Effective Thermal Diffusivity Research Articles

A novel contact thermal resistance model for heat transfer in granular systems: Leveraging the force-heat analogy

A novel particle-to-particle contact thermal resistance model is proposed in this study, based on the concept of analogy between force and heat transfer. The distribution of heat flux on the contact surface is assumed to resemble the distribution of stress, eliminating the issue of temperature singularity at the contact edge. The model is validated against existing theories and experiments, showing good agreement. The combined use of the model and the thermal discrete element method is applied to analyze temperature and effective thermal conductivity distributions in a pebble bed within a high-temperature test unit. The average effective thermal conductivity obtained from thermal conduction is found to be 2.99 and 2.61 W/(m·K) for power inputs of 20 kW and 82 kW, respectively. An increase in the outer wall temperature from 200 °C to 1000 °C results in an approximate 20 % increase in the effective thermal diffusivity.

An upscaling model for simulation of geothermal processes in stratified formations

In stratified porous media, non-uniform velocity between layers combined with thermal conduction across layers causes spreading of the thermal front: thermal Taylor dispersion. Conventional upscaling not accounting for this heterogeneity within simulation grid blocks underestimates thermal dispersion, leading to overestimation of thermal breakthrough time. We derive a model for effective longitudinal thermal diffusivity in the direction of flow, αeff, to represent the effective thermal dispersion in two-layer media. αeff, accounting for thermal Taylor dispersion, is much greater than the thermal diffusivity of the rock itself. We define a dimensionless number, NTC, a ratio of times for longitudinal convection to transverse conduction, as an indicator of transverse thermal equilibration of the system during cold- or hot-water injection. For NTC > 5, thermal dispersion in the two-layer system closely approximates a single layer with αeff. This suggests a two-layer medium satisfying NTC > 5 can be combined into a single layer with an effective longitudinal thermal diffusivity αeff. In application to a geothermal reservoir, one can apply the model to perform upscaling in stages, i.e. combining two layers satisfying the NTC criterion in each stage. The αeff model accounting for the fine-scale heterogeneity within simulation grid blocks would enhance the prediction accuracy of thermal breakthrough time and thus thermal lifetime.

Weakly nonlinear analysis of rotating magnetoconvection with anisotropic thermal diffusivity effect

The influence of anisotropic thermal diffusive coefficient on the stability of the horizontal fluid planar layer rotating about its vertical axis and permeated by the horizontal homogeneous magnetic field is studied. The linear stability analysis is carried out using the normal mode method. The stationary cross/oblique and parallel modes are calculated for different ranges of control parameters arising in the system. The SA (Stratification Anisotropy) parameter, α (the ratio of horizontal and vertical thermal diffusivities), plays a key role in deciding the boundaries between these modes and their instability regions when there is a combination of high and low rotation with weak and strong magnetic fields. The obtained isotropic results coincide with those obtained by pioneers in the literature. The weakly nonlinear behavior of the stationary convective motion in the vicinity of primary instability threshold is studied using the two-dimensional Landau–Ginzburg (LG) equation with cubic nonlinearity. This equation derived using the multiple scale analysis is similar to the one obtained in the literature having different relaxation time, nonlinear coefficient, and coherence lengths. These coefficients are used to study the heat transfer rate. In the case of high rotation, Nusselt number gets decreased from atmospheric (α < 1) to oceanic (α > 1) SA types. The domain for secondary instability of Eckhaus is obtained using the spatiotemporal LG equation and it is observed that the Eckhaus instability region decreases with increasing α.

Prandtl-eyring couple stressed flow within a porous region counting homogeneous and heterogeneous reactions across a stretched porous sheet

The movement of a non-Newtonian incompressible Prandtl-Eyring fluid layer over a stretching/shrinking permeable plate is shown in the current work. The flow is stressed by an additional couple stress, altered by a uniform normal magnetic field, and saturated in a permeable material. Heat transmission is demonstrated using a temperature-dependent heat source, an exponential space-dependent heat source, Joule heating, and viscous dissipations. Momentum, energy, and chemical type concentration equations, coupled with appropriate boundary conditions, support the mathematical structure. The novel aspect of the current work comes from considering species mass transfer using heterogeneous and homogeneous biochemical reactions along with the effects of thermal diffusion in the aforementioned flow. The reaction processes are isothermal with identical coefficients of chemical species. The leading nonlinear partial differential equations are transformed into ordinary differential equations utilizing appropriate similarity transformations. The homotopy perturbation method examines these equations. As a result, the primary objective distributions' analytical solutions are derived, and a graphical representation of them is produced to show the effects of the pertinent physical elements. The heterogeneity reaction serves as a dual role parameter in this diffusion, while the homogeneity reaction has been discovered to be a decreasing factor for particle diffusion. The ability of suction or injection via a permeable surface is observed to reduce the velocity and heat transmission, meanwhile, increasing chemical species condensation.

