Showing papers on "Thermal diffusivity published in 1975"

Radiation loss in the flash method for thermal diffusivity

Abstract: The boundary radiation heat loss method is investigated for the flash method in measuring thermal diffusivity. Present correction procedures for heat loss are tested experimentally using AXM−5Q (POCO) graphite. In addition, a new method for radiation loss correction is presented based solely on the heating portion of the temperature rise curve. Using the new method, corrections of 35% can be made within a ±3% error. The thermal diffusivity results calculated using the adiabatic expression for various portions of the fractional rise curve and for various heat losses are also presented.

Augmentation of heat transport in laminar flow of polystyrene suspensions. I. Experiments and results

Abstract: The effective thermal conductivity of suspensions of 50− and 100‐μ‐diam polystyrene spheres in aqueous sodium chloride or glycerine, flowing in laminar motion, has been shown to be a function of the state of the motion (unlike single‐phase fluids) and has been seen to be as much as 3 times the thermal conductivity of stationary suspensions Detailed experiments have indicated its dependence upon the particle concentration, particle size, shear rate, kinematic viscosity, and thermal diffusivity of the suspending liquid, and tube diameter and length The Graetz solution corresponding to uniform wall temperature was used to determine the value of thermal conductivity in an apparatus calibrated with tap water The over‐all accuracy of the results is within 10%

Thermal resistance of heterostructure lasers

Abstract: A GaAs−GaAlAs heterostructure laser is modeled as a stripe heat source embedded in a layered structure, and an analytic expression is given for the steady−state thermal resistance 〈R〉 of the model. Over the range of typical layer thicknesses and conductivities, and for heat generated uniformly in the active region, 〈R〉 varies between 14 and 31 K/W for a 12×375−μ active region. Four types of heat sinks are shown to contribute an additional 3 to 10 K/W. Design implications are drawn for various properties including layer thicknesses, heat−sink and bond parameters, and radiative heat transfer by spontaneous emission. In disagreement with the common tacit assumption of a unique active−region temperature, it is found that about 40% of the temperature drop within the laser occurs in the active region (center to edge).

Heating of solid targets with laser pulses

Abstract: Analytical and numerical solutions to the heat-conduction equation are obtained for the heating of absorbing media with pulsed lasers. The spatial and temporal form of the temperature is determined using several different models of the laser irradiance. Both surface and volume generation of heat are discussed. It is found that if the depth of thermal diffusion for the laser-pulse duration is large compared to the optical-attenuation depth, the surface- and volume-generation models give nearly identical results. However, if the thermal-diffusion depth for the laser-pulse duration is comparable to or less than the optical-attenuation depth, the surface-generation model can give significantly different results compared to the volume-generation model. Specific numerical results are given for a tungsten target irradiated by pulses of different temporal durations and the implications of the results are discussed with respect to the heating of metals by picosecond laser pulses.

Theory, Measurement, and Application of Thermal Properties of Biomaterials

Abstract: Biomaterials are capable of heat transfer-energy transport by virtue of a temperature gradient. In the case of a living biomaterial, the heat transfer capability of the material can be especially important because the state of life may depend upon the maintenance of a specific temperature. All of the three basic mechanisms of heat transfer, that is, conduction, convection, and radiation, can occur in physical situations involving biomaterials, but of these three, conduction is usually most important in detcrmining the heat transfer within the biomaterial itself. The ability of a biomaterial to transport energy by conduction is best characterized in the steady state by its thermal conductivity, k, and in the nonsteady state by its thermal diffusivity, rx. Because of the complex structure of most biomaterials, there is no adequate microscopic theory, such as that for solids and to a lesser extent for liquids, that allows the direct determination of k and rx of a biomaterial from some fundamental property or properties. For this reason, it is usually necessary to

Application of the Langevin equation to fluid suspensions

Abstract: Brownian motion of particles suspended in a fluid is studied, and expressions derived for the particle diffusivity and velocity autocorrelation function. The theory of thermal noise in a general linear system is applied to both the particles and the fluid in a novel formulation. This enables the recent modification of the Langevin equation to include the effect of fluid inertia to be seen as just a necessary but simple reinterpretation of the original analysis, without introducing the theory of non-Markovian processes.

