Variable Thermal Conductivity Research Articles

Three-level homogeneous model for the study of heat transfer mechanism in metallic nanofluids

To explain heat conduction mechanism in nanofluids, different existing models have been analysed, but none of these can predict exact variation of thermal conductivity with volume concentration. In present study, it is found that the static and dynamic contributions of nanoparticles in the base fluid are responsible for its conductivity. Unfortunately, these both aspects have not been included in any of the existing model. We have modified the Xuan model including effect of shape and size of nanoparticles (static contribution) and allocation of nanoparticles, temperature, junction layer, congregate pattern, and Brownian diffusion (dynamic contribution) to propose a new model which can predict the exact mechanism of thermal conductivity of nfs. We have tested the proposed model to compute the thermal conductivity of Al2O3, ZnO, Ag, CuO, Co3O4 and Fe2O3 based nanofluids at different volume concentrations. Computed results using our proposed model are in very good agreement with experimental data.

The microstructure and physical properties of AlN and SiC ceramics after irradiation with 2 MeV Au ions

The long-term irradiation environment of fusion reactors often induced serious damage to ceramics as the tritium resistant and functional materials. In this work, silicon carbide (SiC) and aluminum nitride (AlN) ceramics were irradiated with the flux of 1 × 1016 ions·cm−2 by 2 MeV Au ions at 773 K. The results of GIXRD indicated a higher volumetric swelling of AlN (0.45%) than that of SiC (0.13%) after ion irradiation.Furthermore, the physical property study investigated the slight variations of mechanical properties (increase ratio of 83.4%) and thermal conductivity (decrease ratio of 7.75%) for irradiated SiC, which were both lower than the data of irradiated AlN (237.4% and 11.97% for increase and decrease ratios respectively). This study provides a useful reference for the selection of tritium resistant and functional materials in nuclear fusion reactors.

Using the decomposition method to solve the fractional order temperature distribution equation: A new approach

Due to its importance in science, finding both exact and approximate solutions to fractional partial differential equations with boundary conditions is important for the research community. The natural decomposition method (NDM), which is based on the natural transformation method (NTM) and the Adomian decomposition method, is modified in this study to produce exact and approximate solutions for boundary value problems (BVPs) of partial differential equations (PDEs) with fractional coefficients. In addition, we present an exact solution to the temperature distribution in a slab constructed of materials with variable thermal conductivity's combined convection–radiation lumped system. We present these findings as numerical tables and graphs that show the convergence and stability rates. The study demonstrates that this approach is effective since it is simple to apply and produces reliable findings. We are the first to use this approach for such applications, as far as we are aware. Additionally, this method is applicable to a sizable class of BVPs for ordinary differential equations (ODEs) and PDEs.

Molecular Weight Segregation and Thermal Conductivity of Polydisperse Wax-Graphene Nanocomposites.

Paraffin waxes are a promising material for heat storage with high energy density. Their low thermal conductivity, which limits the speed of charging and discharging in heat buffers, was previously shown to be improved by adding graphene nanofillers. In the present study, using molecular dynamics simulations, the segregation by molecular weight of polydisperse paraffin near graphene flakes is investigated. In liquid bidisperse paraffin composed of decane and triacontane, an aligned layer containing mainly triacontane was observed next to the graphene. Upon slow cooling, the wax crystallised into distinct layers parallel to the graphene sheet, with much stronger segregation by molecular weight than in the crystallised bidisperse wax without graphene. For polydisperse wax, the segregation effect was much less pronounced. The molten paraffin had a somewhat higher concentration of the longest chains in the first layers next to the graphene, but during crystallisation, the molecular weight segregation was only slightly increased. Measurements of crystallinity using an alternative version of the method developed by Yamamoto showed that the layers of wax were highly aligned parallel to the graphene, both in the solid state with all wax crystallised and in the liquid state with one layer of aligned wax above and below the graphene. Thermal conductivity was increased in planes parallel to the graphene flakes. The strong segregation of chain lengths in the bidisperse wax resulted in clear differences in thermal conductivity in the segregated regions. The less segregated polydisperse wax showed less variation in thermal conductivity.

Thermoelastic interactions in a rotational medium having variable thermal conductivity and diffusivity with gravitational effect

ABSTRACT This research work is related with the investigation of thermodynamical interactions for a rotational thermodiffusive medium including gravitational effect. To differentiate it from existing works, thermal conductivity and mass diffusivity are considered as variables. The governing equations are formulated under Green–Lindsay (G-L) theory by using Kirchhoff's transformation. Thermal and chemical shocks of constant intensity are imposed on the boundary surface. By using normal mode technique, solutions for the variables considered such as displacement, stress, temperature and chemical potential are achieved. Numerical results are computed for copper material. The impacts of variable thermal conductivity, variable diffusivity, rotation and gravity on different physical fields in the medium are depicted through various figures.

