Inhomogeneous Thermal Conductivity Research Articles

Heat transfer analysis of magnetohydrodynamics peristaltic fluid with inhomogeneous solid particles and variable thermal conductivity through curved passageway

PurposeThe particle distribution in a fluid is mostly not homogeneous. The inhomogeneous dispersion of solid particles affects the velocity profile as well as the heat transfer of fluid. This study aims to highlight the effects of varying density of particles in a fluid. The fluid flows through a wavy curved passage under an applied magnetic field. Heat transfer is discussed with variable thermal conductivity.Design/methodology/approachThe mathematical model of the problem consists of coupled differential equations, simplified using stream functions. The results of the time flow rate for fluid and solid granules have been derived numerically.FindingsThe fluid and dust particle velocity profiles are being presented graphically to analyze the effects of density of solid particles, magnetohydrodynamics, curvature and slip parameters. Heat transfer analysis is also performed for magnetic parameter, density of dust particles, variable thermal conductivity, slip parameter and curvature. As the number of particles in the fluid increases, heat conduction becomes slow through the fluid. Increase in temperature distribution is noticed as variable thermal conductivity parameter grows. The discussion of variable thermal conductivity is of great concern as many biological treatments and optimization of thermal energy storage system’s performance require precise measurement of a heat transfer fluid’s thermal conductivity.Originality/valueThis study of heat transfer with inhomogeneous distribution of the particles in a fluid has not yet been reported.

Исследование влияния углеродных нанотрубок на теплопроводные свойства термопаст

Many industrial sectors require efficient heat transfer to ensure safe and efficient operation of equipment. Thermal pastes are materials used to improve heat transfer between different surfaces. Recently, carbon nanotubes (CNTs) have become widely used in the field of electronics and microelectronics due to their unique physical and chemical properties. One of the areas where CNTs can be applied is thermal conductivity. In this study, the effect of carbon nanotubes on the thermal conductive properties of thermal pastes was investigated. In this paper, an experiment was conducted to evaluate the thermal stability and performance improvement of thermal pastes by adding carbon nanotubes. KPT-8 thermal paste with a density of 2.6 g/cm2 and thermal conductivity of 0.7 W/mK was used in the experiment. Thermal pastes are materials used to improve heat transfer between different surfaces. The results of the study showed that thermal paste with CNT additives has higher thermal conductivity than thermal paste without additives. This indicates that CNTs can be effectively used as additives to improve the thermal conductivity of thermal pastes. But when CNTs are used as additives in thermal pastes, certain problems may arise, such as the grouping of CNTs in the mass of the substance. This can lead to an uneven distribution of CNTs in the thermal paste and, as a consequence, to inhomogeneous thermal conductivity. The results emphasize the potential of carbon nanotubes as a promising additive for improving the thermal conductive properties of thermal pastes in various applications.

Modeling of temperature processes in orthotropic boards of radio-electronic devices

The design of electronic equipment that is resistant to external influences is a topical problem. The arrangement of individual circuit parts is modeled in the form of an orthotropic plate. The paper describes the solutions of homogeneous and inhomogeneous thermal conductivity equation for an orthotropic rectangular plate, obtained by the recurrence-operator method. Numerical results are given for two cases of temperature distribution, satisfying zero boundary conditions, during cooling and heating and given initial conditions.

Performance investigation of a two-stage thermoelectric cooler with inhomogeneous materials

A novel model of two-stage thermoelectric cooler with inhomogeneous thermal conductivity in steady-state operating condition is established. The modification of the constant properties model allows controlling the distribution of Joule heat. Considering internal irreversibilities of the thermoelectric cooler, expressions for the cooling capacity, COP, and exergy efficiency are derived. By utilizing numerical methods, the temperature profile along the thermoelectric legs is presented. The optimal operating regions are explored. The COP vs. cooling capacity describing optimal operating regions in inhomogeneity materials are plotted. Meanwhile, the influence of the main parameters such as the variation of thermal conductivity distribution, cold-end temperature and the number of thermoelectric modules on the cooling performance is discussed in detail. Results indicate that the cooling capacity, COP and exergy efficiency are improved compared to those of homo?geneous two-stage thermoelectric coolers when an appropriate inhomogeneous property parameter is applied. The work can provide guidance on design of actual two-stage thermoelectric coolers with inhomogeneous materials.

