Showing papers on "Thermal diffusivity published in 2000"

Helium diffusion from apatite: General behavior as illustrated by Durango fluorapatite

Abstract: High-precision stepped-heating experiments were performed to better characterize helium diffusion from apatite using Durango fluorapatite as a model system. At temperatures below 265°C, helium diffusion from this apatite is a simple, thermally activated process that is independent of the cumulative fraction of helium released and also of the heating schedule used. Across a factor of ∼4 in grain size, helium diffusivity scales with the inverse square of grain radius, implying that the physical grain is the diffusion domain. Measurements on crystallographically oriented thick sections indicate that helium diffusivity in Durango apatite is nearly isotropic. The best estimate of the activation energy for He diffusion from this apatite is E_a = 33±0.5 kcal/mol, with log(D_0) = 1.5±0.6 cm^2/s. The implied He closure temperature for a grain of 100 μm radius is 68°C assuming a 10°C/Myr cooling rate; this figure varies by ±5°C for grains ranging from 50 to 150 μm radius. When this apatite is heated to temperatures from 265 to 400°C, a progressive and irreversible change in He diffusion behavior occurs: Both the activation energy and frequency factor are reduced. This transition in behavior coincides closely with progressive annealing of radiation damage in Durango apatite, suggesting that defects and defect annealing play a role in the diffusivity of helium through apatite.

Analysis of Variants Within the Porous Media Transport Models

Abstract: An investigation of variants within the porous media transport models is presented in this work. Four major categories in modeling the transport processes through porous media, namely constant porosity, variable porosity, thermal dispersion, and local thermal non-equilibrium, are analyzed in detail. The main objective of the current study is to compare these variances in models for each of the four categories and establish conditions leading to convergence or divergence among different models. To analyze the effects of variants within these transport models, a systematic reduction and sensitivity investigation for each of these four aspects is presented. The effects of the Darcy number, inertia parameter, Reynolds number, porosity, particle diameter, and the fluid-to-solid conductivity ratio on the variances within each of the four areas are analyzed. For some cases the variances within different models have a negligible effect on the results while for some cases the variations can become significant. In general, the variances have a more pronounced effect on the velocity field and a substantially smaller effect on the temperature field and Nusselt number distribution

Effective diffusivity as a diagnostic of atmospheric transport: 1. Stratosphere

Abstract: The transport and mixing properties of the isentropic flow in the lower and middle stratosphere are analyzed by using observed winds to advect a tracer on isentropic surfaces in the range 400–850 K The effective diffusivity diagnostic introduced by Nakamura and collaborators is applied to the tracer field in order to identify barriers to transport and mixing regions, and to follow their seasonal evolution Large effective diffusivity corresponds to strong mixing, and small effective diffusivity corresponds to weak mixing, ie, to barriers The effective diffusivity shows, in the winter stratosphere of each hemisphere, the evolution of the vortex-edge barrier and the midlatitude surf zone, and also the extent of any mixing within the vortex At low latitudes in the stratosphere there is a region of low effective diffusivity whose latitudinal width varies with height, broadening substantially from 400 K to 550 K The low values of effective diffusivity in this “tropical-reservoir” region imply little isentropic transport into or out of it There is a strong seasonal cycle to the reservoir, which has different forms at 400 K, 450–600 K, and above 650 K, determined by the relative influences of tropospheric synoptic eddies and stratospheric planetary waves Comparison of effective diffusivity between the Northern Hemisphere winters 1996/1997 and 1997/1998 shows strong differences at low latitudes according to the phase of the quasi-biennial oscillation (QBO) When there are QBO easterlies, there is a broad region of very low effective diffusivity at low latitudes When there are QBO westerlies, there are very low values of effective diffusivity at low latitudes within the westerlies themselves but larger values at their edges

Electrochemical‐Thermal Model of Lithium Polymer Batteries

Abstract: A mathematical model has been developed to study heat transfer and thermal management of lithium polymer batteries. Temperature dependent parameters including the diffusion coefficient of lithium ions, ionic conductivity of lithium ions, transference number of lithium ions, etc., have been added to a previously developed electrochemical model to more completely characterize the thermal behavior of the lithium polymer system. In addition, experimental studies of the discharge behavior and heat generation rate of lithium polymer cells have been conducted. Comparisons between experimental and mathematical results are presented. Finally different thermal management approaches are discussed. © 2000 The Electrochemical Society. All rights reserved.

