Showing papers on "Thermal diffusivity published in 1996"

Moisture Diffusivity Data Compilation in Foodstuffs

Abstract: A review of the recently reported moisture diffusivity experimental data in food materials is presented in this work Values are classified and analysed statistically to reveal the influences of temperature and moisture content Empirical models relating moisture diffusivity with the above variables, along with relevant graphs for various materials, are included Other factors affecting moisture diffusivity prediction are also discussed

Moisture transport in porous building materials

Abstract: The isothermal moisture transport in various porous building materials during absorption and drying was studied by nuclear magnetic resonance (NMR). It is shown that the moisture diffusivity can be determined directly from measured transient moisture profiles. NMR appears to be an accurate and reliable method to determine these moisture profiles. For both water absorption and drying it is found that, within the experimental accuracy, a single unambiguous relation exists between the moisture diffusivity and the moisture content, which is dependent on the type of material. Preliminary absorption measurements on brick/mortar samples suggest that the hydraulic contact between mortar and brick may not be perfect.

Revised model calculations for the thermal histories of ordinary chondrite parent bodies

Abstract: — Recent measurements of ordinary chondrite physical and thermal properties along with new geothermometry studies have provided the necessary parameters for updating a previously proposed model (Miyamoto et al., 1981) for the thermal evolution and internal structure of ordinary chondrite parent bodies. Model calculations assumed a heat source term derived from the decay of 26Al (justification is provided). Differences from the previous model include: varying the thermal diffusivity parameter with increasing temperature (and decreasing porosity), using variable physical and thermal parameters to provide end member models, and incorporating a shortened thermal history of 60 Ma (obtained from new Pb-Pb chronology of phosphates) rather than 100 Ma. Times of isotopic closure in chondrite phosphates overlap the thermal model estimates, and postmetamorphic cooling rates from the model approximately coincide, in both trend and magnitude, with metallographic and fission track cooling rate data. Model calculations attempt to match peak metamorphic conditions in the central portions of these bodies and yield accretion ages between 1.4 to 3.1 Ma after calcium-aluminum inclusion (CAI) formation. Model calculations also predict that both the H and the L chondrite parent asteroids consisted of ∼80% equilibrated and 20% unequilibrated chondritic material and that their original radii ranged from 80 to 95 km.

Effective Moisture Diffusivity Estimation from Drying Data. A Comparison Between Various Methods of Analysis

Abstract: Moisture diffusivity is the most crucial property in drying calculations. Literature data are scarce due to the variation of both experimental measurement techniques and methods of analysis. The effect of using different methods of analysis on the same experimental drying data is examined in this work. Detailed and simplified mathematical models, incorporating moisture diffusivity as model parameter, are applied. It is proved, that significant differences in the calculated values of moisture diffusivity result when different models are used, and probably these differences explain the variation in literature data. Thus, the adoption of a standardised methodology will be of great importance in moisture diffusivity evaluation. The above findings resulted from the application of four alternative models on the drying data of three common food materials, potato, carrot and apple. A typical pilot plant scale dryer with controlled drying air conditions was used for the experiments. The moisture content d...

Mathematical modeling of supercritical extraction of sage oil

Abstract: Modeling of supercritical CO{sub 2} extraction of essential oils from leaves was studied using sage at 90 bar (9 MPa) and 50 C. The fractional separation of the extracts enabled essential oil to be obtained. Four mean sage particle sizes ranging from 0.25 to 3.10 mm were tested. The model proposed was based on differential mass balances performed along the extraction bed. Experimental data suggest that the internal mass transfer was the controlling stage for the extraction process. Different hypotheses were tested on vegetable matter geometry, and their incidence on the model performance was evaluated. The particle shape proved to be a key factor in fitting experimental results, which were fairly good when the conventional spherical geometry was replaced by a realistic slab geometry. Diffusivity of the solute in the solid matrix was used as the only adjustable parameter of the model; its best fit value was 6.0 {times} 10{sup {minus}13} m{sup 2}/s. The effect of the introduction of particle-size distribution into calculations was also tested. To verify if the external mass-transfer mechanisms influence the extraction process, experiments at two different CO{sub 2} flow rates were also performed. Simplified models were also considered, and the extent of approximations was evaluated.

Diffusivity of ozone in water

Abstract: The diffusivity of ozone in water was determined experimentally by measuring the rate of ozone absorption into an aqueous laminar liquid jet. Penetration theory was used to interpret the results. Experiments were conducted over the temperature range of 10 °C to 45 °C. The results for diffusivity were correlated by an Arrhenius function for temperature dependence with a maximum deviation of ±3.3%: D/m2·s-1 = 1.10 × 10-6 exp[−1896/(T/K)].

