Handbook of Turbomachinery Second Edition Revised - Ventech!

The power output of the stage can be determined from impulse–
momentum considerations. Efficiency is shown to be [6]
Where
Z ¼ 2 w
V0
Va
V0
þ Vb
V0
w ¼ the mass of steam flowing per second.
Alternatively,
Z ¼ V2 1 V2 2 þ V2 3 V2 4
V 2 0
ð10Þ
ð11Þ
The theoretical efficiency of stages with reactions is shown in Fig. 27. The
parameter X indicates the fraction of stage energy released in the blades. As
shown in the figure, in a pure impulse stage, where X ¼ 0, the optimum ratio
of W=V0 is 0.5. Optimum ratios for other percent reactions are shown in
Fig. 27. Note that in reality, as the velocity ratio, W=V0, changes, the
fractional reaction will also change. Thus the change in efficiency of an
actual turbine stage efficiency would cut across the velocity ratio curves with
changes in imposed reaction. Rotating steam turbine blades utilized in
impulse type of construction are often referred to as ‘‘buckets.’’
Figure 27 Thermodynamic efficiency of a turbine stage versus velocity ratio for
various bucket/blade reactions.
Copyright © 2003 Marcel Dekker, Inc.