Theory of the photoacoustic effect with solids

Abstract: Chopped light impinging on a solid sample in an enclosed cell produces an acoustic signal within the cell, A theoretical explanation for this effect is presented, providing a quantitative foundation for photoacoustic spectroscopy. Nonradiative de‐excitation of the absorbed light within the solid sample produces a periodically varying heat flow from the sample to the surrounding gas and backing material. The solution to the thermal diffusion equations shows that only a thin boundary layer of gas adjacent to the surface of the sample responds thermally to this periodic heat flow. The boundary layer can then be thought of as acting as an acoustic piston creating the acoustic signal detected in the cell. An explicit formula gives the. magnitude and phase of the acoustic pressure in terms of the optical, thermal, and geometric parameters of the system. Simple approximations for the pressure are given according to the relative magnitudes of the optical absorption length, thermal diffusion length, and thickness ...

The Roughness Length for Water Vapor Sensible Heat, and Other Scalars.

Abstract: An expression is presented for the roughness length of water vapor (or sensible heat, etc) that is the surface value intercept of the straight line resulting from a semi-logarithmic plot of the mean specific humidity (or potential temperature, etc) profile in the dynamic sublayer The derivation is based on the standard assumption of continuity in the mean profile at the interface between the interfacial transfer sublayer and the fully turbulent surface sublayer It is found that the resulting formulation yields similar results for a number of empirical and theoretical equations for the interfacial transfer that are available in the literature The roughness length of any scalar admixture depends not only on the nature of the surface but also on the intensity of the surface shear stress and on the molecular diffusivity and the viscosity In meteorological applications under rough flow conditions this roughness length may be considerably smaller than the aerodynamic roughness length Z0, whereas

On Solving the Flow Equation in Unsaturated Soils by Optimization: Horizontal Infiltration

Abstract: Recently, integral methods have been developed that provide accurate approximations to the diffusion equation with rapidly varying diffusivity, but require in general some numerical iteration. In the present paper, a new analytical approach is developed which yields optimal approximations without any numerical iteration. The method is applied to the problem of one-dimensional horizontal infiltration of water in soils. In that case it is shown that the sorptivity is an invariant, which can be expressed in terms of the sum of two integrals involving the diffusivity. The analytical results are in excellent agreement with numerical calculations for Yolo light clay.

Thermal resistance of heat sinks with temperature-dependent conductivity

Abstract: Kirchhoff's transformation is summarised in a form appropriate for semiconductor-device heat sinks and then illustrated with a brief application to the thermal resistance of a GaAs laser. Under the most common semiconductor boundary conditions his transformation immediately converts the steady-state linear temperature rise (based on a temperature-independent conductivity σ 0 ) of a uniform heat sink of any shape into the nonlinear rise [based on a temperature-dependent conductivity σ ( T ) of any functional form].

Observation of protein diffusivity in intact human and bovine lenses with application to cataract.

Abstract: Using the technique of optical mixing spectroscopy, we have observed the Brownian movement and measured the diffusivity of proteins within whole, intact human and bovine lenses. The magnitude and the temperature dependence of the protein diffusivity in bovine and normal human lenses implies that cold cataract is the result of a first-order phase separation of the protein-water mixture in the lens. The magnitude of the diffusivity of proteins within cataractous human lenses indicates the presence of large aggregates of proteins.

Augmentation of heat transport in laminar flow of polystyrene suspensions. II. Analysis of the data

Abstract: It has been experimentally demonstrated in the preceding paper, I, that heat transport is augmented in the laminar flow of suspensions of 100‐ and 50‐μ polystyrene spheres. The augmentation has been seen to be as much as 200% and depends on the shear rate and several other parameters. Here the experimental data are analyzed by proposing a model based on the particle rotations and the entrained fluid, and by presenting similarity arguments. It is shown that the augmentation in heat transport resides in the inertia of the entrained fluid rotating with the particle which is manifest in two parameters, namely, Reynolds (ωa2/νf) and Peclet (ωa2/αf) numbers, where ω is the angular velocity of the particle, a is the radius of the sphere, and νf and αf, respectively, are the kinematic viscosity and the thermal diffusivity of the suspending fluid. The other parameters on which the augmentation in heat transport depends are the particle volume fraction, tube‐radius–to–particle‐radius ratio, tube‐length–to–particle‐...

