Showing papers on "Constructal law published in 2004"

Constructal theory of flow architecture of the lungs

Abstract: Here we explain the reasons why we have a bronchial tree with 23 levels of bifurcation. Based on Bejan’s Constructal Principle we found that the best oxygen access to the alveolar tissues as well as carbon dioxide removal is provided by a flow structure composed of ducts with 23 levels of bifurcation (bronchial tree) that ends with spaces (alveolar sacs) from where oxygen diffuses into the tissues. The model delivers the dimensions of the alveolar sac, the total length of the airways, the total alveolar surface area, and the total resistance to oxygen transport in the respiratory tree. A constructal law also emerges: the length defined by the ratio of the square of the airway diameter to its length is constant for all individuals of the same species and is related to the characteristics of the space allocated to the respiratory process and determines univocally the branching level of the respiratory tree.

Tree network channels as fluid distributors constructing double-staircase polymer electrolyte fuel cells

Abstract: In this article, constructal tree-like channel networks are introduced as a fuel cell fluid distribution concept, which also optimizes the shape of polymer electrolyte fuel cells. This concept is the main contribution of the article. To perform quantitative calculations based on this concept, a one-dimensional model, accounting for oxygen consumption in the feed channel, oxygen mass transfer between the channel and the backing layer, and oxygen mass transfer through the backing layer to the catalyst layer, is used to predict the polarization curve of a so-constructed polymer electrolyte fuel cell. Pressure drop and pumping power required for the fluid circulation is estimated. Multiobjective genetic search is performed to maximize the net power density with respect to constructal parameters and operating conditions, leading to the optimized tree network. It is found that the use of tree networks instead of the traditional, nonbifurcating serpentine channels in rectangular systems can provide substantially improved cell performance due their intrinsic advantage with respect to both mass transfer and pressure drop. The resulting “double-staircase” shape of the fuel cells differs from the traditional rectangular shape while maintaining simplicity, and it is determined based on the functionality of the flow distribution system.

Constructal multi-scale tree-shaped heat exchangers

Abstract: This paper describes the conceptual design and performance of balanced two-stream counterflow heat exchangers, in which each stream flows as a tree network through its allotted space. The two trees in counterflow are like two palms pressed against each other. The paper develops the relationships between effectiveness and number of heat transfer units for several tree-counterflow configurations: (i) constructal dichotomous trees covering uniformly a rectangular area, (ii) trees on a disk-shaped area, and (iii) trees on a square-shaped area. In configurations (ii) and (iii) each stream flows between the center and the periphery of the area. Configurations (i) and (ii) are trees with minimal resistance to fluid flow. Configuration (iii) is designed by minimizing the length of each duct in the network. The paper reports the formula for the number of heat transfer units in each configuration. Unlike in counterflows formed by two parallel streams, in which the longitudinal temperature gradient is constant, in the counterflow formed by two trees the longitudinal temperature gradient is steeper as one approaches the periphery of the tree canopy. The application of dendritic heat exchangers to devices with maximal transport density is discussed, e.g., electronics cooling, fuel cell architectures, etc.

Thermodynamic optimization of internal structure in a fuel cell

Abstract: This paper shows that the internal structure (relative sizes, spacings) of a fuel cell can be optimized so that performance is maximized at the global level. The optimization of flow geometry begins at the smallest (elemental) level, where the fuel cell is modelled as a unidirectional flow system. The polarization curve, power and efficiency are obtained as functions of temperature, pressure, geometry and operating parameters. Although the model is illustrated for an alkaline fuel cell, it may be applied to other fuel cell types by changing the reaction equations and accounting for the appropriate energy interactions. The optimization of the internal structure is subjected to fixed total volume. There are four degrees of freedom in the optimization, which account for the relative thicknesses of the two (anode and cathode) diffusion layers, two reaction layers and the space occupied by the electrolyte solution. The available volume is distributed optimally through the system so that the total power is maximized. Numerical results show that the optima are sharp, and must be identified accurately. Temperature and pressure gradients play important roles, especially as the fuel and oxidant flow paths increase. The optimized internal structure is reported in dimensionless form. Directions for future improvements in flow architecture (constructal design) are discussed. Copyright © 2004 John Wiley & Sons, Ltd.

Tree-shaped flow structures: are both thermal-resistance and flow-resistance minimisations necessary?

Abstract: 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.

Constructal Networks for Efficient Cooling/Heating

Abstract: Channel networks designed with constructal theory are compared. The efficiency of the networks when used for cooling a uniformly heated surface is compared. Three networks are compared and it is found that the two constructal designs with two and three constructal levels have similar performance. It is shown that for a given pumping power, the constructal designs give a heat transfer coefficient of the surface which is almost a factor of magnitude higher than the one obtained for a parallel channel system.Copyright © 2004 by ASME

Thermal Optimization of a Composite Heat Spreader

Abstract: A theoretical study is undertaken to determine the optimal geometry of a composite heat spreader subjected to cooling by a single phase fluid. Optimal geometry is obtained when heat transfer from the spreader is maximized. Constructal theory is employed, where the fundamental construct for the composite consists of a thin high-thermal conductivity blade in contact with a matrix of lesser conductivity. Following a systematic procedure, a tree-like geometry is built up from this fundamental unit, which increases in surface area with each successive construct and possesses optimal geometry at each construct level. Numerical results are calculated for a carbon-fiber composite in an epoxy matrix. We find the optimal aspect ratio for all constructs to be of the order of 1 for a wide range of prescribed surface area for the fundamental unit.Copyright © 2004 by ASME

Constructal theory: One of new orientations of general thermodynamics optimization

Abstract: The development of constructal theory, which had been one of the new approachs of general thermodynamic optimization, were reviewed. The great benefits of optimizing devices with the theory in thermodynamic optimizing, and the state of the arts of constructal theory and applications in heat transfer, fluid flowing, drying, traffic and transportation, piping net and economy decision-making were summarized. The principle that sorts of forms and contractures of systems and organizations in nature were derived from the permanent struggle for better performance was discussed. The general approach for future research work is to combine the constructal theory with the field synergy theory.

Fuel Cells Constructal Optimization and Research Perspectives

Abstract: The hydrogen economy is a possible alternative to the current oil based global economy. The technology to build and operate fuel cells is well advanced. However, cost is the reason why fuel cells are not being installed wherever there is a need for more power. Therefore, optimization is a natural alternative to reduce cost and make fuel cells increasingly more attractive for power generation. This paper discusses the process of determining the internal geometric configuration of a unit fuel cell for maximum power. The optimization of construction (architecture) starts at the smallest (elemental) fuel cell level. The optimization of system architecture must be subjected to a fixed volume constraint. There are several degrees of freedom in the fuel cell configuration, i.e., the thickness of two gas channels (fuel and oxidant), two diffusion layers and two reaction layers (anode and cathode) and the electrolyte solution space. Research perspectives for fuel cells are presented and discussed.Copyright © 2004 by ASME