Showing papers on "Thermal diffusivity published in 1998"

The observed properties of liquid helium at the saturated vapor pressure

Abstract: The equilibrium and transport properties of liquid 4He are deduced from experimental observations at the saturated vapor pressure. In each case, the bibliography lists all known measurements. Quantities reported here include density, thermal expansion coefficient, dielectric constant, superfluid and normal fluid densities, first, second, third, and fourth sound velocities, specific heat, enthalpy, entropy, surface tension, ion mobilities, mutual friction, viscosity and kinematic viscosity, dispersion curve, structure factor, thermal conductivity, latent heat, saturated vapor pressure, thermal diffusivity and Prandtl number of helium I, and displacement length and vortex core parameter in helium II.

Enhanced Mass Transfer During Osmotic Dehydration of High Pressure Treated Pineapple

Abstract: High pressure pretreatment (100–700 MPa) was applied to enhance mass transfer rates during osmotic dehydration of pineapples and accelerate the process. Experimentally determined diffusivity values, based on a Fickian model, increased fourfold for water and twofold for sugar. Diffusivity values were correlated with pretreatment pressure by an equation of the form D�A exp(�B/P), which suggests that diffusivity would level after an initial increase in pressure. The increase was attributed to breaking-up of cells walls which facilitated the transport of water. Evidence for the extent of cell wall break-up with applied pressure was based on differential interference contrast microscopic examination of tissue. Preliminary experiments on rehydration characteristics showed high pressure pretreated samples did not absorb as much water as controls.

Thermal diffusion effects in hydrogen-air and methane-air flames

Abstract: The influence of thermal diffusion on the structure of hydrogen-air and methane-air flames is investigated numerically using complex chemistry and detailed transport models. All the transport coefficients in the mixture, including thermal diffusion coefficients, are evaluated using new algorithms which provide, at moderate computational costs, accurate approximations derived rigorously from the kinetic theory of gases. Our numerical results show that thermal diffusion is important for an accurate prediction of flame structure. §E-mail address: ern@cermics.enpc.fr ∥E-mail address: giovangi@cmapx.polytechnique.fr

The effect of bubble-mediated gas transfer on purposeful dual-gaseous tracer experiments

Abstract: For air-water gas exchange across unbroken surfaces, the only gas-dependent parameter affecting the transfer velocity is the molecular diffusivity of the transferring species. In contrast, bubble-mediated transfer processes can cause the transfer velocity to depend on both molecular diffusivity and aqueous-phase solubility. This can complicate the analysis of data from dual-gaseous tracer gas transfer experiments. Bubble effects also complicate the estimation of transfer velocities for other gases from the transfer velocity calculated using the dual-tracer data. Herein a method for incorporating the effects of bubble-mediated gas transfer processes on the transfer velocity is presented. This new procedure is used to analyze the data from two recent dual-tracer gas transfer experiments. Transfer velocities that include the effect of bubbles are calculated using the data from two previous oceanic dual-gaseous tracer experiments. Comparing these transfer velocities with transfer velocities calculated by neglecting the effect of bubbles shows that bubble-mediated transfer increased the transfer velocity of helium 3 by 5% at a wind speed of 10.6 m s−1. However, when using the transfer velocities for helium 3 to calculate transfer velocities for carbon dioxide under the same conditions, including the effect of bubbles decreases the transfer velocity of carbon dioxide by 18%. This shows that bubble-mediated transfer does not have a large effect on the analysis of dual-tracer data, but it is important in relating transfer velocities determined using helium 3 and sulfur hexafluoride to transfer velocities of more soluble gases at wind speeds above 10 m s−1.

Thermal Diffusivity of Semitransparent Materials Determined by the Laser-Flash Method Applying a New Analytical Model

Abstract: We have developed an analytical model to determine the thermal diffusivity of nonscattering materials from samples with low optical thickness and opaque boundaries with arbitrary emissivities. The paper outlines the new analytical model and describes measurements on two samples: a microscope slide glass and a high-grade fused quartz plate. Results show that the new model applied to measurements on gold- or graphite-coated samples leads to the same results as if a conventional model is used on gold-coated samples.

