Showing papers on "Thermal diffusivity published in 1977"

Engineering Properties of Snow

Abstract: The general properties of snow are described with a view to engineering applications of data. Following an introduction and a short note on the origins of snow, data are given for fall velocities of snow particles, and for mass flux and particle concentrations in falling snow and blowing snow. Notes on the structural properties of deposited snow cover grain size, grain bonds, bulk density, overburden pressure, and permeability. A section on impurities deals with stable and radioactive isotopes, chemical impurities, insoluble particles, living organisms, acidity, and gases. Mechanical properties are treated only selectively, and the reader is referred to another paper for comprehensive coverage. The selective treatment deals with stress waves and strain waves, compressibility, effects of volumetric strain on deviatoric strain, and specific energy for comminution. The section on thermal properties covers heat capacity, latent heat, conductivity, diffusivity, heat transfer by vapor diffusion, heat transfer and vapor transport with forced convection, and thermal strain. The section on electrical properties opens with a brief discussion on dielectric properties of ice, and proceeds to a summary of the dielectric properties of snow, including dielectric dispersion, permittivity, dielectric loss, and d.c. conductivity. There are also notes on the thermoelectric effect and on electrical charges in falling and blowing snow. The section on optical properties deals with transmission and attenuation of visible radiation, with spectral reflectance, and with long-wave emissivity. The review concludes with some comments on engineering problems that involve snow, and the requirements for research and development.

Analytical optoacoustic spectrometry. Part III. The optoacoustic effect and thermal diffusivity

Abstract: Optoacoustic spectrometry can be employed to obtain data concerning the spectral absorption characteristics and thermal diffusivity of small solid samples. Theoretical expressions have been developed to relate the optical and thermal properties of such samples to the amplitude and phase of the signals observed in optoacoustic spectrometry when using dispersive and non-dispersive techniques. The validity of these expressions has been examined by using transparent and opaque materials with widely differing thermal diffusivities.

Experimental Measurement of Electron Heat Diffusivity in a Tokamak

Abstract: Electron temperature perturbations produced by internal disruptions in the center of the Oak Ridge Tokamak (ORMAK) are followed with a multichord soft-x-ray detector array. The space-time evolution is found to be diffusive in character, but the conduction coefficient determined from a heat-pulse-propagation model is larger by a factor of 2.5-15 than that implied by the measured gross energy-containment time.

Engineering properties of sea ice

Abstract: The paper starts with a consideration of the different fields of engineering which require information on sea ice with the tasks ranging from the design of ice-breaking ships to Arctic drilling platforms and man-made ice islands. Then the structure of the sea ice is described as it influences the observed variations in physical properties. Next the status of our knowledge of the physical properties important to engineering is reviewed. Properties discussed include mechanical properties (compressive, tensile, shear and flexural strengths; dynamic and static elastic moduli; Poisson's ratio), friction and adhesion, thermal properties (specific and latent heats, thermal conductivity and diffusivity, density) and finally electromagnetic properties (dielectric permittivity and loss, resistivity). Particular attention is given to parameters such as temperature, strain-rate, brine volume, and loading direction as they affect property variations. Gaps, contradictions in the data, and inadequacies in testing techniques are pointed out.

Thermal diffusivity and thickness measurements for solid samples utilising the optoacoustic effect

Abstract: A simple apparatus is described that permits heat-flow characteristics of a sample to be determined, and its application to the determination of the thermal diffusivity or thickness of polymer films on a copper substrate is reported. The apparatus utilises the optoacoustic effect in which periodically interrupted radiation is absorbed by a solid sample to give rise to a similar periodic variation of the pressure of the air in contact with the sample surface in a closed cell. The amplitude and phase of the observed optoacoustic signal depend on the thermal diffusivity and thickness of the material irradiated.

