radiation term up to 60 %. In addition, particles modify the structural factor, leading to a reduction up to 22 % in the solid thermal conductivity. The keys to further reduce the thermal conductivity are also discussed.

A coupled model of finite element method and Mie theory for heat transfer inside expanded perlite vacuum insulation panels (VIPs) at high temperatures

The present study introduces the first coupled model of the finite element method and Mie theory that predicts the heat transfer mechanism inside the expanded perlite vacuum insulation panels at a temperature range of 300–800 K. The finite element method is applied to discretise the governing equation of the steady state heat transfer and the three-dimensional Fourier's law is used. COMSOL Multiphysics is the finite element numerical solver which predicts solid and gaseous thermal conductivity inside the VIPs. Moreover, Mie theory predicts the radiative conductivity. Heat transfer radiation is superimposed on the heat transfer through conduction. The model is capable of predicting total thermal conductivity of VIPs at high temperatures ≥ 500 K, which makes it a unique model with clear advantage over previously developed numerical models. The numerical predictions of the developed coupled model are validated experimentally. The predictions of the coupled model are in good agreement with the experimental measurements, which indicate the capability of the developed model to predict the mechanisms of heat transfer at high temperatures. For instance, the values of total thermal conductivity for numerical prediction and experimental measurement, at 557 K, are 14.6 and 14.7±1.2 mW/m/K, respectively, excellently agreeing within ± 8.9 %.

Layered Inorganic Silicate Aerogel Pillared by Nanoclusters for High Temperature Thermal Insulation.

Layered inorganic material, with large-area interlayer surface and interface, provides an essential material platform for constructing new configuration of functional materials. Herein, a layered material pillared with nanoclusters realizing high temperature thermal insulation performance is demonstrated for the first time. Specifically, systematic synchrotron radiation spectroscopy and finite element calculation analysis show that ZrOx nanoclusters served as "pillars" to effectively produce porous structures with enough boundary defect while maintaining the layered structure, thereby significantly reducing solid state thermal conductivity (≈0.32Wm-1 K-1 , 298-573K). Moreover, the layered inorganic silicate material assembled aerogel also exhibits superior thermal insulation performance from room temperature (0.034Wm-1 K-1 , 298K, air conditions) to high temperature (0.187Wm-1 K-1 , 1073K, air conditions) and largely enhanced compressive strength (42kPa at 80% compression), which is the best layered material-based aerogel that has achieved synergistic improvement in thermal and mechanical performance so far. Layered inorganic silicate aerogel pillared by nanoclusters will pave a new avenue for the design of advanced thermal insulation materials under extreme conditions.

Investigating the impact of modeling assumptions and closure models on the steady-state prediction of solar-driven methane steam reforming in a porous reactor

In this work, the effect of different modeling assumptions and closure models — frequently considered in the literature of solar thermochemical reactors and closely-related areas — is investigated. The application of different modeling assumptions and closure models may strongly affect the results accuracy and compromise a meaningful comparison across literature works. The following is herein investigated: (i) the role of upstream and downstream fluid regions to the two-phase reactor region; (ii) relevance of species and gas-phase thermal diffusion mechanisms; (iii) reaction heat accounted for in gas- or solid-phase energy balance equations; (iv) local thermal equilibrium (LTE) vs. local thermal non-equilibrium (LTNE) models; (v) model dimension (one-dimensional vs. two-dimensional axisymmetric models); (vi) local volumetric convection heat transfer correlations; and (vii) effective solid thermal conductivity correlations. The relevance of this work extends well beyond the current application (methane steam reforming in a volumetric solar reactor), since similar models and assumptions have also been widely applied for predicting the performance of volumetric solar absorbers and dry reforming solar reactors. The results show that an upstream fluid region should be considered while applying inlet first-type boundary conditions and diffusion transport in the corresponding governing equations to ensure full-conservation and simultaneously to account for developing profiles upstream the reactor inlet section. For the operating conditions considered, species diffusion and gas-phase heat conduction are particularly relevant at the reactor centerline but with a negligible integral effect. A significant difference in the reactor performance is observed while accounting for the reaction heat from surface reactions in the solid- or gas-phase energy balances — for the lowest inlet gas velocity herein considered, the thermochemical efficiency (methane conversion) is approximately equal to 70.8% (76.3%) and 77.3% (84.7%) assigning the reaction heat to the gas- and solid-phase energy balances, respectively. LTE model results are strikingly different from the results obtained with the LTNE model considering the reaction heat accounted for in the gas-phase energy balance but not significantly different from the results computed with the LTNE model with the reaction heat assigned to the solid-phase energy balance. One-dimensional reactor modeling provided with an average concentrated solar heat flux value results in a similar average performance as that given by a two-dimensional model but fails to predict the high temperatures observed at reactor centerline — for the lowest inlet gas velocity, the difference between the maximum solid temperatures predicted by one- and two-dimensional models is about 340K.

