901 publications from this institution
This is a review of recent engineering developments in thermodynamic optimization, which shed light on a universal design principle that accounts for macroscopic organization in nature. It is shown that the optimal performance of a finite-size system with purpose is always characterized by the equipartition of driving forces or the optimal allocation of material subject to overall constraints. Examples are drawn from natural inanimate systems (river basins, turbulent flow) and animate systems (living trees). It is shown that this principle also governs the architecture of tree networks. Tree networks can be obtained in purely deterministic fashion by minimizing the flow resistance (or the time of travel) between one point and a finite area or a finite volume (an infinite number of points). The shape of each volume element can be optimized for minimal flow resistance. The network is ‘constructed’ by assembling the shape-optimized building blocks, and proceeding in time from the smallest volume element toward larger constructs. In constructal theory small size and shapeless flow (diffusion) come first, and larger sizes and geometrical form (channels, streams) come later.
This paper presents the constrained thermodynamic optimization of a crossflow heat exchanger with ram air on the cold side. The ram-air stream passes through a diffuser before entering the heat exchanger, and exits through a nozzle. This configuration is used in the environmental control systems of aircraft. In the first part of the study the heat exchanger is optimized alone, subject to fixed total volume and volume fraction occupied by solid walls. Optimized geometric features such as the ratio of channel spacings and flow lengths are reported. It is found that the optimized features are relatively insensitive to changes in other physical parameters of the installation. In the second part of the study the entropy generation rate also accounts for the irreversibility due to discharging the ram-air stream into the atmosphere. The optimized geometric features are relatively insensitive to this additional effect, emphasizing the robustness of the thermodynamic optimum.
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In this paper we show that the thermodynamic performance of a gas turbine power plant can be optimized by adjusting the flow rate and the distribution of pressure losses along the flow path. Specifically, we show that the power output has a maximum with respect to the fuel flow rate or any of the pressure drops. The maximized power output has additional maxima with respect to the overall pressure ratio and overall temperature ratio. When the optimization is performed subject to a fixed fuel flow rate, and the power plant size is constrained, the power output and efficiency can be maximized again by properly allocating the fixed total flow area among the compressor inlet and the turbine outlet.
We now turn our attention to the results that constitute the core of modern research on convective heat and mass transfer through porous media. Our objective is not only to organize the compact presentation of these results, but also to explain their origin. We want...
This article elaborates the constructal theory and its link with natural design. Constructal theory is the view that the generation of design (configuration, rhythm) in nature is a universal phenomenon, which is covered by a law of physics known as the constructal law. The constructal law is about direction in time. It provides a broad coverage of “designedness” everywhere, from engineering to geophysics and biology. The constructal law provides the student with strategy for how to pursue and discover design—the configurations or patterns—in both space and time. Constructal theory pushes design thinking closer to science and away from art. It tears down the walls between engineering and natural sciences. Because the configuration-generating phenomenon of “design” has scientific principles that are now becoming known, it is possible to learn where to expect opportunities for discovering new, more effective configurations. How to pursue these discoveries with less effort and time is the chief merit of the constructal law.
In this paper, we show that many features of a heat transfer installation can be deduced from the maximization of the global performance of the greater system that employs the installation. The heat transfer installation is a series of two cross-flow heat exchangers. The greater system is the environmental control system (ECS) of an aircraft. The global performance objective is the minimization of the total thermodynamic irreversibility of the ECS. Several architectural features are deduced from principle: the relative position of the two heat exchangers, their relative sizes, and all the geometric aspect ratios of the two heat exchanger cores. We find that the optimized architecture is insensitive (robust) to changes in some of the external parameters. Robustness is a useful feature because it simplifies the design work. Furthermore, one design that is built can be expected to function at near-optimal levels when the external parameters change. The application of this method of topology optimization to more complex systems is discussed. Copyright © 2001 John Wiley & Sons, Ltd.
Shapes with length/height ratios (l/h) close to 3/2 are everywhere and give the impression that they are being 'designed' to match the golden ratio (φ = 1.618).here I show that these shapes emerge as part of an evolutionary phenomenon that facilitates the flow of information from the plane to the brain, in accordance with the constructal law of generation and evolution of design in nature.The time required by the eyes to scan a rectangular area l × h is minimal when the shape is l/h = V l /V h , where V l and V h are the horizontal and vertical scanning speeds.This special shape is such that the time for scanning long and fast (l/V l ) is the same as the time for short and slow (h/V h ).I also show that V l /V h is approximately 3/2 and that consequently l/h ~ 3/2.This natural evolutionary design is an integral part of the universal constructal design that facilitates the flow of the biosphere, hydrosphere and atmosphere on earth.Vision, cognition and locomotion are features of a single design for movement of animal mass with easier and easier access in time, all over the globe.
In this paper, we show how vascular design controls the cooling and mechanical performance of a solid square slab heated uniformly and loaded with uniform pressure. The vascular structure is a combination of one grid and several peripheral trees. The designs of trees with grid canopies provide greater robustness than purely dendritic designs. The coolant fluid enters the slab from the center or from the rim, cools the slab to an allowable temperature level, and then exits. Numerical simulations show the flow of the fluid, heat, and stresses. The results indicate that the lowest peak temperature and lowest flow resistance can be achieved with radial channels, and the lowest peak stress can be achieved with trees with canopies. The peak stresses are the lowest when the configuration is a hybrid grid and tree design. There is an optimal ratio of the grid length divided by the slab length for each specified fraction of the fluid volume occupied by the radial channels. When the heating is concentrated in a small area, the peak temperature is smaller when the heated spot is closer to the center of the slab.