Thermal characteristics of GaN-on-diamond HEMTs: Impact of anisotropic and inhomogeneous thermal conductivity of polycrystalline diamond

The CVD growth of polycrystalline diamond (PCD) on GaN is a novel and effective approach to improve heat dissipation for high power GaN-based transistors. However, the growth-induced spatial variation in PCD's thermal conductivity makes it difficult to assess the thermal characteristics of GaN-on-diamond devices using bulk diamond properties. By including the depth-dependent anisotropic thermal conductivity of PCD, a finite element thermal simulation is used to study the heat spreading process within a device. The effective thermal conductivity (κeff) of the PCD substrate as “seen” by the device is found to be lower than the average thermal conductivity (κavg) of PCD, suggesting that the low thermal conductivity diamond film during the initial growth has a significant impact on the device thermal resistance. Moreover, κeff is investigated as a function of the thermal boundary resistance between GaN and diamond, the gate pitch of the device as well as the grain boundary conductance and the grain evolution rate of diamond polycrystals. The results provide reliable thermal assessment for the PCD substrate as used in a GaN-on-diamond device and have important implications for thermally optimizing the device layout and engineering the thermal conductivity of PCD heat spreaders for devices.

Evaporation heat transfer characteristics of composite porous wick with spherical-dendritic powders

A composite porous wick with spherical-dendritic powders is proposed in this paper. Both the small pores between the dendritic powders and the pores between the spherical and dendritic powders are formed. Moreover, the inhomogeneous local thermal conductivity may increase the tortuosity of the evaporating interface and provide more evaporating area. Two kinds of materials (copper and nickel) and two kinds of structures (spherical and dendritic powders) are chosen to prepare four composite porous wicks, and the evaporation heat transfer characteristics are studied experimentally. Two evaporation heat transfer modes have been observed. The composite porous wick with spherical-dendritic copper powders has the largest critical heat flux of 15.1 W/cm2. At high heat load, the upper surface temperature of composite porous wick may be equal to or even lower than the vapor temperature. The temperature fluctuation may occur, but not affect the evaporator wall temperature. According to the distribution of the evaporating meniscus in porous wick, the critical heat flux may be predicted. The reasonable design of the pore size distribution and large evaporating rate in small pores can increase the critical heat flux.

Identification of conductivity in inhomogeneous orthotropic media

Purpose The purpose of this paper is to solve numerically the identification of the thermal conductivity of an inhomogeneous and possibly anisotropic medium from interior/internal temperature measurements. Design/methodology/approach The formulated coefficient identification problem is inverse and ill-posed, and therefore, to obtain a stable solution, a non-linear regularized least-squares approach is used. For the numerical discretization of the orthotropic heat equation, the finite-difference method is applied, while the non-linear minimization is performed using the MATLAB toolbox routine lsqnonlin. Findings Numerical results show the accuracy and stability of solution even in the presence of noise (modelling inexact measurements) in the input temperature data. Research limitations/implications The mathematical formulation uses temporal temperature measurements taken at many points inside the sample, and this may be too much information that is provided to identify a space-wise dependent only conductivity tensor. Practical implications As noisy data are inverted, the paper models real situations in which practical temperature measurements recorded using thermocouples are inherently contaminated with random noise. Social implications The identification of the conductivity of inhomogeneous and orthotropic media will be of great interest to the inverse problems community with applications in geophysics, groundwater flow and heat transfer. Originality/value The current investigation advances the field of coefficient identification problems by generalizing the conductivity to be anisotropic in addition of being heterogeneous. The originality lies in performing, for the first time, numerical simulations of inversion to find the orthotropic and inhomogeneous thermal conductivity from noisy temperature measurements. Further value and physical significance are brought in by determining the degree of cure in a resin transfer molding process, in addition to obtaining the inhomogeneous thermal conductivity of the tested material.

Detecting Thermal Cloaks via Transient Effects.