Mechanical and thermophysical properties of thick PYSZ thermal barrier coatings : correlation with microstructure and spraying parameters

Abstract: The thermophysical and mechanical properties of plasma-sprayed thermal barrier coatings are very strongly dependent on the microstructure, and this may be controlled by manipulating the parameters controlling the plasma spray process. A series of coatings was produced by plasma spraying ZrO 2 +8wt%Y 2 O 3 (PYSZ) to a thickness of about 2 mm, some six to eight times thicker then state of the art coatings currently in service. A controlled variation of plasma spray parameters was carried out to produce a range of different microstructures that could be related to material properties and these in turn correlated with the process variables. The particular properties studied were the thermophysical properties, diffusivity and expansion, and the mechanical properties, modulus, strain to failure and flexural strength measured in four point bend.

A new model of thermal diffusion coefficients in binary hydrocarbon mixtures

Abstract: Thermal diffusion is important for the study of composition variations in hydrocarbon reservoirs, and it can either enhance or weaken the separation in mixtures. We present a new model for the prediction of thermal diffusion coefficients in binary mixtures of reservoir fluids using the thermodynamics of irreversible processes. The model needs equilibrium properties of mixtures and energy of viscous flow. Equilibrium properties are obtained from the volume translated Peng−Robinson equation of state, and the energy of viscous flow is estimated from viscosity. The model has been applied to predict thermal diffusion coefficients of several mixtures consisting of nonhydrocarbon and hydrocarbon fluids. Comparisons of theoretical results with experimental data show a good performance of the model except in the near-critical region where all existing models are deficient. In particular, the predicted sign of thermal diffusion coefficients is consistent with experimental observations in systems investigated here, ...

Choosing a thermal model for electrothermal simulation of power semiconductor devices

Abstract: The literature proposes some thermal models needed for the electrothermal simulation of power electronic systems. This paper gives a useful analysis about the choice of the thermal model circuit networks, equivalent to a discretization of the heat equation by the finite difference method (FDM) and the finite-element method (FEM), and an analytic model developed by applying an internal approximation of the heat diffusion problem. The effect of the boundary condition representation and the introduced errors on temperature response at the heat source are studied. This study is advantageous, particularly for large surges of a short time duration.

A comparative study of transport in stellarators and tokamaks

Abstract: Experimental results on transport in stellarators and tokamaks are reviewed in a comparative sense. The objective is to learn about the importance of plasma current and magnetic shear for anomalous transport in high-temperature plasmas. On the basis of scaling expressions, the absolute values and parameter dependences of the confinement time are similar if the plasma current is expressed in terms of magnetic field parameters. The degradation of confinement with heating power is of the same order in both devices and it is difficult to explain it in terms of a diffusivity which depends on temperature or temperature gradient. The density dependence of confinement observed in stellarators has similar features as in ohmically heated tokamak discharges. A linear and a saturated regime can be distinguished. The critical density, at which saturation sets in, has a similar value and it seems to decrease with increasing machine size. Power degradation, transient transport, profile consistency and non-local transport are treated as related problems, which are connected to the question of the temperature dependence of the thermal diffusivity. Results from the various experiments cannot yet be described with a consistent physical picture. However, the importance of non-local effects is established in stellarators and tokamaks, although observed in different types of perturbation experiments. In stellarators and tokamaks, fluctuation measurements in the scrape-off layer are consistent with drift-wave-like turbulence being responsible for anomalous transport. In the core, density fluctuation amplitudes increase together with the diffusivity when the density is increased but the two parameters show opposite trends with increasing heating power. It turns out that transport in the two classes of devices is more alike than it has previously appeared. This indicates that the strong toroidal plasma current, major rational values of the rotational transform inside the plasma or strong magnetic shear are not the central elements of a theoretical model for anomalous transport in fusion plasmas.

