901 publications from this institution
This letter proposes to use entropy generation as a measure of the relative merit of heat transfer augmentation techniques relative to each other and to the heat exchange apparatus in which they may be incorporated. In this context, heat transfer augmentation techniques are viewed as design changes capable of reducing the irreversible destruction of useful energy (exergy) in heat exchange equipment. The entropy generation rate takes into account simultaneously the heat transfer and fluidd friction changes associated with implementing a heat transfer augmentation technique. Using in-tube roughness as an example, the letter shows what specific operating conditions must be met before the destruction of exergy can be reduced via heat transfer augmentation.
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No abstract is provided for this article.
In the present study, we show that the fastest runners and swimmers are becoming not only faster but also heavier, taller and more slender. During the past century, the world record speeds for 100 m-freestyle and 100 m-dash have increased with body mass (M) raised to the power 1/6, in accordance with the constructal scaling of animal locomotion. The world records also show that the speeds have increased in proportion with body heights (H) raised to the power 1/2, in accordance with animal locomotion scaling. If the athlete's body is modeled with two length scales (H, body width L), the (M, H) data can be used to calculate the slenderness of the body, H/L. The world records show that the body slenderness is increasing very slowly over time.
Here we show the fundamental tradeoffs that underpin the design of a distributed energy system with two objectives: the production and distribution of electric power driven by solar heating, and desalinated water produced by consuming solar power. We show analytically that larger solar power plants and desalination plants are more efficient than smaller plants. This phenomenon of economies of scale is countered by the greater losses associated with larger distribution networks. From this conflict emerges the proper allocation of nodes of production of power and water on a territory. We show that as the individual needs of power and water increase in time, the sizes of solar plants and desalination plants increase, and so does the size of the territory served by each power plant. At the same time, the territory served by each desalination plant decreases, and this means that the number of desalination plants allocated to one power plant increases.
This paper shows that it is possible to distribute a finite amount of insulation in an optimal way that minimizes the overall heat transfer rate from a nonisothermal wall to the ambient. The optimal insulation thickness for a plane wall varies as the square root of the local wall-ambient temperature difference. Corresponding variational-calculus results are developed for cylindrical walls covered with insulation. The heat loss reduction associated with using the optimal thickness is greater when the wall is plane, as opposed to cylindrical, and when the wall temperature variation in the x direction has a greater second derivative, d2 T/dx 2. It is shown finally that the best insulation for a single-phase stream suspended in an environment of different temperature is the insulation with uniform thickness.
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By a porous medium we mean a material consisting of a solid matrix with an interconnected void. We suppose that the solid matrix is either rigid (the usual situation) or it undergoes small deformation. The interconnectedness of the void (the pores) allows the flow of...
In this chapter we turn our attention to processes of combined (simultaneous) heat and mass transfer that are driven by buoyancy. The density gradients that provide the driving buoyancy force are induced by the combined effects of temperature and species...
Science holds that the arrow of time in nature is imprinted on one-way (irreversible) phenomena and is accounted for by the second law of thermodynamics. Here I show that the arrow of time is painted much more visibly on another self-standing phenomenon: the occurrence and change (evolution in time) of flow organization throughout nature, animate and inanimate. This other time arrow has been present in science but not recognized as such since the birth of thermodynamics. It is Maxwell's demon. Translated in macroscopic terms, this is the physics of the phenomenon of design, which is the universal natural tendency of flow systems to evolve into configurations that provide progressively greater access over time and is summarized as the constructal law of design and evolution in nature. Knowledge is the ability to effect design changes that facilitate human flows on the landscape. Knowledge too flows.
In this paper we report experimentally the hydraulic and thermal behavior of a balanced counterflow heat exchanger in which each stream flows through a tree-shaped structure covering a circular area. The tree structure is the same on both sides of the heat exchanger: they have three channels reaching/leaving the center, and three branching levels (i.e., 24 channels on the periphery of the circular area). On the hot side, fluid is pumped from the center to the periphery. On the cold side, fluid is pumped from the periphery to the center, and leaves the heat exchanger as a single stream. Two experimental apparatuses were built and tested. In the first design, the body of the heat exchanger was made out of plexiglass and a peripheral plenum was used to collect or distribute the working fluid to the tree structure. The measurements showed that the use of a plenum generates undesirable volumetric flow asymmetries. These lessons led to a second design, which has two major improvements: (i) the heat exchanger core was made out of aluminum and (ii) individual ports (inlets/outlets) were used for each of the peripheral channels. The hydraulic results show a relation between the appearance of volumetric flow rate asymmetries and the bifurcation angles throughout the dendritic structure. The heat transfer results are also discussed.
