Variation Of Thermal Diffusivity Research Articles

Анализ структуры многомасштабного течения в горизонтальном слое жидкости с неоднородными свойствами

The paper provides the study of the flow structure in a horizontal fluid layer with variable thermal diffusivity or with internal heat generation. The media properties are set up to produce a thin boundary sublayer convectively much more unstable than the bulk of the layer.The obtained multiscale flow was studied by computational homology methods and methods based on the theory of dynamical systems. The flows were investigated for the Prandtl number in the range [0.5,10]. The effect of medium properties on the structure of the fluid motion was studied.

Concentration dependent thermo-optic properties of yellow emissive ZnO quantum dots

This paper study the thermal properties of ZnO quantum dots (QDs) synthesized by sol-gel method. The structural and morphological properties were investigated using x-ray diffraction technique (XRD) and transmission electron microscopy (TEM). Thermal properties of ZnO QDs were measured using dual beam mode matched thermal lens technique. The results reveal that the thermal diffusivity of the nanofluid depends on the concentration of ZnO QDs. There is an optimum concentration of ZnO QDs, above which thermal diffusivity increases more than that of the base fluid whereas thermal diffusivity decreases below the optimum concentration. This variation in thermal diffusivity of ZnO QDs nanofluid is explained using absorption and emission spectra.

Wavelet investigation of Soret and Dufour effects on stagnation point fluid flow in two dimensions with variable thermal conductivity and diffusivity

The current analysis is devoted to observing the influence of Soret and Dufour effects on stagnation point fluid flow in two dimensions. The buoyancy effects, variable thermal conductivity and diffusivity are also considered in our study. The governing equations of flow are reduced to a nonlinear ordinary differential equation system via a suitable transformation technique. The numerical simulation is performed by means of a new algorithm based on the Chebyshev wavelet method (MCWM). A detailed assessment of outcomes obtained by MCWM with fourth-order Runge–Kutta technique is available to support our wavelet-based solution. The significant properties and effects of various physical parameters are presented graphically. It is noted that an increase in the values of magnetic parameters cause a decrease in the velocity profile for the assisting case while the behavior of velocity is the opposite for the opposing case. The effects of Soret and Dufour enhanced the temperature and concentration, respectively. The comparison and convergence analysis indicate that the proposed algorithm is an efficient tool and could be extended for other complex nature models in engineering and physics.

Carbon nanoparticles assisted energy transport mechanism in leaves: A thermal lens study

In the world of increasing population and pollution due to carbon emissions, the research for effective utilization of futile diesel soot for fruitful applications has become a necessity for a sustainable development. The contribution to pollution from vehicles and industries due to the aging of engines has caused a crisis. Carbon nanoparticles (CNPs) have been the subject of interest because of their good physical, chemical, and biological properties. The present work investigates the role of CNPs produced by internal combustion engines on the energy transport mechanism among leaf pigments using the sensitive and nondestructive single beam thermal lens technique. The studies reveal the absorption changes by various chlorophyll pigments with the concentration of CNPs sprayed on the leaves. Though for low concentrations CNPs lower the photon absorbance by chlorophyll pigments, the effect gets reversed at higher concentrations. The variation of thermal diffusivity with CNP concentration and its role in the energy transport mechanism among chlorophyll pigments are also studied. It is found that CNP concentrations of 625-2500mg/l are good for better intra-pigment energy transport leading to increased rate of photosynthesis and plant yield and thereby helping in attaining food security. The variation of CNP assisted energy transport among leaf pigments on the production of nicotinamide adenine dinucleotide phosphate (NADPH) and carbohydrates is also studied with ultraviolet (UV) and near-infrared (NIR) spectroscopy.

Analytical study of transient thermo-mechanical responses in a fractional order generalized thermoelastic diffusion spherical shell with variable thermal conductivity and diffusivity

ABSTRACT With the great development of micro-electromechanical devices and wide application of subpicosecond/femtosecond ultrafast laser technology in micro-machining of micro-electromechanical devices, the investigations of interactions between thermodiffusion and deformation become significantly important. To accurate modeling the thermo-elasto-diffusive coupling, a new time fractional-order (i.e. memory-dependent derivative) based model of generalized thermoelastic diffusion for a perfect thermally and isotropic conducting medium which is assumed to have variable thermal conductivity and diffusivity is given in this work. This model is applied to investigate transient responses of a spherical shell subjected to instantaneous thermal and chemical shock loadings at external surface, whilst it is assumed that a permeating substance is in contact with the rigid surface at its internal surface. The solutions in the Laplace transform domain are obtained by using Kirchhoff and Laplace transform techniques and then the numerical inversion process based on fast Fourier transformation is carried out to derive the transient solutions in the time domain. Numerical computations for all the physical quantities considered are implemented and represented graphically. The results show that the variable thermal conductivity and diffusivity as well as memory-dependent parameters have the varying degrees of influences on the structural transient thermo-elasto-diffusive responses.

