Adrian Bejan

Entropy generation minimization: The new thermodynamics of finite‐size devices and finite‐time processes

Entropy generation minimization (finite time thermodynamics, or thermodynamic optimization) is the method that combines into simple models the most basic concepts of heat transfer, fluid mechanics, and thermodynamics. These simple models are used in the optimization of real (irreversible) devices and processes, subject to finite‐size and finite‐time constraints. The review traces the development and adoption of the method in several sectors of mainstream thermal engineering and science: cryogenics, heat transfer, education, storage systems, solar power plants, nuclear and fossil power plants, and refrigerators. Emphasis is placed on the fundamental and technological importance of the optimization method and its results, the pedagogical merits of the method, and the chronological development of the field.

Thermal energy storage : systems and applications

List of Contributors.Acknowledgements.Preface.General Introductory Aspects for Thermal Engineering. Energy Storage Systems. Thermal Energy Storage (TES) Methods. Thermal Energy Storage and Environmental Impact. Thermal Energy Storage and Energy Savings. Heat Transfer and Stratification in Sensible Heat Storage Systems. Modeling of Latent Heat Storage Systems. Heat Transfer with Phase Change in Simple and Complex Geometries. Thermodynamic Optimization of Thermal Energy Storage Systems. Energy and Exergy Analyses of Thermal Energy Storage Systems. Thermal Energy Storage Case Studies.Appendix A -- Conversion Factors.Appendix B -- Thermophysical Properties.Appendix C -- Glossary.Subject Index.

Constructal-theory network of conducting paths for cooling a heat generating volume

Abstract This paper develops a solution to the fundamental problem of how to collect and ‘channel’ to one point the heat generated volumetrically in a low conductivity volume of given size. The amount of high conductivity material that is available for building channels (high conductivity paths) through the volume is fixed. The total heat generation rate is also fixed. The solution is obtained as a sequence of optimization and organization steps. The sequence has a definite time direction, it begins with the smallest building block (elemental system) and proceeds toward larger building blocks (assemblies). Optimized in each assembly are the shape of the assembly and the width of the newest high conductivity path. It is shown that the paths form a tree-like network, in which every single geometric detail is determined theoretically. Furthermore, the tree network cannot be determined theoretically when the time direction is reversed, from large elements toward smaller elements. It is also shown that the present theory has far reaching implications in physics, biology and mathematics.

Constructal theory of generation of configuration in nature and engineering

Constructal theory is the view that the generation of flow configuration is a physics phenomenon that can be based on a physics principle (the constructal law): “For a finite-size flow system to persist in time (to survive) its configuration must evolve in such a way that it provides an easier access to the currents that flow through it” [A. Bejan, Advanced Engineering Thermodynamics, 2nd ed. (Wiley, New York, 1997); Int. J. Heat Mass Transfer, 40, 799 (1997)]. This principle predicts natural configuration across the board: river basins, turbulence, animal design (allometry, vascularization, locomotion), cracks in solids, dendritic solidification, Earth climate, droplet impact configuration, etc. The same principle yields new designs for electronics, fuel cells, and tree networks for transport of people, goods, and information. This review describes a paradigm that is universally applicable in natural sciences, engineering and social sciences.

Heat and mass transfer by natural convection in a porous medium

Abstract This paper reports a fundamental study of the phenomenon of natural convection heat and mass transfer near a vertical surface embedded in a fluid-saturated porous medium. The buoyancy effect is due to the variation of temperature and concentration across the boundary layer. The study contains two parts. In the first part, scale analysis shows that the natural convection phenomenon conforms to one of four possible regimes, depending on the values of buoyancy ratio N and Lewis number Le . The scales of the heat and mass transfer rates are determined for each regime. In the second part of the study, the boundary-layer problem is solved via similarity formulation in the buoyancy ratio range − 5 ⩽ N ⩽ 4 and Lewis number range 1 ⩽ Le ⩽ 100. The similarity solutions confirm the validity of the order-of-magnitude limiting results revealed by scale analysis.

Second-Law analysis in heat transfer and thermal design

This review is devoted to the introduction of second-law analysis in heat transfer and entropy generation minimization in thermal design. The presentation proceeds from the derivation of the Gouy-Stodola theorem, the basis for entropy generation minimization in the conceptual design of heat transfer equipment. Appropriate analytical tools, such as the entropy generation number, are devised for the task of estimating the destruction of available work in the processes involving heat transfer. However, the entropy generation number concept is considerably more general, since it can be used to quantitatively describe the degree of irreversibility of engineering components and processes which do not draw their irreversibility solely from heat transfer. The examples considered in this article range from the irreversibility associated with some of the most fundamental convective heat transfer processes to the minimum irreversibility design of one-dimensional insulations such as the main counterflow heat exchanger of a helium liquefaction plant.

Second law analysis in heat transfer

The second law of thermodynamics is used as a basis for evaluating the irreversibility (entropy generation) associated with simple heat transfer processes. In the first part of this paper, the irreversibility production is analyzed from the local level, at one point in a convective heat transfer arrangement. The second part of the paper is devoted to a limited review of second law analysis applied to classic engineering components for heat exchange. In this category, the paper includes such topics as: heat transfer augmentation techniques, heat exchanger design, and thermal insulation systems. Analytical methods for evaluating and minimizing the irreversibility associated with textbook-type components of heat transfer equipment are presented.

Theory of heat transfer-irreversible power plants

Abstract 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 Ahlborns 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.

The Concept of Irreversibility in Heat Exchanger Design: Counterflow Heat Exchangers for Gas-to-Gas Applications

The thermal design of counterflow heat exchangers for gas-to-gas applications is based on the thermodynamic irreversibility rate or useful power no longer available as a result of heat exchanger frictional pressure drops and stream-to-stream temperature differences. The irreversibility (entropy production) concept establishes a direct relationship between the heat exchanger design parameters and the useful power wasted due to heat exchanger nonideality. The paper presents a heat exchanger design method for fixed or for minimum irreversibility (number of entropy generation units N/sub s/). In contrast with traditional design procedures, the amount of heat transferred between streams and the pumping power for each side become outputs of the N/sub s/ design approach. To illustrate the use of this method, the paper develops the design of regenerative heat exchangers with minimum heat transfer surface and with fixed irreversibility N/sub s/.

The optimal spacing of parallel plates cooled by forced convection

This paper reports the optimal board-to-board spacing and maximum total heat transfer rate from a stack of parallel boards cooled by laminar forced convection. The optimal spacing is proportional to the board length raised to the power 1/2, the property group (μα)14, and (ΔP)−14, where ΔP is the pressure head maintained across the stack. The maximum total heat transfer rate is proportional to (ΔP)12, the total thickness of the stack (H), and the maximum allowable temperature difference between the board and the coolant inlet. Board surfaces with uniform temperature and uniform heat flux are considered. It is shown that the surface thermal condition (uniform temperature vs uniform heat flux) has a minor effect on the optimal spacing and the maximum total heat transfer rate.