Vertical distribution of dilute suspensions in turbulent pipe flow

Abstract: Closed-form expressions are presented for predicting vertical concentration distribution of the dispersed phase in the turbulent core of pipe and channel flow. Experimental data obtained with spherical particles, of mean diameter d50 comparable to the Kolmogorov microscale, are in good agreement with model predictions. The average value of dimensionless lateral particle diffusivity, ζ = e/Ru*, determined from these data is approximately 0.25, that is, fairly close to previously reported measurements with much coarser particles.

Numerical analysis of lava lake cooling models; Part II, Application to Alae lava lake, Hawaii

Abstract: The numerical formulation of Shaw, Hamilton, and Peck (p. 384-414 of this issue) using the method of explicit cell balances was applied to the cooling of Alae lava lake, Hawaii. Temperatures in this 15-m-thick, ponded basalt flow were measured over a 4-yr period from 1 week after it formed in August 1963 at a temperature of 1140 degrees C until temperatures throughout the lake had fallen to below 100 degrees C. Representative temperature profiles in the thicker central part of the lake can be reproduced with average deviations of 21 degrees C using one-dimensional models with a lake thickness of 14.6 m, a constant diffusivity of 0.006 cm 2 /sec, a latent heat of 90 cal/g, and a heat withdrawal of 620 cal/cm2 of measured rainfall using the average measured density of basalt in the lake in the calculations. Average differences between computed and observed temperature profiles were decreased to 2 degrees C by using all the following modifications of the initial model: (1) diffusivity was allowed to vary according to the calculated increase of the heat capacity with temperature, the variations of the measured density of the lava with depth in the lake, and the variations of conductivity with temperature and lava porosity; (2) heat loss to rainwater was computed using the measured density structure of the lake; (3) the lake thickness was increased to 14.9 m, a better approximation of the deepest part of the lake, and the latent heat of the basalt was accordingly decreased; and (4), low conductivity and density values were assigned to one of the cells to simulate the highly vesicular and cavernous zones found at shallow depth in the lake. The best results were obtained using a latent heat of 80 + or - 10 cal/g (from 80 percent crystallization of the lava) and a conductivity that increased with temperature at a rate of 0.06 + or - 0.01 percent of its value at room temperature per degree C. Computations using a two-dimensional model indicate that marginal cooling decreased temperatures in the lake more than 1 degrees C only in the outer 15 m of the lake until a year after the eruption and only in the outer 45 m during the final year of cooling. The values for latent heat and conductivity derived from the thermal modeling are in good agreement with values determined directly in the laboratory and calculated from laboratory data.

The Effective Medium Theory of Diffusion in Composite Media

Abstract: The effective medium theory is developed for diffusion in random composite media. It is demonstrated that the theoretical diffusivity depends crucially on whether concentration or chemical potential is taken to be the field variable for describing transport.

Self-diffusion in copper at “low” temperatures

Abstract: A low-energy argon ion-beam sputtering system was developed and applied to serial sectioning in a radiotracer study of “low”-temperature self-diffusion of copper. Very thin uniform sections with thicknesses between several nm and a few hundred nm were removed from copper single crystals of extremely low dislocation density. Gaussian penetration curves are observed. The self-diffusion coefficient was measured over the temperature range 574 to 905 K and varies between about 3 × 10−19 and 6 × 10−13 cm2 s−1. In conjunction with the high-temperature data of Rothman and Peterson the present data are analysed in terms of mono- and divacancy contributions to the total diffusivity. In the temperature range of the present study the divacancy contribution is almost completely negligible. For the sum of formation and migration enthalpy of a mono-vacancy a best value of 2.04 eV is obtained.

Heat capacity and thermal conductivity from pulsed wire probe measurements under pressure

Abstract: If a metal wire immersed in an electrically insulating medium is heated by a known constant power, the thermal conductivity of the medium may be deduced from the temperature variation of the wire. It is shown that further analysis of this variation also permits the specific heat capacity or the thermal diffusivity to be determined. A nickel wire is used both as a heater and as a temperature sensor. The power is kept constant by electronic means, and the resistance measured by a four-probe technique. Eight temperature values are recorded during the power pulse which has a typical duration of 1 s. This extended method has been tested by studying glycerol in its amorphous and crystalline states in the range 130-300K and at pressures of up to 0.8 GPa.

