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Introduction: Dimensional Analysis—Basic Thermodynamics and Fluid Mechanics

A turbomachine is a device in which energy transfer occurs between a flowing fluid and a rotating element due to dynamic action, and results in a change in pressure and momentum of the fluid. Mechanical energy transfer occurs inside or outside of the turbomachine, usually in asteady-flow process. Turbomachines include all those machines that produce power, such as turbines, as well as those types that produce a head or pressure, such as centrifugal pumps and compressors. The turbomachine extracts energy from or imparts energy to a continuously moving stream of fluid. However in a positive displacement machine, it is intermittent. The turbomachine as described above covers awide range of machines, such as gas turbines, steam turbines, centrifugal pumps, centrifugal and axial flow compressors, windmills, water wheels, and hydraulic turbines. In this text, we shall deal with incompressible and compressible fluid flow machines.

There are different types of turbomachines. They can be classified as: 1. Turbomachines in which (i) work is done bythe fluid and (ii) work is done on the fluid.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved

Figure 1.1 Types and shapes of turbomachines.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved

2. Turbomachines in which fluid moves through the rotating member in axial direction with no radial movement of the streamlines. Such machines are called axial flow machines whereas ifthe flow is essentially radial, it is called a radial flow or centrifugal flow machine. Some of these machines are shown in Fig. 1.1, and photographs of actual machines are shown in Figs. 1.2 –1.6. Two primary points will be observed: first, that the main element is a rotor or runner carrying blades or vanes; and secondly, that the path of the fluid in the rotor may be substantially axial,substantially radial, or in some cases a combination of both. Turbomachines can further be classified as follows: Turbines: Machines that produce power by expansion of a continuously flowing fluid to a lower pressure or head. Pumps: Machines that increase the pressure or head of flowing fluid. Fans: Machines that impart only a small pressure-rise to a continuously flowing gas; usually the gas may be considered tobe incompressible.

Figure 1.2

Radial flow fan rotor. (Courtesy of the Buffalo Forge Corp.)

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved

Figure 1.3 Centrifugal compressor rotor (the large double-sided impellar on the right is the main compressor and the small single-sided impellar is an auxiliary for cooling purposes). (Courtesy of Rolls-Royce, Ltd.)

Figure 1.4Centrifugal pump rotor (open type impeller). (Courtesy of the IngersollRand Co.)

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved

Figure 1.5 Multi-stage axial flow compressor rotor. (Courtesy of the Westinghouse Electric Corp.)

Figure 1.6 Axial flow pump rotor. (Courtesy of the Worthington Corp.)

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved

Compressors: Machines thatimpart kinetic energy to a gas by compressing it and then allowing it to rapidly expand. Compressors can be axial flow, centrifugal, or a combination of both types, in order to produce the highly compressed air. In a dynamic compressor, this is achieved by imparting kinetic energy to the air in the impeller and then this kinetic energy is converted into pressure energy in the diffuser.

To study the performance characteristics of turbomachines, a large number of variables are involved. The use of dimensional analysis reduces the variables to a number of manageable dimensional groups. Usually, the properties of interest in regard to turbomachine are the power output, the efficiency, and the head. The performance of turbomachines depends on one or more of several...
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