Thermal diffusivity and secondary crystallisation kinetics in poly(lactic acid)

The non-steady state technique of laser flash analysis (LFA) was used to record the variation of thermal diffusivity with time during the cold-crystallisation of poly(lactic acid), a grade that contains 4% D stereoisomer. The measured diffusivity data was analysed in terms of the Avrami and Hoffman-Lauritzen models for polymer crystallization and this yielded kinetic data that was comparable to that which originated from DSC. Within the temperature range 88 to 109 °C, mechanistic ‘n’ values of 2.0 ± 0.1 were calculated and a nucleation constant of 6.58 × 10−5 K2 was determined. Measurements of thermal diffusivity were also recorded over a longer time-scale to determine whether thermal diffusivity was a useful indicator of secondary crystallization and this data was analysed using a modified Avrami equation that includes a root-time dependence of the secondary process. Secondary crystallisation rate constants (ks) increased with crystallisation temperature and yielded an activation energy for the secondary crystallisation process of 40 kJ/mol.

Evaluating thermal properties of sugarcane bagasse-based composites by using active infrared thermography and terahertz imaging

This study is devoted to the analysis of impact damage in composite samples by applying techniques of IR and Terahertz imaging. The samples made of high-density homogeneous particleboards of sugarcane bagasse and castor oil polyurethane resin were subjected to impacts with the energy from 5 to 30 J. The impact damage defects were evaluated by applying one- and two-sided thermal non-destructive testing. Both the apparent thermal effusivity and diffusivity were determined to analyse their relationship to impact energy. In the above-mentioned range of impact energies, the thermal effusivity varied from 5 to 18%, while the variations in thermal diffusivity were from 4 to 24%. The algorithm of dynamic thermal tomography was used to demonstrate that predominant subsurface cracking occurred at depths up to 1 mm. In addition, the defect areas were tested by applying the Terahertz technique, whose results qualitatively matched the IR thermographic ones to show that the damaged areas were larger than they appeared visually. However, both inspection techniques have revealed no significant dependence of analysed parameters on impact energy. This is believed to prove that non-uniform composite structure is the decisive factor in producing structural defects under impacting.

Nonhomogeneous porosity and thermal diffusivity effects on stability and instability of double-diffusive convection in a porous medium layer: Brinkman Model

AbstractA model for double-diffusive convection in anisotropic and inhomogeneous porous media has been analysed. In particular, the effect of variable permeability and thermal diffusivity has been studied using the Brinkman model. Moreover, we analyse the effect of slip boundary conditions on the stability of the model. Due to numerous applications in micro-electro-mechanical-systems (MEMS) and other microfluidic devices, such a study is essential to have. Both linear instability analysis and nonlinear stability analysis are employed. We accurately analyse when stability and instability will commence and determine the critical Rayleigh number as a function of the slip coefficient.

Transient thermal characterization of suspended monolayer MoS2

We measure the thermal time constants of suspended single-layer molybdenum disulfide drums by their thermomechanical response to a high-frequency modulated laser. From this measurement, the thermal diffusivity of single-layer ${\mathrm{MoS}}_{2}$ is found to be $1.14\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{2}/\mathrm{s}$ on average. Using a model for the thermal time constants and a model assuming continuum heat transport, we extract thermal conductivities at room temperature between 10 to $40\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. Significant device-to-device variation in the thermal diffusivity is observed. Based on a statistical analysis we conclude that these variations in thermal diffusivity are caused by microscopic defects that have a large impact on phonon scattering but do not affect the resonance frequency and damping of the membrane's lowest eigenmode. By combining the experimental thermal diffusivity with literature values of the thermal conductivity, a method is presented to determine the specific heat of suspended 2D materials, which is estimated to be $255\ifmmode\pm\else\textpm\fi{}104\phantom{\rule{4pt}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for single-layer ${\mathrm{MoS}}_{2}$.

