Entropy Generation Minimization: The Method of Thermodynamic Optimization of Finite-Size Systems and Finite-Time Processes

389 Pages·1995·17.217 MB·English

by  Adrian Bejan

ENTROPY
GENERATION
MINIMIZATION
The Method of
Thermodynamic Optimization
of Finite-Size Systems
and Finite-Time Processes
ADVANCED TOPICS IN MECHANICAL
ENGINEERING SERIES
SERIES EDITOR Frank A. Kulacki, University of Minnesota
Books in this series include
ENTROPY GENERATION MINIMIZATION
Adrian Bejan, Duke University
NONLINEAR ANALYSIS OF STRUCTURES
M. Sathyamoorthy, Clarkson University
FINITE ELEMENT AND COMPOSITE STRUCTURES
ANALYSIS USING MATLAB
Hyochoong Bang, Naval Postgraduate School
MECHANICS OF SOLIDS AND SHELLS
Gerald Wempter, Georgia Institute of Technology
ADRIAN BEJAN
J. A. Jones Professor of Mechanical Engineering
Duke University
The Method of
Thermodynamic Optimization
of Finite-Size Systems
and Finite-Time Processes
CRCPress
Boca Raton New York
Library of Congress Cataloging-in-Publication Data
Bejan, Adrian, 1948-
Entropy generation minimization
Adrian Bejan.
p. em.-(CRC's mechanical engineering series)
Includes bibliographical references and indexes.
ISBN 0-8493-9651-4 (alk. paper)
1. Thermodynamics-Mathematical models. 2. Heat-Transmission-
Mathematical models. 3. Entropy. I. Title. II. Series.
TJ265.B47 1995
621.402'1-dc20 95-11954
CIP
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International Standard Book Number 0-8493-9651-4
Library of Congress Card Number 95-11954
Printed in the United States of America 2 3 4 5 6 7 8 9 0
Printed on acid-free paper
PREFACE
Entropy Generation Minimization (EGM) is the method of thermodynamic
optimization of real systems that owe their thermodynamic imperfection to heat
transfer, fluid flow, and mass transfer irreversibilities. It was identified as a self
*
standing field of research in the book by Bej an ( 1982). The EGM method combines
at the most fundamental level the basic principles of thermodynamics, heat and mass
transfer, and fluid mechanics. As a field, it is the confluence of thermodynamics, heat
transfer, and fluid mechanics (Figure 1); because of its interdisciplinary character,
EGM is distinct from each of these classical disciplines.
As a method of engineering research and education, EGM was designed to make
both thermodynamics and heat transfer more "applicable", i.e., easier to understand,
easier to use, and more effective. I was introduced to the power and rigors of the
method as a student at MIT, in particular by my graduate theses advisor, Prof. Joseph
L. Smith, Jr. Although "good engineering work" has always demanded the simulta-
neous and correct application of the principles of thermodynamics, heat transfer, and
fluid mechanics, I saw a need to bring these principles together systematically into
a method defined by its own rules, language, and promise. Entropy Generation
Minimization is just that - the minimization of thermodynamic irreversibility in
real-world applications by accounting for the finite-size constraints of actual devices
and the finite-time constraints of actual processes.
Entropy
generation
through
heat and fluid flow
Thermodynamics
Figure 1. The interdisciplinary triangle covered by the EGM method (Bejan, 1982).
* Bejan, A. 1982. Entropy Generation through Heat and Fluid Flow. Wiley, New York.
The arrival of a new field of research is announced by the emergence of a number
of simple, fundamental results that hold for entire classes of known and future appli-
cations. Entropy Generation Minimization emerged in the 1970s in engineering: its first
series of fundamental results formed the backbone of the Bejan (1982) book. These
results were obtained by minimizing entropy generation in realistic (heat and fluid
flow) models built for several levels of complexity (Figure 2). The minimum entropy
generation design was determined for each model, as in the case of the optimal
diameter of a duct with flow and heat transfer, or the optimal hot-end temperature of
a power plant with bypass heat leak to the ambient. The approach of any other design
(Sgen) to the limit of "realistic" thermodynamic ideality represented by the design with
minimum entropy generation (Sgen,min) was monitored in terms of the entropy genera-
tion number N = Sgen/Sgen,min > 1, or alternatives of the same ratio.
5
The EGM method is also distinct from exergy analysis. The latter belongs to
pure thermodynamics: to calculate exergy, the analyst needs only the first law, the
second law, and a convention regarding the values of the intensive properties of the
environment. The critically new aspect of the EGM method is the minimization of the
calculated entropy generation rate. To minimize the irreversibility of a proposed
design the analyst must use the relations between temperature differences and heat
transfer rates, and between pressure differences and mass flow rates. He must relate
the degree of the thermodynamic nonideality of the design to the physical character-
istics of the system, namely, to finite dimensions, shapes, materials, finite speeds, and
finite-time intervals of operation. For this he must rely on heat transfer and fluid
mechanics principles, in addition to thermodynamics. Only by varying one or more
Approach
*E>- .ccQ'!l) mE~m VJ :a;c;:~: ; J!!
OViJ Ec0 . EQ) -Q) Q~ )
10- (0.. ) wQ) 'iI5=
LLL
Sgen,min Sgen,min
Refrigeration ldxdydz
plants
Duct
Power plants
cVJ Fin
:0a
.!.I Solar power and Rougheness
a. refrigeration plants
c.
<( Heat exchanger
Storage systems Insulation
Solar collector
Time-dependent
processes Storage unit
Figure 2. The two-dimensional structure of the field of Entropy Generation Minimization.
of the physical characteristics of the system can he bring the design closer to the
operation characterized by minimum entropy generation subject to finite-size and
