Variable Thermal Conductivity Research Articles

Non‐Monotonic Variation of the Low Lattice Thermal Conductivity with Temperature in Penta‐HgO2 Sheet

AbstractInspired by the experimental synthesis of bulk HgO2 with the potential of exfoliation to form a penta‐HgO2 sheet composed entirely of pentagonal motifs, a detailed theoretical study on the lattice thermal conductivity by using first‐principles calculations combined with the unified theory of thermal transport is performed. It is found that the penta‐HgO2 sheet is semiconducting with an indirect bandgap of 1.18 eV and possesses a low lattice thermal conductivity of 2.07 W m−1 K−1 (3.28 W m−1 K−1) along the x (y)‐direction at 300 K. More interestingly, the variation of its thermal conductivity with temperature is non‐monotonic, different from most cases. The phonon dispersion, phonon scattering, and phonon coherence is further systematically investigated to understand the underlying physics. This results suggest that the strong intrinsic anharmonicity resulting from its unique atomic configuration with the buckled structure and the heavy element of Hg leads to a high scattering rate, resulting in the ultralow particle‐like thermal transport of 0.20 W m−1 K−1 (0.01 W m−1 K−1) in the x (y)‐direction, while the narrow average frequency interval and strong phonon linewidth are responsible for the dominant coherent thermal transport and non‐monotonic variation of the low lattice thermal conductivity of the penta‐HgO2 sheet.

A Study on the Combined Effects of Variable Viscosity and Thermal Conductivity on Forced Convection in Bidisperse Porous Medium

This research focuses on exploring the flow and temperature distribution of forced convection in a channel of horizontally placed parallel plate under the existence of a bidisperse porous medium (BDPM) emphasizing the consequences of viscous dissipation. The temperature-dependent thermal conductivity and viscosity are considered while maintaining the momentum slip and constant wall temperature at the channel walls. Analysis of the dynamics of fluid flow and the temperature distributions in both the fluid and solid phases is conducted by following the two-velocity and two-temperature models in this work. The model governing equations are non-dimensionalized following the relevant dimensionless parameters and the study calculates the temperature and velocity profiles for each phase using the Homotopy Analysis Method (HAM). The obtained temperature and velocity are discussed and explained with the help of graphs. Certain fluids exhibit varying behaviors at different temperatures. Therefore, it is crucial to consider the temperature-dependent physical properties of the fluid for specific applications. This highlights the importance of implementing temperature-dependent physical properties in the analysis. Throughout the investigation, it is discovered that the viscosity parameter has a greater influence on velocity as compared to temperature, the same for thermal conductivity on temperature. The impact of physical parameters including skin friction, volume flow rate, and Nusselt number on both plates are also examined in this study. The findings are discussed, and tables displaying numerical values for various physical characteristics are provided for both the liquid and solid phases. The results obtained were also compared with existing literature, revealing a strong alignment between them.

Thermal properties and microstructure of Al–Sn alloys

The Al–Sn alloys feature excellent wear and corrosion resistance, along with good mechanical properties. They are mostly used as bearing materials. However, the application of these alloys as phase change materials (PCMs) for thermal energy storage (TES) has recently been suggested. To assess the adequacy of these alloys in a given area, a detailed evaluation of their thermal properties is required. In the present study, Al–Sn alloys with 11.7, 22.4, 32.8, 41.1, and 53.4 at.% Sn were produced by melting of pure metals. The melt was continuously stirred to avoid segregation, after which casting was carried out in a stainless steel mold. The obtained ingots had a homogeneous microstructure without the appearance of cracks and pores. The thermal diffusivities of the solid Al–Sn alloys in the temperature interval 25–150 °C were measured using the light flash method. The densities at room temperature were measured by the Archimedes method, and the specific heat capacities at different temperatures were calculated using the calculation of phase diagrams (CALPHAD) method. Then the thermal conductivities for studied Al–Sn alloys were obtained by the specific conversion equation. The phase transition temperatures and related heat effects were studied using differential scanning calorimetry (DSC). Variations of thermal conductivity with composition and temperature were determined as well as variation of latent heat of fusion with alloy composition. Moreover, the microstructures and phase compositions of the alloys were examined using scanning electron microscopy (SEM) combined with energy dispersive spectrometry (EDS). The present research results provide important information on the thermal properties and microstructure of the Al–Sn alloys for designing new PCM for thermal energy storage.

