Page 20 - Fluid Mechanics and Thermodynamics of Turbomachinery

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
simulataneously in the fluid.
The definition of a turbomachine as stated above, is rather too general for the
purposes of this book as it embraces open turbomachines such as propellers, wind
turbines and unshrouded fans, all of which influence the state of a not readily
quantifiable flow of a fluid. The subject fluid mechanics, thermodynamics of turbo-
machinery, therefore, is limited to machines enclosed by a closely fitting casing or
shroud through which a readily measurable quantity of fluid passes in unit time.
The subject of open turbomachines is covered by the classic text of Glauert (1959)
or by Duncan et al. (1970), the elementary treatment of propellers by general fluid
mechanics textbooks such as Streeter and Wylie (1979) or Massey (1979), and the
important, still developing subject of wind turbines, by Freris (1990).
Two main categories of turbomachine are identified: firstly, those which 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 there is one type of
pump or turbine best suited to provide optimum conditions of operation. This point
is discussed more fully in the section of this chapter concerned with specific speed.
Turbomachines are further categorised according to the nature of the flow path
throughthepassagesoftherotor.Whenthepathofthethrough-flow iswhollyormainly
parallel to the axis of rotation, the device is termed an axial flow turbomachine (e.g.
1