Effects of residual stress on the performance of plasma sprayed functionally graded ZrO2/NiCoCrAlY coatings

Abstract: Functionally graded ZrO2/NiCoCrAlY coatings were produced by plasma spraying using pre-mixed and spheroidized powders as the feedstock. The microstructure, density, elastic modulus, thermal conductivity/diffusivity, microhardness and coefficient of thermal expansion were found to change gradually through the five-layer functionally graded coatings which was beneficial for the improvement of mechanical and thermal properties of the coatings. The residual stresses of the as-sprayed coatings with different graded layers and different thicknesses, as well as the changes of residual stresses during thermal cycling were simulated by finite element analysis (FEA). Results showed that residual stress was the lowest for the five-layer functionally graded coating compared to that of the duplex coating and three-layer coating with the same thickness, and the residual stresses increased with a decrease in coating thickness. For the coatings with the same thickness, the bond strength and thermal cycling resistance were found to increase with an increase in the number of graded layers which is due to the decrease in the residual thermal stresses. The bond strength of the five-layer functionally graded coating was about twice as high as that of the duplex coating and the number of thermal cycles of functionally graded coating was five times higher than that of the duplex coating. Results also showed that the bond strength decreased with an increase in the coating thickness.

A Computational Study of Molecular Diffusion and Dynamic Flow through Carbon Nanotubes

Abstract: Molecular dynamics simulations are used to study the flow of methane, ethane, and ethylene through carbon nanotubes at room temperature. The interatomic forces in the simulations are calculated using a classical, reactive, empirical bond-order hydrocarbon potential coupled to Lennard−Jones potentials. The simulations show that the intermolecular and molecule−nanotube interactions strongly affect both dynamic molecular flow and molecular diffusion. For example, molecules with initial hyperthermal velocities slowed to thermal velocities in nanotubes with diameters less than 36 A. In addition, molecules moving at thermal velocities are predicted to diffuse from areas of high density to areas of low density through the nanotubes. Normal-mode molecular thermal diffusion is predicted for methane for nearly all the nanotube diameters considered. In contrast, ethane and ethylene are predicted to diffuse by normal mode, single-file mode, or at a rate that is transitional between normal-mode and single-file diffusi...

The glass transition temperature concept in rice drying and tempering : Effect on milling quality

Abstract: Research on rice drying and tempering has shown that high drying temperatures (up to 60 ³ C) and high moisture removal rates (up to 6 percentage points moisture content) can be used without reducing milling quality as long as sufficient tempering at a temperature above the glass transition temperature (Tg) is allowed between drying passes. Using drying air temperatures above the Tg of the rice significantly reduces drying and tempering durations since kernel moisture diffusivity is much higher above Tg. Understanding the effects of glass transition is important in optimizing the drying and tempering processes in terms of overall required drying durations to achieve given moisture removals. The objective of this study was to investigate the effect of Tg on drying rates when using drying air temperatures above and below Tg. Both medium–grain and long–grain rice was harvested during 2000 and dried under various air conditions above and below the Tg of the rice. Results showed that rice dried significantly faster above Tg than below Tg. In addition, high temperature/low relative humidity drying air conditions, which result in a low equilibrium moisture content, apparently caused the surface of the kernel to transition from a rubbery to a glassy state and reduced the drying rate.

Brownian Dynamics simulation of hard-sphere colloidal dispersions

Abstract: The rheology of hard-sphere suspensions in the absence of hydrodynamic interactions is examined by Brownian Dynamics. Simulations are performed over a wide range of volume fraction φ and Peclet number Pe=γa2/D, where γ is the shear rate and D=kT/6πηa is the Stokes-Einstein diffusivity of an isolated spherical particle of radius a and thermal energy kT in a fluid of viscosity η. At low Pe, the viscosity decreases as Pe increases—the suspension shear thins. The first normal stress difference is positive, while the second normal stress difference is negative. Each normal stress difference vanishes at very low Pe and increases in magnitude to an extremum at Pe≈3. The suspension pressure is proportional to kT and is found to grow as Pe2 from its equilibrium value. Long-time self-diffusivities scale as D and grow as Pe is increased in this regime. At Pe≈100, the suspension undergoes a disorder–order transition to a microstructure of hexagonally packed strings aligned in the flow direction, which is accompanie...

Experimental measurement of gas diffusivity in bitumen: Results for carbon dioxide

Abstract: A new technique is developed to measure the diffusivity of gas in bitumen as a function of composition. Results are presented for a carbon dioxide−bitumen system, which is of considerable industrial relevance. The technique employs transient pressure data obtained from a nonintrusive pressure decay experiment at constant temperature and volume. The underlying theory is presented along with a computational algorithm to calculate diffusivity. Using experimental pressure decay data in the range 25−90 °C at 4 MPa, the diffusivity of carbon dioxide in bitumen is calculated. The results are compared with the limited data available in the literature. The approach is straightforward and can be easily applied to other nonvolatile liquid systems.

