Showing papers on "Thermal diffusivity published in 1973"

Theoretical Analysis of Forced Convective Heat Transfer in Regional Ground-Water Flow

Abstract: Among the numerous processes that will cause anomalous temperature distributions in geologic basins is the spatial redistribution of heat by moving ground water. This problem is examined by solving the energy equation for the simultaneous transport of water by hydraulic gradients and heat by forced convection. The factors that affect the temperature distribution in a given basin include the intrinsic properties of the medium and contained fluid—namely, the thermal diffusivity of the solid-fluid complex and the hydraulic conductivity, the water-table configuration, and the ratio of basin depth to basin length. The severity of an anomalous geothermal gradient or temperature measurement depends primarily on the relative magnitude of the ratio of hydraulic conductivity to thermal diffusivity, and on the geometry of the flow field. A dimensionless group may be formulated from these parameters, and provides a relative measure of the simultaneous transport of heat by the bulk motion of the fluid to that by pure conduction. Solutions to the equation itself indicate that convective heat losses in ground-water recharge areas are balanced by convective heat gains in discharge areas. The geothermal gradient accordingly increases with increasing depth in recharge areas, decreases with increasing depth in discharge areas, and is a manifestation of pure conduction at the hinge line separating areas of recharge and discharge.

Optical waveguides formed by thermal migration of ions in glass.

Abstract: We present our results on optical waveguides formed by thermal diffusion of ions in glass. It was found that the peak of the ion-exchanged region can be shifted into the substrate interior by limiting the diffusion process. We also found that low loss films (<0.1 dB/cm) can be fabricated using this process and that the modal losses in these films do not agree with those losses predicted by existing theories. Also, the ion-exchange process has proved to be a simple means for fabricating tapered-edge couplers.

Thermal diffusion measurements using spatially periodic temperature distributions induced by laser light

Abstract: Two light waves with different directions of propagation derived from a pulsed Nd:YAG laser are superimposed in an absorbing sample and generate an interference field. Due to absorption, a spatially periodic temperature distribution occurs, producing a spatial modulation of the refractive index which can be considered a thermal phase grating. The light of an argon laser simultaneously incident on the sample is diffracted by the thermal grating. When excitation is over, the decay time of the diffracted light is measured. From this decay time the thermal diffusivity of the sample is determined. Measurements on colored methanol and glycerin as well as on ruby compare favorably with the results of other authors. The possibility of exciting and detecting temperature waves (second sound) in solids by the method of light‐induced gratings is discussed.

Thermal conductivity of Ga1−xAlxAs alloys

Abstract: This paper presents the results of our measurement of the room‐temperature thermal conductivity of the Ga1−xAlxAs alloy system. The study was motivated by our need to characterize the thermal properties of cw heterostructure lasers, in which the heat generated in the optically active layer must flow through a substantial thickness of Ga1−xAlxAs to reach the heat sink. The measurement was made on LPE layers which were separated from their substrates and fashioned into rectangular bars. A steady‐state heat flow was established in the bars utilizing an argon laser as the heat source, and the conductivity was obtained by measuring the temperature gradient in the bars with thin films of cholesteric liquid crystals. The results indicate that the thermal conductivity of the alloy is well described by an existing theoretical treatment of high‐temperature lattice thermal conductivity of disordered semiconductor alloys. In the Ga1−xAlxAs system, the increased thermal resistivity of the alloy is shown to be a result...

Finite pulse‐time and heat‐loss effects in pulse thermal diffusivity measurements

Abstract: The heat transfer problem associated with pulse thermal diffusivity measurements is analyzed for the cases of (i) triangular pulses whose widths are comparable with the transit time of temperature fronts across a sample (finite pulse width effects), (ii) heat losses from sample faces, and (iii) the simultaneous occurrence of heat losses and finite pulse‐width effects. Methods for the analysis of experimental results which are affected by these conditions are discussed. Tabular and graphical data which facilitate the analysis are given.

Thermal properties of selected masonry unit concretes

Abstract: THE THERMAL CHARACTERISTICS OF 16 CONCRETES HAVE BEEN STUDIES AT ELEVATED TEMPERATURES WITH THE AID OF THERMOGRAVIMETRY, DILATOMETRY, AND "THERMAL PROPERTY TESTS." AN ANALYSIS OF THE RESULTS IS GIVEN FROM THE POINT OF VIEW OF MATERIAL BEHAVIOR UNDER FIRE EXPOSURE. /AUTHOR/

Thermal and elastic properties of silicate oxyapatite crystals

Abstract: The elastic constants, thermal expansion, thermal conductivity, and heat capacity are reported for a new class of laser hosts‐the silicate oxyapatites. For Ca2La8(SiO4)6O2 the five independent elastic constants obtained from sound velocity measurements are C11=1.71, C12=0.62, C33=1.71, C44=0.52, and C13=0.39×1012 dyn/cm2, the thermal expansion coefficients at 300 K are 8.9×10−6 and 6.6×10−6 K−1 for the [1010] and [0001] directions, respectively, and the room‐temperature conductivity is about 0.019 W cm−1 K−1. Sound velocity and thermal expansion data suggest a Debye temperature between 400 and 500 K.

