901 publications from this institution
This paper describes a numerical and theoretical study of the transient natural convection heating of a two-dimensional rectangular enclosure filled with fluid. The heating is applied suddenly along one of the side walls, while the remaining three walls are maintained insulated. It is shown that the process has two distinct phases, an early period dominated by conduction and a late period dominated by convection. The scaling laws for the heat transfer rate and the effectiveness (energy storage fraction) are determined based on scale analysis. These theoretical results are confirmed by numerical experiments conducted in the domain Ra = 103−106, Pr = 7, A = 1, where Ra is the Rayleigh number based on height and initial temperature difference, Pr is the Prandtl number, and A is the height/length ratio of the enclosure. Correlations for heat transfer rate and effectiveness are constructed by comparing the theoretical scaling laws with the numerical results.
This paper shows that in a space filled with heat generating parallel plates and laminar forced convection, the heat transfer density can be increased beyond the level known for parallel plates with optimal spacing. The technique consists of inserting in every entrance region new generations of smaller plates, because smaller plates have thin boundary layers that fit in the unused (isothermal) entrance flow. This technique can be repeated several times, and the result is a sequence of multi-scale flow structures that have progressively higher heat transfer densities. The work consists of numerical simulations in a large number of flow configurations, one differing slightly from the next. The complete optimized architecture and performance of structures with one, two and three plate length scales are reported. Diminishing returns are observed as the number of length scales increases. This method can be used to develop multi-scale nonuniform flow structures for heat exchangers and cooled electronic packages.
This is a fundamental study of how to vascularize a self-healing composite material so that healing fluid reaches all the crack sites that may occur randomly through the material. The network of channels is built into the material and is filled with pressurized healing fluid. When a crack forms, the pressure drops at the crack site and fluid flows from the network into the crack. The objective is to discover the network configuration that is capable of delivering fluid to all the cracks the fastest. The crack site dimension and the total volume of the channels are fixed. It is argued that the network must be configured as a grid and not as a tree. Two classes of grids are considered and optimized: (i) grids with one channel diameter and regular polygonal loops (square, triangle, hexagon) and (ii) grids with two channel sizes. The best architecture of type (i) is the grid with triangular loops. The best architecture of type (ii) has a particular (optimal) ratio of diameters that departs from 1 as the crack length scale becomes smaller than the global scale of the vascularized structure from which the crack draws its healing fluid. The optimization of the ratio of channel diameters cuts in half the time of fluid delivery to the crack.
Filtration is one of the simplest and most effective ways to separate particulate matter from fluid and filters find diverse application. The filter design is a critical aspect because it impacts performance. The constructal theory is about the generation of flow architecture both in engineered and natural flow systems, as evidenced by numerous applications (e.g. Bejan, 2000; Bejan & Lorente 2006). According to constructal law, the system shape and internal flow architecture do not develop by chance, but result from the permanent struggle for better performance and therefore must evolve in time. In this study we apply the constructal law to filter design with a geometry that is optimized for the capture of submicrometre aerosols which are particularly hazardous. Filters operate at low Reynolds numbers. Their principle of operation is this: an air stream containing particles with concentration Cin is forced through a set of collecting elements (e.g. fibres, beds). Particles approaching the collectors are then transferred from the air stream to the collector surface. The global purpose is that particle concentration at the outlet is the smallest possible. This means that the purpose of the flow architecture is to maximize deposition rate and volumetric density. If the collecting elements are very close to each other (e.g., in the small-porosity limit). The corresponding particle transfer density (mel) is (Reis et al., 2006)
This article addresses two questions, why certain animals (frogs, breaststroke swimmers, hovering fliers, jellyfish) push rapidly against the surrounding fluid and then reach forward slowly, and whether this rhythm of propulsion is a manifestation of the universal phenomenon of design evolution in nature. Emphasis is on the distribution of time periods of locomotion in which, during the driving phase of cyclic movement (the motive stroke, phases 1 and 2, in alternating sequence with the dissipative stroke, phase 3), the work is generated (phase 1) and dissipated (phase 2). The relative lengths of the characteristic times t1 and t2 of the phases 1 and 2, are predicted. The relative duration of the proposed three phases of a cycle is the 'rhythm'. The analysis is based on a model of how the effective cross-sections of the stroking body parts impact the surrounding medium, water, or air, and the total power required to account for the kinetic energy losses during phases 2 and 3, which are due to drag forces posed by the surrounding medium. The body configuration (limbs' cross-sections) determines the limbs' velocities that maximize mean power, and the times t1 and t2 within the motive stroke. Emphasis is placed on the freedom to change the evolving design. Freedom is represented in two ways: the number of degrees of freedom in changing the dimensions of the model and its deformation in time, and the effect that evolutionary changes have on the access that the body has to its available space. Freedom to change the locomotion design leads to greater power and speed.
This paper reviews a recent development in the integrative optimization of complex energy systems: the geometric optimization of tree-shaped networks that facilitate volume-point and area-point flows with minimum resistance, time, or cost. This method was originally...
