Publications

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.

Thermally active fabrics and insulations consist of fibers coated with phase-change materials and surrounded by air. This paper constructs a homogeneous porous medium model for melting and solidification in spaces filled with thermally active fibers. Three basic configurations are analyzed numerically: one-dimensional conduction, one-dimensional convection, and two-dimensional natural convection due to heating or cooling from the side. Concrete means for calculating the time of complete melting or solidification are reported as functions of the important dimensionless groups that govern each configuration. It is shown that the change of phase in a space filled with coated fibers and air differs fundamentally from the corresponding phenomena in a porous medium saturated with a phase-change material.

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.

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.

No abstract is provided for this article.

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.

In this paper we describe the thin-film melting of a block of solid phase-change material (the bearing) around a rotating cylinder (the shaft). We determine the relation between the force applied on the shaft and the speed with which the shaft migrates into the bearing, the relation between the applied force and the torque, and the angle between the applied force and the direction of migration into the bearing. The method is based on contact melting theory, which combines the Reynolds thin-film lubrication theory with an analysis of phase-change heat transfer in the melt. The paper addresses three limiting regimes of the contact melting phenomenon: (1) the long bearing with melting due to frictional heating in the melt layer; (2) the short bearing with melting due to frictional heating in the melt layer, and (3) the short bearing with melting due to a temperature difference imposed between the hot cylinder and the cold phase-change material.

This lecture outlines the basis for the entropy generation minimization method, and a series of key applications in power generation, refrigeration, and energy conservation. The lecture begins with a review of the concept of irreversibility, entropy generation, or exergy destruction. The proportionality between exergy destruction and entropy generation is used in the search for improved thermodynamic performance subject to finite-size constraints and specified environmental conditions. Examples are drawn from refrigeration, energy storage systems for sensible heat and latent heat, solar energy, and the generation of maximum power by using a stream of hot gas. It is shown that the physical structure of the system springs out of the process of global thermodynamic optimization subject to global constraints. This principle generates structure not only in engineering but also in physics and biology (constructal theory).

 This paper presents a new concept for generating the multi-scale structure of a finite-size flow system that has maximum heat transfer density–

Constructal theory is applied to the cooling of a disc where heat is uniformly generated. The disc size and the total volume occupied by the ducts (distributing the flow from the centre to the periphery) are constrained. It is shown that when the objective is to minimise the global thermal resistance, the best design is the one built with radial ducts. On the other hand, the minimisation of the pumping power leads to tree-shaped structures. The results show that the two optimisation approaches, thermal and fluid-mechanical, generate results with nearly the same global performance. Yet, when the scale of the problem becomes smaller and smaller and dendritic flows perform better, demonstrating the usefulness and robustness of tree-shaped structures.

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...

1. Natural form, questioning, and theory 2. Mechanical structure 3. Thermal structure 4. Heat trees 5. Fluid trees 6. Ducts and rivers 7. Turbulent structure 8. Convective trees 9. Structure in power systems 10. Structure in time: rhythm 11. Transportation and economics structure 12. Shapes with constant resistance About the author Author index Subject index.

No abstract is provided for this article.

Here we show that the trends in four sports are united by the evolutionary constructal design of all animal locomotion.The trend is toward greater effi ciency in the falling-forward movement of the body, and this is why the evolution of the throwing motion unites team sports (baseball, hockey) with individual sports (golf, boxing, running and swimming).Records during the past 100 years indicate that in these sports the trend has been toward bigger and taller bodies, which possess greater speed of falling forward and throwing, greater force, and greater throwing distance.Equipment technology (golf balls, hockey sticks) has evolved in the same direction.In sum, the evolution of sports is in accord with the constructal law, and constitutes a "laboratory" for witnessing the evolution of biological design in our lifetime.

This paper shows that thermodynamic optimization provides a common theoretical basis for the existence of the finely tuned frequencies of pulsating processes of animals. In respiration and circulation, the minimization of mechanical power consumption subject to finite contact area and mass transfer (metabolic) rate constraints also explains why frequencies decrease as the body size (M) increases. For example, it is shown that the optimal breathing and heartbeat time intervals should increase as M 0.24, which is in excellent agreement with experimental data. It is also shown that the breathing and heartbeat time intervals should be of the same order of magnitude. In ejaculation, the maximization of the mechanical power transmitted to the ejected seminal fluid explains the existence of an optimal bursting time interval. The deterministic method of thermodynamic optimization predicts temporal organization in Nature, and extends thermodynamics to the field of biology.

Here we show that the emergence of scaling laws in inanimate (geophysical) flow systems is analogous to the emergence of allometric laws in animate (biological) flow systems, and that features of evolutionary “design” in nature can be predicted based on a principle of physics (the constructal law): “For a finite-size flow system to persist in time (to live) it must evolve in such a way that it provides easier and easier access to its currents”, meaning that the configuration and function of flow systems change over time in a predictable way that improves function, distributes imperfection, and creates geometries that best arrange high and low resistance areas or volumes. This theoretical unification of the phenomena of animate and inanimate flow design generation is illustrated with examples from biology (lung design, animal locomotion) and the physics of fluid flow (river basins, turbulent flow structure, self-lubrication). The place of this design-generation principle as a self-standing law in thermodynamics is discussed. Natural flow systems evolve by acquiring flow configuration in a definite direction in time: existing configurations are replaced by easier flowing configurations.

We address the fundamental problem of determining the optimal history (regime of operation) of a battery so that the work output is maximum. The essential features of the problem are: (i) the life of the battery is constrained, (ii) the battery has an internal resistance through which it can lose its charge even at open circuit, and (iii) the battery is connected to an electric motor with finite winding resistance. The optimal regime of time-dependent operation is determined based on variational calculus. It is shown that the maximized work output is smaller than the exergy stored initially in the battery, and decreases as the motor resistance increases. The exergy stored in a battery cannot be recovered fully as soon as the resistance of the external circuit is finite, no matter how small.