Directional enhancement and potential reduction of thermal conductivity induced by one-dimensional nanoparticle addition in pure PCMs

To increase the thermal conductivity of organic phase change material (PCM), One-dimensional materials are often used as an additive due to their excellent thermal performance. Many experiments have shown that micro effects have a great influence on the thermal conductivity of composite materials. To clarify the micro effect, numerous molecular dynamics simulations (MD) have been carried out. Previous research has entirely overlooked the thermal conductivity changes of pure PCM in composite systems. Neglecting the thermal conductivity changes of pure PCM is unreasonable, as the mass fraction of nanoparticles is often small. We took paraffin wax (PW) with carbon nanotube (CNT) as an example, and successfully reproduced the phase transition of composite PCM. Then we optimized the details of simulating thermal conductivity, and two processes of replication and movement were taken to get a stable thermal conductivity. A disassembly of thermal conductivity based on heat flux was actualized to investigate the thermal conductivity of pure PCM in composite systems. We investigated the difference in thermal conductivities between solids and liquids and analyzed the underlying mechanism through vibrational changes. The change of thermal conductivity in solids is contributed by CNT itself and the structure of PW; the change of thermal conductivity in liquids is contributed by CNT itself and the interaction of CNT. Among these factors, structure change wouldn't always be beneficial. CNT always induces an axial enhancement of thermal conductivity in pure PCMs. At last, verification was carried out to prove the importance of structure. Our study brought a new view to understanding the behavior of thermal conductivity in composite PCM.

Heat energy transport characteristics of microchannel reactors for hydrogen production by steam-methanol reforming on copper-based catalysts

Numerical simulations are carried out to understand the heat energy transport characteristics of microchannel reactors for hydrogen production by steam-methanol reforming on copper-based catalysts. Enthalpy analysis is performed and the evolution of energy in the oxidation and reforming processes is discussed in terms of reaction heat flux. The effects of solid thermal conductivity, gas velocity, and flow arrangement on the thermal behavior of the reactor is evaluated in order to fully describe the thermal energy change in the reactor. The results indicate that the thermal behavior of the reactor depends upon the thermal properties of the walls. The change in enthalpy is of particular importance in exothermic and endothermic reactions. The net enthalpy change for oxidation and reforming is negative and positive, but the net sensible enthalpy change is always positive in the reactor. The wall heat conduction effect accompanying temperature changes is important to the autothermal design and self-sustaining operation of the reactor. The solid thermal conductivity is of great importance in determining the operation and efficiency of the reactor. The reaction proceeds rapidly and efficiently only at high solid thermal conductivity. The reaction heat flux for oxidation and reforming is positive and negative. The change in flow arrangement significantly affects the reaction heat flux in the reactor. The parallel flow design is advantageous for purposes of enhancing heat transfer and avoiding localized hot spots.

Solute segregation affected by transport properties during a bubble entrapment in upward solidification

Abstract The effects of transport properties on segregation of a dissolvable gas, for example, carbon dioxide in water in the presence of an entrapping bubble during upward unidirectional solidification is numerically investigated. Microporosity and inter-related solute segregation are two common defects detrimental to mechanical properties of components, especially to fatigue performance, strength and ductility. In this work, transport processes between the bubble, liquid and solid affected by mass, momentum, energy, and concentration Eqs. are solved by finite element schemes provided from the commercial COMSOL code. The results find that solute concentration in solid and liquid surrounding the pore increases with decreasing Henry's law constant and gravitational acceleration, and increasing surface tension and solid thermal conductivity. Solute gas concentration on the top free surface is insensitive to that in the pore rather than that in liquid and solid away from the pore. Apex cap becomes non-spherical affecting solidification rate and shape of the solidification front as ambient pressure at the top free surface and gravitational acceleration increase. Contact angle versus location of the solidification front predicted from this work and Abel's Eq. agrees well. Solute segregation encountered by pore formation is controllable.