Bioconvective radiative unsteady Casson nanofluid flow across two concentric stretching cylinders with variable viscosity and variable thermal conductivity

The role of variable viscosity and variable thermal conductivity in nanofluid flows is significant, as they can strongly influence the flow behavior and heat transfer performance of nanofluids. Keeping in mind the importance of these effects of variable characteristics, our motivation in this study is to investigate the role of nonlinear radiative heat flux on unsteady Casson nanofluid flow across two concentric stretched cylinders in the presence of temperature-dependent viscosity and thermal conductivity. A convective condition on the inner cylinder wall is also considered. In addition, the nanofluid is immersed with microorganisms that can swim and move independently. The addition of microorganisms has a significant role in the stability of the nanofluid flow. The influence of thermophoresis and Brownian motion on the flow, heat, mass, and microorganisms are scrutinized by employing the Buongiorno model. The governing flow equations are converted into a system of differential equations engaging similarity transformations, and the bvp4c method is employed to solve it numerically. Engineering parameters are computed and tabulated numerically. The findings demonstrate that as the unsteadiness parameter upsurges, the radial profile intensifies while the axial profile diminishes near the outer cylinder surface. It is also comprehended that both Lewis and bioconvective Lewis numbers reduce the motile density profiles. The accuracy of the presented model is also given in a graphical illustration by comparing it with a published result in a limiting case.

Effect of an excited non-local microelongated semiconductor with variable thermal conductivity on the propagation of photo-thermoelastic waves

Effect of an excited non-local microelongated semiconductor with variable thermal conductivity on the propagation of photo-thermoelastic waves

Probe into the adaptability of local equilibrium/non-equilibrium assumptions in simulating freeze-drying of porous media

A multiphase transport model was formulated to explore the adaptability of local thermal and mass equilibrium/non-equilibrium (LTE/LTNE and LME/LMNE) assumptions in simulating freeze-drying of porous media. Saturated and unsaturated samples with the same initial mass and moisture content but different initial saturations were used in freeze-drying experiments and numerical simulations. Four pairwise combinations between LTE/LTNE and LME/LMNE were tested using the commercial software of COMSOL Multiphysics based on the finite element method. Results showed that the LTE-LMNE assumption achieved excellent agreements between numerical and experimental drying curves with RMSEs being 1.81% for the saturated sample and all less than 1.00% for the unsaturated samples. Based on distributions of saturation and temperature, and variations of effective mass diffusivity and thermal conductivity, the rate-controlling factor was determined to be mass transfer for the saturated material and heat transfer for the unsaturated one. Convective heat transfer played an unimportant role in freeze-drying. The LTE-LMNE based model provided satisfactory predictive capabilities under different conditions. Appropriately increasing the operating temperature could boost the drying rate. Changing the chamber pressure had insignificant effects with drying time differences only below 3%. The smaller the initial saturation, the shorter the drying time. The initially unsaturated frozen material with preformed pores significantly enhanced the freeze-drying process and improved the process economy.

Tracing the Soil Water Flux in Variably Saturated Zones Using Temperature Data

AbstractThe coexistence of water, vapor, and solid phases and the spatiotemporal changes in thermal conductivities in response to variations in water content pose a challenge to the heat‐tracing method used to calculate water flux in variably saturated zones. In this study, a new approach, which adopted finite difference numerical schemes along with the limited‐memory Broyden–Fletcher–Goldfarb–Shanno bounded (L‐BFGS‐B) parameter optimization algorithm, was developed to inversely trace vertical soil water fluxes in variably saturated zones using temperature‐depth time‐series data. The newly developed method did not utilize hydraulic parameters, and only temperature time‐series data were used at the adjacent depths as boundary conditions. The soil water, vapor flux, and spatiotemporal variations in thermal conductivity at a given location were optimized by fitting the temperature time‐series at the target depth using the L‐BFGS‐B algorithm. Further, the method was verified through laboratory experiments under different saturated conditions that were conducted in two columns with saturated and free drainage bottom boundaries, and five sets of different flow rates with a relatively stable flow condition. The soil water flux calculated using the proposed method was consistent with the observed data, thus, verifying the reliability of the approach in applications with relatively stable flow rates. Furthermore, the new method was applied to a field site to estimate the time‐varying soil water flux using the measured temperature data. The trends of the calculated soil water flux at different depths were consistent with the field hydrologic conditions and observations. Thus, this study provides a novel computationally efficient and easy‐to‐implement tool to determine soil water flux using temperature measurements.