Pore and surface diffusion in multicomponent adsorption and liquid chromatography systems

Abstract: A generalized parallel pore and surface diffusion model for multicomponent adsorption and liquid chromatography is formulated and solved numerically. Analytical solution for first- and second-order central moments for a pulse on a plateau input is used as benchmarks for the numerical solutions. Theoretical predictions are compared with experimental data for two systems: ion-exchange of strontium, sodium, and calcium in a zeolite and competitive adsorption of two organics on activated carbon. In a linear isotherm region of single-component systems, both surface and pore diffusion cause symmetric spreading in breakthrough curves. In a highly nonlinear isotherm region, however, surface diffusion causes pronounced tailing in breakthrough curves; the larger the step change in concentration, the more pronounced tailing, in contrast to relatively symmetric breakthroughs due to pore diffusion. If only a single diffusion mechanism is assumed in analyzing the data of parallel diffusion systems, a concentration-dependent apparent surface diffusivity or pore diffusivity results; for a convex isotherm, the apparent surface diffusivity increases, whereas the apparent pore diffusivity decreases with increasing concentration. For a multicomponent nonlinear system, elution order can change if pore diffusion dominates for a low-affinity solute, whereas surface diffusion dominates for a high-affinity solute.

Scanning thermal microscopy: Subsurface imaging, thermal mapping of polymer blends, and localized calorimetry

Abstract: We have used a platinum/10% rhodium resistance thermal probe to image variations in thermal conductivity or diffusivity at micron resolution and to perform localized calorimetry. The probe is used as an active device that acts both as a highly localized heat source and detector; by generating and detecting evanescent temperature waves, we may control the maximum depth of sample that is imaged. Earlier work has shown that subsurface images of metal particles buried in a polymer matrix are consistent with computer simulations of heat flows and temperature profiles, predicting that a 1 μm radius probe in air will give a lateral resolution of ∼200 nm near the surface, with a depth detection of a few μm. We have a special interest in polymer blends, and we present zero‐frequency mode and temperature‐modulation mode thermal images of some immiscible blends in which the image contrast arises from differences in thermal conductivity/diffusivity between single polymer domains. The behavior of domains is observed in real time as the blends are subjected to a slow temperature rise. We have also achieved localized differential thermal analysis of a number of polymers, and recorded events such as glass transitions, meltings, recrystallizations, and thermal decomposition within volumes of material estimated at a few μm3. This opens the way forward towards calorimetric imaging, by which it should be possible to distinguish between different regions undergoing either reversible or irreversible changes as the temperature is varied.

Statistical analysis of the surface circulation of the California Current

Abstract: We use a set of mixed-layer drifting buoy trajectories from the California Current (20°N–40°N) during 1985–1990 to obtain statistically reliable estimates of the mean currents, the mean variance field, and the geographically varying diffusivity, integral timescales, and integral space scales. Typical values for the diffusivity are 1.1–8.7 × 107 cm2 s−1, while the timescales and space scales are 2.1–7.1 days and 16–59 km, respectively. The variance field displays a strong westward gradient out to 125°W, and diffusivity shows a tendency to decrease toward the southwest part of the domain. Significant anisotropy is found in the variance field near the coastal boundary and at 30°N, 130°W, which is the region where the subarctic and northern subtropical fronts approach the California Current. The antisymmetric component of the diffusivity tensor indicates that cyclonic eddies dominate the mesoscale signature of drifters in this region. We seek simple parameterizations to relate the scales of motion of the random velocity field to the diffusivity by testing least squares fits to κ∞ ∝ u02T and κ∞ ∝ u0L, where u02 is the velocity variance. We found no cases for which these two hypotheses could be distinguished. For the meridional component the linear regressions are not successful, which suggests that the meridional departure velocities result from a flow regime that is significantly organized by, for example, waves or coherent structures. A subset of the drifters measured temperature along their tracks, and we use the resultant data to produce the first direct estimates of the horizontal eddy heat flux divergence based on Lagrangian estimates. In addition, we separately compute the “eddy diffusivity” parameterization of the eddy heat flux divergence, ∇ · 〈u′θ′〉 = ∇(κ∇Θ), using our diffusivity estimates and a sea surface temperature climatology. The two independent terms agree well, which provides a measure of reassurance about the diffusivity estimates. The eddy heat flux divergence in the California Current is very small (<5 W m−2) and does not appear to be significant in the long-term heat budget of the upper ocean in this region.

A model for estimating penetration depth of laser welding processes

Abstract: Penetration depth is one of the most important factors critical to the quality of a laser weld. However, no on-line, non-destructive method exists by which to inspect this quantity. Indirect, model-based estimation schemes are feasible solutions for monitoring laser welding processes. In this paper, a model for estimating penetration depth based on a 2D heat conduction model and a conical keyhole assumption is developed. This model relates the penetration depth to the incident power and the Peclet number, which is a function of the welding speed, the keyhole radius and thermal diffusivity. The Peclet number is determined by measuring the weld width on the top surface. The model is validated by a number of laser welds made to join a low-carbon steel hub and plate assembly using a 5 kW laser with different combinations of power and speed. The results show that the proposed model is consistent with the experimental data and is computationally efficient. Therefore, this model is suitable as a basis for model-based, on-line depth estimation schemes.