Recent research on the development of a thermal cloak has concentrated on engineering an inhomogeneous thermal conductivity and an approximate, homogeneous volumetric heat capacity. While the perfect cloak of inhomogeneous κ and inhomogeneous ρcp is known to be exact (no signals scattering and only mean values penetrating to the cloak’s interior), the sensitivity of diffusive cloaks to defects and approximations has not been analyzed. We analytically demonstrate that these approximate cloaks are detectable. Although they work as perfect cloaks in the steady-state, their transient (time-dependent) response is imperfect and a small amount of heat is scattered. This is sufficient to determine the presence of a cloak and any heat source it contains, but the material composition hidden within the cloak is not detectable in practice. To demonstrate the feasibility of this technique, we constructed a cloak with similar approximation and directly detected its presence using these transient temperature deviations outside the cloak. Due to limitations in the range of experimentally accessible volumetric specific heats, our detection scheme should allow us to find any realizable cloak, assuming a sufficiently large temperature difference.

Anisotropic and inhomogeneous thermal conduction in suspended thin-film polycrystalline diamond

While there is a great wealth of data for thermal transport in synthetic diamond, there remains much to be learned about the impacts of grain structure and associated defects and impurities within a few microns of the nucleation region in films grown using chemical vapor deposition. Measurements of the inhomogeneous and anisotropic thermal conductivity in films thinner than 10 μm have previously been complicated by the presence of the substrate thermal boundary resistance. Here, we study thermal conduction in suspended films of polycrystalline diamond, with thicknesses ranging between 0.5 and 5.6 μm, using time-domain thermoreflectance. Measurements on both sides of the films facilitate extraction of the thickness-dependent in-plane (κr) and through-plane (κz) thermal conductivities in the vicinity of the coalescence and high-quality regions. The columnar grain structure makes the conductivity highly anisotropic, with κz being nearly three to five times as large as κr, a contrast higher than that reported previously for thicker films. In the vicinity of the high-quality region, κr and κz range from 77 ± 10 W/m-K and 210 ± 50 W/m-K for the 1 μm thick film to 130 ± 20 W/m-K and 710 ± 120 W/m-K for the 5.6 μm thick film, respectively. The data are interpreted using a model relating the anisotropy to the scattering on the boundaries of columnar grains and the evolution of the grain size considering their nucleation density and spatial rate of growth. This study aids in the reduction in the near-interfacial resistance of diamond films and efforts to fabricate diamond composites with silicon and GaN for power electronics.

Tunable thermal conductivity via domain structure engineering in ferroelectric thin films: A phase-field simulation

Effective thermal conductivity as a function of domain structure is studied by solving the heat conduction equation using a spectral iterative perturbation algorithm in materials with inhomogeneous thermal conductivity distribution. Using this proposed algorithm, the experimentally measured effective thermal conductivities of domain-engineered {001}p-BiFeO3 thin films are quantitatively reproduced. In conjunction with two other testing examples, this proposed algorithm is proven to be an efficient tool for interpreting the relationship between the effective thermal conductivity and micro-/domain-structures. By combining this algorithm with the phase-field model of ferroelectric thin films, the effective thermal conductivity for PbZr1-xTixO3 films under different composition, thickness, strain, and working conditions is predicted. It is shown that the chemical composition, misfit strain, film thickness, film orientation, and a Piezoresponse Force Microscopy tip can be used to engineer the domain structures and tune the effective thermal conductivity. Therefore, we expect our findings will stimulate future theoretical, experimental and engineering efforts on developing devices based on the tunable effective thermal conductivity in ferroelectric nanostructures.

Inhomogeneous thermal conductivity enhances thermoelectric cooling

We theoretically investigate the enhancement of thermoelectric cooling performance in thermoelectric refrigerators made of materials with inhomogeneous thermal conductivity, beyond the usual practice of enhancing thermoelectric figure of merit (ZT) of materials. The dissipation of the Joule heat in such thermoelectric refrigerators is asymmetric which can give rise to better thermoelectric cooling performance. Although the thermoelectric figure of merit and the coefficient-of-performance are slightly enhanced, both the maximum cooling power and the maximum cooling temperature difference can be enhanced significantly. This finding can be used to increase the heat absorption at the cold end. We further find that the asymmetric dissipation of Joule heat leads to thermal rectification.

Light dynamics in materials with radially inhomogeneous thermal conductivity

We study the properties of bright and vortex solitons in thermal media with nonuniform thermal conductivity and homogeneous refractive index, whereby the local modulation of the thermal conductivity strongly affects the entire refractive index distribution. While regions where the thermal conductivity is increased effectively expel light, self-trapping may occur in the regions with reduced thermal conductivity, even if such regions are located close to the material boundary. As a result, strongly asymmetric self-trapped beams may form inside a ring with reduced thermal conductivity and perform persistent rotary motion. Also, such rings are shown to support stable vortex solitons, which may feature strongly noncanonical shapes.

