Showing papers on "Thermal diffusivity published in 1990"
TL;DR: In this article, the authors present new, representative equations for the viscosity and thermal conductivity of carbon dioxide, which are based in part upon a body of experimental data that have been critically assessed for internal consistency and for agreement with theory whenever possible.
Abstract: The paper contains new, representative equations for the viscosity and thermal conductivity of carbon dioxide. The equations are based in part upon a body of experimental data that have been critically assessed for internal consistency and for agreement with theory whenever possible. In the case of the low‐density thermal conductivity at high temperatures, all available data are shown to be inconsistent with theoretical expectation and have therefore been abandoned in favor of a theoretical prediction. Similarly, the liquid‐phase thermal conductivity has been predicted owing to the small extent and poor quality of the experimental information. In the same phase the inconsistencies between the various literature reports of viscosity measurements cannot be resolved and new measurements are necessary. In the critical region the experimentally observed enhancements of both transport properties are well represented by theoretically based equations containing just one adjustable parameter. The complete correlations cover the temperature range 200 K≤T<1500 K for viscosity and 200 K≤T≤1000 K for thermal conductivity, and pressures up to 100 MPa. The uncertainties associated with the correlation vary according to the thermodynamic state from ±0.3% for the viscosity of the dilute gas near room temperature to ±5% for the thermal conductivity in the liquid phase. Tables of the viscosity and thermal conductivity generated by the representative equations are provided to assist with the confirmation of computer implementations of the calculation procedure.
707 citations
TL;DR: In this article, the authors examined the two-dimensional advective and conductive transport of heat in a region of thrust faulting and showed that the steady state temperatures near the fault are reduced by a divisor, S = 1+b(z)Vsinδ)/κ below what they would be with the same heat sources but in the absence of advection.
Abstract: We examine the two-dimensional advective and conductive transport of heat in a region of thrust faulting. Both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor, S=1+b(zƒVsin δ)/κ below what they would be with the same heat sources but in the absence of advection. In this expression zƒ is the depth to the fault, V is the slip rate, δ is the component of the dip of the fault in the direction of underthrusting, κ is thermal diffusivity, and b is a dimensionless factor that is essentially equal to one for most forms of heating. Initial changes in temperatures near the fault are given by T(zƒ,t)=T(zƒ,0)-12Vsinδ∂T(z,0)/∂z. These simple formulae are successful because of the neglible influence of lateral conduction of heat, at least for slip rates of a few mm/yr or more. Two time constants govern the transition from the initial change in temperature to steady state: t1 = uƒ/V, where uƒ is the distance along the fault in the direction of underthrusting from the surface to the depth in question, and t2=zƒ2/κπ2, where zƒ is the depth to the fault. When elapsed times exceed the sum of these two time constants, temperatures differ from their steady state values by only about 10 percent. The numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted. Putain. An application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 MPa and are greater than 30 MPa. Calculations of temperatures appropriate for the Himalaya suggest that shear stresses of 100 MPa on the Main Central Thrust probably are required to account for the Tertiary granites of the region, if melting took place after slip began on the thrust. Similarly, the cut-off in seismicity at a depth of about 15 km in the Himalaya, if due to temperatures exceeding 350° to 450°C, implies a deviatoric stress close to 100 MPa.
373 citations
TL;DR: In this article, the diffusivity of ultrasound in an untextured aggregate of cubic crystallites is studied theoretically with a view towards nondestructive characterization of microstructures, and the covariance is found to obey an equation of radiative transfer for which a diffusion limit is taken.
Abstract: T he diffusivity of ultrasound in an untextured aggregate of cubic crystallites is studied theoretically with a view towards nondestructive characterization of microstructures. Multiple scattering formalisms for the mean Green's dyadic and for the covariance of the Green's dyadic (and therefore for the energy density) based upon the method of smoothing are presented. The first-order smoothing approximation used is accurate to leading order in the anisotropy of the constituent crystallites. A further, Born, approximation is invoked which limits the validity of the calculation to frequencies below the geometrical optics regime. Known result for the mean field attenuations are recovered. The covariance is found to obey an equation of radiative transfer for which a diffusion limit is taken. The resulting diffusivity is found to vary inversely with the fourth power of frequency in the Rayleigh, long wavelength, regime and inversely with the logarithm of frequency on the short wavelength, stochastic, asymptote. The results are found to fit the experimental data.
