Effect Of Heat Conduction Research Articles

The effect of heat conduction on the realization of the primary standard for sound pressure

Pressure reciprocity calibration of microphones provides the basis for primary measurement standards for sound pressure in air. At low frequencies, reciprocity calibration requires that a heat conduction correction be employed to account for energy transfer to and from the bounding surfaces of the close-coupled microphone arrangement. The standard governing reciprocity calibration, IEC 61094-2 : 2009, provides two models for the heat conduction correction: the Low Frequency Solution, and the Broadband Solution. Analysis has revealed significant and unexplained differences in behaviour between the models at very low frequencies, leading to inconsistency in calibration results, which has been quantified. Additionally, both heat conduction solutions given in IEC 61094-2 are simplifications that strictly apply only above their respective lower limiting frequencies. An international comparison on microphone calibration is currently underway that includes measurements below the lower limiting frequencies of the models. In this paper, the origin and nature of the Broadband simplifications have been identified, and estimates of the error given. A flaw in the Broadband theory is identified and its effect quantified. Simplification error for the Low Frequency solution is evaluated, and the full spectrum solution is given. This paper urges caution in the application of the models at low frequency and provides data useful for assessing the contribution to the measurement uncertainty.

Effective heat conduction in hybrid sphere & wire nanodispersions

Heat conductivity of dispersions can be modified by varying shapes of dispersed particles and also by making hybrid dispersions containing particles of different shapes and sizes. Spheres and their agglomerates that we have investigated previously are replaced in this paper by spheres and wires. The method used to derive the formulas for the overall effective heat conductivity is based on the Maxwell homogenization (adapted to hybrid dispersions) followed by a mesoscopic analysis in which heat transfer is regarded as transport of phonons. The mesoscopic formulation provides then also a setting for investigation the role of particle-matrix nanoscale interfaces.

Effective Thermal Conductivity of Spherical Particulate Nanocomposites: Comparison with Theoretical Models, Monte Carlo Simulations and Experiments

The purpose of this work is to study heat conduction in systems that are composed out of spherical micro-and nanoparticles dispersed in a bulk matrix. Special emphasis will be put on the dependence of the effective heat conductivity on various selected parameters as dimension and density of particles, interface interaction with the matrix. This is achieved by combining the effective medium approximation and extended irreversible thermodynamics, whose main feature is to elevate the heat flux vector to the status of independent variable. The model is illustrated by three examples: Silicium-Germanium, Silica-epoxy-resin and Copper-Silicium systems. Predictions of our model are in good agreement with other theoretical models, Monte-Carlo simulations and experimental data.

On Propagation of One Dimensional Small Amplitude Waves in Radiating Viscous and Heat Conducting Gas

In this paper, effect of radiation, heat-conduction and viscosity on propagation of one-dimensional small amplitude waves is investigated. It is shown that there are three distinct modes of propagation viz. (i) Radiation-induced mode, (ii) Modified gasdynamic mode and (iii) Coupled heat-conduction and viscous mode. The dispersion relation is solved both asymptotically and numerically. For very small values of omega, the asymptotic solution predicts the speed of propagation of distriubance as zero, as (isentropic sound velocity) and 0.336 times the isothermal sound velocity. For very large values of omega, the high frequency waves propagate with characteristic speeds of the seventh order operation.

Determining the geotechnical properties of planetary regolith using Low Velocity Penetrometers

Measurements of mechanical and thermophysical properties of planetary surface allow determining many important parameters useful for planetologists. For example, effective heat conductivity or thermal inertia of the regolith can help to better understand the processes occurring in the bodies interior. Chemical and mineralogical composition gives us a chance to determine the origin and evolution of moons and satellites. Mechanical properties of the surface are one of the key factors needed by civil engineers for developing future bases on space bodies.Space missions to planetary bodies highly restrict the payload concerning its mass and power consumption. Therefore, it is quite impossible to use a standard terrestrial technique like the Load Plate Test or Direct Shear Tests to determine the geotechnical parameters of the planetary regolith. Even the Dynamic Cone Penetration (DCP) method, which is frequently used for field testing, does not fit well with the constraints imposed by a space mission. Nevertheless, its operation principle is very similar to that of at the Low Velocity Penetrators (LVP), several of them being currently on their way to planetary bodies (e.g. the MUPUS instrument) or which were developed in the last couple of years (e.g. the CHOMIK instrument or the KRET device).In this paper we present a comparison between DCP method and LVP operation which was observed during several tests campaigns during mole KRET and CHOMIK instrument development. The tests were performed in different planetary analogues: JSC-1A, Chenobi and AGK-2010, Phobos analogue, cometary analogues F1, F2 and F3 (SRC) and dry quartz sand. In the last part of the paper the concept of results׳ interpretation is presented.