An investigation of the heat and mass transfer effects in vertical channels with immersible fluid flow through a porous matrix

AbstractIn this article, we investigated the heat and mass transfer of immiscible fluid flow in symmetric two‐vertical regions with a porous matrix in region‐II. The properties of heat generation and absorption are discussed. The channel, consisting of two vertical regions, is filled with a porous matrix with viscous fluids. In both cases, we considered the fluids that undergo Newtonian heat generation and absorption. It was assumed that the porous matrix's channel wall was thermally stable and electrically non‐conducting. The analytical method is used to solve governing equations. Variations in Brinkman Number (Br), Biot Number (Bi), Grashof (Gr) number, viscosity ratio (m), heat generation, and thermal conductivity are illustrated graphically. And we examined the increase in Grashof number, which has effects on fluid flows in both regions. Similarly, the change of heat and momentum transmission in porous media is greatly affected by the viscosity, channel width, source, and sink. Also, when the Grashof number increases, buoyancy increases, leading to an increase in the fluid flow for heat generation and absorption.

A numerical study on MHD Casson fluid flow in a non-uniform rough channel with temperature-dependent properties using OHAM

This study explores the complex dynamics of magnetohydrodynamic (MHD) Casson fluid in a non-uniform rough channel, focusing on the effects of temperature-dependent viscosity and variable thermal conductivity under no-slip boundary conditions. The study employs an innovative approach by utilising a rough surface with irregular textures to analyse flow patterns and assess drag forces on channel objects. A novel mathematical model, governed by continuity, momentum, and heat transfer equations, is developed and transformed into dimensionless, nonlinear Ordinary Differential Equations (ODEs) using non-dimensional quantities and fundamental assumptions. The Optimal Homotopy Analysis Method (OHAM) is applied to solve these equations to enhance convergence speed and accuracy. The research explores the impact of surface roughness on velocity profiles and temperature distributions under various physical constraints. Numerical simulations are conducted to determine skin friction coefficients and Nusselt numbers. Furthermore, the study examines the influence of confined boluses on fluid flow in diverse physiological conditions. A comprehensive analysis is performed to elucidate the combined effects of surface roughness on fluid passage, including flow separation, pattern alterations, pressure distribution and drop, heat transfer characteristics, and flow resistance. The intricate interplay between temperature-dependent viscosity, varying thermal conductivity, and surface roughness is thoroughly investigated to explain the complex dynamics of MHD Casson fluid movement in non-uniform channels. Implementing a magnetic field over the rough, non-uniform channel is found to provide stability and prevent fluid overflow. This research has significant real-world applications, including soil erosion prevention, blood flow regulation in arteries, and optimisation of hydropower channels and penstocks. By enhancing our understanding of flow dynamics through rough and non-uniform channels, this study contributes valuable insights into both theoretical fluid mechanics and practical engineering applications.

Effect of chemical absorption on Jeffery Fluid Flow in saturated Porous Media with variable thermal conductivity

This study investigates the impact of species absorption of Jeffery fluid flow through a saturated permeable medium with variable thermal conductivity. The dimensional partial nonlinear derivative model controlling the chemical reacting fluid flow is transformed to invariant form. The resulting flow equations are computed numerically using an approximated finite implicit Crack-Nicolson. A chemical absorption of fluid reactant occurs in a boundless vertical device. Computations are performed for different parameters to examine their sensitivity under isothermal temperature conditions. The computed results are offered graphically for qualitative and quantitative insights into the flow behaviour. The obtained outcomes revealed that the fluid flow rate rises with an increasing values of the parameters and, and it damps with upsurge in the values of the parameters, and. The heat transfer field is reduced with enhancing values of the thermal source, viscous dissipation and time, but declines for a boosted value of the terms suction and Prandtl number. Also, the mass transfer field is higher when mass absorption and time are increased, and diminish with an elevate in the values of Schmidt number and chemical reactions.