291 citations
TL;DR: In this paper, the effective moisture diffusivity (D) in starch materials was estimated by the method of slopes of the drying curve and a computer simulation technique, which was obtained on slabs and spherical samples of hydrated and gelatinized starches in an air-dryer operated at 60-100 degrees C and air velocity 2 m/sec.
Abstract: The effective moisture diffusivity (D) in starch materials was estimated by the method of slopes of the drying curve and a computer simulation technique. Drying data (moisture vs time) were obtained on slabs and spherical samples of hydrated and gelatinized starches in an air-dryer operated at 60-100 degrees C and air velocity 2 m/sec. The two methods gave similar results in high-amylopectin starch gels of low porosity, where liquid diffusion might predominate during drying. Considerable differences between the two methods were found in hydrated granular starches and in porous high-amylose gels, where vapor diffusion might be the main water transport mechanism.
278 citations
TL;DR: In this paper, a generalized Maxwell-Stefan (GMS) formulation of irreversible thermodynamics is used to describe surface diffusion of n sorbed species using the GMS formulation of reversible thermodynamics.
250 citations
TL;DR: In this article, improved correlations are described to correlate data on diffusivities D and the related energies of activation, and solubilities S and related heats of solution of gases in polymers.
228 citations
TL;DR: In this paper, a beamexpanding fiber (BEF) for embedding optical devices has been fabricated by utilizing thermally induced Ge diffusion in silica single-mode fibers (SMFs), and the preparation of fiber samples and their heat treatment is described, and the effect of heat on Ge dopant distribution and diffusion is discussed.
Abstract: A beam-expanding fiber (BEF) for embedding optical devices has been fabricated by utilizing thermally induced Ge diffusion in silica single-mode fibers (SMFs). The preparation of fiber samples and their heat treatment is described, and the effect of heat on Ge dopant distribution and diffusion is discussed. Modal-intensity distributions were studied and found to confirm the broadening of the modal field distribution after heat treatment of the fiber. Localized heat treatment to obtain BEFs is considered, and device characteristics are discussed. The BEF can arbitrarily change the spot size of a propagating mode without changing the normalized frequency. >
202 citations
TL;DR: In this paper, a methodology to measure hydraulic diffusivity of rock samples in the laboratory was developed and tested using a pore pressure oscillation superimposed upon the ambient porepressure at one end of the rock sample.
195 citations
01 Jan 1990
TL;DR: In this article, thermal conduction through tissue and its heat capacity is discussed and a variety of methods may be used to measure the thermal properties of tissue samples; the techniques used may be categorized as invasive or noninvasive, and in each case, it may enable steady-state or non-steady-state measurements to be made.
Abstract: This chapter discusses thermal conduction through tissue and its heat capacity. A variety of methods may be used to measure the thermal properties of tissue samples; the techniques used may be categorized as invasive or noninvasive, and in each case, it may enable steady-state or non-steady-state measurements to be made. Also, a set of semi-invasive techniques has been investigated in which temperatures have been measured using cutaneous and subcutaneous thermocouples with surface heat fluxes provided by various non-invasive sources. On the other hand, totally noncontact methods use external radiation to heat tissue and observe the subsequent time-course of skin temperature with a radiometer. The thermal conductivity, k, of tissues at temperatures above freezing may increase while showing a very slight positive temperature coefficient. It is generally recognized that tissues may be considered more accurately for thermal analysis as being composed of water, protein, and fat. Subsequently, thermal conductivity may then be expressed as k = ρ ∑ n = 1 3 k n ω n / ρ n , and ω n are thermal conductivity, density, and mass fraction of the n th component respectively and ρ the density of the composite material. While for temperatures below freezing, the specific heat of tissues, C, varies markedly with temperature in a manner depending strongly on the tissue water content. For the calculation of thermal capacities, the following equation may be used: C = ∑ n = 1 3 ω n C n where ω n is the mass fraction of the n th component and C n its specific heat.