THREE-FLUID, THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC SOLAR WIND MODEL WITH EDDY VISCOSITY AND TURBULENT RESISTIVITY

We have developed a three-fluid, three-dimensional magnetohydrodynamic solar wind model that incorporates turbulence transport, eddy viscosity, turbulent resistivity, and turbulent heating. The solar wind plasma is described as a system of co-moving solar wind protons, electrons, and interstellar pickup protons, with separate energy equations for each species. Numerical steady-state solutions of Reynolds-averaged solar wind equations coupled with turbulence transport equations for turbulence energy, cross helicity, and correlation length are obtained by the time relaxation method in the corotating with the Sun frame of reference in the region from 0.3 to 100 AU (but still inside the termination shock). The model equations include the effects of electron heat conduction, Coulomb collisions, photoionization of interstellar hydrogen atoms and their charge exchange with the solar wind protons, turbulence energy generation by pickup protons, and turbulent heating of solar wind protons and electrons. The turbulence transport model is based on the Reynolds decomposition and turbulence phenomenologies that describe the conversion of fluctuation energy into heat due to a turbulent cascade. In addition to using separate energy equations for the solar wind protons and electrons, a significant improvement over our previous work is that the turbulence model now uses an eddy viscosity approximation for themore » Reynolds stress tensor and the mean turbulent electric field. The approximation allows the turbulence model to account for driving of turbulence by large-scale velocity gradients. Using either a dipole approximation for the solar magnetic field or synoptic solar magnetograms from the Wilcox Solar Observatory for assigning boundary conditions at the coronal base, we apply the model to study the global structure of the solar wind and its three-dimensional properties, including embedded turbulence, heating, and acceleration throughout the heliosphere. The model results are compared with plasma and magnetic field observations on WIND, Ulysses, and Voyager 2 spacecraft.« less

Verification of a model of thermal storage incorporated with an extended lumped capacitance method for various solid–fluid structural combinations

With an extended lumped capacitance method applied to account for the internal heat conduction resistance in a solid (for Biot number larger than 0.1), a general model of thermal energy storage with various solid–fluid structural combinations is presented and verified using numerical results. The thermal energy storage system has a heat transfer fluid (HTF) flowing through a packed bed of solid materials structured in different configurations, such as in the form of solid pebbles, parallel plates, solid rod-bundles, or solids with fluid tubes imbedded through them. The model of energy conservation in the liquid and solid is transient, one-dimensional in nature, due to the introduction of a modified lumped capacitance method that counts for the effect of three-dimensional heat conduction in the solid structures. The computational workload using this modified model is significantly less compared to that of a comprehensive CFD analysis. Numerical results obtained from a CFD analysis of the thermal energy storage in the solid and liquid are used to verify the model. The CFD simulated results of temperatures of HTF are compared with the 1D model results, and they show excellent agreement. In conclusion, the 1D model is recommended as a convenient and accurate tool for general analysis and sizing of thermal energy storage containers that have various solid–fluid structural combinations.

The turbulent rotating-disk boundary layer

The turbulent boundary layer on a rotating disk is studied with the aim of giving a statistical description of the azimuthal velocity field and to compare it with the streamwise velocity of a turbulent two-dimensional flat-plate boundary layer. Determining the friction velocity accurately is particularly challenging and here this is done through direct measurement of the velocity distribution close to the rotating disk in the very thin viscous sublayer using hot-wire anemometry. Compared with other flow cases, the rotating-disk flow has the advantage that the highest relative velocity with respect to a stationary hot wire is at the wall itself, thereby limiting the effect of heat conduction to the wall from the hot-wire probe. Experimental results of mean, rms, skewness and flatness as well as spectral information are provided. Comparison with the two-dimensional boundary layer shows that turbulence statistics are similar in the inner region, although the rms-level is lower and the maximum spectral content is found at smaller wavelengths for the rotating case. These features both indicate that the outer flow structures are less influential in the inner region for the rotating case.

