This article presents a theory of how the melt region advances as an intrusion layer along the top boundary of a solid phase-change material that is heated from the side. The phase-change material fills the pores of a solid matrix. We show that the thickness of the horizontal melt layer increases as x 3 5 , where x is the horizontal distance measured by from the leading edge of the layer. The total length of the intrusion layer increases as t 3 4 , and as Tmax 5 4 . Finite-difference simulations of convection melting in the Darcy-Rayleigh number range of 200–800 agree with the theoretical results. We also show that in a rectangular porous medium heated from the side, the size of the entire melt region is dominated by the melting contributed by the horizontal intrusion layer, if the time is great enough so that the group (Ste Fo) 3 4 is greater than 1.
The objective of the present article is to compare previous experimental data of Gao et al. [20] to the predictions of Bejan and Sciubba's analysis [7] on the optimal spacing for maximum heat transfer from a package of parallel plates. Experimental investigations of the flow and the associated heat transfer were conducted in two-dimensional microchannels in order to test possible size effects on the laws of hydrodynamics and heat transfer and to infer optimal conditions of use from the measurements. The test section was designed to modify easily the channel height e between 1 mm and 0.1 mm. Measurements of the overall friction factor and local Nusselt numbers show that the classical laws of hydrodynamics and heat transfer are verified for e > 0.4 mm. For lower values of e, a significant decrease of the Nusselt number is observed, whereas the Poiseuille number continues to have the conventional value of laminar developed flow. The transition to turbulence is not affected by the channel size. The experimental data were processed by using the dimensionless parameters of Bejan and Sciubba's analysis [7]. For fixed pressure drop across the channel, a maximum of heat transfer rate density is found for a particular value of e. The corresponding dimensionless optimal spacing and heat transfer rate density are in very good agreement with the predictions of Bejan and Sciubba. This article reports the first time that the optimal spacing between parallel plates is determined experimentally. Keywords: microfluidicsheat transfermicrochannelsoptimization
This paper reports two new advances on constructal design, or the generation of flow architecture in the pursuit of global thermodynamic performance subject to constraints. This is thermodynamic optimization with an emphasis on its product: flow geometry, as a mechanism for the achievement of performance under constraints. The first part of the paper relies on this principle in the design of dendritic (tree-shaped) networks that distribute a stream optimally from one source to a disc-shaped area for which the source is the center. One global objective is (i) the minimization of source-area flow resistance, which, when the overall flow rate is fixed, is the same as the minimization of the destruction of exergy. The alternative (ii) is to design the flow architecture such that the flow path followed by each stream is the shortest. Optimal dendritic flow architectures are reported based on both methods. It is shown that the performance of designs (ii) is nearly the same as that of the optimal designs (i). Examples of dendritic patterns of high-conductivity blades used for cooling a disc-shaped body show that the overall thermal resistance of the construct decreases as the size and complexity of the flow architecture increase. The same principle of maximizing global performance can be used to select the optimal sizes of flow components in all complex energy flow systems (e.g., vehicles, animals). The size of a heat transfer surface and the diameter of a tube with fluid are given as examples.
This paper outlines a combined theoretical and numerical study of the mass transfer effected by high Rayleigh number Bénard convection in a two-dimensional saturated porous layer heated from below. The focus of this study is on the Darcy flow, heat transfer and mass transfer scales of a single cell (roll) that exists in the steady two-dimensional convection regime. The numerical solutions are based on the complete governing equations for two-dimensional flow, and cover the Rayleigh number range 50–2000. The numerical results compare favorably with the theoretical conclusions of a scale analysis that is based on the recognition of 1. (i) two temperature difference scales in the cell, 2. (ii) a flow field without horizontal boundary layers, and 3. (iii) thermal top and bottom end-regions that are not slender enough to be boundary layers. Writing Le for the Lewis number, the overall mass transfer rate or Sherwood number is shown to scale as Le 1 2 Ra 7 8 if Le > Ra 1 4 , as Le2 Ra 1 2 if Ra− 1 4 < Le < Ra 1 4 , and as O(1) if Le < Ra− 1 4 . The transition from the Darcy flow to the inertia-dominated Forschheimer flow and the scales of the Forschheimer regime are discussed in the closing section.