This paper reviews recent constructal-theory advances: the optimal distribution of discrete heat sources cooled by laminar natural convection. Three scenarios are investigated: (i) many small heat sources mounted on a vertical wall, (i) a few small finite-size heat sources mounted on the side wall of a two-dimensional enclosure, and (iii) one heated area on the wall of a vertical diverging or converging channel with chimney flow. In (i) and (ii), the optimally distributed heat sources are not equidistant. In (iii), the geometry changes by varying the space between the walls, the distribution of heating along the walls, and the angle between the two walls. Numerical simulations in the Rayleigh number range 105≤RaH≤107 show that for maximal heat transfer rate density it is better to install heated sections at the channel entrance. The optimal angle between the two walls is approximately zero when RaH is large. The robustness of flow architectures with optimized distribution of heat sources is discussed.
In this paper we consider the fundamental problem of how to heat a stream to a specified exit temperature such that the overall fuel consumption is minimal. As illustration, we consider metal slabs that move at constant speed through a very slender enclosure with fixed total volume and arbitrary (nonuniform) distribution of cross-sectional area (heat transfer contact area). The heating is provided by a large number of heaters, which are distributed arbitrarily along the enclosure. The combustion gases flow in the x direction, which is oriented against the direction of the metal stream. The heat transfer is by convection. We show that minimal heat consumption is achieved when the heaters and the heat transfer contact area are distributed nonuniformly. The density of heaters per unit length must decrease as x −0.8 toward the entrance of the metal stream, and the heat transfer contact area must increase in proportion with x. These features suggest that the metal must move not as a single stream but as a tree-shaped flow. The metal enters in several parallel streams, which serve as tributaries to larger streams, leading to a single stream that exits at the specified temperature.
No abstract is provided for this article.
This paper shows that the internal structure (relative sizes, spacings) of a fuel cell can be optimized so that performance is maximized at the global level. The optimization of flow geometry begins at the smallest (elemental) level, where the fuel cell is modelled as a unidirectional flow system. The polarization curve, power and efficiency are obtained as functions of temperature, pressure, geometry and operating parameters. Although the model is illustrated for an alkaline fuel cell, it may be applied to other fuel cell types by changing the reaction equations and accounting for the appropriate energy interactions. The optimization of the internal structure is subjected to fixed total volume. There are four degrees of freedom in the optimization, which account for the relative thicknesses of the two (anode and cathode) diffusion layers, two reaction layers and the space occupied by the electrolyte solution. The available volume is distributed optimally through the system so that the total power is maximized. Numerical results show that the optima are sharp, and must be identified accurately. Temperature and pressure gradients play important roles, especially as the fuel and oxidant flow paths increase. The optimized internal structure is reported in dimensionless form. Directions for future improvements in flow architecture (constructal design) are discussed. Copyright © 2004 John Wiley & Sons, Ltd.
No abstract is provided for this article.
No abstract is provided for this article.
This paper details the generation of multi-scale flow structure in a package with heat sources. The model is based on abandoning two common assumptions (1) the assumption that there are many channels (components) in the package, which is traditionally made to exploit simplifications due to symmetry, and (2) the assumption that the heat-generating components have negligible thickness. Numerical simulations document the flow stagnation and separation generated by blunt heat sources, and the effect of these flow phenomena on the optimized internal flow structure. The effect of asymmetric thermal boundary conditions on the optimal spacing between heat-generating plates is significant. If the package has some channels with symmetric boundary conditions, and some with asymmetric boundary conditions, then the optimal structure has multiple spacings. The effect of freedom on design performance is documented by optimizing competing configurations that have different numbers of degrees of freedom.
No abstract is provided for this article.
In this paper we optimize the performance of several classes of simple flow systems consisting of T- and Y-shaped assemblies of ducts, channels and streams. In each case, the objective is to identify the geometric configuration that maximizes performance subject to several global constraints. Maximum thermodynamic performance is achieved by minimization of the entropy generated in the assemblies. The boundary conditions are fixed heat flow per unit length and uniform and constant heat flux. The flow is assumed laminar and fully developed. Every geometrical detail of the optimized structure is deduced from the constructal law. Performance evaluation criterion is proposed for evaluation and comparison of the effectiveness of different tree-shaped design heat exchangers. This criterion takes into account and compare the entropy generated in the system with heat transfer performance achieved.
A series of numerical experiments concerning the phenomenon of natural convection in a two-layer composite system heated from below is reported. The composite system consists of a fluid layer bounded from below by a porous bed saturated by the same fluid. The numerical solutions are based on the full equations for time-dependent flow, and document the main features of the flow at Rayleigh numbers several orders of magnitude above critical value. The ranges covered by the numerical solutions are 102–106 for overall Rayleigh number, 10−7–10−4 for Darcy number, and 0.2–1 for the height to length ratio of the composite system
Various studies on request patterns in P2P networks have confirmed the existence of the interest-based clusters [11] and [12]. Some P2P networks that exhibit the small-world phenomenon contain clusters of peers that frequently communicate with one another [17]. The...
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