178 citations
TL;DR: In this article, the authors derived a new infiltration equation that takes into account the possibility of an infinite diffusivity near saturation, using the example of two soils (clay and coarse sand).
Abstract: We derived a new infiltration equation that takes into account the possibility of an infinite diffusivity near saturation. Using the example of two soils (clay and coarse sand), we showed that this new infiltration equation has a sound physical basis. In particular, all parameters used are true soil properties that are constant with time and independent of the water depth imposed as a surface boundary condition. Compared with analytical, numerical, and experimental results, the equation shows a great precision (σ2 The present law introduces a significant improvement over the law obtained in part 1 of this series dealing with ponded infiltration by introducing the physical effect of an infinite diffusivity at saturation.
139 citations
TL;DR: In this article, the authors have modelled the properties of porous, single-crystal membranes for Langmuir's isotherm assuming that entry to and exit from the crystal proceeds via an externally adsorbed layer and that for intracrystalline diffusion.
Abstract: Properties of porous, single-crystal membranes have been modelled for Langmuir's isotherm assuming that entry to and exit from the crystal proceeds via an externally adsorbed layer. The potential moderating role of surface processes upon steady flow depends strongly upon temperature and upon the difference in activation energy for escape from the crystal to the externally adsorbed layer and that for intracrystalline diffusion. The moderating role decreases with increasing membrane thickness. Where steady flow is reduced by surface processes the apparent diffusivity found from this flow is less than the true intracrystalline diffusivity by an amount which increases the smaller the crystal. If along one-dimensional channel systems there are occasional partial blockages with associated abnormally high energy barriers, quantitative extension of the model shows potentially large flow reductions, independent of membrane thickness. Provided surface processes have in comparison a negligible effect steady flow will give the correct average intracrystalline diffusivity. Mixture separation has been considered in partially blocked or unblocked crystal membranes in the Henry's law range.
TL;DR: In this article, the authors reviewed and discussed data and information on the thermophysical properties of ice, snow, and sea ice, including thermal conductivity, specific heat, density, thermal diffusivity, latent heat of fusion, thermal expansion, and absorption coefficient.
Abstract: The paper reviews and discusses data and information on the thermophysical properties of ice, snow, and sea ice. These properties include thermal conductivity, specific heat, density, thermal diffusivity, latent heat of fusion, thermal expansion, and absorption coefficient. The available data are shown graphically for convenience in conjunction with the recommended correlation equations.
TL;DR: Using two different photoacoustic techniques for a two-layer system of variable thickness, the thermal diffusivity and the thermal conductivity are completely determined, based upon the effective-sample model widely used in heat transfer problems as discussed by the authors.
Abstract: Using two different photoacoustic techniques for a two-layer system of variable thickness, we show that the thermal diffusivity and the thermal conductivity are completely determined, based upon the effective-sample model widely used in heat-transfer problems. A procedure to establish a standard photothermal technique for measuring both the thermal diffusivity and the thermal conductivity is also discussed.
TL;DR: In this article, a Monte Carlo model for simulating final-stage sintering has been developed, which incorporates realistic microstructural features (grains and pores), variable surface difusivity, grain-boundary diffusivity and grain boundary mobility.
Abstract: A Monte Carlo model for simulating final-stage sintering has been developed. This model incorporates realistic microstructural features (grains and pores), variable surface difusivity, grain-boundary diffusivity, and grain-boundary mobility. A preliminary study of a periodic array of pores has shown that the simulation procedure accurately reproduces theoretically predicted sintering kinetics under the restricted set of assumptions. Studies on more realistic final-stage sintering microstructure show that the evolution observed in the simulation closely resembles microstructures of real sintered materials over a wide range of diffusivity, initial porosity, and initial pore sizes. Pore shrinkage, grain growth, pore breakaway, and reattachment have all been observed. The porosity decreases monotonically with sintering time and scales with the initial porosity and diffusivity along the grain boundary. Deviations from equilibrium pore shapes under slow surface diffusion or fast grain-boundary diffusion conditions yield slower than expected sintering rates.