Investigation of Macro-Thermal Contact Resistances in the Cathode of PEM Fuel Cells

Polymer Electrolyte Membrane (PEM) fuel cells are electrochemical energy conversion devices, and have been a main focus of study in the area of renewable energy systems in the past decade. With the potential to generate electricity with zero local greenhouse gas emissions, when fed hydrogen and oxygen from air, PEM fuel cells also have other promising advantages, such as supplying a high power to volume ratio, operating with little to no noise and being able to quickly reach steady state conditions [1]. Although the technology is promising, before PEM fuel cells can reach widespread adoption, effective thermal management must be achieved in order to reach the performance and durability levels required for commercialization.The cathode of the PEM fuel cell, which is composed of the Gas Diffusion Layer (GDL), Micro-Porous Layer (MPL), and Catalyst Layer (CL), houses various processes which affect the performance of the cell. While the inlet fuel and oxidant gases are typically heated, heat is also produced from electrochemical reactions and water phase change, resulting in temperature gradients that can affect the conditions of the cell. These exothermic reactions generate temperature gradients, which in turn affect the cell’s relative humidity levels, water saturation levels, and the cell’s reaction kinetics [2]. Pathways for effective heat conduction are vital for avoiding excessive condensation of water from cooling, or material dry-out from overheating. It is therefore crucial to have a way of measuring how effectively thermal energy can be transferred out of the CL, the region of generation, through the MPL, and finally out of the GDL, in order to have better control of parameters involved in future improvements to cell components.Present thermal conductivity models provide insight for analyzing thermal energy transfer in the through-plane and in-plane directions of the GDL, but do not consider interfacial mechanics at the micro-level caused by surface roughness, waviness, and irregularities. Therefore in order to provide a more realistic representation of the through-plane thermal conductivity of the GDL of a PEM fuel cell, a model will be developed which incorporates information regarding fibre surface morphology measured using atomic force microscopy [3], and will also incorporate polytetrafluoroethylene (PTFE), and binding materials into the thermo-mechanical analysis. The thermal resistance network will be constructed using stochastic relationships derived statistical expressions regarding the contacts in the GDL found in a previous study [2], and it will be informed with experimental data obtained via micro-computed tomography [4]. Secondly, the thermal contact resistance between the MPL and CL will be determined using a modification of Greenwood’s Rough Contact Model. The surface structures of the MPL and CL will be analyzed using a combination of scanning electron microscopy, atomic force microscopy, and micro-computed tomography, from which analysis will be conducted to determine the thermal contact resistance of the interface.A realistic thermal conductivity model representing surface features of carbon fibre at the nanoscale will be presented. The model will incorporate PTFE and binding materials in the thermal analysis, for various compressive loads and GDL sizes/types, from which preliminary results are shown in Figure 1. Also, the thermal contact resistance of the MPL|CL interface will be explored providing insight into a more realistic thermal model of the macro-thermal resistances within the cathode of the PEM fuel cell. Figure 1. Through-plane thermal conductivity with porosity for untreated GDL considering surface features of carbon fibre.

A 1.5D model of the steady-state thermal structure of detachment folds

We present a 1.5D steady-state model of thermal perturbations in the core of detachment anticlines. The model incorporates the effects of heat conduction, advection by folding, radiogenic sources, and conversion of work into heat by viscous dissipation. We work out analytical solutions of model's equations for heat flow and temperature field for the case of low amplitude detachment folds growing in a thick homogeneous stratigraphy.Our analytical solutions for the simplest case of no heat sources show that detachment folds develop a thermal boundary layer. Conduction of heat dominates inside the layer, whereas advection of heat by folding prevails outside the boundary layer. We also find that heat flow in a growing detachment fold decays exponentially with the square of the thickness of the detached stratigraphic section. When heat sources are included into the problem we find that radiogenic heat generates thermal anomalies at the crest of the anticlines whereas viscous dissipation produces thermal anomalies at the core of the detachment fold.We analyze three cases of geological interest: detachment folds growing in evaporitic sequences, marine carbonates, and shaley sediments. For a typical uplift rate of 2 mm/yr conduction dominates in the first class of rocks. Viscous dissipation dominates in folds growing in limestone due to its high viscosity. Advection and the generation of heat by radiogenic isotopes dominate the growth of detachment folds in shale. Additionally, the cooling histories of Lagrangian particles embedded in the fold for the last two cases are analyzed. The core of the fold is characterized by rapid cooling whereas the limbs cool at slower rates than the core. Thus, the development of detachment anticlines may either accelerate maturation of source rocks and expand the maturation window or halt it by removing source rocks from the generating window.

