Principles of naval engineering - Historic Naval Ships Association

Chapter 12-PROPULSION STEAM TURBINESThe rate of doing work on the blade mayalso be derived from consideration of thethermal energy and kinetic energy enteringthe system and leaving the system. Althoughthe actual derivations are not given here, itmay be of interest to note the relationshipsexpressed in the following equations:2 2W(Vi - V2)Wk = W (hi - h2) + 50, 000"discovered" by Newton, it is interesting tonote that the first reaction turbine— and, indeed,perhaps the first steam engine of anykind ever made—was developed by the Greekmathematician Hero about 2000 years ago.This turbine, shown in figure 12-8, consistedof a hollow sphere which carried four bentnozzles. The sphere was free to rotate on thetubes that carried steam from the boiler, below,to the sphere. As the steam flowed outandBtu per second^^ri, - W(Ri - R^ + RJ W(V? - v|)b 50, 000Btu per secondThe pressure and velocity changes that occurin the nozzle and in the blades of an impulseturbine are shown in figure 12-6. As may beseen, the pressure is the same at the entranceand at the exit of the blade; the only pressuredrop occurs in the nozzle. Figure 12-7 showsa section of an impulse turbine rotor, with theblades in place.Theory of Reaction TurbinesReaction turbines, as their name implies,are moved by reactive force rather than by adirect push or impulse. Although we commonlythink of reactive force as having beenNOZZLE147.94Figure 12-5.— Velocity diagram for impulseblading.through the nozzles, the sphere rotated rapidlyin a direction opposite to the direction of steamflow.Reaction turbines used in modern timesutilize the reactive force of the steam in quitea different way. In a modern reaction turbine,there are no nozzles as such. Instead, the bladesthat project radially from the periphery of therotor are formed and mounted in such a waythat the spaces between the blades have, in crosssection, the shape of nozzles. 2 Since theseBLADE;:^«- V5147.93Figure 12-4.— Curved impulse blade.The distinction between actual nozzles and the bladingwhich serves the purpose of nozzles in reaction turbinesis mechanical rather than functional. The previousdiscussion of steam flow through nozzles appliesequally well to steam flow through thenozzle-shaped spaces between the blades of reactionturbines.323