An effective thermal conductivity model for the three-phase porous media based on the numerical simulation

Heat conduction in the porous media (PM) with gas-liquid-solid three-phase has been numerically investigated. The validation of the adopted numerical method is performed by comparing the effective thermal conductivity (ETC) obtained from the simulation results with the experimental data. The results show that the ETC of the PM increases with increasing solid volume fraction (ϕs ) and liquid saturation degree (Sr). Sr has a weaker effect on the ETC compared to ϕs . Similar variation laws of the ETC with Sr and ϕs are found for systems with different solid and liquid thermal conductivity. In this work, the PM is simplified as a circuit network. The modeling for the ETC is performed by defining a weighted harmonic parameter (η) to connect the serial and parallel models. η is closed as a function of Sr, ϕs and the thermal conductivity of the solid and liquid phases. The predictability of the model is evaluated. The results demonstrate that the model can provide accurate estimations for the ETC of the three-phase PM.

Numerical analysis of combined heat transfers through concrete hollow bricks in a hot climate of Morocco

The aim of this study is to determine the thermal conductance of concrete hollow bricks, which is crucial for assessing the energy efficiency of buildings. The study focuses on three types of concrete hollow bricks that are frequently used to build walls in Morocco. The three modes of heat transfer by convection, conduction and radiation are taken into account. The thermal behavior of the three types was simulated using a computational model created with the finite volume method. The effect of internal surfaces emissivity and solid wall thermal conductivity on overall heat transfer is investigated using practical values of the thermal excitations. Simulation results of the three types were compared and showed that the concrete hollow brick of Type (C) is the best performing configuration. Indeed, it reduces the thermal conductance ( U-value) by 40% compared to hollow brick of Type (B), which will contribute to improve the thermal resistance of the building walls.

Guarded Parallel-Plate Instrument for the Determination of the Thermal Conductivity of Gases, Liquids, Solids, and Heterogeneous Systems

In the present study, a new version of a guarded parallel-plate instrument (GPPI) for the absolute determination of the thermal conductivity under steady-state conditions is presented. It is based on an existing concept of an automated instrument and includes several modifications with the aim of further improving the achievable accuracy and experimental uncertainty while reducing the measurement time. These modifications mainly include improvements to the guard system in the upper hot part, which lead to a further suppression of parasitic heat fluxes, and the integration of flow circulation systems for the active temperature control in the periphery of the instrument. By using relatively large values for the temperature difference between the heating and cooling plates and for the sample thickness, expanded (k = 2) measurement uncertainties between (2 and 3)% could typically be achieved for the thermal conductivity. For mean temperatures from (283 to 358) K and atmospheric pressure, the measurement results agree well with reference data for liquid water, toluene, and ethylene glycol as well as air, polytetrafluorethylene (PTFE), and stainless steel 316. In addition, the applicability of the new GPPI for the determination of the effective thermal conductivity of heterogeneous systems was demonstrated using water-based nanofluids containing dispersed silicon dioxide, titanium dioxide, or copper oxide nanoparticles.

Hydrogen charging in AX21 activated carbon-PCM-metal foam-based industrial-scale reactor: Numerical analysis

A computational fluid dynamics (CFD) model based on the mass, momentum, and energy conservation equations is used to simulate the industrial-scale hydrogen storage in activated carbon (AC: AX-21) bottle surrounded by a layer of metal foam (MF)-phase change material (PCM: RT 22 HC) latent thermal unit. The aim is to boost the recuperation (storage) of the heat liberated from the H₂‘s exothermic (adsorption). Following the model's validation, several numerical simulations were performed to evaluate the impact of various physical features of the MF (copper), at variable porosity (ε = 0.80–1.0), on heat transfer in the hole reactor. Independently of the MF porosity, the AC-bed temperature suddenly increased (from 295 K), reaching a maximum (at about 500 s) and decreased afterward. The temperature's intensity was dependent on the probe's position in the AC-bed (i.e. several positions were studied). The H2 concentration in the AC-bed may reach 25 mmol/g. The PCM-based unit received the discharged heat from the AC-bed slowly, but with PCM incorporation in MF, the heat was quickly recovered (stored). Thanks to the MF-induced PCM's increased thermal conductivity, the heat flux, the heat transfer coefficient, and the melting rate of the PCM were all accelerated (during the charging phase) several times by decreasing the MF porosity (increasing MF mass) from 1 (pure PCM) to 0.8. The temperature and the melting contours throughout the absorption process (up 10,000 s) supported all these findings. It is obvious from the results of the current study that improving the reactor geometry and the solid bed's thermal conductivity (thermal performance) is necessary for an effective hydrogen storage system.