Consequences of Thermal Diffusion and Chemical Reaction on Mixed Convection MHD Casson Fluid through Porous Media with Inclined Plates

In this present article, we analyzed the effects of Thermal diffusion and chemical reaction on nonlinear mixed convection MHD flow of viscous, incompressible and electrically conducting fluid past an inclined porous channel under the influence of thermal radiation and chemical reaction. The transformed conservation equations are solved analytically subject to physically appropriate boundary conditions by using two term perturbation technique. The numerical values of fluid velocity, fluid temperature and species concentration are displayed graphically whereas those of skin friction coefficient, rate of heat transfer and rate of mass transfer at the plate are presented in tabular form for various values of pertinent flow parameters. It is observed that the velocity is decreased with increasing magnetic field parameter. The resultant velocity and concentration has enhances with increasing thermal diffusion parameters. The study is relevant to chemical materials processing applications.

Macroscopic transport characteristics in packed bed with a multi-physics inversion strategy for solid breeding blanket of HCCB TBM

The solid breeding blanket with typical packed bed structure is a key component for tritium breeding and thermal conversion in future fusion reactors. The pore scale model for multi-physical field solution of solid breeding blanket is fine but very expensive. The simplified packed-bed scale model is more favored in engineering especially for the system-level calculation of the blanket, but it needs to be inputted appropriate macroscopic transport parameters. In this work, the multi-physics calculations of blanket in Chinese HCCB TBM (Helium-Cooled Ceramic-Breeder Test Blanket Module) at pore scale and packed-bed scale were performed. It was found that the packed-bed scale model with traditional macroscopic transport parameters is not accurate enough, especially the prediction deviation of tritium concentration distribution could reach about 25 %. Hence, a novel multi-physics inversion strategy was proposed to predict the macroscopic transport parameters of the packed bed under the multi-field coupling conditions. The inversion results show that the convection of helium significantly enhances the heat and tritium diffusion in the packed bed, and the effective thermal conductivity and effective diffusivity could be increased by 18 % and 46 %, respectively. Then based on the results by multi-physics inversion strategy, the modified effective thermal conductivity and effective diffusivity correlation models for solid breeding blanket packed bed are constructed. With the modified correlation models, the relative prediction deviations of pressure drop, temperature distribution and concentration distribution are within 2.5 %, 0.6 % and 10 % respectively at different inlet velocities (0.05 m/s ∼ 0.25 m/s) and different inlet temperatures (300 K ∼ 900 K). The study in this paper would assist the thermal-hydraulics analysis of the solid breeding blanket in the fusion reactor.

Heat and Mass Transfer in Unsteady Radiating MHD Flow of a Maxwell Fluid with a Porous Vertically Stretching Sheet in the Presence of Activation Energy and Thermal Diffusion Effects

The aim of this study is to analyze the effects of activation energy and thermal diffusion an unsteady MHD reactive Maxwell fluid flow past a porous stretching sheet in the presence of Brownian motion, Thermophoresis and nonlinear thermal. The non-linear partial differential equations that govern the fluid flow have been transformed into a two-point boundary value problem using similarity variables and then solved numerically by fourth order Runge–Kutta method with shooting technique. Graphical results are discussed for non-dimensional velocity, temperature and concentration profiles while numerical values of the skin friction, Nusselt number and Sherwood number are presented in tabular form for various values of parameters controlling the flow system. The present study is compared with the previous literature and found to be in good agreement.

