Enhanced Heat Transport Research Articles

Non-Fourier heat and mass transport enhancement by hybrid nanofluid-flow over a non-linearly stretchable surface having variable thickness

This article uses non-classical Fick's law, non-Fourier's law, and conservation laws for mass and thermal transport. The hybrid nanoparticles Cu and Al2O3 are considered. The new correlations among the thermo-physical properties of base fluid, Cu and Al2O3 are coupled with simplified nonlinear mathematical models. The resulting models are solved numerically by the finite element method (FEM). The linear shape functions are chosen for the approximation of residual equations. This approximation leads to a nonlinear algebraic system that is linearized by the Picard scheme. The numerical results are ensured to be grid-independent, and convergence is analyzed. The results are validated, and an excellent agreement is obtained between available benchmarks and current outcomes. Thermal solutal relaxation phenomena are responsible for a significant reduction in the transport of heat and mass in Newtonian fluids. These non-Fourier's and non-classical Fick's laws are capable of capturing thermal and solutal elastic phenomena, respectively. Cu and Al2O3 simultaneously act as good conductors of heat, and their simultaneous dispersion in base fluid results in a significant rise in thermal conductivity. Numerical experiments have shown that the transport of heat can be optimized by simultaneous suspension of Cu and Al2O3.

Impingement of vortex dipole on heated boundaries and related thermal plume dynamics

The profound influence of an externally induced vortex dipole on thermal plume dynamics is numerically studied for varying Rayleigh numbers (Ra) employing the Bhatnagar–Gross–Krook collision model-based lattice Boltzmann method with a double distribution function approach. This study is extended to vortex dipole impingement with different types of heated bottom boundaries of two-dimensional domain, such as flat, “V-shaped,” and “inverted-V-shaped.” The vortex dipole impingement with the heated boundaries generates secondary vortices, which in turn produce vortex-driven thermal plumes, thereby advancing plume generation. The subsequent merging of the plumes enhances heat transport and leads to a continuous plume ascent. The presence of convex corners facilitates flow separation and also gives rise to the formation of secondary vortex dipoles, thereby significantly impacting the continuous generation of jet-like plumes when compared to concave configurations. The lack of an external vortex in pure buoyancy-driven flows produces less pronounced jet-like plumes and a relatively low Nusselt number. The boundary types and Ra significantly influence the vorticity production, resulting in higher enstrophy and palinstrophy for convex boundaries compared to flat and concave ones. A lower Prandtl number increases secondary vortices and corner rolls, leading to larger velocity gradients, higher thermal diffusivity, resulting in increased kinetic energy and thermal dissipation rates. The increased cell height enhances heat transfer at the top boundary due to improved heat convection from the slanted boundary and influence of early dipole impingement. Furthermore, kinetic energy dissipates in the dipole-driven flows and increases in the buoyancy-dominated flows.

Nonlinear convective transport of nanofluid over an inclined plate situated in a Darcy–Forchheimer porous medium with viscous dissipation and varying thermo-physical features

This article presents a theoretical analysis of the effects of varying thermo-physical properties, chemical reactions, and activation energy on the quadratic convective transport of nanofluids. The flow is created through an inclined plate in a Darcy–Forchheimer porous medium. The nanofluid model includes thermophoresis and Brownian motion to account for nanoparticle movement. The original flow equations are transformed using similarity transformations, and the resulting dimensionless equations are solved with a multi-domain spectral collocation approach. The effects of physical parameters on flow variables, heat and mass transfer coefficients are discussed. Comparisons between vertical and inclined plates reveal that inclination improves thermal performance but decreases velocity and engineering-related quantities. Fluctuating viscosity and nonlinear convection speed up fluid flow, whereas temperature rises with variable thermal conductivity and Biot number. Variable fluid properties, nonlinear convection, and convective heat conditions enhance heat transport coefficients. Activation energy boosts nanoparticle volume fraction profiles but decreases mass transport rates.