Contact Heat Conductivity Under Conditions of High-Temperature Heat Transfer in Fibrous Heat-Insulating Materials

A mathematical model for calculating the effective heat conductivity of fi brous materials at high temperatures and variable density has been suggested and substantiated. Consideration is focused basically on accounting for heat conduction in the solid phase of a fi brous material. The calculated results were compared with experimental data that demonstrated a good accuracy of the proposed model. The effective heat conductivity of a fi brous heat-insulating material has been calculated as a function of density for a number of temperatures within the range from 200 to 1000 o C.

Effective heat conduction in dispersion of wires

We derive a formula for the heat conductivity coefficient of dispersions of wires in a homogeneous matrix. Such formula is particularly useful for thermoelectric applications. The method used to derive this type of formula in Behrang et al. [J. Appl. Phys. 114, 014305 (2013)] for spherical particles is adapted to generally oriented wires of a finite length. Both diffuse and specular scatterings on the wire-matrix interface are considered. The results obtained previously from numerical solutions of the phonon kinetic equation under the assumption of diffuse scattering agree with predictions based on the formula.

Continuum mechanical modeling of laser‐pulsed heating in polycrystals: A multi‐physics problem of coupling diffusion, mechanics, and thermal waves

AbstractThis contribution introduces a geometrically nonlinear, continuum thermomechanical framework for pulsed laser heating in crystalline matter: a physical process which is characterized by a non‐Fourier like heat propagation and defect diffusion. The key objective of this work is to derive the highly nonlinear and strongly coupled system of governing equations describing the multi‐physical behavior from fundamental balance principles. A general form for the Helmholtz energy is proposed and the resulting constitutive laws are derived from logical, thermodynamically consistent argumentation. The approach adopted to derive the governing equations is not entirely specific to laser induced heating, rather it encompasses a wide range of applications wherein heat conduction, species diffusion, and finite elastic effects are coupled. The present theory is thus applicable to the generality of models for thermal and mechanical waves: an area of increasing research interest. A numerical example is presented for the fully coupled, nonlinear and transient theory.

Transient behavior of co-current parallel flow three-fluid heat exchanger

Transient behavior of co-current parallel flow three-fluid compact heat exchangers with the effect of two-dimensional longitudinal heat conduction through the separating sheet and axial dispersion in fluids has been investigated numerically by using the Gauss–Seidel iterative technique for step excitation provided to hot fluid inlet temperature. The results reveal that the performance of the heat exchanger is affected when two-dimensional longitudinal conduction in separating sheets and axial dispersion in fluids are considered.

Axial heat conduction effects in the entrance region of laminar duct flows: Correlations for the local Nusselt number

Axial heat conduction effects within the fluid can be important for duct flows if the Prandtl number is low (liquid metals) or if the hydraulic diameter of the channel is very small (micro-channels). Exact analytical solutions for the extended Graetz problem for such situations are well known and documented in literature. In the present paper suitable correlations are developed for the local Nusselt number in the thermal entrance region in a parallel plate channel and in a pipe. Correlations are given for different thermal boundary conditions and for axial positions before (x<0) and after (x>0) the jump in wall temperature or wall heat flux.

Performance Analysis of Parabolic Trough Collector in Hot Climate

In most existing buildings cooling and heating loads lead to high primary energy consumption and, consequently, high CO2 emissions. These can be substantially decreased with suitable energy concepts using appropriate integrated renewable systems. In the present study a numerical model is developed to study the effect of different collector parameters and operating conditions on the performance of parabolic trough solar collector (PTSC) in Kuwait climate. The proposed model takes into consideration the thermal interaction between absorber-envelope, and envelopeenvelope for thermal radiation losses which have been neglected in existing models. A review of the equations for convective heat transfer losses was performed as well and new equations were developed and used in the present model. The effects of heat conduction in the collector tube wall and mixed convection in the inner tube, which have been neglected in previous studies, are included in the proposed model. In addition, a case study is carried out adapting parabolic trough collectors to satisfy nominal space heating load, water heating load and cooling load of a typical Kuwaiti dwelling. Finally, the environmental impact of solar heating and cooling systems under Kuwait climate conditions is investigated. Present results indicate that convection loss from the absorber tube to supporting structures is the largest among the other losses (conduction and radiation). Also, at noon time PTSC has the smallest angle of incidence and the highest efficiency and when the annulus between the receiver surface and the glass envelope is Original Research Article British Journal of Applied Science & Technology, 4(14): 2038-2058, 2014 2039 In vacuo, conduction and convection across the annulus are effectively eliminated. In addition, space heating load and domestic water heating load can be completely provided by PTSC. The minimum required collector area is about 82 m to supply cooling loads of a typical residential house under all climatic conditions in Kuwait. A CO2 emission reduction of about 12.3tonne/year can be achieved as a result of adapting parabolic trough solar collector to a typical Kuwaiti dwelling.

