Showing papers on "Critical radius published in 1986"

Critical radius of endotherms

Abstract: The critical radius effect for insulation, well known in the engineering literature, was used by other authors to explain the lack of insulation on newborn endotherms. If that effect existed in small animals, they would lose less heat if nude than if fur or feathers were present. We show 1) that the previous analysis, although incomplete, yields the same result as a solid insulation model with the required sophistication and 2) that a proper model of fur is a porous media model. Neither of two porous media versions yield a critical radius effect. No critical radius effect occurs because simultaneous heat transfer by conduction and radiation makes it impossible to obtain the required logarithmic increase in thermal resistance with increasing insulation radius in a porous medium.

Cascade-induced fluctuations and the transition from the stable to the critical cavity radius for swelling

Abstract: Recently, a cascade diffusion theory was developed to understand cascade-induced fluctuations in point defect flux during irradiation. Application of the theory revealed that such fluctuations give rise to a mechanism of cascade-induced creep that is predicted to be of significant magnitude. Here we extend the investigation to the formation of cavities. Specifically, we explore the possible importance of cascade-induced cavity growth excursions in triggering a transition from the gas-content-dictated stable radius to the critical radius for bias-driven growth. Two methods of analysis are employed. The first uses the variance of fluctuations to assess the average effect of fluctuations. The second is based on the fact that in a large ensemble of cavities, a small fraction will experience larger than average excursions. This prospect is assessed by estimating upper limits to the processes. For the conditions considered, it is concluded that cascade-induced fluctuations are of minor importance in triggering the onset of swelling in a population of stable gas-containing cavities.

Critical radius for nucleation

Abstract: The free energy of formation and the critical radius for homogeneous nucleation of a spherical nucleus in supercooled liquid, at given temperature and ambient pressure, are determined, taking fully into account surface area, curvature, and pressure effects. The specific heats and densities of the two phases are allowed to be different and all thermophysical properties are temperature dependent. In the simple case in which classical nucleation theory is valid, the results predict a critical radius of about 40% larger than the classical value, and an activation energy barrier of almost three times larger than the classical value. 8 refs.

Analysis of Critical Radius Problems

Abstract: THE PURPOSE OF this paper is not to reiterate the classic critical radius problem but to extend the analysis to develop relationships for predicting optimal insulation values. This type of analysis has not been carried out in the literature. Predictions of the thickness required for reduced heat transfer from critical radius surfaces are presented in this paper for cylinders and spheres. An interesting application of the critical radius phenomena, as it applies to electronic circuitry, is discussed. As is well known, the installation of insulation to a curved surface may not always have the desired effect of reducing the heat transfer. In some cases the added insulation may actually mcrease the rate of heat loss because of the critical radius problem. This phenomenon occurs because, while the addition of insulation introduces a thermal resistance to the flow of heat, it also increases the exterior surface area. This increases the convective heat flow from the surface.

Theory of Ostwald ripening in a two-component system

Abstract: When a two-component system is cooled below the minimum temperature for its stability, it separates into two or more immiscible phases. The initial nucleation produces grains (if solid) or droplets (if liquid) of one of the phases dispersed in the other. The dynamics by which these nuclei proceed toward equilibrium is called Ostwald ripening. The dynamics of growth of the droplets depends upon the following factors: (1) The solubility of the droplet depends upon its radius and the interfacial energy between it and the surrounding (continuous) phase. There is a critical radius determined by the supersaturation in the continuous phase. Droplets with radii smaller than critical dissolve, while droplets with radii larger grow. (2) The droplets concentrate one component and reject the other. The rate at which this occurs is assumed to be determined by the interdiffusion of the two components in the continuous phase. (3) The Ostwald ripening is constrained by conservation of mass; e.g., the amount of materials in the droplet phase plus the remaining supersaturation in the continuous phase must equal the supersaturation available at the start. (4) There is a distribution of droplet sizes associated with a mean droplet radius, which grows continuously with time. This distribution function satisfies a continuity equation, which is solved asymptotically by a similarity transformation method.

Optically Thin, Adiabatic Electron-Positron Pair Wind

Abstract: Steady spherically symmetric e+e- pure pair plasma wind is inves­ tigated by taking into account the pair creation and annihilation and by adopting the equations of state which are valid in the full range of tem­ perature. The effects of equations of state adopted are demonstrated. For the relativistic Maxwell-Boltzmann gas, the adiabatic sound speed is dependent only on the temperature. In the high temperature regime, the adiabatic index is 4/3 and among two solutions near critical points (surfaces) one is of the acceleration type and the other is of the deceleration type, while both solutions are of the deceleration type in the low temperature regime, where the adiabatic index is 5/3. Thus the behavior of solutions changes drastically when kTo/mc 2 '" 0.2, To being the temperature at critical points. As temperature and number density rapidly decrease outward, the pair reaction rates also drop. Therefore, the pair reaction does not occur effectively during the flow time scale outside the critical radius. Thus electrons and positrons, ejected from a relativistic plasma which is confined near the center of active galactic nuclei, may be kept to be almost decoupled.

Field Emission Microscopic Study of Thermal Field- and Condensation-Induced Growth Forms of Crystal Tips

Abstract: Owing to their small dimensions, conducting crystal tips with curvature radii of 10-6-10-4 cm, usually studied by field electron microscopy (FEM) techniques [1], are single-crystalline even if made from polycrystalline wire. As the crystal tip is heated, its surface atoms become mobile, with the mobility increasing with temperature. At temperatures sufficient for an appreciable probability of two-dimensional sublimation of the atoms located at the edges of close-packed planar atomic networks, the tip is blunted (the tip material is transported to its lateral surface). This process, controlled by the Laplace pressure pγ = 2γ/r, slows down drastically when a certain curvature radius, r, of the tip is reached [2, 3]; this radius is a function of temperature and surface tension γ (for example, in tungsten tips the critical radius is about 10-4 cm).