Asymptotic analysis of MHD chemically reacting boundary layer flow of Jeffrey hybrid nanofluid

AbstractFluids with enhanced heat transport characteristics are essential for efficient convection heat transportation. Hybrid nanofluids have demonstrated their effectiveness as viable substitutes for conventional heat transport fluids. This study explores the heat and mass exchange occurring within a chemically reactive, unsteady boundary layer flow of a copper oxide‐multi‐walled carbon nanotubes (CuO‐MWCNTs)/ethylene glycol Jeffrey hybrid nanofluid. Additionally, the influence of heat source/sink effects in a hydromagnetic environment is carefully added. The study employs a non‐Newtonian flow model and incorporates the Arrhenius activation energy for analysis. The hybrid nanofluid consists of a base fluid, ethylene glycol, enriched with copper oxide nanoparticles and multi‐walled carbon nanotubes. The governing coupled non‐linear partial differential equations are transformed into ordinary differential equations using similarity transformations, considering appropriate free stream, and wall boundary conditions; then, the Shooting method is employed to solve the resulting ordinary differential equations (ODEs) in MATLAB. The graphical and numerical outcomes are studied for various parameter combinations. The graphs illustrate the numerical results for the CuO‐MWCNTs/ethylene glycol hybrid nanofluid. These results are comprehensively discussed to analyze the influence of different thermo‐fluidic parameters on the Jeffrey hybrid nanofluid's heat, mass, and flow characteristics. The skin friction, Nusselt number, and Sherwood number are provided in a numerical table that displays the alterations of these parameters across various parameter values. As the Jeffrey fluid parameter rises, the Nusselt number and skin friction escalate, while the Sherwood number diminishes. Conversely, as the Deborah number rises, the Nusselt number and skin friction decline, but the Sherwood number increases. A comparative analysis with published results confirms the consistency of the present results.

The Impact of Roughness and Partitions in Thermal Convection to Enhance the Heat Transport

Abstract In this investigation, we explore the profound impact of adiabatic partitions on heat transport within a square Rayleigh-Bénard (RB) convection enclosure characterized by surface roughness on the lower hot and upper cold plates. The surface irregularities take the form of a rectangular base and a triangular apex. Our study employs two-dimensional direct numerical simulations spanning the Rayleigh number (Ra) range of 106–108 and a Prandtl number (Pr) of 1. Adiabatic partitions, strategically positioned between successive roughness, serve as the focal point of our exploration. Remarkably, our findings reveal a substantial enhancement in heat transport with the introduction of a partition board between consecutive roughness elements. As we escalate the number of partitions from one to four, the heat flux experiences a notable augmentation, reaching 2.3 times that of the classical square RB configuration. This enhancement is attributed to the breakdown of large-scale rolls into multiple rolls, a phenomenon intensified by the increased partition height. Further intriguing observations unfold as we investigate the interplay of surface roughness and partitions. Configurations featuring roughness with partitions exhibit an impressive 2.7 times heat flux enhancement compared to the classical square RB setup. The complex interplay of heat transport improvement is closely connected to optimizing the distance between the conduction plate and the partition. Through meticulous analysis, we identify that the optimal gap facilitates heightened local velocity, effectively thinning the thermal boundary layer and consequently augmenting the overall heat flux. In essence, our study sheds light on the synergistic effects of adiabatic partitions and surface roughness in the context of RB convection. The observed enhancements in heat transport underscore the potential for tailored design strategies involving partitions and surface modifications to optimize thermal performance in diverse applications.

Improved slip mechanism and convective heat impact for ternary nanofluidic flowing past a riga surface

This study of electro-magneto-hydrodynamics has great significance due to its numerous applications like chromatography, fluid pumping, micro coolers and fluid stirring thermal reactors and flow regulation in fluidics systems. Subject to upper functions on electrical magnetic field, the consequences of electromagnetic initiation into ternary nanofluids flow through the Riga plate were noticed. Furthermore, this ternary hybrid nanofluid flow was based on the effect on slip condition, uniform heat source, convective energy and thermal radiation. The ternary hybrid nanofluid was built from the scattering of silver, copper and copper oxide nanoparticles by this base fluid blood. This phenomenon had been formed by the model of the system in partial differential equations; it was made easy in the dimensionless nonlinear structure of ordinary differential equations by employing comparison substitutions. This result on the acquired set of the differential equations was simulated over the bvp4c method. It has been discovered that the ternary hybrid nanofluid velocity is essentially lower along the differing numbers of permeable media, although it amplifies along the upshot on the Hartmann number. Moreover, an enhanced heat transport rate of up to 14% was marked for the triple nanoparticle nanofluid by relating it to another nanofluid and establishing an excellent behavior on triple nanoparticle nanofluids.