Variable-viscosity flows in channels with high heat generation

Abstract: This paper presents a similarity solution for plane channel flow of a very viscous fluid, whose viscosity is exponentially dependent upon temperature, when heat generation is very large. A dimensionless formulation of the problem involves two length scales (the depth h and length l, respectively, of the channel), one velocity scale (the mean velocity V of the fluid along the channel), the thermal conductivity k, thermal diffusivity k and viscosity V of the fluid, and the temperature coefficient b of the viscosity. From these, two important dimensionless groups arise, the Graetz number (Gz = Vh2/kl) and the Nahme–Griffith number (G = μ V2b/k). In the case of steady flow with G−1 [Lt ] Gz−1 [Lt ] 1 a thin thermal boundary layer of thickness proportional to Gz−½ arises at each wall with an even thinner shear layer, detached from the wall and embedded in the thermal boundary layer, of thickness proportional to Gz−½(ln G)−1, coinciding with the region of maximum temperature (ln G)/b. The similarity variable is (Pe½y/x½) where Pe is the Peclet number (Vh/k) and y and x are measured away from and along (either) boundary wall. The analogous unsteady uniform flow solution is also given.

Thermal Conductive Properties of Wood, Green or Dry, from -40 deg to +100 deg C: A Literature Review.

Abstract: : This literature review was conducted in connection with a study on heat transfer in frozen logs. A combination of data by two researchers on specific heat and thermal conductivity and diffusivity in the radial direction of wood, at various temperatures and moisture contents, is discussed and compared with data from other sources. Limited information found for the tangential and longitudinal directions is also included in the report. In addition, a data set of average thermal diffusivities in the radial direction of wood is provided for temperature intervals involving complete thawing. (Author)

Pyroelectric, dielectric and thermal properties of TGS, DTGS and TGFB

Abstract: The pyroelectric coefficient, dielectric constant, specific heat and thermal diffusivity of TGS, DTGS and TGFB have been measured as a function of temperature. At a given Tc — T, the deuteration seems to increase the pyroelectric coefficient by a factor 1.35, with a smaller increase in the dielectric constant. For TGFB a good Curie law of the dielectric constant has never been observed. The anomalies at Tc of both the specific heat and thermal diffusivity are much larger for TGFB than for TGS and DTGS. This is consistent with the different temperature variations of both the pyroelectric coefficient and dielectric constant. A discussion is given of the interest of DTGS and TGFB versus TGS, for pyroelectric detection.

Vertical infiltration in dry soil

Abstract: An approximate method is proposed to integrate the ordinary differential equations governing the terms in Philip's (1957) series formulation of the water content profile. The method is related to the weighting solution used earlier (Brutsaert, 1976) for the sorption problem, and it is intermediate between the quasi-steady state method and the sharp front method. In a comparison with an available exact solution (Brutsaert, 1968b) it is shown that the derived solution is usually accurate to within less than 1%. The water content profile and the infiltration rate can be expressed concisely in terms of soil physical parameters by making use of suitable expressions for the soil water diffusivity and the capillary conductivity. As an illustration, the infiltration rate is calculated for the Averjanov-Irmay capillary conductivity and the author's three-parameter power function diffusivity. Inspection of two extreme cases in this calculation finally leads to a new infiltration equation in closed form which is valid not only for short but also for large time of infiltration.