Thermal conductivity of the hole-doped spin ladder system Sr14-xCaxCu24O41

Abstract: The thermal conductivity of the spin-1/2 ladder system Sr14-xCaxCu24O41 ( x = 0, 2, and 12) has been measured both along ( kappa(c)) and perpendicular to ( kappa(a)) the ladder direction at temperatures between 5 and 300 K. While the temperature dependence of kappa(a) is typical for phonon heat transport, an unusual double-peak structure is observed for kappa(c)(T). We interpret this unexpected feature as a manifestation of quasi-one-dimensional magnon thermal transport mediated by spin excitations along the ladders.

Imaging Superficial Tissues With Polarized Light

Abstract: Objective: Polarized light can be used to obtain images of superficial tissue layers such as skin, and some example images are presented. This study presents a study of the transition of linearly polarized light into randomly polarized light during light propagation through tissues. Study Design/Materials and Methods: The transition of polarization was studied in polystyrene microsphere solutions, in chicken muscle (breast) and liver, and in porcine muscle and skin. The transition is discussed in terms of a diffusion process characterized by an angular diffusivity (radians 2 /mean free path) for the change in angular orientation of linearly polarized light per unit optical path traveled by the light. Results: Microsphere diffusivity increased from 0.031 to 0.800 for diameters decreasing from 6.04 mm to 0.306 mm, respectively. Tissue diffusivity varied from a very low value (0.0004) for chicken liver to an intermediate value (0.055) for chicken and porcine muscle to a very high value (0.78) for pig skin. Conclusion: The results are consistent with the hypothesis that birefringent tissues randomize linearly polarized light more rapidly than nonbirefringent tissues. The results suggest that polarized light imaging of skin yields images based only on photons backscattered from the superficial epidermal and initial papillary dermis because the birefringent dermal collagen rapidly randomizes polarized light. This anatomic region of the skin is where cancer commonly arises. Lasers Surg. Med. 26:119‐129, 2000. © 2000 Wiley-Liss, Inc.

Measurements of Thermal Conductivity and Electrical Conductivity of a Single Carbon Fiber

Abstract: In this paper, the thermal conductivity of a single carbon fiber under different manufacturing conditions is measured using the steady-state short-hot-wire method. This method is based on the heat transfer phenomena of a pin fin attached to a short hot wire. The short hot wire is supplied with a constant direct current to generate a uniform heat flux, and both its ends are connected to lead wires and maintained at the initial temperature. The test fiber is attached as a pin fin to the center position of the hot wire at one end and the other end is connected to a heat sink. One-dimensional steady-state heat conduction along the hot wire and test fiber is assumed, and the basic equations are analytically solved. From the solutions, the relations among the average temperature rise of the hot wire, the heat generation rate, the temperature at the attached end of the fiber, and the heat flux from the hot wire to the fiber are accurately obtained. Based on the relations, the thermal conductivity of the single carbon fiber can be easily estimated when the average temperature rise and the heat generation rate of the hot wire are measured for the same system. Further, the electrical conductivity of the single carbon fiber is measured under the same conditions as for the thermal conductivity using a four-point contact method. The relation between the thermal conductivity and electrical conductivity is further discussed, based on the crystal microstructure.

Parameter estimations for measurements of thermal transport properties with the hot disk thermal constants analyzer

Abstract: The objective of this work is to improve measurements of transport properties using the hot disk thermal constants analyzer. The principle of this method is based on the transient heating of a plane double spiral sandwiched between two pieces of the investigated material. From the temperature increase of the heat source, it is possible to derive both the thermal conductivity and the thermal diffusivity from one single transient recording, provided the total time of the measurement is chosen within a correct time window defined by the theory and the experimental situation. Based on a theory of sensitivity coefficients, it is demonstrated how the experimental time window should be selected under different experimental situations. In addition to the theoretical work, measurements on two different materials: poly(methylmethacrylate) and Stainless Steel A 310, with thermal conductivity of 0.2 and 14 W/mK, respectively, have been performed and analyzed based on the developed theory.