Heat transport during drop impact onto a heated wall covered with an electrospun nanofiber mat: The influence of wall superheat, impact velocity, and mat thickness

Nanofiber surface coating is a promising method for the enhancement of heat transfer during spray cooling. In the present work, the drop dynamics as well as local and overall heat transfer during single drop impact onto a heated wall covered with a polyacrylonitrile (PAN) nanofiber mat are investigated to obtain insight into the mechanisms governing the heat transport enhancement. The influence of wall superheat, drop impact velocity, and mat thickness on the hydrodynamics and heat transfer from the heated wall to the fluid is studied. The experiments were conducted inside a temperature-controlled test cell with a pure vapor atmosphere maintained with refrigerant FC−72 (perfluorohexane). The temperature field at the solid–fluid interface was observed with a high-speed infrared camera, and the heat flux field was derived by solving a three-dimensional transient heat conduction equation within the substrate. The presence of the nanofiber mat on the heater surface suppresses the drop receding phase due to the pinning of the contact line at the end of the spreading phase. Two different heat transfer scenarios are observed depending on the wall superheat and drop impact velocity: scenario (I), in which the liquid drop completely penetrated through the porous nanofiber mat and made contact with the heater surface; and scenario (II) in which the vapor produced inside the pores of the nanofiber mat prevented the liquid drop from touching the heater surface. In scenario (I), an up 450% heat flow enhancement during the sessile drop evaporation stage has been observed. In scenario (II), a 80% heat flow enhancement at this stage has been registered.

Magnetized hybrid nanofluid flow across a wedge: A computational three-dimensional study

The boundary layer fluid flow past a wedge surface occurs in various industrial processes, including wire drawing, metal foaming, polymer extraction, fiber processing, and in determining the dynamic properties (shear stress, drag) of working fluids. Theoretically, the boundary layer three-dimensional flow of a magnetized hybrid nanoliquid over a wedge surface is studied with viscous dissipation and radiation effects. The flow is due to moving wedge surface and power-law free-stream velocities. The current study unifies three different classical flow problems, namely, the Blasius flow, the Hiemenz flow, and the Falkner-Skan flow. Single-phase description of hybrid nanofluid is considered, and the problem is kept realistic by considering the experimental thermophysical properties of the nanomaterial. The governing system consists of conservation of mass, momentum, and the energy equations for hybrid nanofluids. Prandtl’s boundary layer approximations are applied to study the viscous dominant fluid regimes. The entropy equation and the Bejan number are also derived. Numerical computations of self-similar equations are obtained to explore the rheological and the heat transport features against the physical parameters. The Nusselt number and the drag on the wedge surface are computed and analyzed. Of the three classical flow problems, the maximum thickness of the momentum layer occurs for the Blasius flow problem. A minimum drag force on the wedge surface occurs for lower Hartmann number values. Furthermore, an increase in the volume fraction of the hybrid nanoparticles leads to an enhanced heat transport. The use of carbon nanotubes which has exceptional characteristics are taken into consideration in this study. This study also demonstrates the comparison on three-different classical flow problems (Blasius flow, Hiemenz flow, and Falkner-Skan flow).

Thermal bridging effect enhancing heat transport across graphene interfaces with pinhole defects

Interfacial thermal conductance (G) is of critical significance to the efficient thermal management of two-dimensional (2D) material devices. Despite the importance of defects engineering in tuning G, the mechanisms of heat transport across defective graphene interfaces remain unclear. In this study, we have identified and demonstrated the origins of the enhanced heat transport across pinhole defective graphene through our ultrafast pump-probe measurements and molecular dynamics (MD) simulations. In prior work, the enhanced G across defective graphene interfaces was commonly attributed to the improved overlap of phonon density of states (DOS) or interfacial coupling strength. We, however, observe a remarkable 2.5-fold increase in heat conduction and a distinctly different temperature dependence of G for defective graphene with the same superstrate and substrate material, compared to pristine graphene. In contrast, interfaces with different superstrate and substrate materials show only minor changes (<15%) in thermal conductance after introducing defects in the graphene layer. Through careful analysis of our MD simulations for various defective graphene interfaces, we conclude that, contradictory to common beliefs, it is the formation of direct contact thermal bridge, instead of the improvement in the phonon DOS overlap or interfacial coupling strength, that has a decisive effect and promotes heat transport across defective graphene interfaces. The differing impacts of the thermal bridge on G of defective graphene sandwiched by layers of the same and different materials are primarily attributed to the varying coupling strength between the substrate and superstrate. Our work provides important insights and physical understandings of the mechanisms of heat conduction across defective graphene interfaces.