The 3ω technique for measuring dynamic specific heat and thermal conductivity of a liquid or solid

Abstract: We show how to measure dynamic specific heat and thermal conductivity of a solid or liquid sample using the 3ω technique, which is an ac‐modulation method where we use a heater simultaneously as the sensor. By varying the width of the heater relative to the thermal decay length, one can choose the proper regime to measure thermal conductivity or specific heat. The technique is applied to window glass and the results confirm the validity of the method. Experimental results for potassium dihydrogen phosphate crystal demonstrate the first‐order transition at the Curie point, and the dynamic specific heat of supercooled liquid potassium–calcium nitrate is shown.

Turbulent Thermal Diffusion of Small Inertial Particles

Abstract: A new physical effect, turbulent thermal diffusion, is discussed. This phenomenon is related to the dynamics of small inertial particles in incompressible turbulent fluid flow. At large Reynolds and Peclet numbers the turbulent thermal diffusion is much stronger than the molecular thermal diffusion. It is shown that inertia of particles under certain conditions can cause a large-scale instability of spatial distribution of particles. Inertial particles are concentrated in the vicinity of the minimum (or maximum) of the mean temperature of the surrounding fluid depending on the ratio of material particle density to that of the surrounding fluid.

Thermal conductivities of molten metals: Part 1 Pure metals

Abstract: Thermal conductivity data for molten metals, published since the review of Touloukian et al. in 1970, are collated and evaluated. Where possible recommended values are given. Where availability of data permits, the Wiedemann-Franz-Lorenz equation relating electrical and thermal conductivities has been assessed. It has been found to be valid for most pure metals at their melting point.

The influence of bubble plumes on air-seawater gas transfer velocities

Abstract: Laboratory results have demonstrated that bubble plumes are a very efficient air-water gas transfer mechanism. Because breaking waves generate bubble plumes, it could be possible to correlate the air-sea gas transport velocity kL with whitecap coverage. This correlation would then allow kL to be predicted from measurements of apparent microwave brightness temperature through the increase in sea surface microwave emissivity associated with breaking waves. In order to develop this remote-sensing-based method for predicting air-sea gas fluxes, a whitecap simulation tank was used to measure evasive and invasive kL values for air-seawater transfer of carbon dioxide, oxygen, helium, sulfur hexafluoride, and dimethyl sulfide at cleaned and surfactant-influenced water surfaces. An empirical model has been developed that can predict kL from bubble plume coverage, diffusivity, and solubility. The observed dependence of kL on molecular diffusivity and aqueous-phase solubility agrees with the predictions of modeling studies of bubble-driven air-water gas transfer. It has also been shown that soluble surfactants can decrease kL even in the presence of breaking waves.

Thermal diffusivity of mantle minerals

Abstract: The thermal diffusivity tensors at ambient pressure and temperature of three silicate mineral phases abundant in the upper mantle (San Carlos olivine [Mg0.89Fe0.11]2SiO4, Kilbourne Hole orthopyroxene [Mg1.63Fe0.17Ca0.04Mn0.01] [Cr0.01 Al0.12] [Si1.89Al0.11]O6 and a garnet of intermediate composition Py51Al32Gr16Sp1 are reported. The extension to high pressure and temperature of the experimental technique employed here is discussed and, for olivine, data at high pressure are also reported. The diffusivity in the two orthorhombic minerals is highly anisotropic, the components of the tensor along the a, b, and c crystallographic axes, in units of mm2/sec, being [2.16 1.25 1.87] in the case of olivine and [1.26 1.05 1.66] in the case of the orthopyroxene. The isotropic thermal diffusivity in garnet is 1.06 mm2/ sec. The experimental uncertainty is approximately 2%. The pressure dependence of thermal diffusivity is approximately 4% per GPa. The relation of thermal to elastic anisotropy is briefly considered. A model incorporating elastic anisotropy, anharmonicity described by acoustic Gruneisen parameters, Brillouin zone structure, and the increased phase volume for the scattering of short wavelength phonons provides a qualitatively reasonable description of the thermal diffusivity anisotropy. Since both olivine and orthopyroxene are aligned by flow deformation processes, the upper mantle is expected to be thermally anisotropic.