Diffusion of Light Hydrocarbons in 5A Zeolite

Abstract: Experimental diffusivity data are presented for the diffusion of ethane, ethylene, propane, propylene, cyclopropane, n-butane, 1-butene, cis-2-butene, and trans-2-butene in Linde 5A zeolite. It is ...

Some physical properties of two amorphous metallic alloys

Abstract: Room‐temperature values of the thermal‐expansion coefficient, thermal diffusivity, thermal conductivity, density, Young's modulus E, and the temperature coefficient (1/E) dE/dT are presented for two splat‐cooled amorphous alloys of composition (in atomic percent) Pd82Si18 and Fe75P15C10. Values of E were obtained from the flexural resonant frequencies of cantilever mounted reeds; the thermal properties were derived from internal friction measurements giving the height and location of the thermoelastic ``transverse thermal current'' peak. Compared to crystalline alloys, the amorphous samples are poorer thermal conductors but exhibit a comparable expansion behavior. In view of the small density difference between the amorphous and crystalline states, the most striking difference observed was the lower elastic modulus of the amorphous alloys. The present results supplement a growing body of evidence that materials in the amorphous condition may have a Young's or shear modulus which is typically 20–40% less t...

Tunneling states and the low-temperature thermal expansion of glasses

Abstract: A recent theoretical model, based on tunneling states, for the heat capacity and thermal conductivity of amorphous solids below 1 K is extended to include thermal expansion. The calculated Gruneisen constant γ is sensitive to the details of the tunneling states, but the model provides an explanation for the large negative values of γ observed in vitreous silica below 4 K.

The Viscosity and Thermal Conductivity Coefficients of Dilute Nitrogen and Oxygen

Abstract: The viscosity and thermal conductivity coefficients of dilute oxygen and nitrogen are discussed and tables of values are presented for temperatures between 80 and 2000 K. The oxygen viscosity tables are estimated to be accurate to two percent for temperatures up to 400 K and four percent above that temperature; the nitrogen viscosity tables are estimated to be reliable to one percent in the range 100–1000 K, increasing to two percent above 1000 K and below 100 K. The error assigned to the thermal conductivity is three percent below 400 K and five percent above 400 K for both gases. The tables were calculated from the appropriate kinetic theory equations using the m‐6–8 model potential with nonspherical contributions. The approximations to the equations are discussed. It is emphasized that the available data for oxygen viscosity are generally poor and that the thermal conductivity data for both oxygen and nitrogen cannot be considered reliable at high temperatures. No oxygen data exist for temperatures above 1500 K.

Theoretical Analysis of Wankel Engine Combustion

Abstract: A one-dimensional model for. the spark ignition Wankel engine combustion is presented, The model is based on the use of a turbulent diffusivity for heat and mass transfer by which the motion of the finite-thickness, unsteady turbulent flame is calculated. A one-step second order reaction rate equation is used and wall heat transfer losses are included. The instantaneous chamber pressure and the local and instantaneous gas temperature, density, and velocity are calculated. Results are presented for a typical engine design. Initial results of two parametric studies are also reported, but their generalization is not undertaken, at this point, due to the complexity of the process.

Properties of Aluminum and Aluminum Alloys

Abstract: : ;Contents: Thermal conductivity; Specific heat; Thermal radiative properties; Thermal diffusivity; Thermal expansion; Electrical resistivity.

Measurement of Moisture Diffusivity of Wet Swelling Systems

Abstract: A method for estimating the diffusivity function of a saturated swelling system is examined. The method is based on an equation linking the sorptivity, the diffusivity function, and the flux-concentration relation of Philip (1973) and uses appropriate values of the flux-concentration relation to set limits to the diffusivity function. For the material used in the experiments demonstrating the procedure, the calculation errors were no more than 7.5 percent. The method permits correction if this order of accuracy is insufficient. The results suggest that the conditions for which Smiles and Rosenthal (1968) solved their diffusion equation were not realized and their Dm and D results are therefore suspect.