The Thermal Transport Properties of Polymers

Abstract: Values of thermal diffusivity and thermal conductivity are needed for heat-flow calculations, for the determination of structure-property relationships, and for material selection and comparison. However, all aspects are hampered by a dearth of reliable data and anything more than a superficial glance at the literature is apt to be discouraging for the uninitiated. Hardly any thermal diffusivity data exist, and the reported values of thermal conductivity show very large scatter. The present state of confusion can be seen, for example, in Figures 1 and 2, which show the reported thermal conductivity values for polystyrene and gum natural rubber. Not only do the values differ at some temperatures by more than 100%, and in the case of rubber by almost 300%, but different trends are indicated throughout the temperature range. Discrepancies of this size cannot be due to sample variations, and they give some indication of the experimental difficulties associated with thermal property measurements.

Nonlinear thermal convection in an elasticoviscous layer heated from below

Abstract: The linear and nonlinear stabilities of a horizontal layer of an elasticoviscous fluid, whose stress-rate-of-strain relations are due to Oldroyd (1958), are studied. In the linear theory it is already shown that steady convection (the situation generally referred to as the exchange of stability) is preferred for all relevant values of the Prandtl number (which is the ratio of the kinematic viscosity to the thermal diffusivity). The study of nonlinear effects for slightly supercritical Rayleigh number (which measures the temperature contrast across the layer) shows that plane disturbances for the case where the exchange of stability is valid and plane or centred disturbances for the case of overstability are governed by equations similar to those derived by Hocking, Stewartson & Stuart (1972) for plane Poiseuille flow. The influence of elasticity is to give rise to a burst only when the principle of exchange of stability is valid and provided certain conditions relating to the elastic parameters of the fluid are satisfied. The effect of the adiabatic temperature gradient is also discussed. It is shown that it stabilizes the layer in the linear theory. However, in the nonlinear theory it can destabilize the layer if the ratio the mean temperature of the layer to the temperature difference across the layer is large enough. For most practical purposes it does not influence the conditions necessary for a burst to occur.

Effects of Grain Size and Microcracking on the Thermal Diffusivity of MgTi2O5

Abstract: The laser flash technique was used to study the effect of grain size on the thermal diffusivity of polycrystalline MgTi2O5 from 25° to 800°C. Microcracking decreased the thermal diffusivity by as much as a factor of two with the decrease increasing with increasing grain size. When specimens were heated then cooled, the thermal diffusivity exhibited a horizontal flat figure-eight hysteresis. This hysteresis, which appeared to be a function of the thermal history, was attributed to a balance between crack healing, or closure, at high temperatures and the growth of existing cracks or the formation of new cracks during cooling.

The Effect of Temperature and Moisture on the Thermal Properties of Rapeseed

Abstract: THERMAL conductivity, thermal diffusivity and bulk density of rapeseed were measured at tem-peratures ranging from -26 °C to +19 °C and moisture contents ranging from 1 to 20 percent wet basis. The line heat source method was used to determine thermal conductivity and diffusivity. Specific heats were calcu-lated from the measured thermal conductivity, diffusivity, and bulk density. The thermal diffusivity and specific heat were found to be roughly the same as for other grains. However, the thermal conductivity is appre-ciably lower than for other grains, probably due to the spherical shape of the seeds. Increases in moisture content produce increases in thermal conductivity and specific heat except that low temperatures combined with high moisture contents apparently produce ice crystals, which causes a sharp drop in the values.

The lethality‐fourier number method: confidence intervals for calculated lethality and mass‐average retention of conduction‐heating, canned foods

Abstract: The accuracy associated with a calculated lethality or mass-average retention is dependent on the accuracy of the variables needed for the calculation. This study was undertaken to determine the effect of biological variability in destruction rates of microorganisms and quality factors and in thermal diffusivity on accuracy of thermal process calculations. Estimates of the distributions of these biological parameters were used in a Monte Carlo procedure to generate distributions of calculated lethality and mass-average retention. From these distributions, the error in thermal process calculations could be determined. Variation in parameters describing destruction rates and thermal diffusivity of the food can result in standard deviation for the F-value of 10–15s of the F-value at small Fourier numbers (dimensionless heating time). At larger Fourier numbers (≧0.7) the standard deviation is approximately 5–8%. For mass-average retention, the maximum standard deviation is approximately 5%.