Levenberg-Marquardt technique analysis of thermal and concentration storage in cone-disk apparatus with neural network-enhancement

An artificial intelligence-driven neural network with the Levenberg-Marquardt method (NN-LMM) has been developed to investigate the thermal and mass transfer characteristics of a liquid flowing through the conical gap in a cone-disk apparatus. The disk and cone can either remain static or rotate at changing angular speeds, with discussion for heat transfer influenced by cosmic emission. The Rosseland resemblance is employed to compute heat diffusion in this study. To monitor variations in mass removal on the exterior, thermophoresis effects are appropriated into consideration. By applying appropriate transformations, the partial differential equations (PDEs) governing this problem are converted into typical ones. A reference dataset for NN-LMM is generated, covering various influential model variations, simulating scenarios using the Lobatto III-A numerical method. The purpose of employing the Levenberg-Marquardt technique in the analysis of thermal and concentration storage in a cone-disk apparatus, enhanced by neural networks, lies in optimizing the modeling and prediction accuracy of complex fluid dynamics and heat transfer phenomena. This technique, coupled with neural network enhancement, aims to address challenges associated with traditional analytical methods by offering a robust and efficient approach to modeling intricate processes in fluid dynamics and heat transfer. The novelty of utilizing the Levenberg-Marquardt technique, along with neural network enhancement, lies in its ability to effectively capture and simulate the dynamic interactions between thermal and concentration storage in the cone-disk apparatus. By leveraging the power of neural networks to learn complex patterns and relationships from data, combined with the optimization capabilities of the Levenberg-Marquardt algorithm, this approach offers improved accuracy and predictive capabilities compared to conventional methods. It is analyzed that the mass transport field for Nt decreases as temperature ratios increase. This phenomenon occurs because the thermophoretic force becomes more pronounced at higher temperatures, causing more particles to move towards the apparatus. Further it is seen that stationary cone and a rotating disk exhibit enhanced heat transport with increasing radiation parameter values. This suggests that elevating the radiation parameter leads to more efficient heat transfer in this specific model. Highest flow intensity observed around the cone. This reference data is subjected to testing, validation, and training processes to fine-tune the approximate solution to meet the desired results. The accuracy, stability, capacity, and robustness of NN-LMM are verified through mean squared error (MSE)-based fitness curves, regression plots, error histograms and absolute error assessments. A comparative analysis demonstrates the accuracy of the proposed solver, with absolute errors in the range of 10-9 to 10-6 for all influential parameters results.

Heat transfer analysis for 3d ternary hybrid nanofluid flow with MHD and non-fourier flux impact over a linearly stretching surface: Response surface optimization

BackgroundCombining different-shaped nanoparticles enhances the mechanism of heat transfer. Nanoparticles can be spherical, cubic, rod, tube, helical, triangular, hexagonal, oval, or prismatic, and they are frequently combined with the base liquid. Biology and industry are two academic fields that are significantly impacted by nanotechnology. The use of hybrid nanoparticles to enhance heat transport in a working fluid has piqued the curiosity of numerous experts. Ternary hybrid nanofluids are frequently used in heat transfer applications because of have higher thermal conductivity than ordinary fluid, especially as heat exchangers. MotivationIn contemporary times, the amalgamation of liquids has assumed paramount significance in diverse domains including healthcare, production, naval academies, aerosol particle processing, instrument design, and so on. The non-Fourier flux and magnetic impact in three-dimensional flow characteristics using the linear Roseland approximation. Furthermore, ternary solid nanoparticles in a variety of densities and forms were included. Aim and objectiveThe primary goal of the research is in a three-dimensional rectangular closed domain stretching surface with Magnetic effect, linear thermal radiation and non-Fourier flux to analyze the heat and velocity transfer rate with different cases like Case-1: AA7072+SWCNT+MWCNT with PEG−water a base fluid, Case-2 Fe3O4+Diamond+TiO2 considered along as PEG−water a base fluid ternary hybrid nanofluid. Methodology: By transforming the three-dimensional model's governing PDEs into nonlinear ODEs. The ODE45 solver in MATLAB was utilized to construct the graphical representations of the numerical solution findings. Results & conclusionsIt is investigated how well different flow and temperature parameters function when changed. The analysis is also done on simulation findings for friction and heat transmission for different values. And transfer rate are higher in case-2 Fe3O4+Diamond+TiO2 than in case-1 AA7072+SWCNT+MWCNT with base fluid PEG−water. A collection of statistical and mathematical techniques for problem modelling and analysis is known as response surface methodology, or RSM. To guarantee that the response gets close to the desired maximum or minimum value, factorial variable settings are optimised using processes included in RSM. Residual R2 in Case-1 92.65%, Case-2 91.90% will represent the accuracy of the model.