Analysis of axisymmetric thermoelastic diffusive half-space with variable material properties using hyperbolic two-temperature model

The aim of the current study is to investigate the variable thermal conductivity and diffusivity impact on the transient response of an axisymmetric thermodiffusive elastic half-space under axisymmetric thermal, mechanical, and concentration loads in the light of hyperbolic two-temperature generalized fractional thermoelastic diffusion model. With the help of Kirchhoff’s transform, the governing equations are converted into linear form. The resulting equations are solved by imposing the combined Laplace–Hankel transform. In converted space, the solutions for different field quantities are presented in compact form. The copper material is considered for numerical computation. By employing a mathematical inversion procedure, the solutions are converted into the original space. Numerically computed solutions for various physical quantities are illustrated in the form of graphs to show the impact of variable thermal conductivity and diffusivity, hyperbolic two temperature, and fractional parameters. Validation of the obtained results is also presented.

Contribution of hall and ion slip effects with generalized mass and heat fluxes with entropy analysis on three-dimensional Prandtl model

A major challenging proffered study rely on the inquisitive optimization of entropy together with thermal as well as mass transportation towards the chemically reactive 3-D Prandtl liquid under consideration of applied magnetic field, variable thermal conductivity and also diffusivity, hall current together with ion slip features, viscous dissipation and heat generation. Moreover, in present research study, model such as Cattaneo Christov heat flux was implemented in order to explore the thermal relaxation characteristics. By employing correspondence transformations, the system of modeled equations modified into non-linear ODEs system. The Optimal Homotopy Analysis methodology (OHAM) was adopted to solve the proffered problem. The influential arising constraints demeanor within both velocities (horizontal as well as vertical), temperature and concentration profiles were discussed and also shown graphically. Moreover, outcomes of diverse parameters were also examined and presented graphically within the entropy formation rate and also Bejan number. The effects of implanted factors across the drag force, the rate of thermal as well as mass transportation are accessible in current study via tables and validate the obtained results. Both velocity profiles (horizontal as well as vertical) decreased for increment in Hartman number whilst opposite demeanor seen for other considered constraints. Temperature profile drops for Prandtl number and elastic constraints whereas enhanced for other influential considered parameters whilst concentration profile augmented with Prandtl number. Present problem novelty relies on the modeling of comprehensive system of equations which modified into nonlinear ODEs. In order to obtain the solution numerically, technique such as Optimal Homotopy was opted.

Influence of gravitational on a rotating nonlocal thermoelastic medium with thermal variable conductivity

The present study discussed the effect of rotation and gravitational on a nonlocal two-temperature thermoelastic solid with variable thermal conductivity. The problem is solved using the dual-phase-lag model, coupled theory (CD), and Lord-Shulman (L-S) theory with memory-dependent derivatives. Using suitable non-dimensional variables and normal mode analysis is considered to solve the problem. MATLAB software is used for numerical calculations. Comparisons are made with the results of three different theories predicted in the absence and presence of the gravity field and rotation. Comparisons are made with the results of three different theories predicted for different values of a nonlocal parameter. It is found that the physical quantities are affected by the rotation, gravity field, and the nonlocal parameter.

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.

Physical characteristics of variable thermal conductivity and MHD flow across a continually stretched sheet

The primary purpose of this paper is to investigate the flow and thermal properties of a continuously stretching sheet. To determine temperature, variable thermal conductivity is also encountered. Furthermore, the flow problem considers the convective circumstances of heat and mass transfer. By lowering the number of independent components, the governing equations are reduced into non-dimensional types, which are then numerically solved using the RKF-4 method and shooting methodology. A visualization evaluation of the entangled flow properties is done for velocity concentration and temperature distributions. It is found that the fluid concentration is discovered to be more significant in the [Formula: see text] case, followed by the [Formula: see text] and [Formula: see text] cases. Furthermore, the efficiency of the [Formula: see text] the [Formula: see text] parameter’s shifting values control field.