TL;DR: In this paper, a laser pulse, when focused on a metal sample, produces characteristic elastic waveforms, which depend on whether thermo-elastic or ablative/evaporative mechanisms dominate the generation process.
Abstract: A laser pulse, when focused on a metal sample, produces characteristic elastic waveforms, which depend on whether thermoelastic or ablative/evaporative mechanisms dominate the generation process. In the thermoelastic regime, with an unconstrained surface, the predominant axial displacement is opposite to the direction of propagation (negative), but there is typically a small transient positive displacement. This precursor is not predicted by elastic point source models, but is predicted by models including thermal diffusion. A recent formulation of pulsed photoacoustic generation is used to show how the precursor arises from interaction of the thermal and elastic modes at the illuminated surface.
TL;DR: In this paper, a method for simultaneous or consecutive determination of the specific heat, thermal diffusivity, the dielectric constant, and the pyroelectric coefficient of thin polysilicon films is described.
Abstract: A method is described that enables the simultaneous or consecutive determination of the specific heat, the thermal diffusivity, the dielectric constant, and the pyroelectric coefficient of thin pyroelectric films. The sample is heated by the absorption of intensity modulated light at one surface. The pyroelectric current and the transient temperatures of the sample surfaces are recorded as a function of the modulation frequency of the chopped light. The transient temperature recording is performed via thin‐film bolometers. Analysis procedures for the determination of the specific heat, the thermal diffusivity, and the spatially varying pyroelectric coefficient are introduced and discussed. A simulation of the performance of integrated pyroelectric ir sensors on silicon chips is performed by a coupling of the pyroelectric material to a heat sink. It is shown that the response of pyroelectric ir sensors integrated on silicon chips is influenced by heat wave interference effects. Experimental results are giv...
TL;DR: Dans l'intervalle des frequences ou l'echantillon est thermiquement epais, l'amplitude du signal and the phase peuvent differencier les sources de chaleur de recombinaison non radiative rapide and lente responsables du signal photo acoustique.
Abstract: The photoacoustic signal of two semiconductor samples is investigated as a function of the modulation frequency in a heat-transmission configuration. It is shown that, in the frequency range where the sample is thermally thick, the signal amplitude and phase can single out the different fast and slow nonradiative recombination heat sources responsible for the photoacoustic signal. The characterization of the thermal and the carrier transport properties is discussed and some practical procedures for this purpose are also outlined.
TL;DR: In this article, the authors used regular regime theory to evaluate the effective moisture diffusivity of a commercial white bread, plain sheet muffin, and baking powder biscuit as a function of moisture content based on desorption experiments.
Abstract: Regular regime theory was used to evaluate the effective moisture diffusivity of a commercial white bread, plain sheet muffin, and baking powder biscuit as a function of moisture content based on desorption experiments. Volume shrinkage during drying was also monitored. The existence of regular regime periods in desorption processes for porous baked products was experimentally verified. Effective moisture diffusivity at temperatures between 20 and 100°C ranged from 2.5×10−5 to 5.5×10−3 cm2/s in the moisture range of 0.1–0.7 g of H20/g of solid for bread, 9.35×10−6 to 9.7×10−4 cm2/s in the moisture range of 0.1–0.65 g of H20/g of solid for biscuit, and 8.4×10−6 to 1.54×10−3 cm2/s in the moisture range of 0.1–0.9 g of H20/g of solid for muffin. The effect of temperature on effective moisture diffusivity was adequately modeled by the Arrhenius relationship. Activation energies for bread, biscuit, and muffin were found to be independent of moisture content and were 51, 51, and 55 kJ/mol, respectively. Mathematical models to relate the effective moisture diffusivity to temperature and moisture content were developed.
TL;DR: The nature of proton defects in oxides is described briefly in this paper, and literature data for the concentration, diffusivity, and conductivity of protons in oxide are reviewed and discussed.
TL;DR: In this article, the thermal conductivity, diffusivity and emissivity of a sandwich-system of low pressure chemically vapour-deposited SiO2 and Si3N4 films were investigated in the temperature range 80 −400 K.
TL;DR: In this article, an analytical model that describes hydrate dissociation under thermal stimulation in porous media is presented, and the model views the dissociation as a process in which gas and water are produced at a moving dissociation boundary.