finite-time constraints.
Writing this Preface takes me back to September 1978, and my first days as an
assistant professor at the University of Colorado. Having done most of my EGM
work as a graduate student at MIT (in cryogenics) and as a postdoctoral fellow in
Berkeley (in heat transfer), I had seen the generality of the method and the new field
that was taking shape. To place EGM squarely on the map of engineering research
and education I knew that I had to present it in such a way that it could be taught as
a self-standing method from one generation of engineers to the next. This is why in
September 1978 I proposed a new graduate course (ME 551, Thermodynamic De-
sign), which I then taught every year starting with the spring semester of 1979. The
notes developed for that course became the Bejan (1982) book.
The EGM field has experienced tremendous growth during the 1980s and 1990s.
The objective of this new book is to place this growth in perspective. Special emphasis
is given to chronology and the relationship between the newer work and the work that
got the field started. The growth of the EGM field is due to three developments: ( 1) the
diversification of the topics covered in the engineering literature after the energy
policies of the late 1970s, (2) the lifting of the Iron Curtain, and the absorption of old
and new work contributed by previously unrecognized practitioners of the method, and
(3) the contributions that appear now in the physics literature under the title of "finite
time" thermodynamics, where finite time is a reference to rate processes such as heat
transfer. It is worth stressing that Entropy Generation Minimization is a transparent and
general way of saying thermodynamic optimization, which includes as specific appli-
cations the maximization of power in power plants, the minimization of power con-
sumption in refrigerators and heat pumps, the maximization of refrigeration load and
cryogen production in low-temperature refrigerators and liquefiers, and many other
design objectives in a long list of devices.
The mid-1970s expansion of EGM from cryogenic engineering into the wider
field of heat transfer led to the diversification and two-dimensional structure shown
in Figure 2. Before the applications to heat transfer, the EGM method had been
applied to total (complete) systems such as refrigeration plants (e.g., Figure 6.4) and
power plants (e.g., Problem 2.2). At that time, the approach was to recognize "over
the top" one or more of the irreversibility mechanisms, and to declare the "rest" of
the system irreversibility free. Success depended fully on the analyst's intuition, as
the irreversible compartments were hung on the system model like decorations on a
Christmas tree. There was not a solid connection between the resulting model and the
hardware of the real system, especially with regard to the compartment that was
declared "reversible".
The introduction of EGM in heat transfer began with recognizing that (a)
systems are made of actual components, (b) each component may contain large
numbers of one or more elemental features, and (c) each elemental feature owes its
irreversibility to processes that occur at the differential level. The chosen approach
was to minimize entropy generation in a fundamental way at each level, starting from
the simple and proceeding toward the complex. The chosen approach was fundamen-
tal, i.e., science not art.
The main point is that thermodynamically optimized elemental features work
toward decreasing the irreversibility of components, and that optimized components
are desirable from the point of view of reducing the irreversibility of the total system.
Important to note is that when a component or elemental feature is imagined sepa-
rately from the larger system, the quantities assumed specified at the points of
separation act as constraints on the optimization of the smaller system. The principle
of thermodynamic isolation (page 125) must be kept in mind during the later stages
of the optimization procedure, when the optimized elements and components are
integrated into the total system, which itself is optimized for minimum total cost in
the final stage.*
Figure 2 outlines the two-dimensional structure of the EGM field. On the vertical
we see partial listings of specific applications. The horizontal shows the approach to
the optimized total system: this can be made from the right, as in post-1975 heat
transfer, or from the left, by focusing directly on the total system. This two-dimen-
sional structure was evident in the material covered in Bejan (1982). The present
treatment shows that the same structure is in place today, of course, in a considerably
more diversified and in-depth form.
As in Bejan (1982), the mission of this book is to bridge the gap between
thermodynamics, heat transfer, and fluid mechanics. It is to make each of these
disciplines easier to grasp and use in solving real problems and is pursued with equal
vigor in the Solutions Manual that accompanies the text. The mission is to demon-
strate that there is one point of view that unites these seemingly separate disciplines,
and, when viewed as a whole, the subject is surprisingly simple. Entropy Generation
Minimization is changing the way we think and apply Thermodynamics.
Adrian Bejan
Durham, North Carolina
* Bejan, A., Tsatsaronis, G. and Moran, M. 1996. Thermal Design and Optimization, Wiley, New York.
OTHER BOOKS BY ADRIAN BEJAN:
Entropy Generation through Heat and Fluid Flow, Wiley, 1982.
Convection Heat Transfer, Wiley, 1984.
Advanced Engineering Thermodynamics, Wiley, 1988.
Convection in Porous Media, with D. A. Nield, Springer-Verlag, 1992.
Heat Transfer, Wiley, 1993.
Convection Heat Transfer, 2nd Edition, Wiley, 1995.
Thermal Design and Optimization, with G. Tsatsaronis and M. Moran, Wiley, 1996.