901 publications from this institution
The parallel structure of the fields of natural convection in fluids and natural convection in porous media is reviewed in terms of the most basic scales of flow, heat and mass transfer. In each field, the review includes single vertical boundary layers, enclosures, heat and mass transfer in the presence of temperature and concentration buoyancy effects, Benard flow and new or still unexplored directions of research. In particular, an original scale analysis of the nonDarcy regime in a porous layer heated from below shows that the Darcy-nonDarcy transition occurs when the Darcy-Rayleigh number Rak exceeds O(Prp), where Prp is an important new dimensionless group called “the porous medium Prandtl number.” In the nonDarcy regime the Nusselt number scales as Rak ½ times Prp ½.
This letter outlines the analogy which exists between inviscid jets and elastic columns in axial compression. It is shown that straight inviscid jet columns possess the property of sinusoidal infinitesimal buckling. The buckling wavelength scales with the transversal dimension of the jet. The repeated buckling and breakup of the jet column is responsible for the observed whiplash motion of turbulent jets. The buckling theory predicts correctly the natural frequency of the whiplash motion and the Reynolds number for the laminar-turbulent transition in free jet flow.
This article emphasizes on the role engineering plays in the existence and working of any civilization. The power of engineering is that it provides the foundation for further advancements. That enables access to natural resources, the access increases the potential for human freedom and creativity, and that freedom allows for the creation of new engineering advances. The engineered contrivances are diverse—coming in many sizes and configurations, being as ancient as rope or as new as self-driving vehicles—and at first sight appear complicated, disorganized. Access to more power—that is, the ability to move across space, in opposition to the forces that oppose motion—due to the application of engines touched off an explosion in new contrivances. It is only when access to the fruits of engineering is severed, due to earthquakes and hurricanes or accidents and acts of despotism that most people become aware of the monumental role that engineering plays in our lives today. Today, biomedical engineers are working to improve and extend lives. They are part of an engineering tradition that extends for millennia.
This paper outlines a direct route to the construction of effective tree-shaped flow structures. Dendritic flow structures dominate the design of natural and engineered flow systems, especially in thermal and fluid systems. The starting point is the optimization of the shape of each elemental area or volume, such that the length of the flow path housed by the element is minimized. Proceeding toward larger and more complex structures – from elements, to first constructs, second constructs, etc. – the paper develops tree-shaped flow structures between one point and a straight line, one point and a plane, a circle and its center, and a point and many points distributed uniformly over an area. In the latter, the construction method is applied to a fluid flow configuration with laminar fully developed flow. The constructions reveal several features that are supported by empirical observations of natural tree-shaped flows: asymmetry, flow rate imbalance, pairing or bifurcation, angles between branches, and Y-shaped constructs that lie in a plane. It is shown that these basic features are necessary because of “packing”, i.e., assembling optimized elements into a fixed space, and filling the space completely. For the flow between an area and one point, the best elemental shape is the regular hexagon. It is shown that the emergence of string-shaped links that connect two or more elements are necessary features, which are also required by packing. Strings cover some of the inner zones of the tree network, particularly the inner zones of large and complex trees. Dichotomous Y-shaped constructs dominate the tree structure, especially the peripheral zones of the tree canopy. The practical importance of the simplified design method is discussed.
This article presents the solutions to two fundamental problems in the design of the insulation for a refrigerator. The first problem is how to incorporate the new generation of non-CFC materials in a composite layer that also contains polyurethane foam. The article shows how to select the thickness of each sublayer in the composite so that the total cost is minimized over the lifetime of the refrigerator. The second problem addresses the distribution of a given amount of insulation material around the refrigerator, for example, over the freezer and cooler sections. The article reports the optimal insulation thicknesses for the freezer and cooler sections so that the overall heat leak into the cold space is minimum. Notes Address correspondence to Prof. Adrian Bejan, Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708-0300.
Technical Briefs Scales of Melting in the Presence of Natural Convection in a Rectangular Cavity Filled With Porous Medium P. Jany, P. Jany 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. Jany 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. May 1988, 110(2): 526-529 (4 pages) https://doi.org/10.1115/1.3250522 Published Online: May 1, 1988 Article history Received: January 13, 1987 Online: October 20, 2009
The term “mass transfer” is used here in a specialized sense, namely the transport of a substance that is involved as a component (constituent, species) in a fluid mixture. An example is the transport of salt in saline water. As we shall see below,...
here I show that the pattern of generation of science is in disaccord with the pattern of admission to the academy.The contrast between the two patterns is illustrated with two rankings: the most highly cited authors in all US engineering and the members of the US National academy of Engineering.Only 2.7% of the academy members are highly cited.This discrepancy raises the question of how the academicians have succeeded in this mode of self-organization.The answer is evident when we examine two drawings: (1) the numbers of highly cited authors versus the ranks of their institutions, and (2) the numbers of academy members versus the ranks of their institutions.Drawing ( 1) is a natural (Zipf) distribution, which is virtually the same as the hierarchy and scaling of all freely morphing flow structures in nature (e.g.city sizes on the landscape, tree sizes in the forest, and river sizes in the basins).Drawing (2) fails the natural (Zipf) test, and reveals that the flow to the academy is not free, that is, not for everybody.
The objective of the Journal of Mechanics is to provide an international forum to foster exchange of ideas among mechanics communities in different parts of world.The Journal of Mechanics publishes original research in all fields of theoretical and applied mechanics.The Journal especially welcomes papers that are related to recent technological advances, such as micro/nanomechanics, medical and biological systems, and microscale heat transfer.The contributions, which may be analytical, experimental or numerical, should be of significance to the progress of mechanics.
