No abstract is provided for this article.
No abstract is provided for this article.
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 paper documents the effect of thermal expansion on a vascularized plate that is heated and loaded mechanically. Vascular cooling channels embedded in a circular plate provide cooling and mechanical strength. The coolant enters the plate from the center and leaves after it cools the plate to an allowable temperature limit. The mechanical strength of the plate decreases because of the embedded cooling channels. However, cooling the plate under an allowable temperature level decreases the thermal stresses. The mechanical strength of the plate which is heated and loaded mechanically at the same time can be increased by inserting cooling channels in it. The mechanical and thermofluid behavior of a vascularized plate was simulated numerically. The cooling channel configurations that provide the smallest peak temperature and von Mises stress are documented. There is one cooling channel configuration that is the best for the given set of boundary conditions and constraints; however, there is no single configuration that is best for all conditions.
This paper documents the geometric optimization of an assembly of staggered vertical plates that are installed in a fixed volume. The heat transfer is by laminar natural convection. The objective is to maximize the overall thermal conductance between the assembly of plates and the surrounding fluid. The geometric parameters that vary are the horizontal spacing between adjacent columns of plates, the number of plates mounted in each vertical column, the plate dimensions, and the stagger between columns. The optimization is performed numerically and experimentally in the range Pr = 0.72 and 103 ≤ RaL ≤ 106, where RaL is the Rayleigh number based on the vertical dimension of the assembly (L). The results for the optimal horizontal spacing are correlated using formulas derived from the theory of the intersection of asymptotes (Bejan, 1984, 1995).
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 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.
Physics is concise, simple, unambiguous, and constantly improving. Yet, confusion reigns in the field especially with respect to complexity and the second law of thermodynamics. In this paper, we step back and take a look at these notions—their meaning and definition—on the background provided by nature and thermodynamics. We review the central concepts and words that underpin the physics of evolutionary design today: information, knowledge, evolution, change, arrow of time, pattern, organization, drawings, complexity, fractal dimension, object, icon, model, empiricism, theory, disorder, second law, the “any” system in thermodynamics, morphing freely, and the constructal law. We show, for example, that information is not knowledge, fractal dimension is not a measure of complexity, and pattern is not a live flow architecture. Drawings, as physical means to facilitate the flow of knowledge, are subject to the natural tendency toward design evolution. Complexity, organization, and evolution in nature are most powerful and useful when pursued as a discipline, with precise terms, rules, and principles.
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.
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.
We start with the simplest case, that of zero flow through the fluid-saturated porous medium.For an equilibrium state the momentum equation is satisfied if 6.1 $$- \nabla P + \rho fg = 0$$...
The universal phenomenon of evolution consists of change after change in flow configuration in a time direction that is perceptible to the observer. This reality clashes with the doctrine of precise optima, minima, and maxima, now rigidly in place because of calculus and computational simulations of all kinds of flowing and changing configurations. With two dissimilar examples, access on an area (a human settlement) and along a line (animal locomotion), it is shown that even a 1-percent imperfection in performance is accompanied by a sizable bandwidth of freedom to hit the ‘target’, that is, an easily accessible design with close to perfect performance. The evolutionary designs reveal the physics behind the phenomenon of diminishing returns in the vicinity of the mathematical optimum. In evolution what works is kept.
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...
No abstract is provided for this article.