In this paper we explore the use of vascular design that provides cooling and mechanical strength at the same time. We illustrate the concept with a circular plate vascularized with embedded channels. The cooling fluid enters to the plate from the center or from the rim, and leaves after it cools the plate down to an allowable temperature level. The vascular cooling channels also affect the mechanical strength of the plate. We simulated numerically the thermofluid and mechanical behavior for three different structures; radial, dendrites with one pairing level and dendrites with two pairing levels. We found that for a given set of conditions (applied pressure difference, coolant inlet position, and number of the cooling channels) there is one configuration that is best; however, there is no single configuration that is best for all conditions.
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.
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.
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 the examples of forced and natural convection discussed until now, the fluid that flowed through the pores did not experience a change of phase, no matter how intense the heating or cooling effect. In the present chapter we turn our attention to situations in...
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.
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.
Porous and Complex Flow Structures in Modern Technologies represents a new approach to the field, considering the fundamentals of porous media in terms of the key roles played by these materials in modern technology. Intended as a text for advanced undergraduates and as a reference for practicing engineers, the book uses the physics of flows in porous materials to tie together a wide variety of important issues from such fields as biomedical engineering, energy conversion, civil engineering, electronics, chemical engineering, and environmental engineering. Thus, for example, flows of water and oil through porous ground play a central role in energy exploration and recovery (oil wells, geothermal fluids), energy conversion (effluents from refineries and power plants), and environmental engineering (leachates from waste repositories). Similarly, the demands of miniaturization in electronics and in biomedical applications are driving research into the flow of heat and fluids through small-scale porous media (heat exchangers, filters, gas exchangers). Filters, catalytic converters, the drying of stored grains, and a myriad of other applications involve flows through porous media. By providing a unified theoretical framework that includes not only the traditional homogeneous and isotropic media but also models in which the assumptions of representative elemental volumes or global thermal equilibrium fail, the book provides practicing engineers the tools they need to analyze complex situations that arise in practice. This volume includes examples, solved problems and an extensive glossary of symbols.
This paper outlines a completely deterministic (“constructal”) theory of why quasi‐similar street patterns exist, how they form, and how they grow in time. The function of the street network is to connect a finite area to a single destination point. The new idea is that the network of streets evolves in time , by starting with the optimization of the shape of the smallest area element that is serviced by the network. Next, the optimized area elements are assembled into a larger area element which is again optimized for shape. This sequence of optimization & organization is repeated in finite‐size steps, toward larger quasi‐similar assemblies. The optimization consists of minimizing the travel time between each point of a finite area and a common point of destination. The network is constructed (optimized, organized) in time. Every single geometric feature of the network is the result of pure, deterministic theory: the shape of each area element, the shape of each new (larger) assembly, the optimal number of parts in each assembly, the relative orientation of successive streets, and the optimal width of each street.
Since we have dealt with natural convection and forced convection in some detail, our treatment of mixed convection can be brief. It is guided by the review paper by Lai, Prasad, and Kulacki (1991). We start with a treatment of boundary layer flow on heated plane...
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.
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...
We consider the problem of cooling a two-dimensional heat generating conducting volume with one heat sink, such that the smallest features of the internal structure are so small that the conventional description of conduction breaks down. The effective thermal conductivity exhibits the “size effect,” and is governed by the smallest structural dimension, which is comparable with the mean free path of the energy carriers. According to the constructal method, the development of the internal cooling structure proceeds from small to large, in steps of geometric optimization and assembly. This starts at the elemental level, where there is only one high-conductivity layer for collecting and evacuating the heat. The shape of the smallest volume can be optimized for minimal thermal resistance. Next, a first construct is formed by optimizing the number of assembled elements and the internal geometric features of the assembly. The method is repeated at the second construct level, where several first constructs are grouped so that their global thermal resistance is minimal. The construction reveals an internal multiscale structure shaped as a tree, where the spaces between the smallest branches are ruled by nanoscale heat transfer. It is shown that the transition from regions with nanoscale heat transfer to regions with conventional heat transfer is governed not only by the smallest dimensions, but also by heterogeneity (relative amounts of high- and low-conductivity materials).