Investigation of moving trapezoidal and exponential fins with multiple nonlinearities

In this article, we study moving longitudinal and exponential fins with variable thermal conductivity, thermal emissivity, and heat transfer coefficient. The problem is solved numerically using shooting technique. To compare the performance of trapezoidal fins with exponential fins, we introduce a novel idea of volume adjusted fin heat transfer rate, fin effectiveness, and fin efficiency. The result shows that heat transfer from the exponential fin is higher than from the trapezoidal fin. It is observed that the capability of an exponential fin is higher than the trapezoidal fin. With increasing thermal conductivity and radiation-conduction number as well as convection-conduction number, the fin’s efficiency increases. The results illustrate the effect of heat transfer coefficient, thermal emissivity, radiation-conduction and convection-conduction parameters, dimensionless radiation, and convection sink temperatures on the fin’s performance.

Application of cooling processes for the impact of variable thermal conductivity on the MHD flow of kerosene-based nanofluid

This paper studies the recent flow phenomena based upon the kerosene-based nanofluid through vertical parallel channel for the consideration of the variable thermal conductivity. The free convection of electrically conducting MHD fluid due to the interaction of inertial drag along with thermal radiation, heat source and the dissipative heat enriches the study as well. The structural behavior of the physical quantities is useful in recent applications like the cooling processes in refrigerators, semi-cryogenic rocket engine, etc. because of the practical application of the kerosene-based nanofluid. Also, the models associated with the thermophysical properties such as viscosity and thermal conductivity for the choice of volume concentration are favorable for the thermal enhancement. The designed model for the transport phenomena is transformed to its non-dimensional form for the suitable choice of similarity variables and the set of equations are handled by traditional numerical technique. Further, the involvement of the physical parameters and their physical significance is described briefly for the appropriate values within the specified range. Finally, the important outcomes of the study are that nanoparticle concentration has vital role in decelerating the fluid velocity and the fact is due to the heavier density and the inclusion of porous matrix is useful to retard the wall thickness at both the ends. Moreover, the increasing concentration favors in enhancing shear rate at the first wall whereas near the second wall, the impact is opposite.

Photo Thermal Diffusion of Excited Nonlocal Semiconductor Circular Plate Medium with Variable Thermal Conductivity

To examine the effects of the nonlocal thermoelastic parameters in a nanoscale semiconductor material, a novel nonlocal model with variable thermal conductivity is provided in this study. The photothermal diffusion (PTD) processes in a chemical action are utilized in the framework of the governing equations. When elastic, thermal, and plasma waves interact, the nonlocal continuum theory is used to create this model. For the main formulations to get the analytical solutions of the thermal stress, displacement, carrier density, and temperature during the nanoscale thermo-photo-electric medium, the Laplace transformation approach in one dimension (1D) of a thin circular plate is utilized. To create the physical fields, mechanical forces and thermal loads are applied to the semiconductor’s free surface. To acquire the full solutions of the research areas in the time-space domains, the inverse of the Laplace transform is applied with several numerical approximation techniques. Under the impact of nonlocal factors, the principal physical fields are visually depicted and theoretically explained.

Vieta-Lucas Collocation Technique for Examination of the Flow of Casson Fluid over a Slippery Stretching Sheet Which Is Impacted by Thermal Slip, Ohmic Dissipation, and Variable Thermal Conductivity

This work aimed to present the influence of the magnetic field and Ohmic dissipation on the non-Newtonian Casson fluid on a vertical stretched sheet to numerically solve the problem. Here, the variable thermal conductivity is taken as a linear function of temperature. Electric fields, thermal slip, and viscous dissipation effects are taken into consideration. A collection of physical conditions on the sheet’s enclosing wall and the momentum and heat transport processes are expressed as partial differential equations (PDEs). Some of the similarity transformations are used to convert the collection of PDE into a system of ordinary differential equations. This system is numerically treated by implementing the Vieta-Lucas spectral collocation method. Some observations are made for the investigation of method convergence. The effect of some different parameters on the velocity and temperature profiles is graphically represented. Additionally, this area of study has significant practical applications in a variety of industries, including paper production, thermal power generation, nuclear reactors, cooling of metallic sheets, glass fiber, and lubrication.