A Comparative Study of Newtonian and Non-Newtonian Nanofluids with Variable Thermal Conductivity Over a 3-D Stretching Surface

This study presents a comprehensive numerical and statistical analysis of the flow, heat/mass transfer management of Newtonian and non-Newtonian nanofluid over a bidirectional Darcy-Forchheimer stretching sheet. The external effects of MHD, Joule heating, thermal radiation, heat generation/absorption, Brownian motion, thermal diffusion and chemical reaction are taken into account. It is presumed that the thermal conductivity of fluid varies linearly with temperature. The non-linear coupled P.D.Es are converted into nonlinear coupled O.D.Es using similarity transformation. These equations are solved using MATLAB by implementing four-stage Lobatto IIIa formula and the outcomes of numerous flow parameters are presented graphically. In addition to numerical investigations, a comprehensive statistical analysis is performed using R-software to evaluate the sensitivity of key input parameters towards variable thermal conductivity. The values of local wall friction, local wall heat flux, and wall mass flux for various parameters are tabulated. The study reveals that the heat transmission is significant for dilatant fluids (156.8%) when compared to the pseudoplastic fluids (113.8%). Enriching the values of the Brownian motion parameter suppresses the molecular diffusion while a contrary nature is observed for the thermal diffusion parameter. Further, the mass transfer coefficient shows a very strong negative correlation with variable thermal conductivity parameter for Shear thinning fluids, whereas for Newtonian and Shear thickening fluids it shows a very strong positive correlation.

Effects of Diffusion Thermo and Thermal Diffusion on an Unsteady MHD Dissipative Squeezing Flow of Casson Fluid Over Horizontal Channel in the Presence of Radiation and Chemical Reaction

This study analyses the impacts of Diffusion thermo and thermal diffusion on the magneto hydrodynamic (MHD) squeezing Casson fluid flow through a porous medium under the slip condition with viscous dissipation the presence of chemical reaction and thermal radiation. The flow is produced when two plates are compressed together in close proximity to one another. Using similarity variables may successfully convert partial differential equations (PDEs) to ordinary differential equations (ODEs). The shooting technique was used to perform the numerical analysis, which entailed solving the competent governing equations with dominating parameters for a thin liquid layer. This was done to determine the results of the study. It is essential to evaluate the numerical results in light of previously conducted research to validate the current answers. According to the results, an increase in the distance between the two plates leads to a rise in the velocity and the wall shear stress. The velocity diminishes due to an increase in the Hartmann and Casson parameters, whereas the reverse behaviour has been observed in the case of temperature and concentration. The opposite behaviour has been observed in temperature with increasing Diffusion thermo and thermal diffusion parameters.

Analysis of the effect of surface coating and texturization on the estimation of effective thermophysical properties

This article aims to investigate the influence of surface modifications on the effective thermophysical properties of textured carbon steel and coated hard metal. The novelty of this work lies in the simultaneous estimation of thermal diffusivity and thermal conductivity via measurements using one active heating surface and Bayesian inference to solve the inverse heat conduction problem. To achieve this, an experimental setup was developed where samples were partially heated on an active surface, and the temperature was measured at two different points on the surface to estimate the thermal properties. The method was applied to two different thermal models using the same set of experimental data. Thus, the thermophysical properties were determined with and without the Markov Chain Monte Carlo technique using the Metropolis-Hastings sampling algorithm. Both techniques proved suitable for estimating the thermophysical properties. It is noteworthy that texturization and coating did not significantly modify the effective thermal diffusivity of the analyzed samples. However, there were considerable changes in the effective thermal conductivity, with an increase of approximately 13 % in the textured carbon steel sample and a reduction of approximately 11 % in the coated hard metal sample. The expanded uncertainty of the estimated effective thermophysical properties was less than 14 %. These results reflect low dispersion and good accuracy of the experimental technique used.

Variable thermal conductivity and mass diffusivity effects in a free convective flow of doubly stratified non-darcian porous medium over a vertical plate

The research in this article is carried out to study incompressible and unsteady free convective flow on a semi-infinite isothermal vertical plate in a doubly stratified non-Darcian porous media with variable mass diffusivity and variable thermal conductivity. The governing non-linear partial differential equations of flow were calculated by applying an implicit finite difference scheme of Crank-Nicolson type. Various parametric impacts on concentration profiles, temperature, velocity, as well Sherwood number, Nusselt number and skin friction, were examined and presented in graphs. It is examined that there exists a significant temperature decrease for high Darcy number in stratified fluids. Also, it is detected that the presence of stratification produces a considerable drop in skin friction while increases the mass and heat transfer rate. Comparison of current outcomes well agreed with the available solutions.

