901 publications from this institution
This paper is a proposal to vascularize a solid wall with transverse tree-shaped channels with fluid flow from side to side. The purpose of the fluid flow is to intercept in counterflow the solid heat conduction caused by an intense heat flux that lands on the exit plane of the wall. Tree architectures with 1–4 levels of bifurcation are simulated numerically, as a conjugate (convection and conduction) heat flow phenomenon in three dimensions. The effect of local pressure losses at junctions and entrances is included. The numerical work covers the Reynolds number range 10–550, svelteness number range 2.2–11.5, wall porosity range 0.01–0.1, and pressure drop number range 3×105–1010. The objective of the tree flow is to keep down the peaks of hot spot temperatures and the fraction of the volume occupied by high temperatures while using minimum pumping power. It is shown that the tree design is very effective, and that there is an optimal number of bifurcation levels for a specified porosity and pressure drop number. Tree designs are more effective than designs with parallel channels when the pressure drop number and the porosity are sufficiently large.
No abstract is provided for this article.
NothingEconomies of scale moves unless it is driven. The environment resists the movement. The flier and the swimmer experience “drag”. The skin of the fish and the airplane fuselage experiences “friction,” and the impala and the...
No abstract is provided for this article.
Diverse measurements of heat transfer by Benard convection in a fluid saturated porous layer are correlated by considering only the Darcy and Forschheimer asymptotes of the phenomenon. Scale analysis suggests that in the Forschheimer limit the Nusselt number must increase as (Ra Prp ) 1 2 , where Ra is the Darcy-Rayleigh number based on height and Prp is an entirely new group called “the porous medium Prandtl number.” The transition from the Darcy regime to the Forschheimer regime occurs at Ra ∼ Prp. The heat transfer correlation is Nu = {( Ra 40 )n + [c(RaPrp) 1 2 ]n} 1 n with n = −1 65 , c = 1896.4 and Prp = (H/bK)Prm , where H, b, K and Prm are the height, Forschheimer constant, permeability and Prandtl number based on the thermal conductivity of the porous medium.
This paper reports similarity solutions for vertical boundary layer natural convection near a solid wall adjacent to a fluid-saturated porous medium, in the case where the pore Reynolds number is high enough for the Darcy flow model to break down. Based on scaling arguments, it is shown that the departure from the Darcy flow model is dictated by the dimensionless number G = (v/K)(bgβΔT)− 1 2 , with G → 0 representing the high pore Reynolds number limit, and with G → ∞ representing the Darcy flow limit. Similarity solutions are reported for flow and heat transfer in the following cases: (1) the nonDarcy limit G = 0 near an isothermal wall and near a constant-heat-flux wall, and (2) the intermediate regime G = 0(1) for natural convection near an isothermal wall. It is shown that the heat transfer results differ fundamentally from those known from traditional Darcy flow studies.
This paper extends to electrical machines the thermodynamics and heat transfer optimization approach that has been developed for heat engines. Four models of thermodynamically irreversible electrical motors with heat transfer to the ambient are proposed and optimized: general reversible motor in series with its resistance, induction motor with transformer, synchronous motor with transformer and direct current motor. The conversion efficiency at maximum power is 1 2 . When, as in specific applications, the operating temperature of the windings must not exceed a specified level, the power output is lower and the efficiency higher. In the case of an electrical power generator it is shown that the generator and the heat engine that drives it can be optimized separately for maximum power.
Die Arbeit berichtet über eine Lösung mit finiten Differenzen der natürlichen Gegenströmung in einem horizontalen adiabaten Kanal mit k
This paper describes an experimental and analytical study of the phenomenon of heat transfer by natural convection in a rectangular enclosure fitted with an incomplete internal partition. The experiments were carried out in a water-filled enclosure with adiabatic horizontal walls and vertical walls maintained at different temperatures. Heat transfer measurements and flow visualization studies were conducted in the Rayleigh number range 109–1010, for aperture ratios h/H = 1, 1/4, 1/8, 1/16, and 0, where h and H are the height of the internal opening (above the partition) and the height of the enclosure, respectively. It is demonstrated that the aperture ratio h/H has a strong effect on both the heat transfer rate and the flow pattern. The second part of the study consists of an asymptotic analysis of the same phenomenon, valid in the limit of vanishing Rayleigh numbers. The flow and temperature fields in this limit are reported graphically for H/L = 0.5, Pr = 0.71 and 0.3 < h/H < 0.7, where L and Pr are the enclosure length and the Prandtl number, respectively.
In this paper we consider the optimization of the shape of a cavity that intrudes into a solid conducting wall. This intrusion may be regarded as a “negative fin”, i.e., the outside-in version of a conductive fin the shape of which is to be optimized. The objective is to minimize the global thermal resistance between the solid and the cavity. The cavity is rectangular, with fixed volume and variable aspect ratio. The cavity shape is optimized for two sets of thermal conditions for the solid wall: uniform heat generation, and uniform heat flux on the outer surfaces of the solid wall. The optimized cavity shape is practically independent of the thermal conditions. The cavity shape is optimal when it penetrates the conducting wall completely. In the second part of the paper we optimized a more complex intrusion: a cavity shaped as a T. The performance of the T-shaped cavity is superior to that of the finger-shaped cavity optimized in the first part of the paper.
