Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

CHAPTER 1
Introduction: Dimensional
Analysis: Similitude
If you have known one you have known all. (TERENCE, Phormio.)
Definition of a turbomachine
We classify as turbomachines all those devices in which energy is transferred
either to, or from, a continuously flowing fluid by the dynamic action of one or
more moving blade rows. The word turbo or turbinis is of Latin origin and implies that
which spins or whirls around. Essentially, a rotating blade row, a rotor or an impeller
changes the stagnation enthalpy of the fluid moving through it by either doing positive
or negative work, depending upon the effect required of the machine. These enthalpy
changes are intimately linked with the pressure changes occurring simultaneously in
the fluid.
In the earlier editions of this book, open turbomachines, such as wind turbines, propellers
and unshrouded fans were deliberately excluded, primarily because of the conceptual
difficulty of properly defining the mass flow that passes through the blades.
However, despite this apparent problem, the study of wind turbines has become an
attractive and even an urgent task, not least because of the almost astonishing increase
in their number. Wind turbines are becoming increasingly significant providers of electrical
power and targets have even been set in some countries for at least 10% of power
generation to be effected by this means by 2010. It is a matter of expediency to now
include the aerodynamic theory of wind turbines in this book and so a new chapter has
been added on the topic. It will be observed that the problem of dealing with the indeterminate
mass flow has been more or less resolved.
Two main categories of turbomachine are identified: firstly, those that absorb power
to increase the fluid pressure or head (ducted fans, compressors and pumps); secondly,
those that produce power by expanding fluid to a lower pressure or head (hydraulic,
steam and gas turbines). Figure 1.1 shows, in a simple diagrammatic form, a selection
of the many different varieties of turbomachine encountered in practice. The reason
that so many different types of either pump (compressor) or turbine are in use is because
of the almost infinite range of service requirements. Generally speaking, for a given set
of operating requirements one type of pump or turbine is best suited to provide optimum
conditions of operation. This point is discussed more fully in the section of this chapter
concerned with specific speed.
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