Diffusivity of helium in fused silica

Abstract: A statistical mechanical model of diffusion is applied to helium diffusion in fused silica. The number of gas atoms moving solubility sites is related to the number of atoms in those sites. The number of atoms in the excited state, between two adjacent solubility sites, relative to the number occupying solubility sites can be described by their partition functions. The process of diffusion is treated as a random walk process which results in a diffusion equation in terms of the temperature, fundamental constants, and material parameters. The model is compared to data reported in the literature on He diffusion in fused silica in order to determine the unknown parameters. The model also shows that absolute rate theory is not able to account for the nonlinearity observed in logD vs 1/T for this system.

Isotopic thermal diffusion factors of Ne, Ar, Kr, and Xe from column measurements

Abstract: The isotopic thermal diffusion factors of Ne, Ar, Kr, and Xe were determined as a function of temperature from separation measurements carried out at low temperature differences in a precisely constructed thermal diffusion column. The apparatus and techniques were similar to those used in previous work with Ne–Xe mixtures. The results, which were obtained for average temperatures in the range from 323 to 438°K, were in good agreement with the data of other investigators for Ne and Ar. Partial agreement was obtained for Kr, but for Xe the column results are substantially lower than published values derived from swing separator experiments. The Dymond‐Alder and the Bobetic‐Barker intermolecular potential energy models for argon were applied in reduced form to neon, krypton, and xenon. It was found that the thermal diffusion factors calculated on the basis of either potential were in good agreement with the experimental results. The data appear to follow a unique corresponding‐state relationship between the reduced thermal diffusion factor and the reduced temperature. This relationship, in turn, is closely predicted from calculations based on the Dymond‐Alder potential.

Self-diffusion in liquid tin and indium over extensive temperature ranges.

Abstract: The capillary-reservoir technique has been used to study the diffusion of Sn113 and Sb125 in liquid tin from 665°to 1746°K, and the self-diffusion of In114 from 621° to 1378°K. Multichannel scintillation radiotracer analyses were used to determine the total amount of diffusate migrating either into or out of a capillary tube. A definite wall effect was observed for In114 at 775°K. On the basis of the broad temperature range diffusivity data obtained, experimental criteria were established for very accurate measurement and calculation of liquid metal diffusivities using the capillary-reservoir technique. These criteria include: selection of the capillary tube material and diameter, determination of the magnitude of the △l effect, and careful identification of the radioisotopes employed in the determination of the diffusivities. It was found that the diffusion data do not fit any of the available theoretical models over the complete temperature range, nor any empirical relations of the formD =AT n. The observed nonlinear variation of logD vs logT was correlated with the disappearance of a second structure in the liquid state of tin.

Measurement of mutual diffusion coefficients and thermal diffusivities by quasi‐elastic light scattering

Abstract: Quasi-elastic light scattering techniques have been employed to measure the mutual diffusion coefficient DAB as a function of concentration in eight binary mixtures and the thermal diffusivity x in nine pure liquids and two binary mixtures. The mass diffusivities obtained are typically accurate to 3% while thermal diffusivities are known to be accurate to 5%; both types of values are in substantial agreement with the available bulk values. Under most circumstances light scattering is found to offer distinct advantages over the standard techniques for determining mass and thermal diffusivities.

Liquid phase mass transfer in ion exchange based on the hydraulic radius model

Abstract: Applying the hydraulic-radius model to the flow in a packed bed, ion exchange liquidphase mass transfer is analysed and liquid-phase effective diffusivity, De'', based on the model is compared with the diffusivity, De, based on the film model. The maximum difference between these values was about 5% for the range of diffusivity ratio a>1 and 16% for a<1. Therefore, even if the value of De is used as an approximation, no significant error will appear. Furthermore, experimental results were correlated by the use of individual diffusivity for isotopic ion exchange and De'' for mutual ion exchange. These data satisfy the equationJ*'' ={(1-e)/e}1/3 k*Sc2/3£/us=1.85Re''-2/3, when Re''<100.

Effect of Alkali Oxides on the Diffusion of Helium in a Simple Borosilicate Glass

Abstract: Helium permeation rates, diffusivities, and solubilities in a simple borosilicate glass containing alkali oxides were measured. The major effect of increasing Na2O content from 2 to 13 mol% is a systematic decrease in permeation rate and diffusivity, indicating that the Na+ ion is essentially filling space in the glass network. At 7 mol% alkali oxide, as the alkali-ion size is increased, the changes in diffusivity and activation energy indicate that from Li+ to Na+ to K+, the larger ion more effectively plugs the network interstices. The size distribution of interstices in the base-glass network is such that ions larger than K+ are too large to simply fill the network and consequently tend to spread the network as well.