Influence of the Precipitation of Secondary Phase on the Thermal Diffusivity Change of Al-Mg2Si Alloys

Al-Si-Mg alloys are investigated to determine the relationship between changes in the thermal diffusivity and precipitation behavior of the Mg2Si phase with various contents of Mg2Si and aging treatment conditions. The samples were solid solution-treated and then quenched with water (80 °C). Aging treatments were implemented at temperatures ranging from 180 to 240 °C for 5 h. The precipitation behavior of Mg2Si was observed using a heat flow curve using differential scanning calorimetry analysis. The thermal diffusivity of Al-Mg2Si alloy was affected by the precipitation of the Mg2Si phase, particularly in the meta-stable β phase. In the temperature range of precipitation occurrence, the thermal diffusivity of the alloy increased with the temperature when the precipitation of the meta-stable β phase of the sample was incomplete. However, at the same temperature, the samples in which precipitation had completed did not have any increased thermal diffusivity. The thermal diffusivity of the samples decreased when the meta-stable Mg2Si phase had dissolved in the matrix. The precipitation and dissolution of Mg2Si mainly affected the variation of thermal diffusivity in Al-Si-Mg. In contrast, the stable Mg2Si phase was not affected by changes in thermal diffusivity at a high temperature.

Pharmacological application of the thermal lens technique: a thermal diffusivity study

The photothermal phenomenon has emerged as a potential tool for the nondestructive evaluation of thermal and optical properties of materials. Thermal analysis of drugs is an unavoidable part of preformulation study, as temperature variations can induce structural changes of the constituents of drugs. Techniques based on photothermal phenomena are highly sensitive, as only the absorbed radiation contributes to the signal. Periodic illumination and subsequent nonradiative de-excitation generate thermal lens signals of various types within and around the sample. Variation of thermal diffusivity with a concentration of the commonly used drug terbutaline is studied through the single-beam thermal lens technique. The ultraviolet–visible spectrum of the drug shows strong absorption around 500 nm, which suggests the possible wavelengths that can be used for the study. It is found that concentration of the drug in liquid form decides its thermal stability, as its thermal diffusivity varies with concentration. The study gives information about the optimum value for the concentration of the drug noted above for which the chance of thermal stability is high.

Spatial decay bounds in a channel flow of 2‐D Boussinesq fluid with variable thermal diffusivity

In this paper, the authors investigate the flow of an incompressible Boussinesq fluid in a semi‐infinite plane channel. An explicit spatial decay estimate in terms of the distance from finite end of the channel is obtained from an integro‐differential inequality for an energy integral. To make the result explicit, an upper bound for total energy is also obtained. The results of this paper may be thought of as a version of Saint‐Venant principle.

Variations in thermo-optical properties of neutral red dye with laser ablated gold nanoparticles

We have investigated the thermal and optical properties of neutral red dye incorporated with different weight percentage of gold nanoparticles prepared by laser ablation method. Optical absorption studies confirmed the production of spherical nanoparticles and also the interactions of the dye molecules with gold nanoparticles. The quenching of fluorescence and the reduction in the lifetime of gold incorporated samples were observed and was due to the non-radiative energy transfer between the dye molecules and gold nanoparticles. Dual beam thermal lens technique has been employed to measure the heat diffusion in neutral red with various weight percentage of gold nano sol dispersed in ethanol. The significant outcome of the experiment is that, the overall heat diffusion is slower in the presence of gold nano sol compared to that of dye alone sample. Brownian motion is suggested to be the main mechanism of heat transfer under the present conditions. The thermal diffusivity variations of samples with respect to different excitation power of laser were also studied.

Chemically reactive species in squeezed flow through modified Fourier’s and Fick’s laws

The squeezing flow of a Newtonian fluid with variable viscosity over a stretchable sheet embedded in Darcy porous medium is addressed. Cattaneo-Christov double diffusion models are adopted to disclose the salient features of heat and mass transport via variable thermal conductivity and variable mass diffusivity instead of conventional Fourier’s and Fick’s laws. Further, the concept of heat generation/absorption coefficient and first-order chemical reaction are also imposed to illustrate the characteristics of heat and mass transfer. Highly nonlinear computations are developed in dimensionless form and analyzed via the homotopic technique. The variation of flow parameters on velocity, concentration, and temperature distributions are sketched and disclosed physically. The results found that both concentration and temperature distributions decay for higher solutal and thermal relaxation parameters, respectively. Moreover, a higher chemical reaction parameter results in the reduction of the concentration field whereas the temperature profile enhances for a higher heat generation/absorption parameter.

Thermophysical characterization of polymers according to the temperature using a periodic method

In this paper, we developed a method for measuring thermophysical properties of materials as a function of the temperature using a periodic thermal excitation. This method was tested on three polymers (PVDF, HDPE, PA6). The comparison of specific heat capacity values obtained by this method with those of the literature and those obtained by differential scanning calorimetry is used to validate this measurement method for this kind of material. Moreover, the obtained variation of thermal conductivity and thermal diffusivity of Polyamide 6 versus temperature was compared to measurements performed using a commercial Hot-Disk™ device.