Anomalous and non-monotonic strain dependent thermal conductivity of typical negative thermal expansion material ScF3

It is well known that the lattice thermal conductivity of the vast majority of bulk materials increases under compressive strain and decreases under tensile strain. In this paper, taking the typical negative thermal expansion material ScF3 as an example, we systematically investigate the effect of strain on the phonon transport properties by means of first-principles calculation combined with the Boltzmann transport equation. The results show that the thermal conductivity of ScF3 exhibits an anomalous strain dependence, where the thermal conductivity decreases under compressive strain while increasing as the tensile strain is smaller than −4.2 GPa. When the tensile strain increases further (larger than −4.2 GPa), however, the thermal conductivity of ScF3 decreases with tensile strain, presenting a non-monotonic behavior. Through analyzing the phonon mode information, we demonstrate that such anomalous and non-monotonic strain dependent thermal transport in ScF3 mainly originated from the intrinsic character of the negative Grüneisen parameter (negative thermal expansion). Moreover, the root mean square displacement is also utilized to further demonstrate the variation of phonon anharmonicity and thermal conductivity of ScF3 with strain. The findings shed light on the underlying association between thermal expansion and thermal conductivity and provide a feasible strategy for searching for materials with anomalous strain dependent thermal conductivity.

Influence of thermophoretic deposition and viscous dissipation on magnetohydrodynamic flow with variable viscosity and thermal conductivity

AbstractIn this study, we numerically explore the impact of varying viscosity and thermal conductivity on a magnetohydrodynamic flow problem over a moving nonisothermal vertical plate with thermophoretic effect and viscous dissipation. The boundary conditions and flow‐regulating equations are converted into ordinary differential equations with the aid of similarity substitution. The MATLAB bvp4c solver is used to evaluate the numerical solution of the problem and it is validated by executing the numerical solution with previously published studies. The impacts of several factors, including the magnetic parameter, Eckert number, heat source parameter, thermal conductivity parameter, stratification parameter, Soret, Dufour, Prandtl number, and Schmidt number are calculated and shown graphically. Also, the skin friction coefficient, Nusselt number, and Sherwood number are calculated. Fluid velocity, temperature, and concentration significantly drop as the thermophoretic parameter and thermal stratification parameter increases. As thermal conductivity rises, it is seen that the velocity of the fluid and temperature inside the boundary layer rise as well. Also, the Soret effect drops temperature and concentration profile. The applications of this type of problem are found in the processes of nuclear reactors, corrosion of heat exchangers, lubrication theory, and so forth.

The new correlation for viscosity of synthesized viscoelastic-based nanoliquid using functionalized MWCNT: Stability, thermal conductivity, and rheology

In different industries, non-Newtonian fluids, especially viscoelastic fluids, are processed with variable transport properties, such as viscosity. Thus, it is of great importance for design and engineering purposes to determine the relationship between transfer properties such as viscosity and thermal conductivity coefficients. Even though there have been more studies on nanofluids, they are all based on three components of viscoelastic fluids that are inadequate. Thus, in this research, to begin with, multiwalled carbon nanotubes (MWCNT) have been chemically functionalized with COOH groups in order to obtain f-MWCNT nanoparticles. A series of tests were conducted to characterize the synthesized f-MWCNT nanoparticles, including X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FT-IR), dynamic light scattering (DLS), and transmission electron microscopy (TEM). Further, three components, viscoelastic-based nanoliquids (VNLs) have been prepared using (i) polyacrylamide, glycerol, and water as the base liquid, and (ii) carboxylic-functionalized multiwalled carbon nanotubes (f-MWCNTs) as the nanoparticles. In addition, a study of the stability of the VNL (i + ii) solution has been conducted by examining the variations in thermal conductivity (TC) as well as viscosity of the VNL at various time intervals. Results showed that 0.2 wt% of f-MWCNT in VNL was stable for at least 40 days. f-MWCNT in VNL (0.2 wt%) had a size distribution less than 50 nm. Furthermore, the investigation of the effect of f-MWCNT concentrations in VNL on TC at different temperatures showed that the thermal conductivity variation of VNL as a function of f-MWCNT mass fraction was nearly constant (1.25 W/m.K). A novel correlation was also proposed for the shear viscosity of the VNL, which was well supported by the experimental data. Lastly, we investigated the viscoelastic properties of the VNL and determined that the storage and loss modulus, as well as the first normal stress difference, increased with an increasing mass concentration of f-MWCNT.