Heat flux driven ion turbulence

Abstract: This work is an analysis of an ion turbulence in a tokamak in the case where the thermal flux is fixed and the temperature profile is allowed to fluctuate. The system exhibits some features of self-organized critical systems. In particular, avalanches are observed. Also the frequency spectrum of the thermal flux exhibits a structure similar to the one of a sand pile automaton, including a 1/f behavior. However, the time average temperature profile is found to be supercritical, i.e., the temperature gradient stays above the critical value. Moreover, the heat diffusivity is not the same for a turbulence calculated at fixed flux than at fixed temperature gradient, with the same time averaged temperature. More precisely the diffusivity at fixed temperature is found to be larger in the edge and smaller close to the heat source.

Thermal conductivity determinations of thermal barrier coatings

Abstract: The thermal diffusivity and conductivity of several free-standing thermal barrier coatings (TBCs) and coatings bonded to superalloy substrates were investigated using the laser flash technique. The results show good agreement between conductivity values for the free-standing and bonded TBC layers when certain parameters are within acceptable ranges. The results of a sensitivity analysis to identify the critical parameters are presented. It was shown that layered samples consisting of many alternate layers can be treated as though they are homogeneous with conductivity values equivalent to those calculated from rules of mixtures. The effects of heat treatment on the conductivity of plasma-sprayed TBCs are discussed.

Eddy-mean flow decomposition and eddy-diffusivity estimates in the tropical Pacific Ocean: 1. Methodology

Abstract: [1] Eddy diffusivity of the surface velocity field in the tropical Pacific Ocean was estimated using satellite-tracked drifting buoys (1979 through mid-1996). The tropical Pacific surface current system is characterized by nonstationarity, strong meridional shear, and an energetic mesoscale velocity field. Eddy diffusivity may be defined as the integral of the autocovariance of Lagrangian eddy velocities, requiring both stationary and homogeneous statistics of the eddy field. Eddy velocities were obtained by removing a splined mean field to eliminate mean shear from observations binned (1) spatially to group data that have similar dispersion characteristics and (2) temporally to create stationary eddy statistics. Zonal diffusivity estimates are up to ≈7 times larger than meridional diffusivity estimates in the high eddy energy regions. This anisotropy is associated with the meridional mesoscale wave motion (i.e., by equatorial and tropical instability waves) that increases eddy variance but does not lead to a proportional increase in water parcel diffusion because of the coherent character of the trajectory motion, at least for initial time lags. Simple autoregressive models of first and second order are used to describe and classify the resulting eddy statistics. An independent confirmation of the diffusivity estimate in the central/eastern Pacific was obtained by comparing tracer flux divergence computed from a parameterization using diffusivity estimates of our analysis with that from direct eddy Reynolds stress flux divergence. Our results show that diffusivity can be estimated for regions not considered previously either because of sparse data or the complexities of the velocity field.

Thermal diffusion in binary mixtures of smooth, nearly elastic spheres with and without gravity

Abstract: A Revised Enskog theory, valid to second order in the Enskog approximation, is used to characterize the amount of thermal diffusion in a steady, fully developed flow of a binary mixture of hard, inelastic spheres. The dependence of the thermal diffusion factor on mixture volume fraction, mole fraction, radii ratio, and material density ratio is explored in the presence and absence of gravity.

The vanishing effect of molecular diffusivity on turbulent dispersion : implications for turbulent mixing and the scalar flux

Abstract: In 1921 G. I. Taylor introduced (with little discussion) the notion that the dispersion of a conserved passive scalar in a turbulent flow is determined by the motion of fluid particles (independent of the molecular diffusivity). Here, a hypothesis of diffusivity independence is introduced, which provides a sufficient condition for the validity of Taylor's approach. The hypothesis, which is supported by DNS data, is that, at high Reynolds number, the mean of the scalar conditional on the velocity is independent of the molecular diffusivity. From this hypothesis it is shown that (at high Reynolds number) the conditional Laplacian of the scalar is zero. This new result has several significant implications for models of turbulent mixing, and for the scalar flux. Primarily, a model of turbulent scalar mixing that is independent of velocity is inconsistent with the hypothesis, and gives rise to a spurious source or (more likely) sink of the scalar flux.