Effects of variable thermal diffusivity on the structure of convection

The structure of multiscale convection in a thermally stratified plane horizontal fluid layer is investigated by means of numerical simulations. The thermal diffusivity is assumed to produce a thin boundary sublayer convectively much more unstable than the bulk of the layer. The simulated flow is a superposition of cellular structures with three different characteristic scales. In contrast to the largest convection cells, the smaller ones are localised in the upper portion of the layer. The smallest cells are advected by the larger-scale convective flows. The simulated flow pattern qualitatively resembles that observed on the Sun.

Concentration dependent variation of thermal diffusivity in highly fluorescent carbon dots using dual beam thermal lens technique

Thermal and optical properties of fluorescent carbon dots (CDs) prepared from citric acid anhydrous as the precursor and ethylenediamine as the passivation agent by microwave assisted synthesis technique is explored in the present study. Optical absorption and emission spectra of carbon dots reveal the presence of surface states with an absorption peak around 350 nm and emission around 450 nm. Fourier Transform Infra-Red (FT-IR) spectra also indicate the presence of surface states. Transmission electron microscopy images of these nano particles indicate an average particle size of 5.8 nm. Dual Beam thermal lens (TL) spectrometry, a highly sensitive photothermal technique, is employed to measure the thermal diffusivity of the carbon dots. Thermal diffusivity is found to decrease from 0.135 m2/sec to 0.0365 m2/sec as the weight percentage of carbon dots are increased from 0.0005 g/15 ml to 0.005/15 ml which can be attributed to the enhanced scattering of thermal waves with particle density in the medium. Studies reveal carbon dots to be promising candidates as thermal insulating materials.

Thermo physical properties of Jute, Pineapple leaf and Glass fiber reinforced polyester hybrid composites

The objective of this study is to examine the variation in thermal conductivity, specific heat capacity and thermal diffusivity of composites with respect to fiber content and temperature. The fiber content in the composites was varied from 0.18 to 0.42 by volume. The results revealed that the thermal conductivity of composite decreased with increase in fiber content and the quite opposite trend was observed with respect to temperature. The specific heat capacity of the composite as measured by differential scanning calorimeter showed similar trend as that of the thermal conductivity. It was observed that the thermal diffusivity of composite was influenced by the fiber content, and the variation of thermal diffusivity with temperature was not so significant.

VISUALIZATION ANALYSIS OF NATURAL CONVECTION IN A POROUS ENCLOSURE USING THE CHEBYSHEV SPECTRAL COLLOCATION METHOD WITH TEMPERATURE-DEPENDENT THERMAL DIFFUSIVITY

We studied the effects of temperature-dependent thermal diffusivity and aspect ratio on natural convection in a rectangular porous cavity. Thermal diffusivity is assumed to vary as a linear function of temperature.We used the Chebyshev spectral collocation method to solve coupled nonlinear partial differential equations. The domain of the problem is discretized by Chebyshev–Gauss–Lobatto collocation points, and time derivatives are discretized by a semi-implicit scheme with finite-difference approximation. We executed the treatment of variable thermal diffusivity using array multiplication in the computation and used the matrix diagonalization method to solve discretized equations. Results from the current study are compared with those available in the literature. Numerical solutions are visualized using the isotherm, streamline, and heatline for different values of Rayleigh number, thermal diffusivity coefficient, and aspect ratio. For the first time, the visualization tool of heatline is applied to a porous enclosure with temperature-dependent thermal diffusivity. On the basis of these results, the heat-transfer process across the enclosure was analyzed. We found that variable thermal diffusivity has a considerable influence on flow pattern and corresponding heat transfer in the enclosure.

Study on thermo physical properties of hemp, jute and glass fiber reinforced polyester composites

The objective of this study is to examine the variation in thermal conductivity, specific heat capacity and thermal diffusivity of composites with respect to fiber content and temperature. The fiber content in the composites was varied from 18 to 36 by volume percentage. The results revealed that the thermal conductivity of composite decreased with increase in fiber content and the quite opposite trend was observed with respect to temperature. The specific heat capacity of the composite as measured by differential scanning calorimeter showed similar trend as that of the thermal conductivity. It was observed that the thermal diffusivity of composite was influenced by the fiber content, and the variation of thermal diffusivity decreased with increase in temperature at maximum volume of fiber.