Significance of variable thermal conductivity and suction/injection in unsteady MHD mixed convection flow of Casson Williamson nanofluid through heat and mass transport with gyrotactic microorganisms

AbstractThe present paper explores a two‐dimensional mixed bio‐convective unsteady viscous hydro‐magnetic Casson Williamson nanofluid flow model with heat and mass transport incorporating motile microorganisms towards a stretchy spinning disc. The flow concept is accomplished by rotating a stretched disc with a time‐varying angular velocity. By applying a magnetic field normal to the axial direction, a magnetic interaction is taken into consideration. The Casson Williamson nanofluid contains nanosized particles suspended with swimming motile microorganisms and the rotation of the disc is exhibited by buoyancy forces, thermophoresis, suction/injection, zero mass flux conditions, variable thermal conductivity, Joule heating and so forth. The obtained flow narrating differential equations of the model are transformed into ordinary differential system. This is accomplished by simulating boundary value problems using the shooting technique using the ‘ND‐Solve’ approach included in the Mathematica software (Mathematica 12). The implications of the engaged parameters such as Williamson fluid parameter, Casson fluid (CF) parameter, thermophoresis parameter, Brownian motion parameter and so forth, on both axial and radial velocities, temperature, concentration of nanoparticles and microorganisms are explained by means of graphical and tabular constructions. This paper's validity has been confirmed and its findings align with those of other previously published papers. Furthermore, it is found that both the axial and radial velocity profiles are seen to be diminishing functions of the CF parameter. The identified observation may have theoretical implications for a number of engineering procedures, solar energy systems, biofuel cells and extrusion system improvement. Moreover, this work finds application in micro‐fabrication techniques and the chemical industry.

Numerical Analysis of MHD Casson Fluid Flow over a Stretching Surface with Chemical Reaction and Variable Thermal Conductivity

The motivation of the current article is to explore the Numerical analysis of MHD Casson fluid flow over a Stretching surface. Three dimensional, non-linear of an incompressible fluid flow in the presence of chemical reaction and variable thermal conductivity is considered. The mathematical model effectively describes the current flow analysis. The Governing equations are simplified with the help of boundary layer approximations. Non-linear coupled equations for momentum, energy and mass transfer are tackled with MATLAB bvp4c. The behavior of various non-dimensional parameters which are involved in the derived set of equations are described and explained in detailed through graph.

Effect of variable thermal conductivity in semiconducting medium underlying an elastic half-space

Effect of variable thermal conductivity in semiconducting medium underlying an elastic half-space

Numerical investigation of slip effects on heat and mass transfer in a vertical channel with immiscible micropolar and viscous fluids of variable viscosity

AbstractThis study investigates the fluid flow, heat, and mass transfer phenomena within a vertical channel containing two immiscible fluids, with a particular focus on slip effects. These effects include no slip, velocity slip, thermal slip, and multiple slips, each analyzed with appropriate boundary conditions. The study thoroughly examines key characteristics, such as variations in thermal conductivity and viscosity. Using a sixth‐order Runge–Kutta numerical method implemented using Mathematica, the study achieves precise solutions for complex scenarios. The detailed results show how the various slip mechanisms and relevant parameters interact with each other in complex ways. These findings are useful for both theoretical understanding and application in real‐life engineering situations. This study also gives important information about how fluid flow, heat transfer, and mass transfer change under different slip effects. It looks at these effects and shows how they change visually. It also carefully calculates and analyzes engineering parameters like the Nusselt number, shear stress, and Sherwood number using bar charts, showing how they affect and behave. The study resulted in velocity slip having a minimal impact on temperature, whereas thermal slip resulted in higher temperatures. Both velocity and thermal slip conditions simultaneously result in the lowest temperatures.