Investigation of surface deformation during drying of thin polymer films due to Marangoni convection

The investigation of surface deformation of thin polymer films (methanol–poly(vinyl acetate) solution with 67wt.% methanol) due to inhomogeneous drying is presented. Of interest are the onset of fluid instability and the influence of drying kinetics in a system where surface tension depends on temperature and composition. An experimental method is used that correlates a virtual shift of a random dot pattern, located on the downside of a glass substrate, to a deformation of the free surface of a transparent film. Instability in the fluid is forced over a structured composite substrate with inhomogeneous thermal conductivity. We show that due to the inhomogeneous heat transfer a long-wave instability mode is triggered resulting in directed mass transport. To evaluate the driving forces, 1D simulations of the non-isothermal drying process have been performed. Furthermore the influence of drying kinetics on the developing surface topology is discussed.

Design of square-shaped heat flux cloaks and concentrators using method of coordinate transformation

A square-shaped heat flux cloak and a square-shaped heat flux concentrator have been designed theoretically according to the invariance symmetry of steady state thermal conductive equation. The direction of heat flux in these devices can be modulated as desired. Using the method of coordinate transformation, the inhomogeneous and anisotropic thermal conductivity in the transformation region have been acquired. Two-dimensional finite element simulations were performed to confirm the theoretical results.

Time dependent thermoelectric performance of a bundle of silicon nanowires for on-chip cooler applications

With finite element simulation, the time dependent thermoelectric performance of silicon nanowires (SiNWs) is studied systematically. Short response time has been observed in SiNW cooler which decreases with increasing of the number of SiNWs. Moreover, the impacts of inhomogeneous thermal conductivity distribution in one bundle on the cooling temperature have been studied. The cooling temperature decreases due to the existing of unexpected high thermal conductivity SiNW. This impact can be suppressed in large system. Our results provide a comprehensive performance analysis of SiNWs for on-chip thermoelectric cooler applications.

Reconstruction of depth profiles of thermal conductivity of case hardened steels using a three-dimensional photothermal technique

A method of retrieving thermophysical depth profiles of continuously inhomogeneous materials is presented both theoretically and experimentally using laser infrared photothermal radiometry. This method represents the three-dimensional (3D) extension of earlier one-dimensional thermal-wave inverse-problem techniques for reconstructing inhomogeneous thermal-conductivity or diffusivity depth profiles. A 3D theoretical model suitable for characterizing solids with arbitrary continuously varying thermophysical property depth profiles and finite (collimated or focused) laser beam spotsize is developed. A numerical fitting algorithm to retrieve the thermophysical profile was demonstrated with three case hardened steel samples. The reconstructed thermal conductivity depth profiles were found to be well anticorrelated with microhardness profiles obtained with the conventional indenter method.

Theory of motion in an optical radiation field for sublimating spherical aerosol solid particles with inhomogeneous thermal conductivity

Theory of motion in an optical radiation field for sublimating spherical aerosol solid particles with inhomogeneous thermal conductivity

Effect of the Sublimation Coefficient on the Photophoresis of Aerosol Particles with Inhomogeneous Thermal Conductivity

An analytical expression for the velocity of photophoresis of a large spherical solid aerosol particle is obtained, which takes into account evaporation (sublimation) of the particle material and its inhomogeneity with respect to thermal conductivity.

On the accuracy of depth-dependent thermal conductivity retrieval in photoacoustic experiments

We investigate the intrinsic error sensitivity of photoacoustic experiments in obtaining reliable and accurate information on the inhomogeneous thermal conductivity below the surface. The analysis is based on thermal wave properties and does not depend on any approximate inversion technique. As an example, the theory is applied to the important class of monotonic profiles. The theory can help us in choosing the lower frequency limit in an actual experiment.

Thermal waves in materials with inhomogeneous thermal conductivity: An analytical approach

An exact asymptotic expression for thermal waves in an inhomogeneous semi-infinite condensed matter sample at high frequencies is derived. The theory also allows for a fast and accurate retrieval of subsurface thermal conductivity profiles from surface temperature values in response to incident modulated radiation.