Comparative analysis of unsteady flow of induced MHD radiative Sutterby fluid flow at nonlinear stretching cylinder/sheet: Variable thermal conductivity

Unsteady incompressible Sutterby fluid model is considered. The comparative results of stretching cylinder and sheet have been studied. The induced magnetic field is considered in the presence of thermal slip. Darcy resistance and viscos dissipation impacts have been studied. The thermal conductivity of liquid has considered as variable under the thermal radiation. The governing model of presence study have been developed under boundary layer approximation in term of partial differential equations. The differential equation become dimensionless by means of transformations. The dimensionless system have been solved through numerical procedure. Involving factor of physical influence presented in the form of tabular as well as graphical. Fluid temperature boosted up due to larger values of variable thermal conductivity. Physically, the thermal conductivity of liquid increased as well as variable thermal conductivity liquid enhanced as the temperature of fluid improved. The momentum layer thickness of stretching cylinder achieved mover than stretching sheet. The fluid velocity curves achieved higher thickness for boosting values of Darcy resistant factor. The velocity fluid curves revealed declining by increasing the Sponginess factor. Fluid temperature boosted up due to larger values of Eckert number.

Interaction of variable diffusion coefficients with electrokinetically regulated peristalsis of Carreau-Yasuda nanofluid

In this analysis, the interaction of variable diffusion coefficients with electrokinetically modulated peristaltic motion of a radiative Carreau-Yasuda (CY) nanofluid is communicated. Here, ethylene glycol is used as the base fluid, and graphene nanoplatelets are considered nanoparticles. Unlike many existing studies, the thermophoretic diffusion term appearing in the equation of temperature and concentration is considered a variable term instead of manipulating their average values to obtain a more precise estimation of the conduct of the nanofluid. The Carreau-Yasuda (CY) model is considered to represent the non-Newtonian features of nanofluids. A mathematical model is developed under the effects of Joule heating, variable thermal conductivity, mixed convection, thermal radiation, electric field, Brownian motion, viscous dissipation, magnetic field, and thermophoresis assumptions. The entropy equation involving a variable diffusion coefficient and thermal conductivity is constructed using the second law of thermodynamics. The acquired dimensionless equations are numerically resolved with the help of built-in function. The graphical results indicate that due to Joule heating characteristics, the temperature profile increases. Manipulation of radiation and thermal conductivity parameters can improve the thermal efficiency and productivity of the system. An electroosmotic parameter causes an augmentation in the Bejan number. The nanofluid flow decreases at a higher electroosmotic velocity. The nanofluid chosen makes it possible to envisage industrial applications, for example in the field of heat extraction, without compromising mechanical performance.

Investigation of Mhd Non-Newtonian Nanofluid Stagnation-Point Flow with Variable Transport Properties and Multislip Effects: An Application to Solar Radiation

Heat and mass transfer performance of Casson nanofluid for both non-conducting (m=0), electrically conducting (m≠0) fluids with solar radiation effects in stagnation point flow is considered. In this model, entropy, irreversibility, and multi slip impacts over a shrinking, static, and stretching sheet are investigated. To minimize the energy used in the solar system, it is important to monitor the processes of heat and mass transfer in the solar radiation process. The slips boundary conditions acts as a closure of the fluid velocity, mass, and heat transfer differential equations. The equations obtained are solved numerically via Galerkin Weighted Residual Method (GWRM). In the limiting sense, the present results conform with the existing work. The Behaviors of the flow physical quantities, temperature, concentration, and velocity for distinct values of the applicable dimensionless numbers are demonstrated with tables and graphs. The results reveal that, for a theoretical account of thermal boundary layers, Prandtl number serves as a variable. Furthermore, higher values of variable thermal conductivity have a significant influence on the skin friction coefficient than the case of constant variable thermal conductivity even when the fluid viscosity is assumed to be variable. The structure of the new method can be applied to the development of oil production.

Entropy Generation in Peristaltic Transport of Hybrid Nanofluids with Thermal Conductivity Variations and Electromagnetic Effects.

Entropy generation in peristaltic transport of hybrid nanofluid possessing temperature-dependent thermal conductivity through a two-dimensional vertical channel is studied in this paper. The hybrid nanofluid consists of multi-walled carbon nanotubes mixed with zinc oxide suspended in engine oil. Flow is affected by a uniform external magnetic field, hence generating Lorentz force, Hall and heating effects. Given the vertical orientation of the channel, the analysis accounts for mixed convection. To study heat transfer in the current flow configuration, the model considers phenomena such as viscous dissipation, heat generation or absorption, and thermal radiation. The mathematical modeling process employs the lubrication approach and Galilean transformation for enhanced accuracy. The slip condition for the velocity and convective conditions for the temperature are considered at the boundaries. The study analyzes entropy generation using the Homotopy Analysis Method (HAM) and includes convergence curves for HAM solutions. Results are presented using graphs and bar charts. The analysis shows that higher Brinkman and thermal radiation parameters result in higher temperatures, while higher thermal conductivity parameters lead to reduced entropy generation and temperature profile. Additionally, higher Hall parameter values decrease entropy generation, while an increased Hartman number improves entropy generation.