Electrohydrodynamic instability in a thin fluid layer with an electrical conductivity gradient

Abstract: The onset of electrohydrodynamic motion associated with the imposition of an electric field across a thin layer of liquid has been investigated for the case in which the electrical conductivity varies linearly over the depth of the layer. The variation of the conductivity is due to concentration gradients in the charge-carrying solutes and its spatiotemporal evolution is represented by a convective-diffusion equation. When the viscous relaxation time is long compared to the time for charge relaxation, the analysis reveals that the neutral stability curves for the layer can be characterized by three dimensionless parameters: Rae≡deE02Δσ/μKeffσ0, an electrical Rayleigh number; Δσ/σ0, the relative conductivity increment; and α, the transverse wave number of the disturbance. Here d is the thickness, e is the dielectric constant, and μ is the viscosity of the layer, E0 is the applied field strength at the lower conductivity boundary, and Keff is an effective diffusivity associated with the Brownian motion of the charge-carrying solutes. With stress-free boundaries, at which the electrical conductivity and current are prescribed, the critical Rae is 1.416×104 at a critical transverse wave number of 1.90 when Δσ/σ0 is 8. As Δσ/σ0 increases, the critical Rae increases and shifts to slightly shorter wavelength disturbances; the critical imposed field strength, however, passes through a minimum because the lower-conductivity boundary exerts a considerable stabilizing influence in the presence of steep conductivity gradients. For Δσ/σ0≲8, the critical Rayleigh number increases as Δσ/σ0 decreases and the layer is only sensitive to long wavelength disturbances (α<0.1) for Δσ/σ0 below 4. Similar trends were obtained for liquid layers with other boundary conditions; e.g., rigid boundaries and constant potential boundaries.

High-precision gravimetric technique for determining the solubility and diffusivity of gases in polymers

Abstract: An in situ gravimetric technique, employing an electrobalance, is described for determining the solubility and diffusivity of gases in polymers over extended ranges of temperature and pressure. Solubilities of CO2 in polystyrene at 35°C were measured as a test case; the results are in excellent agreement with the literature values determined by the pressure decay method. Solubility and diffusivity results are also reported for PVC-CO2 at 35°C and for PS-1,1,1,2-tetrafluoroethane at 30, 90, and 120°C. A comparison with other studies shows the in situ method to be more efficient and precise than the ones based on weighing the gas-saturated polymer under ambient conditions. The kinetics of gas sorption were analyzed in terms of two data reduction techniques to derive diffusion coefficients. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2025–2032, 1998

Starch columns: Analog model for basalt columns

Abstract: Desiccation of starch-water mixtures produces tensile-crack patterns which appear to be interesting, but largely unknown study objects for fracture mechanics, structural geology, and volcanology. This paper concentrates on columnar jointing and on columns in starch. Starch columns have polygonal cross sections and are very similar to basalt columns. They are produced by lamp drying starch specimens with dimensions of several centimeters and have diameters in the millimeter range. The columns develop behind a crack front which propagates from the surface into the interior. The experiments, supported by X ray tomograms, show that polygonal regularity of the crack pattern is not present at the surface but develops during penetration. This transition is steered by a minimum-fracture-energy principle. The analogy between basalt cooling and starch desiccation is far reaching: water concentration in starch is analogous to temperature in basalt, both quantities obey diffusion equations, water loss is equivalent to heat loss, the resulting contraction stresses have similar dependences on depth and time, and in both cases the material strength is exceeded. The starch experiments show that column diameters are controlled by the depth gradient of water concentration at the crack front. High (low) gradients are connected with thin (thick) columns. By analogy, a similar relation with the temperature gradient exists for basalt columns. The (normalized) starch gradients are about 3 orders of magnitude larger than the (normalized) gradients in basalt. This explains why starch columns are much thinner than basalt columns. The gradients are so different, because the crack front speeds differ by a factor of about 10: after 3 days the speed is about 10 mm/d in starch but about 100 mm/d in basalt [Peck, 1978]. The speed difference, in turn, results from the difference of the diffusion constants: the hydraulic diffusivity of starch is 2 orders of magnitude lower than the thermal diffusivity of basalt.