Toward Understanding Polar Heat Transport Enhancement in Subglacial Oceans on Icy Moons

AbstractThe interior oceans of several icy moons are considered as affected by rotation. Observations suggest a larger heat transport around the poles than at the equator. Rotating Rayleigh‐Bénard convection (RRBC) in planar configuration can show an enhanced heat transport compared to the non‐rotating case within this “rotation‐affected” regime. We investigate the potential for such a (polar) heat transport enhancement in these subglacial oceans by direct numerical simulations of RRBC in spherical geometry for Ra = 106 and 0.7 ≤ Pr ≤ 4.38. We find an enhancement up to 28% in the “polar tangent cylinder,” which is globally compensated by a reduced heat transport at low latitudes. As a result, the polar heat transport can exceed the equatorial by up to 50%. The enhancement is mostly insensitive to different radial gravity profiles, but decreases for thinner shells. In general, polar heat transport and its enhancement in spherical RRBC follow the same principles as in planar RRBC.

Scale effect of micro ribs on the turbulent transport in an internal cooling channel

Owing to the limited supply and pressure margin in the air system, a cooling technique providing efficient heat transfer with lower flow loss is highly desirable for gas turbine blades. Microscale ribs have promised to be a potential cooling candidate. In this work, large eddy simulations are implemented to reveal the scale effect of micro ribs on the near-wall turbulent transport in a cooling channel. Considering a mechanistic study and practical applications, both single-rib and rib-array arrangements are studied with a wide range of dimensionless viscous-scaled rib heights involving the entire boundary layer. The results indicate that the rib-induced destruction and regeneration of coherent structures are, respectively, responsible for the weakened momentum transport and enhanced heat transport in the near-wall region. Using tiny ribs, regenerated quasi-streamwise vortices are mainly located in the buffer layer. The resulting turbulence burst greatly enhances wall heat transfer while keeping a lower flow loss due to the weak form drag. Regenerated hairpin vortices using tall ribs are activated in the log-law layer and intensively interact with mainstream. Along with improved wall heat transfer, the significant form drag results in a remarkably high flow loss. Accordingly, heat transfer and flow loss show different dependencies on the rib height, which contributes to an optimum height interval of ribs (e+ = 20–40) located in the high buffer and low log-law layer for maximizing the overall performance. Furthermore, for the rib-array scheme, adequate inter-rib spacing is essential to achieve turbulence regeneration for enhancing near-wall heat transport.

Investigating hybrid nanoparticles for drug delivery in multi-stenosed catheterized arteries under magnetic field effects

This groundbreaking study pioneers the exploration of the therapeutic implications of a constant magnetic field simultaneously with hybrid nanoparticles on blood flow within a tapered artery, characterized by multiple stenosis along its exterior walls and a central thrombus, employing three-dimensional bio-fluid simulations. In addition, a magnetized catheter is inserted into the thrombus to increase the therapeutic potential of this novel method. The flow condition under consideration has applications in targeted medication distribution, improved medical device design, and improved diagnostics, as well as in advancing healthcare and biomedical engineering. Our investigation primarily aims to optimize blood flow efficiency, encompassing key parameters like pressure, velocity, and heat fluctuations influenced by diverse geometric constraints within the stenotic artery. Precise solutions are obtained through the finite element method (FEM) coupled with advanced bio-fluid dynamics (BFD) software. Hybrid nanoparticles and magnetic fields impacted pressure and velocity, notably reducing pressure within the stenosis. Convective heat flux remained uniform, while temperature profiles showed consistent inlet rise and gradual decline with transient variations. This approach promotes fluid flow, and convection within stenosed arteries, enhances heat transport, evacuates heat from stenotic regions, and improves heat dispersion to surrounding tissues. These findings hold promise for targeted therapies, benefiting patients with vascular disorders, and advancing our understanding of complex bio-fluid dynamics.

