Chapter 12- PROPULSION STEAM TURBINES 



the velocity. By causing an increase in velocity, 

 therefore, the nozzle causes an increase in the 

 kinetic energy of the steam. Thus it is clear 

 that our last equation has actually described 

 the purpose of a nozzle by equating the decrease 

 in thermal energy with the increase in kinetic 

 energy. 



BASIC PRINCIPLES OF TURBINE DESIGN 



In essence, a turbine may be thought of as 

 a bladed wheel or rotor that turns when a jet 

 of steam from the nozzles impinges upon the 

 blades. The basic parts of a turbine are the 

 rotor, which has blades projecting radially 

 from its periphery^ a casing , in which the 

 rotor revolves; and nozzles, through which 

 the steam is expanded and directed. As we 

 have seen, the conversion of thermal energy 

 to mechanical kinetic energy occurs in the 

 nozzles. The second energy conversion— that 

 is, the conversion of kinetic energy to work- 

 occurs on the blades. 



The basic distinction to be made between 

 types of turbines has to do with the manner in 

 which the steam causes the turbine rotor to 

 move. When the rotor is moved by a direct 

 push or "impulse" from the steam impinging 

 upon the blades, the turbine is said to be an 

 impulse turbine. When the rotor is moved by the 

 force of reaction, the turbine is said to be a 

 reaction turbine. 



Although the distinction between impulse 

 turbines and reaction turbines in a useful one, 

 and one which is followed in this text, it should 

 not be considered as an absolute distinction in 

 real turbines. An impulse turbine utilizes both 

 the impulse of the steam jet and, to a lesser 

 extent, the reactive force that results when the 

 curved blades cause the steam to change di- 

 rection. A reaction turbine is moved primarily 

 by reactive force, but some motion of the rotor 

 is caused by the impact of the steam against 

 the blades. 



Theory of Impulse Turbines 



In discussing the manner in which kinetic 

 energy is converted to work on the turbine 

 blades, it is necessary to consider both the 

 absolute velocity of the steam and the relative 

 velocity of the steam— that is, its velocity 

 relative to the moving blades. The following 

 symbols will be used in the remainder of this 

 discussion: 



Vi = absolute velocity of steam at blade 



entrance 

 V2 = absolute velocity of steam at blade 



exit 

 R^ = relative velocity of steam at blade 



entrance 

 Rg = relative velocity of steam at blade 



exit 

 Vb = peripheral velocity of blade 



Let us consider, first, a theoretical elemen- 

 tary impulse turbine such as the one shown in 

 figure 12-3, The blades of this imaginary tur- 

 bine are merely flat vanes or plates. As the 

 steam jet flows from the nozzle and impinges 

 upon the vanes, the rotor is moved. 



Assuming that there is no friction as the 

 steam flows across the blade, Rl must be 

 equal to Vj^ - Vb and R2 must also be equal 

 to Vi - Vb, since theoretically there is no 

 change in velocity as the steam flows across 

 the blade. 



It will be apparent that, in order to convert 

 all of the kinetic energy into work, it would be 

 necessary to design a blade from which the 

 steam would exit with zero absolute veolcity. 

 This blade would be curved in the manner 

 shown in figure 12-4, and the jet of steam would 

 enter the blade tangentially rather than at an 

 angle. As we shall see, the shape of this blade 

 very closely approximates the shape of the 

 blades used in actual impulse turbines; in a 

 real turbine, however, the steam enters the 

 blade at an angle, rather than tangentially. 



When this curved blade is used, the di- 

 rection of the steam is exactly reversed. The 

 relative velocity of the steam at the blade 

 entrance, Ri, is again Vi - Vb and R2 is 

 again Vj _ Vb. Since the direction of flow is 

 reversed, however, absolute velocity of the 

 steam at blade exit is now 



V2 = (Vl - Vb) 

 = Vi - 2Vx, 



As previously noted, the absolute velocity 

 of the departing steam (V2) should ideally be 

 zero. Therefore, by transposition of the above 

 equation, 



2Vv 



321 



