70592_01

70592_01 - Introduction: Dimensional Analysis: Similitude I...

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Introduction: Dimensional Analysis: Similitude If you have known one you have known all. (TERENCE, Phonnio.) 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 impelZer 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 occumng 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 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 axialflow turbomachine (e.g. 1
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2 Fluid Mechanics, Thermodynamics of Turbomachinery FIG. 1 .l. Diagrammatic form of various types of turbomachine. Figure l.l(a) and (e)). When the path of the through-jow is wholly or mainly in a plane perpendicular to the rotation axis, the device is termed a radialflow turbomachine (e.g.
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This note was uploaded on 04/05/2011 for the course MAE 402 taught by Professor Drivan during the Spring '11 term at FIT.

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70592_01 - Introduction: Dimensional Analysis: Similitude I...

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