Mode-mismatched thermal lens spectrometry for thermo-optical properties measurement in optical glasses: a review

Abstract: In this work, the application of thermal lens spectrometry (TLS) to study thermo-optical and spectroscopic properties of optical glasses is described. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations of the method as compared with conventional measurements. The technique is applied to determine the thermal diffusivities, temperature coefficient of optical path length changes, d s/ d T , and the fluorescence quantum efficiencies of several glasses such as fluorides, chalcogenides, chalcohalides, soda lime and low silica calcium aluminosilicate. For some of these glasses, the effect of glass composition on thermo-optical properties was studied. For aluminosilicate glasses was observed an 8% decrease of the thermal diffusivity with the increase of Nd2O3 doping. Five kinds of fluoride glasses were studied and it was observed that fluorindate and fluoaluminate present better thermo-optical properties compared to fluorozirconate glasses: thermal diffusivity ∼ 20% higher and d s/ d T ∼ 50% lower (in modulus). All fluoride glasses had negative d s/ d T and all the other glasses a positive d s/ d T . For fluoride glasses, the TLS measurements were performed up to the glass transition temperature ( T g ). Near T g thermal diffusivity decreases and d s/ d T increases, both by one order of magnitude. The TLS was used to determine fluorescence quantum efficiency and concentration quenching in Nd3+-doped glasses (aluminosilicate and fluorozirconate). These results were compared with the Judd–Ofelt calculations.

Relative dispersion in the subsurface North Atlantic

Abstract: Pair statistics are calculated for subsurface floats in the North Atlantic. The relative diffusivity (the derivative of the mean square particle separation) is approximately constant at large scales in both eastern and western basins, though the implied scale of the energy-containing eddies is greater in the west. But the behavior at times soon after pair deployment is quite different in the two basins; in the west the diffusivity grows approximately as distance to the 4/3 power, consistent with an inverse turbulent cascade of energy (or possibly of mixing superimposed on a mean shear), but in the east the diffusivity grows more slowly, as for instance in simple stochastic systems. Exponential stretching, expected in an enstrophy cascade, is not resolved in any region; however, this may reflect only that the present pair separations are too large initially.

Theoretical model of thermal diffusion factors in multicomponent mixtures

Abstract: Unlike molecular diffusion, neither measured thermal diffusion coefficients nor the theoretical framework exist for the estimation of thermal diffusion coefficients in nonideal multicomponent mixtures. This work derives a theoretical model for thermal diffusion coefficients in ideal and nonideal multicomponent mixtures, based on the thermodynamics of irreversible processes and the molecular kinetic approach incorporating explicit effects of nonequilibrium properties, such as the net heat of transport and molecular diffusion coefficients, and of equilibrium properties of the mixture, which are determined by the Peng-Robinson equation of state. An interesting feature of this model is that in nonideal multicomponent mixtures thermal diffusion coefficients depend on molecular diffusion coefficients, while in binary mixtures they do not. The model successfully describes thermal diffusion factors of binary mixtures for which experimental data are available, even those in extreme nonideal conditions and close to the critical point. Since experimental data on thermal diffusion factors in multicomponent hydrocarbon mixtures are not available, testing the model's accuracy was not possible. The model, however, successfully predicted spatial variation of composition in a ternary mixture of nC{sub 24}/nC{sub 16}/nC{sub 12}, providing an indirect verification. The six-component mixture of C{sub 1}/C{sub 3}/nC{sub 5}/nC{sub 10}/nC{sub 16}/C{sub 2} shows significant dependency of thermal diffusionmore » factors on the distance to the critical point. It also demonstrates for the first time that there is no need to adopt a sign convention for thermal diffusion coefficients in binary and higher mixtures. The thermodynamic stability analysis shows that when the thermal diffusion coefficient is positive, the component should go to the cold region in a binary mixture.« less

Thermal-wave resonator cavity design and measurements of the thermal diffusivity of liquids

Abstract: A liquid-ambient-compatible thermal wave resonant cavity (TWRC) has been constructed for the measurement of the thermal diffusivity of liquids. The thermal diffusivities of distilled water, glycerol, ethylene glycol, and olive oil were determined at room temperature (25 °C), with four-significant-figure precision as follows: (0.1445±0.0002)×10−2 cm2/s (distilled water); (0.0922±0.0002)×10−2 cm2/s (glycerol); (0.0918±0.0002)×10−2 cm2/s (ethylene glycol); and (0.0881±0.0004)×10−2 cm2/s (olive oil). The liquid-state TWRC sensor was found to be highly sensitive to various mixtures of methanol and salt in distilled water with sensitivity limits 0.5% (v/v) and 0.03% (w/v), respectively. The use of the TWRC to measure gas evolution from liquids and its potential for environmental applications has also been demonstrated.