Bulk Amorphous Alloys : preparation and Fundamental Characteristics

Abstract: 1. History of Bulk Amorphous Alloys 2. High Glass-Forming Ability and its Dominant Factors 3. Compositional Dependence of the Dominant Factors for GlassFormation, Tx and Tg/Tm. 3.1. Ln-Al-(Ni,Cu) Systems 3.2. Mg-Ln-(Ni,Cu) Systems 3.3. Zr-Al-TM Systems 3.4. Pd-Cu-Ni-P System 4. Continuous Cooling Transformation Behavior of SupercooledLiquid and Critical Cooling Rate for Glass Formation 5. Preparation Methods of Bulk Amorphous Alloys. 5.1. High-Pressure Die Casting Method. 5.2. Water Quenching Method 5.3. Copper Mold Casting Method 5.4. Arc Melting Method 5.5. Unidirectional Zone Melting Method 5.6. Suction Casting Method 6. Compositional Dependence of Maximum Sample Thickness (tmax). 6.1. La-Al-(Co,Ni,Cu) Systems 6.2. Zr-Al-(Co,Ni,Cu) Systems 6.3. Zr-Al-Ni-Cu-TM (TM = Transition Metals) Systems 7. Structure of Bulk Amorphous Alloys. 7.1. Mg-La-Ni System 7.2. La-Al-Ni System 7.3. Zr-Al-Ni System 7.4. Zr-Y-Al-Ni System 8. Structural Relaxation and Thermal Stability. 8.1. Structural Relaxation 8.2. Glass Transition 8.3. Two-Stage Glass Transition 8.4. Crystallization Behavior 9. Physical Properties. 9.1. Density 9.2. Specific Heat 9.3. Viscosity 9.4. Atomic Diffusivity 9.5. Electrical Resistivity 9.6. Thermal Expansion

Multi-Scale Microstructural Modeling of Concrete Diffusivity: Identification of Significant Varibles

Abstract: The ability to predict the expected chloride diffusivity of a concrete based on its mixture proportions and field-curing conditions would be of great benefit both in predicting service life of the concrete and in developing durability-based design codes. Here, a multi-scale microstructural computer model is applied to computing the chloride diffusivities of concretes with various mixture proportions and projected degrees of hydration. A fractional factorial experimental design has been implemented to study the effects in the model of seven major variables: water-to-cement (W/C) ratio, degree of hydration, aggregate volume fraction, coarse aggregate particle size distribution, fine aggregate particle size distribution, interfacial transition zone thickness, and air content. Based on this experimental design, W/C ratio, degree of hydration, and aggregate volume fraction have been identified as the three major variables influencing concrete diffusivity in the model. Following identification of the significant variables, a response surface design has been executed and least squares regression used to develop a simple equation for predicting chloride ion diffusivity in concrete based on these three parameters. This simple equation essentially summarizes the complicated simulations involved in computing the model response. Finally, simulations have been conducted to examine the extent of the surface layer in cast-in-place concrete, where the local aggregate volume fraction near the surface is less than that to be found in the bulk of the concrete.

Sensitivity of the GFDL Modular Ocean Model to Parameterization of Double-Diffusive Processes

Abstract: The effect of double-diffusive mixing on the general circulation is explored using the GFDL MOM2 model. The motivation for this comes from the known sensitivity of the thermohaline circulation to the vertical diffusivity and the earlier work of Gargett and Holloway, who studied the effects of a simple nonunity ratio between heat and salt diffusivities in a GCM. In this work, a more realistic, yet conservative, parameterization of the double-diffusive mixing is applied, with the intensity depending on the local density ratio Rρ = αTz/βSz. A background diffusivity is used to represent non-double-diffusive turbulent mixing in the stably stratified environment. The numerical model is forced by relaxation boundary conditions on both temperature and salinity at the sea surface. Three control experiments have been carried out: one with the double-diffusive parameterization (DDP) determined by the local density ratio, one with constant but different diffusivities for heat and salt as previously considere...