Industrial-scale for hydrogen charging in activated carbon (AX-21)-phase change material-nano-oxides systems

As a plausible solution for the issue of heat management during hydrogen adsorption on the activated carbon bed, in the present paper, the effects of different metal oxides nanoparticles (MgO, TiO2, CuO, or ZnO), scattered into a phase change material (PCM), have been investigated over the adsorption and dormancy phases in a large-scale bottle. This study has been conducted using Fluent software, in which a computational fluid dynamics (CFD) model (based on finite volume simulations) was implemented. This model has been validated via the experimental and theoretical results available in literature. Throughout the duration of the charging process, it has been found that the presence of nanoparticles in PCM body does not affect the thermal performance of the solid bed of activated carbon,. However, the melt fraction of PCM is steeply increased to 20 % in the range ~ 0–1300 s, whereas over 1300 s, the melting process of paraffin is amortized until the end of the dormancy phase (attaining ~ 30–35 % of liquid PCM). This trend is observed either in the presence or in the absence of metal oxides, where 16 % of enhancement in the liquid fraction of PCM is retrieved in the presence of nanoparticles (5 %) at the end of the dormancy phase (10,000 s). As a result, The nano-PCM system could be a proposed pathway for recovering the heat generated from the H2 adsorption and then replacing the fluid cooling system (cost). However, it was recommended that future research works should be focused on the improvement of the solid bed thermal conductivity in addition to the enhancement of the reactor geometry via new innovative solution pathways.

Simulation of soil thermal conductivity based on different schemes: An empirical comparison of 13 models

Thermal conductivity is a key soil property widely used for agricultural production, land surface processing research, and geothermal resource development, among others. Although the rapid and accurate determination of soil thermal conductivity (λ) has been a hot topic in recent years, there is still no unified model for the different soil types of soil. Furthermore, the lack of data on thermal conductivity and soil properties leads to errors in parametric models of thermal conductivity. In order to overcome the data shortage, a comprehensive λ dataset of 2972 items was established and 10 influential parameters on thermal conductivity were identified in this study. Based on this, an empirical comparison was made between four classical parametric models and nine machine-learning models with and without an intelligent optimization algorithm was carried out. Of all the methods, the ensemble machine-learning methods perform better in λ simulations. The XGBoost model has better simulation accuracy and generalization capability. Soil moisture properties are the key parameters in performing λ simulations, while the soil texture-related properties such as bulk density and solid thermal conductivity, along with the sand content, also play an important role. The results of this study can provide basic thermal conductivity data and a parameterization scheme for referencing in land surface processing research.

Pore-scale numerical simulation and LTNE analysis for fully-developed forced convective heat transfer in packed beds of mono-sized rough spheres covering near-wall and core regions

This work presents a novel method to acquire the effective convection heat transfer coefficients on the interfaces of wall-fluid contacts and sphere-fluid in both the near-wall region and core region of sphere-packed beds. Pore-scale numerical simulations are first conducted for different porosities pertaining to two regular body-centered cubic and face-centered cubic packings, packing lattice angles, inflow velocities, and ratios of the thermal conductivities of the solid and fluid. A finite-thickness metal reactor wall is included in the pore-scale simulation to make the heating boundary conditions more realistic. The outer surface of the metal wall is subjected to a uniform heat flux. The heat then passes through the metal wall and dispenses into the solid and liquid phases of the packed bed through the wall-sphere and the wall-fluid interfaces at the inner wall according to the physical mechanisms. Within the reactor, the heat transfer mechanism is separately considered in the near-wall region and the core region due to the intrinsic differences in the flow pattern. For each packing, different inflow angles are examined and exhibit different flow patterns and heat transfer performances. After the whole domain is hydrodynamically and thermally fully-developed, the local effective convection heat transfer coefficients can then be calculated. These coefficients reflect the realistic local convection either on the reactor wall or on the sphere-fluid interface. These convection heat transfer coefficients, along with the effective solid thermal conductivity through the contacting spheres obtained in our previous work, are implanted into a modified two-equation LTNE model. This modified LTNE model yields linear longitudinal fully-developed temperature distributions within the reactor having discrepancies in the temperature slopes less than ±10% from those by the pore-scale numerical simulations. In contrast, the conventional model even fails to reach linear distributions of the solid- and liquid-phase temperatures, which are expected for fully-developed packed-bed flows.

