Page 53 - Entrophy Analysis in Thermal Engineering Systems

CHAPTER FOUR

The common source of entropy

increase

4.1 Introduction
Perhaps the entropy-heat relation was an obvious fact among the sci-
entists of the 19th century. On page 162 of Maxwell’s Theory of Heat [1],
entropy is described as “when there is no communication of heat this quan-
tity [entropy] remains constant, but when heat enters or leaves the body the
quantity increases or diminishes.” He further noted on page 194 that
“entropy is a quantity such that without a change in its value no heat can
enter or leave the body.” In a 1907 thermodynamics textbook authored
by Bryan [2], an English mathematician, a list of irreversible processes is
given, which, as articulated by the author, can be categorized as the examples
of irreversible conversion of work into heat. Friction between two rough
surfaces, collision of two imperfectly elastic body, expansion of a gas into
vacuum, and retardation of a fluid motion due to viscosity are some of
the examples discussed by Bryan. The mechanical work that is lost in all
these processes is converted into heat generated internally as explained by
the author.
The proposition that we would like to put forward is that: Thermal effect
is the only source of entropy generation. Alternatively, only irreversible pro-
cesses that include a passage of heat may lead to a generation of entropy. It is
important to realize that not necessarily every natural process may involve a
flow of heat or thermal effect, e.g. aging, change in potential energy, moon
orbiting around the earth, and earth around the sun. What we aim to accom-
plish in this chapter is to show that if processes like pressure drop, free expan-
sion, and mixing of ideal gases lead to the production of entropy, it is due to
the thermal effect involved in these processes.

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