Theoretical Analysis for Freeze-Drying of Initially Unsaturated Porous Material

To improve the process economy,freeze-drying of initially unsaturated frozen material from aqueous solution is put forward in the present investigation.A novel heat and mass transfer model of freeze-drying with hygroscopic effect was derived on the basis of a new adsorption-desorption relationship.The finite-volume method was adopted in solving the governing equations.Mannitol,a typical pharmaceutical excipient,was selected as the solute in aqueous solution to be dried. Experimental results show that the freeze-drying process can be enhanced indeed using the initially unsaturated frozen material,and drying time decreases with increase in the initial porosity,e0(1-S0).There is the shortest drying time in correspondence with about 0.30-0.35of initial saturation, S0.According to the profiles of temperature and saturation,heat and mass transfer mechanism was discussed and the drying rate-controlling factor was analyzed.Inspection on the effective mass diffusivity,KS and the effective heat conductivity,λ+KT ΔH displays that the rate-controlling factor would change from mass transfer to heat transfer with increase in instantaneous porosity,e0(1-S).

Transient Natural Convection Flow of Thermomicropolar Fluid of Micropolar Thermal Conductivity along a Nonuniformly Heated Vertical Surface

The unsteady free convection boundary layer flow of a thermomicropolar fluid along a vertical plate with effect of micropolar heat conduction has been investigated. The governing equations are transformed into a new form using a method of transformed coordinates. We then use an explicit finite difference scheme to solve the transformed equations. Here, the governing equations have been reduced to the forms that are valid for entire, small, and large time regimes, by using stream-function formulation. The results obtained for the above mentioned three time regimes are compared and found to be in excellent agreement. Moreover, the effects of the physical parameters such as the viscosity parameter, K, and the heat conduction parameter, α*, are presented in terms of the transient shear stress, couple stress, and surface heat transfer coefficient as well as transient velocity profiles, angular velocity profiles, and temperature profiles.

Effects of axial heat conduction, property variation and parasitic heat in-leakon performance of compact plate-fin heat exchangers

Modern helium liquefaction/refrigeration systems employ compact plate-fin heat exchangers having very high effectiveness (>0.95). Performance of such systems is a strong function of effectiveness of heat exchangers used. The calculation of heat exchanger effectiveness in such cases needs considerations of several secondary parameters apart from basic fluid film resistance. In the present paper, the combined effects of secondary parameters like axial heat conduction through heat exchanger matrix and parasitic heat in-leak from the surroundings has been studied numerically. Large temperature changes in cryogenic heat exchangers may result in correspondingly larger changes in fluid properties and metal matrix conductivity, which shall be taken care of during numerical calculations. Numerical model developed in the present work is based on the one given in the literature. Numerical technique to solve the system of equations is implemented in MATLAB®. Real properties of helium at each node are evaluated using HEPAK®, which is linked to the developed code. Using this model, performance of heat exchangers is studied at four different temperature levels of (300-77)K, (77–20)K, (20-8)K and (10-5.1)K. The results highlight the effects of each of the above secondary parameters.

Investigation of Evaluation Criterion of Axial Wall Heat Conduction in Tube Laminar Flow and Heat Transfer

The effect of axial wall heat conduction in circular tube laminar flow and heat transfer has been investigated under constant temperature boundary conditions. Three-dimensional numerical simulations are performed with different wall thicknesses, wall materials, tube lengths as well as a wide Peclet number range from 7 to 7000. Three parameters including the modified axial conduction number (M), the so-called axial conduction number (Mc) and the temperature gradient number (β) are comprehensively studied. Under the thermal boundary condition of constant wall temperature, the temperature gradient number is introduced to evaluate the importance of the axial wall heat conduction.