Linear Model for Two-Layer Porous Bed Suspended with Nano Sized Particles

Two immiscible fluids flows are materialized in science and technology; the combined convection of the two immiscible fluids in a square conduit is reviewed in this study. The nanofluid and pure viscous fluid which do not mix are discussed, and both layers saturated with a porous matrix have different permeabilities. The Dupuit–Forchheimer and Tiwari–Das models are applied to outline the permeability of the layer and nanofluids, respectively. The finite difference method is utilized to find the solutions of conservation equations along with suitable boundary and interface conditions. The boundary condition for the velocity is no slip at all the boundaries, while continuity of velocity and shear stress are used at the interface. The left and right walls are kept at constant but different temperatures, the top and bottom walls are isolated, and the continuity of temperature and heat flux is assumed at the interface. Grashof number, Brinkman number, Darcy number, inertia parameter, permeability ratio, solid volume fraction, thermal conductivity and viscosity ratios, different nanoparticles, and various base liquids of the two-layered fluids are engaged. The velocity is depleted by the inertia and viscosity ratio while it is accelerated with the Darcy and Grashof numbers. The energy distribution was not modulated significantly with any of the dimensionless numbers. Using copper nanoparticles doped in mineral oil and ethylene glycol produced the peak momentum. Diamond nanoparticles dropped in water catalysis showed the best heat transfer rate.

Influence of thermal conductivity on transient mixed convection in a vented cavity with a hollow cylinder and filled with CNT-water nanofluid

This study is aimed to perform a numerical time-dependent investigation thermal conductivity effect of the annular cylinder within a vented cavity using CNT based-water nanofluid. For demonstrating the effect of thermal conductivity, four distinct hollow cylinder materials such as Ks = 0.5(Plastic tiles), Ks = 0.84(Clay tiles), Ks = 1.1(Concrete tiles), and Ks = 2(Slate tiles) are introduced together with a suitable variation of dimensionless time (0 ≤ τ ≤ 1). The governing equations of the model with associated boundary conditions is solved using finite element based Galerkin's weighted residual method. Different contour plots for thermal and flow field transformation and mean Nusselt number, mean fluid temperature, bulk convective field temperature, temperature gradient, pressure gradient, vortices, and fluid velocity magnitude are presented for qualitative and quantitative thermal performance analysis. With the decrease of solid thermal conductivity, 27.3% thermal transport enhancement is noted from the heated surface of the cylinder. However, a 16.3% increase in the bulk fluid temperature has been recorded with the increase in cylinder conductivity. The numerical outcomes from this investigation propose a better thermo-fluid efficiency compared to the existing methodology which can be suggestive to engineers and researchers for designing heat exchangers, heat pipes, and other thermal systems.

Large-Scale Fabrication of High-Performing Single-Crystal SiC Nanowire Sponges Using Natural Loofah

This study presents large-scale fabrication of single-crystal SiC nanowire (SiCnw) sponges through a carbothermal reduction approach using agricultural residue loofah, which is believed to be a perfect alternative to traditional petrochemical materials as the carbon source in industrial fabrication of SiC nanowires, without additional chemical C sources or catalysts. The single-crystal ultralong SiCnw are observed to have diverse morphologies with diameters of 50–400 nm, which possess a significant anisotropic and hierarchical microstructure with planar-aligned bamboo joints. A roughly 2 nm thick amorphous SiO2 layer is identified wrapping the crystalline 3C-SiC. The underlying vapor–solid mechanism is systematically investigated via thermodynamic calculations on the formation and interactions of SiO, CO, and SiC. Noteworthily, the fabricated loofah-based SiCnw sponge exhibits prominent high-temperature performance after thermal insulation tests of alcohol lamp flame and butane blowtorch flame, which is attributed to the stacking faults, SiO2 layer, SiC/SiO2 interface of the nanowires, and the overlength pathway in the porous aerogel–sponge domain synergetically acting as a phonon barrier to enhance phonon scattering, prolong phonon diffusion, and eventually reduce solid thermal conductivity in the sponge architecture. Our current work has great potential to be applied in the eco-friendly industrial preparation of high-performing SiCnw as thermal protection materials in harsh environments.