The Diffusion of Ion‐Implanted Arsenic in Silicon

Abstract: In order to characterize implanted‐diffused As layers in Si and to develop general processing information, impurity profiles were determined by secondary ion mass spectrometry (SIMS) and differential conductivity measurements. An analysis of these profiles is given which has yielded information regarding the diffusion of As and the electrical quality of these implanted‐diffused layers. It is shown that implanted‐diffused As profiles with can be described by a Chebyshev polynomial approximation to the diffusion equation with concentration‐dependent diffusivity. The diffusion of As is not dependent upon the furnace ambient, but As pile‐up within 200–400A of the Si surface does occur during diffusion in an oxidizing atmosphere. It is also shown that implanted‐diffused As layers show higher electrical activity for diffusion temperatures below 1100°C than layers diffused from chemical sources. For implanted As layers in which the peak concentration is greater than the solubility limit, the fraction of electrically active As increases at a rate proportional to .

A dynamic method for catalyst diffusivities

Abstract: It is shown that a pulse-response technique can be used with the Wicke-Kallenbach type of diffusion cell to determine effective diffusivities in catalyst pellets. The diffusivity is a function of the retention time of the pulse of diffusing gas in the pellet. Experimental values obtained with an alumina pellet agreed well with expected results for this type of material.

The mutual diffusion coefficient for binary mixtures of water and the isomers of propanol

Abstract: Measurements of the mutual diffusion coefficient of the n -propanol-water and isopropanol-water systems have been performed at a pressure of 10 5 Pa by means of a flow technique. The results extend over the complete composition range for the mixtures and over the temperature range 25-65 °C. The reported diffusivities have an estimated uncertainty of + 2.5% Both the systems studied exhibit the minimum in the diffusivity as a function of composition at constant temperature which is characteristic of alcoho-lwater mixtures. In addition, a local maximum in the diffusivity occurs along an isotherm for alcohol-rich mixtures; this type of behaviour has not been observed hitherto. The data are compared with other transport and equilibrium properties for the mixtures, and with a simplified form of the statistical mechanical theory.

Thermal diffusivity measurement by a radial heat flow method

Abstract: A method is presented whereby the thermal diffusivity of a solid is measured by observing the temperature excursion which results from a radial flow of heat. The radial heat flow is produced by the instantaneous deposition of energy on a disk region of one surface of a planar specimen. Experimental results which utilized this technique are presented for Armco iron and they are compared to results based on an axial heat flow method with a radial heat flow correction.

On Cellular Cloud Patterns. Part 1: Mathematical Model

Abstract: The relationship of “open” or “closed” cellular cloud patterns to large-scale sinking or rising motion is investigated. In particular, it is shown that the open cell patterns typically found behind cold fronts can be determined by a large-scale sinking motion of a convectively unstable layer. The mathematical model treated is one in which a layer of Boussinesq fluid between two conducting porous boundaries is given a uniform vertical velocity w0. The linear stability problem for small γ=w0/κ, where κ is the thermal diffusivity and d the depth of the layer, is solved for a critical Rayleigh number Rc. The solutions for the flow field for this linear problem are infinitely degenerate. Steady finite-amplitude solutions of the nonlinear Boussinesq equations are obtained by a double expansion of the fields in powers of γ and an amplitude ϵ. The stability of the nonlinear solutions is investigated and it is shown that for a certain range of Prandtl numbers, (i) for γ>0, only hexagonal cells with upward...

Transport Processes in Composite Media

Abstract: Transport processes in two types of composite media were investigated In one type, exemplified by hydrogen diffusion in steel with microvoids present, short-lived trapping reactions result in a lowering of the effective or overall diffusion coefficient The other type of composite consists of randomly mixed parts or phases having no interfacial resistance or trapping The percolation properties of such systems are reviewed, and thermal conductivity, self- diffusivity, and gas permeability in certain porous solids are analyzed in the light of percolation theory

Thermal conductivity of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) near the metal-insulator transition

Abstract: The thermal conductivity of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) has been determined through a simultaneous measurement of the specific heat and thermal diffusivity using an ac method. A steplike anomaly has been observed which coincides in temperature with anomalies in the specific heat and electrical conductivity. Interpreting ths step to be due to the loss of the electronic contribution to the thermal conductivity at the metal-insulator transition, we show that the Lorentz number is very close to the free-electron value for conductivity enhancement factors in the range 10-30. A mean-field calculation of the thermal current due to paraconductive fluctuations demonstrates that this contribution is negligible. The conclusion follows, assuming the free-electron Lorentz number to be appropriate for this material, that all of the observed electrical conductivity is the result of normal heat-carrying electrons.