Thermal and mechanical properties of AlSi7Mg matrix syntactic foams reinforced by Al2O3 or SiC particles in matrix

Materials with complex inner structure can be challenging to characterise in an effective way. The effective material parameters significantly depend on the structure, and thus on the interface properties. In the present paper, we focus on metal matrix syntactic foams, and we attempt to determine their effective thermal parameters. We supplement the results with their mechanical properties as well. For thermal characterisation, we use the heat pulse experiment to study their transient thermal response. We observed deviation from Fourier's law in multiple situations, and thus we propose an evaluation procedure for such experimental data using the Guyer–Krumhansl heat equation. We provide an iterative procedure to efficiently find the effective thermal diffusivity values provided by the Guyer–Krumhansl and Fourier equations based on the measured transient response. Furthermore, we studied the effective parameters, the specific heat, mass density, thermal conductivity and thermal diffusivity. We found that the standard estimations significantly overestimate the thermal conductivity. Furthermore, we propose a method for deducing a more reliable thermal diffusivity and thermal conductivity based on the transient temperature history, i.e., we propose to estimate Fourier's thermal diffusivity based on averaging the parameters of the Guyer-Krumhansl equation according to the observed diffusive time scales. We also found that for such metal matrix syntactic foams, even with knowing the constituents, the contact thermal resistance makes more challenging to provide a prior estimate for the effective thermal conductivity, but this difficulty can be resolved using proper effective thermal diffusivity values.

Electron-phonon coupling dictates electron mean free paths and negative thermal diffusion in metals

The spatiotemporal dynamics of the fundamental energy carriers in metals underpin the efficiency of a wide array of technologies. Although much progress in our understanding of the temporal response of electronic relaxation processes following an external field perturbation has been achieved, the spatial relaxation dynamics of electrons after excitation remains unclear. Here, we employ a scanning ultrafast microscopy with high spatial resolution to directly measure the mean free paths of electrons in different metals. We show that the strength of electron-phonon coupling not only dictates the mean free paths, but also controls a peculiar spatial shrinking of the electronic temperature profile immediately after the electrons have thermalized with the ‘colder’ lattice. More specifically, from our experimental tracking of the spatial relaxation profiles, a clear observation of an effective negative thermal diffusion is apparent for metals with weaker electron-phonon coupling, while for metals with stronger electron-phonon coupling, the negative thermal diffusion effect is not observable. Our spatiotemporal modeling based on the two-temperature approach provides evidence that although negative diffusion occurs universally for materials with two different species that have varying diffusivities and are thermodynamically coupled, effective negative diffusion is masked for cases with stronger coupling between the species.

Investigation of Hall current and thermal diffusion effects on unsteady MHD mixed convective Jeffrey fluid flow over an inclined permeable surface with chemical reaction

Investigation of Hall current and thermal diffusion effects on unsteady MHD mixed convective Jeffrey fluid flow over an inclined permeable surface with chemical reaction

Effect of thermal diffusivity on Rayleigh waves in human tissue

This work examines the nature of human tissue under the context of dual phase lag thermoelastic diffusion theory using the principle of propagation of Rayleigh wave along the stress free, thermally insulated/isothermal, and impermeable/isoconcentrated conditions. The governing equations for the tissue are formed by considering the third-order approximation of Taylor’s expansion for the modified Fourier’s law of heat conduction and modified Fick’s law of mass diffusion in terms of phase lag parameters. The irrational secular equations of Rayleigh waves are derived, which are then rationalized to extract their complex roots. The phase speeds and the attenuation coefficients are evaluated from these complex roots. The variation of these phase speeds and attenuation coefficients are plotted graphically against angular frequency for two significant modes. The behavioral patterns of displacement components, temperature, and concentration fields are computed and are expressed graphically. The paths of the particle motion are represented graphically. Some results of published articles are recovered from the present formulation to validate our results.

Increasing contrast in water-embedded particles via time-gated mid-infrared photothermal microscopy.

The transient dynamics of photothermal signals provide interesting insights into material properties and heat diffusion. In a mid-infrared (mid-IR) photothermal microscope, the imaging contrast in a standard amplitude imaging can decrease due to thermal diffusion effects. It is shown that contrast varies for poly-methyl 2-methylpropenoate (PMMA) particles of different sizes when embedded in an absorbing medium of water (H2O) based on levels of heat exchange under the water absorption resonance. Using time-resolved boxcar (BC) detection, analysis of the transient thermal dynamics at the bead-water interface is presented, and the time decay parameters for 500 nm and 100 nm beads are determined. Enhanced (negative) imaging contrast is observed for less heat exchange between the water and bead, as in the case for the 100 nm bead. For the 500 nm bead, boxcar imaging before heat exchange starts occurring, leads to an increase of the imaging contrast up to a factor of 1.6.

