Page 17 - Bird R.B. Transport phenomena

2 Chapter 0 The Subject of Transport Phenomena

motions and interactions are responsible for viscosity, thermal conductivity, and
diffusion.

The main aim of this book is to give a balanced overview of the field of transport phe-
nomena, present the fundamental equations of the subject, and illustrate how to use
them to solve problems.
There are many excellent treatises on fluid dynamics, heat transfer, and mass trans-
fer. In addition, there are many research and review journals devoted to these individual
subjects and even to specialized subfields. The reader who has mastered the contents of
this book should find it possible to consult the treatises and journals and go more deeply
into other aspects of the theory, experimental techniques, empirical correlations, design
methods, and applications. That is, this book should not be regarded as the complete
presentation of the subject, but rather as a stepping stone to a wealth of knowledge that
lies beyond.

§0.2 THREE LEVELS AT WHICH TRANSPORT
PHENOMENA CAN BE STUDIED

In Fig. 0.2-1 we show a schematic diagram of a large system—for example, a large piece
of equipment through which a fluid mixture is flowing. We can describe the transport of
mass, momentum, energy, and angular momentum at three different levels.
At the macroscopic level (Fig. 0.2-1я) we write down a set of equations called the
"macroscopic balances," which describe how the mass, momentum, energy, and angular
momentum in the system change because of the introduction and removal of these enti-
ties via the entering and leaving streams, and because of various other inputs to the sys-
tem from the surroundings. No attempt is made to understand all the details of the
system. In studying an engineering or biological system it is a good idea to start with
this macroscopic description in order to make a global assessment of the problem; in
some instances it is only this overall view that is needed.
At the microscopic level (Fig. 0.2-1 b) we examine what is happening to the fluid mix-
ture in a small region within the equipment. We write down a set of equations called the
"equations of change," which describe how the mass, momentum, energy, and angular
momentum change within this small region. The aim here is to get information about ve-
locity, temperature, pressure, and concentration profiles within the system. This more
detailed information may be required for the understanding of some processes.
At the molecular level (Fig. 0.2-1 c) we seek a fundamental understanding of the mech-
anisms of mass, momentum, energy, and angular momentum transport in terms of mol-

Q = heat added to system

Fig. 0.2-1 (я) A macro-
scopic flow system contain-
W - ~ Work done on the system by ing N and O ; (b) a
m
the surroundings by means 2 2
of moving parts microscopic region within
the macroscopic system
containing N and O ,
2
2
which are in a state of flow;
(c) a collision between a
molecule of N and a mole-
2
cule of O .
2