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.

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.

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.

Turbomachines are further categorised according to the nature of the flow path through the passages of the rotor. When the path of the through-flow is wholly or mainly parallel to the axis of rotation, the device is termed an axial flow turbomachine (e.g.Figure 1.1(a) and (e)).

When the path of the through-flow is wholly or mainly in a plane perpendicular to the rotation axis, the device is termed a radial flow turbomachine (e.g. Figure 1.1(c)).

Mixed flow turbomachines are widely used. The term mixed flow in this context refers to the direction of the through-flow at rotor outlet when both radial and axial velocity components are present in significant amounts. Figure 1.1(b) shows a mixed flow pump and Figure 1.1(d) a mixed flow hydraulic turbine.

One further category should be mentioned. All turbomachines can be classified as either impulse or reaction machines according to whether pressure changes are absent or present respectively in the flow through the rotor. In an impulse machine all the pressure change takes place in one or more nozzles, the fluid being directed onto the rotor.

The Pelton wheel, Figure 1.1(f), is an example of an impulse turbine. The main purpose of this book is to examine, through the laws of fluid mechanics and thermodynamics, the means by which the energy transfer is achieved in the chief types of turbomachine, together with the differing behaviour of individual types in operation.

Methods of analysing the flow processes differ depending upon the geometrical configuration of the machine, whether the fluid can be regarded as incompressible or not, and whether the machine absorbs or produces work. As far as possible, a unified treatment is adopted so that machines having similar configurations and function are considered together.

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