Page 6 - Entrophy Analysis in Thermal Engineering Systems

Preface

It was about two centuries ago that Nicolas L eonard Sadi Carnot, a French
military engineer, presented an influential treatise. Although remained
unappreciated for a decade, it provided a profound basis for investigations
of his successors and the advancement of the Science of Thermodynamics.
Carnot’s research on the theory of heat engines was itself founded upon the
caloric theory, empirical findings of his predecessors, and philosophical rea-
soning. The invalidity of the notion of heat as an indestructible matter had
become obvious among the pioneers by the mid-19th century. There were
compelling experimental evidences supporting the equivalence of heat and
work, the first main principle of the Mechanical Theory of Heat, according
to which heat can be produced by expenditure of work and vice versa.
Unlike the first main principle whose statement and formulation can
readily be understood by a student of an average intelligence, concepts like
entropy originated from the second main principle of the Mechanical
Theory of Heat appear to be challenging, perhaps, for everyone who has
undertaken an introductory class on the subject. Such concepts are invented
through a formulation of the second law of Thermodynamics. However, the
analytical formulation of the second law is not a mere expression of the
experimental observations—that heat cannot be converted completely into
work, or heat cannot spontaneously transfer from a cooler to a warmer body.
It involves a hypothetical concept, reversibility, which may only be realized
in an imaginary process; which may be regarded as a preliminary source of
difficulty in understanding the entropy-related concepts.
Today, after over 150 years of invention of entropy by Clausius, still
there remain confusions surrounding the concept of entropy and the
phenomenon of entropy increase. One may find a variety of interpretations
or descriptions for entropy such as arrow of time, measure of disorder, chaos,
wastefulness, and energy dispersal. On the other hand, some argue that
understanding of entropy is only possible through statistical mechanics.
The first question may cross a curious mind is: Why is not there a universally
agreed interpretation for entropy yet? It has a simple definition dS¼dQ/T,a

differential of S (entropy) is equal to the differential of Q (heat) divided by T
(temperature of body). The explanation given by Clausius as the inventor of
entropy is that S represents the transformational content of a body like U that
denotes its (internal) energy content. All we know nowadays about Clausius

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