Intelligent computing for the electro-osmotically modulated peristaltic pumping of blood-based nanofluid

The study delves into a novel application of artificial neural networks within the biomedical realm, specifically focusing on the peristaltic pumping of ionized blood-infused nanofluid using the Casson model. The investigation takes into account the impacts of electrokinetic forces, chemical reaction, thermal radiation and variable thermal conductivity. The endorsed nanofluid model encompasses both thermophoretic and Brownian diffusions. The Nernst-Planck and Poisson equations are used to characterize electroosmotic process. The mathematical model is then numerically solved using NDSolve in Mathematica. The variation in relevant hemodynamic characteristics concerning key flow parameters is explored through numerous graphical representations. Two distinct artificial neural networks have been developed to estimate values for rates of mass and heat transfer. The Levenberg-Marquardt algorithm is used as the training algorithm in network models with multi-layer perceptron architecture. Graphical inspection of the results demonstrates a decrease in blood flow with an elevated Helmholtz-Smoluchowski velocity. A lower pressure gradient is recorded with a higher electroosmotic parameter. Furthermore, an augmentation in the thermal conductivity parameter leads to a reduction in heat transfer rate at the wall. The concentration profile demonstrates an increase with a rise in the chemical reaction parameter. The results obtained from the neural network models exhibit an ideal harmony with the target values. The developed neural network models demonstrated the ability to predict rates of heat and mass transfer values with average deviations of −0.01% and 0.004%, respectively.

Measurement and theoretical analysis of effective thermal conductivity of lanthanum pentanickel powder beds for hydrogen storage in different particle sizes and bed porosities

Particle diameter and bed porosity change due to the expansion and contraction of metal hydride particles during hydrogen absorption and desorption, which leads to variations in the effective thermal conductivity of metal hydride beds. Therefore, the effective thermal conductivity of the metal hydride beds under different bed porosities and particle diameters must be measured for the design of metal hydride-based hydrogen storage tanks. In this study, the effective thermal conductivities of four kinds of lanthanum pentanickel (LaNi5) powder beds with different bed porosities and particle diameters were measured in various gas atmospheres with gas pressure varying from 0.1 to 4.0 MPa. The volume mean diameters of the four kinds of LaNi5 powders equal 335.2, 196.4, 147.7, and 23.7 μm, respectively. Subsequently, the effects of bed porosity and particle diameter on the effective thermal conductivities were analyzed through the modified Zehner–Schlünder–Damköhler model considering the Smoluchowski effect. Experimental results show that particle diameter and bed porosity influence markedly the effective thermal conductivity of lanthanum pentanickel (LaNi5) powder beds. However, the findings of theoretical analysis based on the modified Zehner–Schlünder–Damköhler model reveal the considerably weaker effect of particle diameter on the effective thermal conductivity under identical bed porosity compared with the measurement results. Such a finding was observed because the variation in particle diameter inevitably changed bed porosity during the effective thermal conductivity measurement, which also affected the effective thermal conductivity of the beds. In addition, decreasing bed porosity and increasing particle diameter can remarkably improve the heat transport via the gas–solid mixture pathway, which improved the effective thermal conductivity of the beds and thus explained the experimental results.

Influence of activation energy and variable thermal conductivity on magneto-convective nonlinear thermo-radiative Casson nanofluid containing motile microorganisms over a sinusoidal surface