Mechanisms of Diffusion thermo and Thermal diffusion on MHD Mixed Convection Flow of Casson fluid over a vertical cone with porous material in the presence of thermophoresis and a Brownian motion

In this present article, we analyzed the Effects of Diffusion thermo and Thermal Diffusion on magnetohydrodynamic (MHD) mixed convection flow for Casson nanofluid is deliberated a vertical cone with porous material. The modeled equations are transformed into a set of non-linear ODEs by employing similar transformable variables. These equations are then solved numerically using the shooting method, through the fourth-order Runge–Kutta integration procedure. Effects of some prominent physical parameters, such as diffusion thermo, Prandtl number, thermophoresis parameter, and magnetic parameter on the velocity, temperature, and concentration profiles are discussed graphically and numerically. Numerical calculations and graphs are used to illustrate the important features of the solution on fluid flow velocity, heat, and mass transfer characteristics under different quantities of parametric circumstances entering into the problem. Moreover, we computed the physical variables such as the coefficient of shear stress, rate of heat, and mass transfer. To establish the veracity of our present results, we compared them to previously published research and found substantial concordance.

Continuous strain tuning of oxygen evolution catalysts with anisotropic thermal expansion

Compressive strain, downshifting the d-band center of transition metal oxides, is an effective way to accelerate the sluggish kinetics of oxygen evolution reaction (OER) for water electrolysis. Here, we find that anisotropic thermal expansion can produce compressive strains of the IrO6 octahedron in Sr2IrO4 catalyst, thus downshifting its d-band center. Different from the previous strategies to create constant strains in the crystals, the thermal-triggered compressive strains can be real-timely tuned by varying temperature. As a result of the thermal strain accelerating OER kinetics, the Sr2IrO4 exhibits the nonlinear lnjo - T−1 (jo, exchange current density; T, absolute temperature) Arrhenius relationship, resulting from the thermally induced low-barrier electron transfer in the presence of thermal compressive strains. Our results verify that the thermal field can be utilized to manipulate the electronic states of Sr2IrO4 via thermal compressive strains downshifting the d-band center, significantly accelerating the OER kinetics, beyond the traditional thermal diffusion effects.

Numerical modeling and statistical analysis of the effect of thermomechanical properties on residual stresses in feldspar‐based ceramic materials after thermal tempering

AbstractThis study focuses on the investigation of the impact of the thermomechanical properties of ceramic materials on residual stresses generated after the thermal tempering process. A sodium feldspar matrix was chosen for numerical simulations, with variations in its thermomechanical properties of up to ± 20%, to encompass a range of properties exhibited by other ceramic materials. The simulations demonstrated that the viscosity of the material had the most significant influence on residual stress generation, while the coefficient of thermal expansion, modulus of elasticity, and effective diffusivity also contributed to residual stress formation. A statistical analysis based on a 24 factorial design revealed that increasing residual compressive stress on the surface was achievable by increasing the coefficient of thermal expansion and modulus of elasticity, as well as the viscosity, while reducing the effective thermal diffusivity. Finally, a mathematical model was proposed to relate the thermomechanical properties of the ceramic material to the residual stresses observed on the surface following thermal tempering.

Impact of thermal diffusion and diffusion-thermal effects on MHD Maxwell fluid subject to Newtonian heat and mass conditions over a stretching cylinder

Current research's main focus is to incorporate the novel idea of Newtonian flux condition in MHD stretching flow of Maxwell liquid with Stefan blowing. An analysis is done to explore the heat and mass transfer characteristics of MHD Maxwell fluid towards a stretching cylinder, including thermal diffusion and diffusion-thermal energy implications in the presence of activation energy. Polar coordinates (z, r) in the shape of a cylinder are used to model the issue. To convert controlling PDEs into ODEs, similarity the variables are used. The governing equations of motion that result are tackled via the notorious R-K 4th order approach. Through graphical portrayal, the impacts of various factors, including curvature, magnetics, Stefan blowing, reaction, and activation energy parameters, are studied. Furthermore, the validation of numerical findings is prepared by benchmarking with previously determined limiting conditions, and we launch a wonderful communication with these results.