Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGIn other words, maximum work is obtainedfrom a reversing blade when the blade velocityis exactly one-half the absolute velocity of thesteam at the blade entrance.^ The maximumamount of work obtainable from a reversingblade is twice the amount obtainable from theflat vane shown in figure 12-3.In actual turbines, it is not feasible toutilize the complete reversal of steam in theblades, since to do so would require that thenozzle be placed in a position that would alsobe swept by the blades— an obvious impossibility.Furthermore, if the steam enteredthe blade tangent ially it would not be carriedthrough the turbine axially (longitudinally). However,it is only the tangential component of thesteam velocity that produces work on the turbineblades; hence the nozzle angle is made assmall as possible.As we know, the work done on the blade mustequal the total energy entering minus the totalenergy leaving. The velocity diagram for animpulse turbine shown in figure 12-5 providesa way of determining the work done on theblade in terms of the various velocities andangles.The tangential component (which is the onlycomponent that produces work) of the velocityof the entering steam isVj cos awhich is also equal toVb + Ri cos bThe tangential component of the velocity ofthe departing steam isfrom Vj^ cos a, the resultant velocity may beexpressed asor, alternatively, asVj^ cos a - V2 cos cRj cos b + R2 cos dAssuming a steam flow of W pounds persecond,andWWmass per secondforceTherefore, the force on the blade isWF, = — (Ri cos b + R2 cos d) pounds__W(Yj^ pQg 3 _y^ cos c) poundsWork is force through distance. Therefore,the rate of doing work on the blade isWkb =FfaVb= W(R^ c°^b+R2 cos d) Vjj foot-pounds^ per second= g-^'^icos a - V2 cos c) Vjj footpoundsper secondwhich isV2 cos calso equal toVjj - R2 cos dNOZZLE V,"steam jetSince entering and leaving velocities areopposed to each other if V2 cos c is in the samedirection as V^ cos a, and supplementary toeach other if V2 cos c is in the opposite directionThis statement assumes, of course, that the nozzleIs tangential to the blades. In actual impulse turbines,the maximum amount of work is done when the bladespeed is one-half the cosine of the nozzle angle timesthe absolute velocity of the entering steam.BLADES147.92Figure 12-3.—Elementary impulse turbine.322