Abstract: An analytical model that describes hydrate dissociation under thermal stimulation in porous media is presented. The model views the dissociation as a process in which gas and water are produced at a moving dissociation boundary. The boundary separates the dissociated zone containing gas and water from the undissociated zone containing the hydrate. A similarity solution to the conservation equations is derived, and results are presented in graphical forms that are useful in numerical computations. In particular, heat fluxes, temperature profiles, and gas pressure distributions are presented for two cases that simulate saturated-steam and hot-water thermal stimulation. A parametric study showed that the dissociation rate is a strong function of the thermal properties of the system and the porosity of the porous medium. The energy efficiency of the dissociation process, defined as the ratio of the heating value of the gas produced relative to the heat input, was also computed. For hydrate thermal stimulation, an energy efficiency value of about nine was found, which appears encouraging for natural gas production from hydrate.
TL;DR: In this paper, the collision integrals for the interactions of N(4S0) + N( 4S0), O(3P) + O( 3P), and O(4P + O 3P were computed from a semiclassical formulation of the scattering using the best available representations of all of the potential energy curves needed to describe the collisions.
Abstract: Accurate collision integrals for the interactions of N(4S0) + N(4S0), O(3P) + O(3P), and N(4S0) + O(3P) are reported. These are computed from a semiclassical formulation of the scattering using the best available representations of all of the potential energy curves needed to describe the collisions. Spectroscopic curves and other accurate measured data are used where available; the results of accurate ab initio electronic structure calculations are used to determine the remaining potential curves. The high-lying states are found to give the largest contributions to the collision cross sections. The nine collision integrals needed to determine transport properties to second order are tabulated for translational temperatures in the range 250-100,000 K. The viscosity, thermal conductivity, diffusion coefficient, and thermal diffusion factor for a gas composed of nitrogen and oxygen atoms in thermal equilibrium have been calculated. It is found that the second-order contribution to the transport properties is small. Graphs of these transport properties for various mixture ratios are presented for temperatures in the range 5000-15,000 K.
TL;DR: In this article, a coarse resolution, primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability, and the experiments suggest that it may be possible to calibrate the rate of deepwater formation of general circulation models, without affecting the poleward heat transport, by varying the magnitude of the vertical diffusivity below the thermocline.
Abstract: A coarse resolution, primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability. A case with constant diffusivity is compared with a case in which the diffusivity is inversely proportional to the local Brunt-V frequency. The stability-dependent parameterization of vertical diffusivity yields a poleward heat flux similar to that of a small, constant diffusivity. However, this parameterization increases the mean temperature in the deep ocean by about 0.8°C and the strength of the meridional circulation by over 40%. In addition, the stability-dependent diffusivity is found to increase the stratification in the deep ocean. The experiments suggest that it may be possible to calibrate the rate of deep-water formation of general circulation models, without affecting the poleward heat transport, by varying the magnitude of the vertical diffusivity below the thermocline. The explicit...
TL;DR: In this article, thermal diffusivity data transverse to the fiber direction for composites composed of a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon-coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix-fiber interface.
Abstract: Experimental thermal diffusivity data transverse to the fiber direction for composites composed of a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon-coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix-fiber interface. Calculations of the interfacial thermal conductances indicate that at 300 C and 1-atm N2, more than 90 percent of the heat conduction across the interface occurs by gaseous conduction. Good agreement is obtained between thermal conductance values for the oxidized composite at 1 atm calculated from the thermal conductivity of the N2 gas and those inferred from the data for the effective composite thermal conductivity.
TL;DR: In this article, the photopyroelectric effect has been used to measure simultaneously specific heat (c), thermal conductivity (k) and thermal diffusivity (α) at low temperatures.
Abstract: The photopyroelectric effect has been used to measure simultaneously specific heat (c), thermal conductivity (k) and thermal diffusivity (α) at low temperatures. A calibration procedure which allows the use of a pyroelectric transducer at low temperatures is described. Simultaneous measurements of c, k, and α over a high T
c superconducting phase transition are reported.