Here we show that the heat transfer between a pipe assembly and the soil during the annual temperature cycle of a heat pump depends on the configuration of the flow system. We rely on constructal design to find the flow architecture (spacings, shapes) such that the heat transfer between the assembly and the ground is increased. The flow configuration changes freely, and its design is evolutionary. The better shapes change gradually from slender to square as the volume fraction occupied by the flow assembly increases. The heat transfer performance increases as the depth of the structure decreases, but the depth has a negligible effect on the shape of the structure. The results also show that the heat transfer performance increases as the configuration of the ground volume and the buried structure evolves to the most slender shape possible.
The discussion of the ideal conversion of enclosed thermal radiation revolves around three different theories, whose earliest proponents were Petela, Spanner, and Jeter, respectively. Each of these theories has been accused to be incorrect. The objective of this paper is to provide a bird’s-eye-view of the relationship between these three theories. It is shown that the theories are individually correct, and that they complement (rather than contradict) one another. The present “unification” consists of answering two important questions: i) What is the origin of the assumed blackbody radiation system? ii) What is the ultimate fate of the blackbody radiation system? It is shown that the supply of high-temperature isotropic radiation postulated by all three theories is the result of a heat input from a high-temperature heat reservoir. When referenced to the same high-temperature heat input, Spanner’s ideal work output must always be smaller than Petela’s, and, in turn, Petela’s work output must be smaller than Jeter’s.
The development of complex cellular systems such as the vertebrates requires the availability of large amounts of oxygen for the metabolic needs of the cells. The respiratory and circulatory systems are the specialized and hierarchically organized flow systems that...
The constructal law of generation of flow structure is used to predict the main features of global circulation and climate. The flow structure is the atmospheric and oceanic circulation. This feature is modelled as convection loops, and added to the earth model as a heat engine heated by the Sun and cooled by the background. It is shown that the dissipation of the power produced by the earth engine can be maximized by selecting the proper balance between the hot and cold zones of the Earth, and by optimizing the thermal conductance of the circulation loops. The optimized features agree with the main characteristics of global circulation and climate. The robustness of these predictions, and the place of the constructal law as a self-standing principle in thermodynamics, are discussed. Copyright © 2005 John Wiley & Sons, Ltd.
The constructal theory of the origin of geometrical form in natural flow systems is used to predict the formation of crack patterns in solids subjected to volumetric cooling by convection. The approach is purely theoretical (deterministic), because it starts from the principle of geometric minimization of resistance to flow, and leads to the existence of optimal distances between successive cracks and, consequently, optimal crack widths. The analytical part of the paper is based on the method of intersecting the asymptotes (many cracks vs. few cracks), and anticipates several natural features that had not been explained previously: cracks are denser when the convective cooling effect is more intense and/or the initial departure from equilibrium is greater, and the loops are close to round (or square) in two-dimensional lattices of cracks. These trends are further illustrated by using a one-dimensional numerical conduction model with equidistant parallel-plate cooling channels.
It is shown that the phenomenon of natural convection driven in a porous medium by a cold plate facing upward or by a warm plate facing downward consists of a finite-length boundary layer flow chopped off by the sharp edges of the plate. The heat and fluid flow features of the boundary layer are determined analytically employing scale analysis and integral analysis. The overall heat transfer rate between porous medium and plate is found to vary as Nu/Ra1/3 = 0(1), where Ra is the Darcy-modified Rayleigh number based on plate half-length. The boundary layer features of the flow and the heat transfer effected by it are confirmed in the Ra range 100–700 by numerical solutions of the complete partial differential equations.
The void spaces of a porous structure can be interconnected or not. In the latter case the void spaces are disconnected “inclusions” that do not allow a flow to permeate through the porous structure. When the inclusions are large, the flow inside each...
This article reports the results of a numerical study of heat transfer to the air permeating through hair strands covering a flat surface. First, the surface is modeled as isothermal, and second the surface is assumed covered with uniform heat flux. For both models, the nivestigation develops dimensionless charts for calculating the overall heat transfer rate through a flat surface of finite size. The heat transfer rate is the sum of the direct heat transfer from the surface to air flow and the heat conducted away by the hair strands. In each model, the overall Nusselt number emerges as a function of four dimensionless groups. When the surface is sufficiently long, the constant flux-surface Nusselt number is up to 20% greater than the Nusselt number of the corresponding isothermal surface.
Globalization, security infrastructure and energy sustainability can be designed based on a scientific principle. In this book, these objectives are approached based on constructal theory, which means to design such projects as global 'flow' architectures that are 'alive' with movement of personnel, equipment, information, education, etc. Constructal Human Dynamics, Security and Sustainability highlights the progress made during the NATO Advanced Research Workshop held in Evora, Portugal in May 2008. This workshop brought together social scientists with physicists, engineers and biologists. Together they addressed main topics such as human dynamics viewed as natural phenomena of design generation, flow networks for distribution and collection, large-scale construction projects (e.g., airports, waste storage), logistics, decontamination, energy supply routes, distributed energy systems, water resources management, environmental security sustainability and globalization. The chapters selected for this book represent the interdisciplinary approach and team atmosphere that emerged in Evora.
This paper is a Second-Law study of thermal insulation systems consisting of two 1-dim. insulations in parallel. It is shown that the parallel-insulation model applies to power and refrigeration systems exposed to large absolute temperature ratios. The insulation design which conserves maximum exergy (available work) is determined for two classes of parallel-insulation systems: in one class, the two parallel insulations are cooled continuously by two streams, in the other class they are cooled partly (intermittently) by one stream of single-phase fluid. The study shows that, in contradiction to previously published results, the continuous-cooling method is thermodynamically superior to the partial-cooling method.