The activation energy of convective-radiative nanoliquids with sinusoidal walls has garnered significant academic interest in recent years. This focus arises due to the pivotal applications of nanoparticles in thermal engineering, industrial processes, and medical sciences. This article investigates the magnetohydrodynamic heat transfer characteristics of Casson nanofluids containing microorganisms influenced by an oscillatory surface. The study incorporates temperature-dependent viscosity and utilizes Buongiorno’s mathematical model to account for nonlinear thermal radiation, Brownian motion, and thermophoretic effects from using nanomaterials in Casson fluids. These effects enhance thermal conductivity, improving the heat transportation phenomenon while considering gyrotactic microorganism density distributions. The boundary layer flow problem is formulated using partial differential equations and associated boundary conditions, subsequently numerically simulated using the bvp4c function. The investigation explores the thermal behavior of nanoparticles in the presence of bioconvection phenomena, Lorentz force, chemical reactions, and activation energy. As a result, this model addresses variable thermal conductivity and bioconvection aspects in Casson nanofluids over a sinusoidal surface. Numerically computed results for the Nusselt number, motile density, and Sherwood number provide insights into several essential features. The findings reveal that an increase in the Casson fluid parameter correlates with a decreased velocity profile. The assumptions of variable thermal conductivity and temperature-dependent viscosity prove more effective in raising the temperature of nanoparticles. The nanoparticle concentration profile rises noticeably for smaller Schmidt number Sc values when the activation energy parameter is higher than larger Sc values. Additionally, the presence of microorganisms in Casson nanofluids leads to increased temperatures due to an elevation in the bioconvection Rayleigh number. Moreover, heightened oscillating frequency parameters decrease nanoparticle concentration and microorganism density. Including activation energy and oscillating frequency parameters further enhances the understanding of nanoparticle concentration and microorganism density dynamics, offering valuable contributions to theoretical research and practical applications across multiple interdisciplinary fields.

Chemically reactive non-Newtonian fluid flow through a vertical microchannel with activation energy impacts: A numerical investigation

This work examines the second law analysis of an electrically conducting reactive third-grade fluid flow embedded with the porous medium in a microchannel with the influence of variable thermal conductivity, activation energy, viscous dissipation, joule heating, and radiative heat flux. A suitable non-dimensional variable is included into the governing equations to transform them into an ensemble of equations that are devoid of dimensions. The acquired equations are then tackled using the Runge Kutta Felhberg 4th and 5th order (RKF-45) approach in conjunction with the shooting methodology. Through comparison with the current results, the numerical results are verified, which provides a good agreement. From the present outcomes, it is established that the entropy generation is supreme for the viscous heating constraint, variable thermal conductivity, Frank Kameneski, heat source ratio parameter and third-grade fluid material constraint. The Bejan number boosts up with larger values of activation energy, and Frank Kameneski constraint and the decreasing nature is noticed for increasing third-grade material parameter, viscous heating parameter. With magnetism, the fluid’s velocity slows down because of a resistive force. A similar impact in the channel on velocity is noticed for larger third-grade fluid, activation energy parameter, and Frank-Kameniski parameters and increasing behavior is noticed for variable thermal conductivity, and permeability parameter. Further, it is cleared that the variable thermal conductivity assumption in the channel plate leads to a significant under prediction of the irreversibility rate.

Thermal transport and topological analyses of the heat-carrying modes and their relevant local structures in variously dense amorphous alumina

Engineering the thermal conductivities of amorphous materials is important for thermal management of various semiconducting devices. However, controlling the heat carriers—long-range propagating propagons and short-range hopping diffusons—in disordered lattices is difficult because the carriers are strongly correlated with lattice disorder. To clarify the relationship between lattice disorder and heat conduction, we must simultaneously investigate the important local structures hidden in a disordered system and the microscopic transport characteristics of propagons and diffusons. Here, we explore the variations in spectral thermal conductivity and the relevant local structures in amorphous alumina (a-Al2O3) at different densities by performing the spectral thermal transport and persistent homology analyses. As the density increases, the thermal conductivity of the high-frequency diffusons linearly increases but those of the propagons and low-frequency diffusons remain constant. The density increase enhances the local strain, thereby increasing the mean free paths of the high-frequency diffusons. The density of states competes with diffusivity, lowering the sensitivity of the density response to the thermal conductivity of low-frequency heat carriers. Furthermore, from the obtained topological features of the connections between the oxygen atoms, we inferred that the collapsed network of six-coordinated AlO6 octahedron clusters underlies the transport of high-frequency diffusons. Besides revealing the conductive pathways of heat-carrying modes in disordered lattices, topology-assisted spectral thermal transport analysis is useful for tailoring the thermal conductivities of amorphous materials.