Computational insights into shape effects and heat transport enhancement in MHD-free convection of polar ternary hybrid nanofluid around a radiant sphere

The control and management of energy and their associated issues are increasingly recognized as one of mankind’s greatest challenges in the coming years to keep pace with the surge in industrialization and technology. Free convection optimizes the heat transfer processes in energy systems like solar collectors and power plants, reducing energy consumption and increasing system effectiveness. Further, studying and analyzing critical factors like magnetic fields, thermal radiation, and the shape of nanoparticles can assist in the control of fluid motion and improve the efficiency of heat transfer processes in a wide range of real-world applications, such as the power sector, aerospace applications, molten metal, nuclear power, and aeronautical engineering. This study aims to scrutinize the thermal performance of a magneto tri-hybrid polar nanoliquid flowing over a radiative sphere, considering the nanosolids’ shape. The single-phase model is developed to acquire the problems governing equations, and the hybrid linearization spectral collection approach is utilized to approximate the solution. The present findings reveal that blade-shaped nanosolids exhibit the highest thermal conductivity ratio when incorporated into the base fluid, whereas spherical nanosolids exhibit the lowest ratio. Volume fraction and thermal radiation factors have an effective role in raising fluid velocity and thermal performance. The magnetic and microapolar factors significantly suppress fluid velocity and energy transfer. As the volume fraction factor increases, the average percentage improvement in convective heat transfer for Al2O3 + Cu + MWCNT/kerosene oil compared to Al2O3 + Cu + graphene/kerosene oil approximately ranges from 0.8 to 2.6%.

Cattaneo-Christov heat and mass fluxes model of Casson fluid employing non-Fourier double diffusion theories with ion slip and Hall effects

It is vitally important to comprehend non-Newtonian fluids flow behaviour from an industrial perspective. Non-Newtonian fluids are needed for a number of industrial and technical processes, including the production of paper, photographic films, and polymer sheet extrusion. There are several industrial applications for heat and mass transfer. However, thermal and solute relaxation time phenomena can't be predicted by classical heat and mass transfer laws (Fourier and Fick laws). This work aims to apply the modified Ohm law to thermal and mass transfer which are based on Fick's and generalized Fourier principles, respectively. The current study examines the three-dimensional Darcy-Forchheimer flow of non-Newtonian fluid “Casson fluid” via porous medium with ion slip and Hall effects across a stretched sheet. Thermophoresis and Brownian motion aspects are also considered. The diffusion phenomena are modelled using the Boungrino model. The modified Buongiorno model (MBM) for nanofluids is used to investigate the enhancement of heat transport. Together with practical nanofluid properties, it covers the mechanics of thermo-migration and random motion of nanoparticles. By adding the appropriate similarity transformations, this collection of (PDEs) that reflect the mathematical model is transformed into an (ODEs) system, which it is subsequently resolved utilizing the Lobatto IIIA technique's powerful computing capabilities via Matlab software. Data visualisations and numerical examples are presented considering many physical restrictions affect the fluctuation of velocities, temperatures, concentration, dimensionless shear stress, Nusselt number, and Sherwood number. According to certain findings, the existence of Hall and ion slip effects, and other factors contribute to an increase in horizontal velocity distribution. Furthermore, Fourier's law produces a wider temperature distribution than the Cattaneo-Christov heat flux model.

