Handbook of Turbomachinery Second Edition Revised - Ventech!

this discontinuity is neglected in the pressure force computations, erroneous
results will be obtained. In the case of Fig. 22, if the stage outlet pressure is
substantially above atmospheric, equal disk face static pressures (as might
be the case with short blades) would mean that the only unbalanced pressure
force would exist axially across the disk at the diameter of the hot-gas seal.
From this one could draw the conclusion that the placement and diameter of
seal packages could be used as tools for control of pressure force
components. This is, in fact, the case and is a fairly common practice in
turbomachinery design.
Figure 23 represents the next step up in complexity of the turbine
pressure force problem. The turbine depicted here is a two-stage impulse
turbine, with interstage leakage minimized through the use of an interstage
labyrinth seal. The turbine rotor is independently supported on its own
bearings, as before. Although the diameter of the labyrinth seal is normally
set so as to minimize interstage leakage, its diameter also influences the
turbine disk pressure forces in the manner described earlier.
The turbine momentum force is due (as the name implies) to the
change in axial momentum of the working fluid as it traverses the blade row.
In an impulse stage, this change in axial momentum is due mainly to bladerow
losses that cause deceleration of the fluid in the blade row. The
momentum force can be expressed by the conservation of linear momentum:
SF ¼ q
0
1
Z Z Z
Z Z
@ ~V rdWA
þ ~V r ~V ?~n dA
qt
Where
CV
CV ¼ integration over an entire control volume.
CS ¼ integration over a control surface.
Figure 23 Two-stage pressure-compounded impulse turbine.
Copyright © 2003 Marcel Dekker, Inc.
CS