Apparatus for the measurement of thermal properties of biomaterials

Abstract: Appropriate heating and temperature sensing means are immersed in a medium the thermal conductivity, k, and thermal diffusivity, α, of which are to be determined both in the presence of and in the absence of a flow of fluid therethrough. The method and system used for making such measurements operate in accordance with a thermal model of the heating means and the medium wherein the heating means is treated as a distributed thermal mass and wherein heat conduction occurs in a coupled thermal system which comprises both the heating means and the adjacent region of the medium which surrounds such heating means. The thermal conductivity of the medium is determined as a function of the temperature difference between the temperature of the unheated medium and the temperature of the heated medium, of the resistance of the heating means, of the power applied to the heating means, of the thermal conductivity of the heating means and of the radius of a spherical heating means having a volume equivalent to that of the actual heating means. In accordance with various embodiments of the invention the temperature difference and heating means resistance are maintained constant or are permitted to vary with time over the time period of measurement. Measurements of the thermal conductivity in the presence and in the absence of fluid flow permit a further measurement to be made of the rate of flow, ω, of a fluid which is flowing through the medium.

The Thermal Conductivity and Diffusivity Of Green River Oil Shales

Abstract: Introduction Thermal decomposition of the essentially insoluble organic matter in oil shales is the easiest way to obtain shale oil Proper planning of both surface and subsurface thermal oil-shale recovery operations requires values of the thermal conductivity and diffusivity of oil shales, There are few publications reporting thermal conductivities of oil shale from the Green River formation Gavin and Sharpl reported three values for the thermal conductivity of an oil-shale sample assaying 427 gal/ton, Their reported values were obtained over the temperature range from 77° to 167°F, but the direction with respect to the bedding plane in which the measurement: were taken was not indicated Thomasz presented thermal conductivity data of an oil shale assaying 30 gal/ton and at a mean temperature of 104”F, finding values parallel to the bedding plane to be 30 percent higher than those normal to it and finding essentially no effect of stress levels of 1,000 psi and above Tihen et al’ report on the thermal conductivity and diffusivity at room temperature of unconfined raw, retorted, and burned oil shales assaying from 8,6 to 58,6 gal/ton Correlations for thermal conductivity are given for temperatures as high as 1,100”F while considering the anisotropic character of the oil shales Though bolted, samples developed cracks during heating, suggesting those results may be more applicable to rubbled pieces in subsurface cavities or large surface retorts than to the calculation of heat losses from a process zone in subsurface thermal operations This report presents thermal conductivity data of confined Green River oil shales over a wide range of temperature, fluid pressure, axial stress, and kerogen content, The bulk of our measurements were carried out on oil shales as they are found in nature; that is, they were not previously retorted At the higher temperatures, however, our reported values include the effects of at least some decomposition of the organic matter Thermal conductivities of burned (organicfree) oil shales were measured on only two samples The effectsof kerogen content, fluid pressure, axial stress, temperature, orientation, heating time, and mineral composition on the thermal conductivities of oil shales were investigated These basic rcsu[ts were then used together with specific gravity and a correlation of specificheat with oil yield by Fischer assay and te:ilperature to obtain calculated thermal diffusivities for each sample

Calculation of Supersaturation Profiles in Thermal Diffusion Cloud Chambers.

Abstract: The maximum supersaturation in a diffusion cloud chamber as a function of the temperature difference between the two plates has been calculated under various conditions. It is shown that the assumption of linear profiles for the temperature and vapor pressure is a very good one, and that the effect of thermal diffusion is negligible for a water-air mixture.

Measurement of thermal properties of nuclear materials by laser flash method

Abstract: The present status and prospects of the laser flash method for the measurement of thermal properties of nuclear materials are reviewed. Special emphasis is placed on the progress seen in experimental techniques for obtaining precise data on heat capacity and thermal diffusivity from 80 to 1,000 K. In this temperature range, the heat capacity and the thermal diffusivity of ceramic nuclear materials can be determined within a precision of ±2%. The data on heat capacity and thermal conductivity determined by the laser flash method are presented and discussed in respect of such nuclear materials as U02 U4O9 ThO2 BeO, ThO2-UO2 ThO2-Ce02 UO2-ZrO2 ThO2-Be0, UC, UN, US, UP, UC1-xNx and UP1-xSx.