Enhanced heat transport in amorphous silicon via microstructure modulation

Amorphous materials are extensively utilized in electronic devices due to their exceptional optoelectronic properties. However, the disorder nature of their structure leads to a low thermal conductivity, which hampers effective thermal management of electronic devices. Understanding the correlation between microstructures and thermal transport in amorphous materials is crucial for modulating their thermal conductivity (TC). In this work, we performed molecular dynamics simulations in conjunction with an accurate machine learning potential to generate various amorphous Si (a-Si) structures through an atomic deposition procedure. The results demonstrate that increasing the substrate temperature during deposition procedure or conducting high-temperature secondary annealing significantly improves the short-range and mid-range ordering of a-Si. This improved structural ordering reduces atomic stresses and consequently boosts the TC of a-Si by more than 40 %. The spectral TC analysis reveals that elevated substrate temperature and annealing temperature primarily improve the TC of modes below 8 THz, corresponding to propagation and diffusion modes. Our work provides not only a strategy to tune the TC of amorphous materials via the deposition process, but also a deeper understanding of heat transport mechanisms in these materials.

A comparative study of twisted and straight fins in enhancing the melting and solidifying rates of PCM in horizontal double-tube heat exchangers

This study explores the melting and solidifying behaviour of phase change materials (PCM) in horizontal double-tube heat exchangers equipped with straight and twisted fins. Three-dimensional numerical simulations employing the enthalpy-porosity approach were conducted. RT35, which acts as the PCM, was enclosed in the inner tube, while water circulated as the heat transfer fluid (HTF) in the outer tube. The impacts of fin configuration (twisted versus straight), number of pitches, thickness, and length on the heat storage and recovery rates were analyzed during the melting and solidifying modes. The results demonstrated enhanced heat transport and phase change rates by incorporating fins, with twisted fins outperforming straight fins. Specifically, the charging and discharging rates improved by approximately 10% and 14% with twisted fins compared to straight fins, and by 15% and 40% relative to the finless case, respectively. The heat transfer rates showed direct correlations with the number of pitches, thickness, and length of the twisted fins. The twisted fins generated a swirling PCM flow, reducing thermal resistance. The study provides important guidelines on optimising twisted fin parameters like length, thickness, and orientation to improve the performance of thermal storage systems across renewable energy, waste heat recovery, and related applications.

Effect of air layer location and depth on redistribution of convective heat transfer induced by energy source in a sandwiched porous-air-porous enclosure free from temperature difference at external boundaries

The study deals with nonlinear convection in a three-layered porous-air-porous enclosure with zero temperature difference at the external impermeable thermally conductive boundaries. The upper and lower porous matrices are internally heated with a uniform energy source. To describe redistribution of heat energy between the top and bottom surfaces of the enclosure, we introduce a relative heat transfer coefficient qr that has the conduction and convection parts. The former part tends to unity in the fully filled porous domain. The symmetry of heat transfer from the inner area towards the outer surfaces breaks down if one adds an intermediate air layer with low thermal conductivity. Two sandwiched systems which have equal depth ratio d but distinct air layer location are considered. In system 1, the air layer is located in the upper unstably stratified half of the enclosure. In system 2, this layer is in the lower stably stratified half. The total value of qr always increases due to penetrative convection. At d = 0.1, local convection in system 1 is easily initiated in contrast to the hard-to-generate large-scale convection in system 2 due to a strong destabilization. The latter most effectively enhances heat transport though the top surface with increasing the supercriticality.

Entropy optimization in mixed convective boundary layer flow of CNTs Casson nanoliquid through a vertical cylinder moving with nonlinear velocity

This article inspects entropy generation and mixed convection boundary layer flow of carbon nanotubes (CNTs) Casson nanoliquid via semi-infinite vertical cylinder which moves with nonlinear velocity in Darcy-Forchheimer porous medium. Appropriate similarity variables have been employed to convert the original partial differential equations into ordinary differential equations that have been solved using overlapping grid spectral quasilinearisation method (SQLM). Comparison of accuracy, convergence and stability between the two methods is made. Rate of entropy generated, flow fields and engineering quantities are discussed for different embedding parameters. It is revealed that single-wall CNTs are more efficacious to improve heat transport features than multi-wall CNTs. The fluid flow and thermal dispersion processes improve with consideration of curved surface, CNTs, non-Newtonian fluid and injection of the fluid. The curvature surface and suction of the fluid contribute towards growth of skin friction factor and rate of thermal transference. Entropy generated expands by accounting for viscous nature of the fluid, strong nonlinearity, suction, non-Newtonian fluid, convective boundary condition and CNTs. The study finds applications in various processes, which are massively impacted by heat transport enhancement and high porosity. Boundary layer flow and heat transfer analysis through cylinders are relevant to various metallurgical and engineering solicitations.