Introduction to Thermal System Design. Thermodynamics, Modeling, and Design Analysis. Exergy Analysis. Heat Transfer, Modeling, and Design Analysis. Applications with Heat and Fluid Flow. Applications with Thermodynamics and Heat and Fluid Flow. Economic Analysis. Thermoeconomic Analysis and Evaluation. Thermoeconomic Optimization. Appendices. Index.
The solution to the problem of maximizing the extraction of exergy from a stream of hot gas showed that the hot stream must be cooled in a counterflow heat exchanger with optimal imbalance of capacity rates, i.e. with an optimal capacity rate on the cold side. This paper outlines the first few steps toward making this solution practical, by combining the optimized counterflow with conventional components for compressing and expanding the cold stream. In the first part of the paper, the cold stream is compressed in an isothermal compressor, expanded in an adiabatic turbine, and discharged into the ambient. In the second part, the cold stream is compressed in an adiabatic compressor. Both designs are optimized with respect to two degrees of freedom, the capacity-rate imbalance of the counterflow, and the pressure ratio maintained by the compressor. The effect of other constraints is documented, e.g. heat transfer area size, hot gas initial temperature and compressor and turbine efficiencies. This study shows the tradeoff between simplicity and increased performance, and outlines the path for further conceptual work on the extraction of exergy from a hot stream that is being cooled gradually. Copyright © 2001 John Wiley & Sons, Ltd.
This paper documents the fundamentals of heat storage by melting a phase-change material inside a cylindrical enclosure with a vertical heated pipe on its axis. The phenomenon is studied analytically and numerically. The analysis consists of the scale analysis of the early stages of natural convection in the liquid, and an analysis of the entire duration of the melting process. The numerical simulations cover the entire process, and validate all the features predicted by theory.
This paper reports a numerical and analytical study of time dependent storage of energy by melting a phase change material in a cylindrical tank. In the first part of the study the heater was a vertical spiral tube positioned coaxially with the tank. In the second part, the heater consisted of two coaxial spirals positioned on the same axis as the cylinder. In accord with the philosophy of constructal design, the heat and fluid flow configuration was endowed with degrees of freedom in order to morph toward better overall performance for a fixed amount of heating material. The paper documents the effect of the geometrical features such as helix diameter and pitch on the overall performance.
This numerical study documents the effect of internal flow obstructions on heat transfer through a 2-dim. porous layer heated from the side. Three types of flow obstructions have been considered. Horizontal diathermal partitions were found to decrease the heat transfer rate in cases where the overall heat transfer was dominated by convection. In the conduction-dominated regime, horizontal diathermal partitions were found to increase the heat transfer rate slightly. Horizontal adiabatic partitions, on the other hand, were found to increase the heat transfer rate when the dominating mechanism is convection. Finally, vertical diathermal partitions were shown to reduce the heat transfer rate by roughly 50% in the convection-dominated regime. Numerical heat transfer results are reported in the domain 0.1 < L/H < 2 and 50 < RaH < 1000, where L/H is the length/height ratio and RaH is the Darcy-modified Rayleigh number based on height.
This essay traces the evolution of thermodynamics from its origins to ad hoc applications of thermodynamic optimization (entropy generation minimization) and the principle-based generation of flow configuration in nonequilibrium systems (constructal theory). Geophysical and biological flow systems evolve in one direction, toward configurations that flow more easily. This evolutionary process is like an animated movie in which existing flow designs are replaced by designs that offer greater flow access. This paradigm fuels a new attitude toward globalization and sustainability: the natural way to bring the less advanced areas into the flow of things is to allow the vascular systems of goods, people and ideas to bathe the whole earth more and more freely. Constructal theory shows that freedom is good for design, and that the future belongs to vascularized architectures with increased svelteness and optimal distribution of imperfection. Copyright © 2008 John Wiley & Sons, Ltd.
The observed degree of thermodynamic imperfection of existing power plants is explained based on a steady-state power plant model the irreversibility of which is due to three sources: the hot-end heat exchanger, the cold-end heat exchanger and the heat leaking through the plant to the ambient. While maximizing the instantaneous power output, it is shown that in addition to Curzon and Ahlborn's optimum temperature ratio there exists also an optimum balance between the sizes of the hot- and cold-end heat exchangers. A graphic construction for pinpointing the optimum location of the power plant on the absolute temperature scale is presented. The efficiency (first or second law) is maximum when the total investment is split optimally between the external conductance and internal thermal resistance built into the power plant. The efficiency data on existing power plants fall in the domain anticipated theoretically. Some of the trade-offs revealed by the theory are illustrated further by the analysis of an ideal Brayton cycle power plant.
Enclosures heated from the side are most representative of porous systems that function while oriented vertically, as in the insulations for buildings, industrial cold-storage installations, and cryogenics. As in the earlier chapters, we begin with the most...
The modern evolution of long distance running and swimming is documented statistically: body mass (M), height (H), slenderness (S), and winning speed (V).In long distance running (10,000 m), M, H, and S are decreasing: these trends contradict the trends in short distance running (100 m).In long distance swimming (1,500 m freestyle), the trends are similar to short distance (100 m freestyle): H and V are increasing.The parallel trends in long versus short distance swimming, and confl icting trends in long versus short distance running are due to dehydration, which is limiting only in long distance running.The speed records ratio running/swimming for long distance sports is decreasing at the same rate as for short distance sports.Running and swimming are subject to speed 'ceilings' (V max ) dictated by physics: the current record speeds in running and swimming are close to max 1 V . 2
This is a review of a new class of fundamental results concerning the optimal spacing of parallel heat generating boards arranged in a stack of fixed volume. The cooling is by single-phase laminar flow, in natural convection or forced convection. Several board models will be considered: smooth boards with uniform temperature or uniform heat flux, flush mounted heat sources, protruding heat sources, and a board cooled in its own (insulated) parallel plate channel.
