Publications

Maximization of energy delivery is the fundamental problem in solar collector thermal design. This paper examines the trade-off between the storage and the immediate use of solar exergy, with the objective of maximising the long-term exergy output from a solar collector installation. It is demonstrated that a trade-off exists, and that for maximum exergy output the collector and the collector-user interaction must conform to a well-defined pattern. It is also shown that the practice of operating collectors at constant temperature, regardless of time of day, is responsible for a sizeable and steady exergy loss

In this paper we use two developments to illustrate our progress in “design with constructal theory” [1]. The first is the development of smart materials with embedded vasculatures that provide multiple functionality: volumetric cooling, selfhealing, enhanced apparent (effective) thermal conductivity, and mechanical strength. Vascularization is achieved by using tree-shaped (dendritic) flow architectures. We show that as length scales become smaller, dendritic vascularization provides dramatically superior volumetric bathing than the use of bundles of parallel microchannels. A novel dendritic architecture has trees that alternate with upside down trees. In addition to flow access to the entire volume, trees offer improved robustness in flow operation. The second development is the distributing of energy systems over a given territory. The distribution of heating is used as an example. The architecture emerges from the balancing of the losses concentrated in the production centers and the losses distributed along the conduits that distribute and collect every thing that flows on the landscape. In sum, flow architectures are derived from principle, in accordance with constructal theory, not by mimicking nature.

In this paper we document the relationship between complex flow architecture and global performance for assemblies of heat pumps coupled thermally with the ground through a single U-shaped loop with circulating fluid. The assemblies vary according to heat pump numbers, sizes and locations along the loop. They are classified in a systematic way, and their performance is documented in three classes of designs: assemblies of heat pumps of the same size, heat pumps distributed equidistantly, and large numbers of heat pumps distributed almost continuously on a long loop. The work is based on numerical simulations, and on an analysis that holds in the limit of heat pumps distributed continuously. The relationship between flow architecture and global performance (heat transfer density) serves as guide for the energy design of high-density urban settlements in the future.

Convection in Porous Media, 4th Edition, provides a user-friendly introduction to the subject, covering a wide range of topics, such as fibrous insulation, geological strata, and catalytic reactors. The presentation is self-contained, requiring only routine mathematics and the basic elements of fluid mechanics and heat transfer. The book will be of use not only to researchers and practicing engineers as a review and reference, but also to graduate students and others entering the field. The new edition features approximately 1,750 new references and covers current research in nanofluids, cellular porous materials, strong heterogeneity, pulsating flow, and more.

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.

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...

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.

The purpose of this paper is to examine how rib configurations and spar configurations influence flying wing stability. Flying wing aircraft exhibit enhanced flutter characteristics when stresses flow smoothly through the wing. We prevent stress strangulation through spar cross-sections by changing the configuration in the plunge direction. We employ and develop computer programs Gmsh, Variational Asymptotic Beam Sectional Analysis, MATLAB scripts, and Nonlinear Aeroelastic Trim and Stability of High Altitude Long Endurance Aircraft. The configurations are designed by considering the same material, mass, and flight conditions. The results indicate that the design with the smoother stress distribution through the wing has a higher flutter speed. It is shown that the σ11 and Von-Misses stress distributions have an important effect on the stability of a flying wing aircraft.

This note describes an experimental study of the damaging effect of hydrogen bubbles on the effectiveness of the thymol blue velocity measurement method. Specifically, we document the effect of flow velocity, cathode voltage, and cathode diameter on bubble formation and the quality of the thymol blue pattern. The time of bubble damage, i.e., the interval preceding the destruction of the blue pattern, was measured and reported as a function of flow velocity, cathode voltage, and cathode diameter.

This paper describes several fundamental trade offs that govern the optimization of cooling techniques for heat generating electric devices. Five basic cooling configurations are optimized. It is shown that in forced air cooling above room temperature the fan power requirement is minimum when the heat transfer contact area is optimized to a value that is of the order of 102 A f, where A f is the flow cross-sectional area. This optimal contact area is independent of whether the coolant is mixed or unmixed inside the heat generating enclosure. In air cooling with natural updraft, the heat generation rate (or amount of electronics, or overall thermal conductance) is maximum when the heat transfer contact area is of order 102 A f. In forced convection cooling at cryogenic temperatures, the refrigerator power requirement has minima with respect to the cold-end (refrigeration load) temperature and the heat transfer contact area. The optimal heat transfer area is again of the order of 102 A f.

This paper reviews recent progress on constructal theory and design. The emphasis is on the development of multi-scale, nonuniformly distributed flow structures that offer increased compactness (e.g., heat transfer density). Examples are counterflow heat exchangers with tree-shaped hot and cold streams, and tree architectures on a disc. Every flow system has a property called svelteness (Sv), which is the ratio between its external (global) length scale and its internal length scale ( V 1 / 3 ), where V is the volume occupied by all the ducts. Emphasis is placed on the development of simple strategies for decreasing the computational cost required by the development of such structures. The generation of multi-scale flow configurations is a process that can be projected on a diagram having global performance on the abscissa and degrees of freedom on the ordinate. This process rules the development (evolution) of all flow configurations for systems with global objective, global constraints and freedom to morph.

The natural circulation of near-4°C water in a vertical cavity heated from the side was studied experimentally in a 0.74-m-tall enclosure. The results of a scale analysis were used in order to correlate the overall wall-to-wall heat transfer rate measured experimentally in the Rayleigh number range 108–1011. An analytical similarity solution was obtained for the slender enclosure limit. In that limit the flow pattern is one of incomplete vertical penetration, with stagnant maximum-density water filling the lower end of the vertical enclosure.