This paper presents a series of examples in which the global performance of flow systems is optimized subject to global constraints. The flow systems are assemblies of ducts, channels and streams shaped as Ts, Ys and crosses. In pure fluid flow, thermodynamic performance maximization is achieved by minimizing the overall flow resistance encountered over a finite-size territory. In the case of more complex objectives such as the distribution of a stream of hot water over a territory, performance maximization requires the minimization of flow resistance and the leakage of heat from the entire network. Taken together, these examples show that the geometric structure of the flow system springs out of the principle of global performance maximization subject to global constraints. Every geometric detail of the optimized flow structure is deduced from principle. The optimized structure (design, architecture) is robust with respect to changes in some of the parameters of the system. The paper shows how the geometric optimization method can be extended to other fields, e.g., urban hydraulics and, in the future, exergy analysis and thermoeconomics.
No abstract is provided for this article.
This paper describes an approach to assertion classification and an empirical study on the impact this task has on phenotype identification, a real world application in the clinical domain. The task of assertion classification is to assign to each medical concept mentioned in a clinical report (e.g., pneumonia, chest pain) a specific assertion category (e.g., present, absent, and possible). To improve the classification of medical assertions, we propose several new features that capture the semantic properties of special cue words highly indicative of a specific assertion category. The results obtained outperform the current state-of-the-art results for this task. Furthermore, we confirm the intuition that assertion classification contributes in significantly improving the results of phenotype identification from free-text clinical records.
This paper addresses the fundamental problem of maximizing the heat transfer rate density in a fixed volume in the limit of decreasing length scales. In this limit boundary layers disappear, optimized channels are no longer slender, and existing results for optimal spacings break down. Three configurations are optimized analytically based on the intersection of asymptotes method: volumes filled with parallel-plates channels, volumes filled with uniformly distributed spheres, and volumes filled with parallel plates and porous structure in each parallel-plates channel. The small-spacings asymptote is for slow Poiseuille and, respectively, Darcy flow. The large-spacings asymptote is based on heat transfer that approaches pure conduction around bodies immersed in a stationary medium. The geometric results are the optimal flow channel size, the optimal porosity of the assembly, and the maximized heat transfer rate density. The latter increases sharply as dimensions become smaller. This trend, and the method of optimizing flow architecture to achieve maximal heat transfer density, are essential in the continuing miniaturization of heat transfer devices.
No abstract is provided for this article.
This paper is an application of the constructal method to the discovery of the optimal distribution of discrete heat sources cooled by laminar natural convection. The global objective is to maximize the global conductance between the wall and the fluid, or to minimize the hot-spot temperatures when the total heat generation rate and global system dimensions are specified. Two scenarios are investigated: (i) a large number of small heat sources mounted on a vertical wall facing a fluid reservoir, and (ii) a small number of finite-size heat sources mounted on the inside of the side wall of a two-dimensional enclosure. It is shown that the optimal distribution is not uniform (the sources are not equidistant), and that as the Rayleigh number increases the heat sources placed near the tip of a boundary layer should have zero spacings. In both (i) and (ii), the optimal configuration of the wall with discrete sources is generated by the pursuit of maximal global performance subject to global constraints.
Numerical calculation from the full differential equations for convection in an unbounded region is expensive, and hence approximate solutions are important. For small values of the Rayleigh number Ra, perturbation methods are appropriate. At large values of Ra...
The English (original) version is published by Doubleday, Random House, Inc., New York, and in Canada by Random House of Canada, Ltd., Toronto. The vernissage of the Romanian version was on February 21, at the Romanian Academy, in Bucharest. The book has 296 pages, 60 figures, and it is shaped and structured in ten chapters: The Birth of Flow; The Birth of Design; Animals on the Move; Witnessing Evolution; Seeing Beyond the Trees and the Forest; Why Hierarchy Reigns; The Fast and Long Meets the Slow and Short; The Design of Academia; The Golden Ratio, Vision, Cognition, and Culture; The Design of History. The book is a story of science. It is about the emergence and the evolution of design patterns of “organization” in nature, and about how this universal phenomenon is condensed into one single law of Physics: the Constructal Law, discovered by Adrian Bejan. The story covers the entire territory of nature, from animate and inanimate phenomena to human, social and technology phenomena. The constructal law unifies these all, reveals their deep connection, and entrench them firmly in Physics. Quite so: biology, economy, evolution of sports and machines, they all belong to Physics. Why? Because everything that moves anywhere in nature obeys the few laws of Physics, for instance, the second law of mechanics, and the laws of thermodynamics: energy balance and irreversibility. This book is about a new law, the constructal law – a natural tendency that was not recognized until recently as a unifying phenomenon in Physics: organization, configuration, freedom of evolution, and life. Unavoidably, any scientific text has a biographical nuance. In fact, the research is, in the first place, about the researcher, or it is not original. This is why the story returns often to the Danube, the delta, the olympiads of mathematics, higher level basketball, and the author’s unique chance to hear his parents telling him about liberty at a time when such lessons was putting them in great an permanent danger. For more information on the Constructal Law visit the site: www.constructal.org.
Technical Briefs Convection in the Cavity Between Two Rollers: the Effect of Thermal Boundary Conditions P. A. Litsek, P. A. Litsek Promon Engenharia, Praia do Flamengo 154, Rio De Janeiro, RJ 22210 Brazil Search for other works by this author on: This Site PubMed Google Scholar Z. Zhang, Z. Zhang Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 Search for other works by this author on: This Site PubMed Google Scholar A. Bejan A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information P. A. Litsek Promon Engenharia, Praia do Flamengo 154, Rio De Janeiro, RJ 22210 Brazil Z. Zhang Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 A. Bejan Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 J. Heat Transfer. Feb 1991, 113(1): 249-251 (3 pages) https://doi.org/10.1115/1.2910536 Published Online: February 1, 1991 Article history Received: November 3, 1989 Revised: July 2, 1990 Online: May 23, 2008