Deep-Water Properties and Surface Buoyancy Flux as Simulated by a Z-Coordinate Model Including Eddy-Induced Advection

Abstract: A parameterization for the adiabatic transport effect of eddies is introduced into a World Ocean model based on the Bryan-Cox code. The model topography is only lightly smoothed and retains realistic sill depths and representation of continental shelves commensurate with the model's 1.6° latitude by 2.8° longitude resolution. The parameterization allows the model to be run without horizontal diffusivity (though only under certain conditions as discussed). A first (control) run features a typical horizontal diffusivity and no eddy-induced transport. A second run features the eddy-induced transport scheme and zero horizontal diffusivity. Substantial changes occur between the runs in subsurface temperature, salinity, and density throughout the model ocean. The most profound changes occur in the deep ocean and feature a marked increase in density in the second run associated with a substantial decline in temperature (especially in the Atlantic) and an increase in salinity (especially in the Southern,...

Blood flow cooling and ultrasonic lesion formation.

Abstract: This article examines lesion formation using focused ultrasound and demonstrates how blood flow may affect lesion dimensions using a theoretical model. The effects of blood flow on temperature distributions during ultrasonic lesioning are examined for both regional cooling by the microvasculature and localized cooling due to thermally significant vessels. Regional cooling was critically assessed using two models: the Pennes bioheat transfer equation and the scalar effective thermal conductivity equation. Localized cooling was modeled by adding an advective term in the heat diffusion equation in regions enclosed by thermally significant vessels. A finite difference approach was used to solve the basic equations of heat transfer in perfused tissues in cylindrical coordinates. The extent of the lesioned tissue was determined by the accumulated thermal dose at each location. The size of the lesion was then calculated from the boundaries of the thermal isodose curves generated by the simulations. The results were compared to published in vivo lesion data in rat liver. It was shown that even for short ultrasound exposure times (approximately 8 s), blood flow may play an important role in the thermal dose distribution.

Modeling diffusion and reaction in soils : III. Predicting gas diffusivity from the Campbell soil-water retention model

Abstract: Improved prediction of gas diffusivity in soils is essential to the development of better gas transport and fate models. Empirical equations analogous to three well known capillary tube models for unsaturated hydraulic conductivity, based on the Campbell soil-water retention function, were used to p

Influence of the structural changes during alkaline cooking on the thermal, rheological, and dielectric properties of corn tortillas.

Abstract: Basic data on thermal, dielectric, and infrared spectra properties of cooked tortillas were investigated with the aim of understanding the role of lime, Ca(OH) 2 , that is incorporated during the alkaline cooking process. The data are further supplemented by X-ray diffraction and texture measurements. The changes in the thermal diffusivities, as measured by the photoacoustic technique, as well as changes in texture, X-ray patterns, dielectric, and infrared spectra properties are presented as a function of the lime concentration. The results show strong evidence that there is a marked concentration of lime at 0.2% above and below where there are different behavior patterns of the measured properties of tortilla. In particular, the results of the study of dielectric constants and thermal diffusivity provide strong evidence for an enhancement of the starch crosslinking taking place at a lime content of 0.2 %.

Amorphous (Mo, Ta, or W)-Si-N diffusion barriers for Al metallizations

Abstract: M–Si–N and M–Si (M=Mo, Ta, or W) thin films, reactively sputtered from M5Si3 and WSi2 targets, are examined as diffusion barriers for aluminum metallizations of silicon. Methods of analysis include electrical tests of shallow-junction diodes, 4He + + backscattering spectrometry, x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and secondary-ion-mass spectrometry. At the proper compositions, the M–Si–N films prevent Al overlayers from electrically degrading shallow-junction diodes after 10 min anneals above the melting point of aluminum. Secondary-ion-mass spectrometry indicates virtually no diffusivity of Al into the M–Si–N films during a 700 °C/10 h treatment. The stability can be partially attributed to a self-sealing 3-nm-thick AlN layer that grows at the M–Si–N/Al interface, as seen by transmission electron microscopy.

Natural convection heat transfer from helicoidal pipes

Abstract: An experimental investigation is reported on natural convection heat transfer in air from the outer surface of constant heat flux helicoidal pipes with vertical and horizontal orientations. The temperatures along the flow direction and peripheral direction of the tube wall were measured. The test Rayleigh numbers range from 4 x 103 to 1 x 10 s for vertical coils and 5 x 103 to 1 X 10s for horizontal coils. The local and average Nusselt numbers are evaluated and correlated. For the vertical case, the results are compared with single horizontal cylinders. It is found that the heat transfer from the first turn is almost the same as that of the single horizontal cylinder. Because of the tube curvature, the heat transfer coefficient on the outer coil wall (i/r = 77/2) is higher than that on the inner coil wall (i/r = 377/2) in the middle turns of the coil. For the horizontal orientation, the results are well correlated with the tube diameter as the characteristic length. The local heat transfer characteristics are discussed as well. The overall average Nusselt number of the horizontal coil is higher than that of the vertical coil in the laminar region.