This is a study of the single-cell natural convection pattern that occurs in a “stably heated” corner in a fluid-saturated porous medium, i.e., in the corner formed between a cold horizontal wall and a hot vertical wall situated above the horizontal wall, or in the corner between a hot horizontal wall and a cold vertical wall situated below the horizontal wall. Numerical simulations show that this type of corner flow is present in porous media heated from the side when a stabilizing vertical temperature gradient is imposed in order to suppress the side-driven convection. Based on numerical solutions and on scale analysis, it is shown that the single cell corner flow becomes increasingly more localized as the Rayleigh number increases. At the same time, the mass flow rate engaged in natural circulation and the conduction-referenced Nusselt number increase. Numerical results for the flow and temperature fields and for the net heat transfer rate are reported in the Darcy-Rayleigh number range 10–6000.
Constructal theory regards the generation of flow configuration as a natural (physics) phenomenon, and attributes this phenomenon to a physics principle (the constructal law): For a flow system to persist in time (to survive), it must evolve in such a way that it provides easier and easier access to the currents that flow through it. Special among the engineered flow architectures derived from the constructal law are the tree-shaped (dendritic) designs. They are invading technological domains in which they were not used previously (manufacturing, electronics cooling, fuel cells). In this paper we report a fundamental study of how to vascularize a volume so that fluid flow and function (e.g., cooling, sensing, maintenance, repair, healing) reaches every point of the material. The examples are architectures that deliver healing fluid to all the crack sites that may occur randomly through the material. In one concept, a grid of interconnected channels is built into the material, and is filled with pressurized healing fluid. It is shown that the optimization of the ratio of channel diameters cuts in half the time of fluid delivery to the crack. In the second concept, one stream flows steadily through the material and bathes it volumetrically. The steam enters through one point, distributes itself as a river delta, reconstitutes itself as a river basin, and exits through one point. In the third, the solid body is vascularized with trees that alternate with upside-down trees. The flow through all the trees is in one direction, from one side of the body to the other. It is shown that the choice of the tree-tree configuration has a decisive impact on the global performance of the vascularized composite.
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,...
Pattern formation and self-organization are phenomena that occur across the board, in animate and inanimate systems. In this paper, we rely on the constructal law to explain the generation of patterns (shapes, structures) in aggregates of organisms–pedestrian crowds and stony corals. In pedestrian crowds a variety of patterns are often observed, from ‘chaotic’ appearances to spontaneous organization in lanes of uniform walking direction. Stony corals and other organisms also present intraspecific variability in shape. We show that flow systems develop in time patterns which provide easier access to the nutrients and space, within a set of constraints imposed by each situation. Flow systems have the freedom to morph their shape in search for architectures that allows them to have greater access to the space that they inhabit. We identify the mechanisms allowing pedestrians to evolve in space and time. We also show that stony corals may develop branched or spherical shapes, depending on which shape performs best in response to the environmental conditions. The constructal law allows systems with complex internal flows to be described and understood for a unified view.
Experimental observations of the shape of the laminar and transition sections of a confined coaxial jet show that the laminar length (L) is always approximately ten times greater than the wavelength of the first sinusoidal deformation of the flow (λ). This discovery coincides with the L∼ 10λ proportionality noted in an earlier study of the laminar sections of air plumes rising above concentrated heat sources. A search of the literature shows that the L∼ 10λ proportionality characterizes other boundary layer-type flows, for example the flow over a flat plate and the plane mixing layer. Using the confined jet and the round plume as examples, it is shown also that the L∼ 10λ proportionality can be derived theoretically.
This article shows that the sudden end of economic expansion (movement, wealth) emerges as a natural, physical feature of the spreading movement, which has access to power (money), freedom to morph, and power storage (savings) for future movement on even greater areas. The movement is driven by power generation, which is interspaced with power savings on the same area. The theory is constructed systematically from the physical basis of economics concepts (money, savings, time, and bubbles) to a physics model that accounts for the time-dependent spreading of movement on an area. Previous study has shown that physics accounts for the proportionality between the annual wealth (GDP) of a population and the annual consumption of fuel to generate power for that population. The present theory extends this view to the more realistic situation where every movement in society (wealth and fuel consumption) is time dependent.
Here we document the effect of flow configuration on the heat transfer performance of a helically shaped pipe embedded in a cylindrical conducting volume. The helix is wrapped on an imaginary cylinder. Several configurations of helices with fixed volume of fluid are considered. We found the optimal spacings between the helical turns such that the volumetric heat transfer rate is maximal. Next, we extended the study by varying the volume (length) of the embedded pipe. We found that the optimized features of the heat transfer architecture are robust with respect to changes in several physical parameters. We compared the performance for both helical 3D and 2D designs. We found that the 2D designs offer greater heat transfer density than the 3D designs.