TL;DR: In this paper, the authors compared tracer diffusivity data obtained in glasses vs. liquids without considering the effects of the transition of the glass structure from an unrelaxed, disequilibrium glass to a relaxed, equilibrium liquid.
Abstract: Experiments are reported on the variations in moisture absorption properties of an epoxy resin produced by sudden changes in temperature. Where the temperature change occurred when only a little water had been absorbed, the change in moisture absorption was determined by the change in diffusivity. Where a temperature change occurred when near saturation, a reverse thermal effect was observed. An attempt has been made to understand the results in terms of thermodynamics. Results obtained for the swelling changes accompanying sudden changes in moisture content suggest that it is the more tightly bound water in the polymer which takes part in the reverse thermal effect.
TL;DR: It is shown that the relaxation of this {ital extended} perturbation is consistent with the power-balance estimates of the local thermal diffusivity.
Abstract: Measurements on the TFTR tokamak of the electron-temperature-profile evolution and soft-x-ray emissivity on a fast (10-\ensuremath{\mu}sec) time scale during a sawtooth crash show that significant heat is deposited beyond the mixing (or reconnection) radius within 200 \ensuremath{\mu}sec following a sawtooth crash. This extended region in which electron heat is redistributed during the sawtooth crash substantially complicates the determination of heat transport properties from the subsequent heat pulse propagation. It is shown that the relaxation of this extended perturbation is consistent with the power-balance estimates of the local thermal diffusivity.
TL;DR: In this article, a two-dimensional Monte Carlo simulation procedure has been used to investigate the effect of the initial pore size on the microstructural evolution and the kinetics of final-stage sintering.
Abstract: A two-dimensional Monte Carlo simulation procedure has been used to investigate the effect of the initial pore size on the microstructural evolution and the kinetics of final-stage sintering. The sintering time scales with rt/Dgb and the grain-growth time scales with ri/D,,,. Pores are found to ef- fectively pin the grain boundaries from the beginning of final-stage sintering at a porosity of 6 = 0.09 until 6 = 0.03. For 6 I 0.03, the remaining pores do not effectively retard grain-boundary migration and normal grain growth occurs. Small pores were found to be less effective at retarding grain growth than expected on the basis of a simple grain- growth pinning model. The mean pore size was found to be N THE preceding paper,' hereafter referred to as part I, we I have described a two-dimensional model for final-stage sin- tering and performed computer simulations using Monte Carlo techniques. In this model, both grains and pores are mapped onto a regular lattice and the resulting microstructure evolves in a self-consistent manner via grain growth and pore shrink- age, as dictated by the energetics and the kinetics of the system. By an appropriate choice of the nearest-neighbor interactions, grain-boundary and surface energies may be as- signed separately. Similarly, the rate constants associated with grain-boundary diffusivity, surface diffusivity, and grain- boundary mobility may also be independently assigned. The simulated microstructure bears close resemblance to those observed in cross sections of experimental samples during final-stage sintering. Features such as pore shrinkage, grain growth, pore migration, pore breakaway, and pore reattach- ment are observed in the simulations. It is also found that the sintering rate decreases monotonically with time and that it scales linearly with grain-boundary diffusivity- although de- viations from the equilibrium shape of pores can slow down sintering somewhat. Although the dependence of the sintering rate on grain- boundary diffusivity is important and self-evident, it is also of great interest to explore the interrelationship between microstructural parameters, such as pore size, grain size, and porosity. In particular, it would be desirable to establish a quantitative correlation between the initial microstructure
TL;DR: In this paper, the use of a pulsed laser source for the generation of elastic waves in materials is investigated, taking into account optical penetration into the material, and the shape of the precursor signal is strongly dependent on the optical and thermal properties of the material.
Abstract: The use of a pulsed laser source for the generation of elastic waves in materials is investigated, taking into account optical penetration into the material. Under appropriate conditions, a significant feature of the laser‐generated elastic waveform is a precursor (sharp spike) signaling the arrival of the longitudinal wave. The shape of this precursor signal is strongly dependent on the optical and thermal properties of the material. This paper shows that the observed precursor can be understood through the use of models that account for optical penetration and thermal diffusion into the material.