Handbook of Turbomachinery Second Edition Revised - Ventech!

Fig. 25. The nature of the flow in this cavity is somewhat reminiscent of the
distribution over a turbine disk, and load predictions are subject to
essentially the same uncertainties as the turbine disk problem. Again, Refs. 9
and 10 can provide us with some useful insight.
The fluid momentum thrust associated with inducers is due mainly to
their tapered geometry. Contemporary high-performance inducers generally
have tapered hubs, and they may or may not have tapered tips. This results
in an increase in the fluid axial momentum across the inducer. This taper is
shown clearly in Fig. 25.
Centrifugal pumping elements are easily the most popular pumping
elements in use in rocket turbomachinery. As such, their axial thrust
characteristics are of great interest. Figure 26 shows a cross section of a
‘‘generic’’ centrifugal pumping element and some of the forces acting on it.
As in the case of the elements discussed previously, axial forces are
generated due to fluid axial momentum changes as well as pressure forces.
Referring to the figure, the pressure force F 1 associated with the impeller
inlet area and pressure is readily calculable. The force F2 is associated with
the change in pumped fluid axial momentum. Using the conservation of
momentum, this is also calculable, although great care needs to be exercised
with regard to the nature of the fluid exit velocity. Although a radial-flow
impeller is shown in the figure, this is not the case with many machines. A
series of higher specific speed impellers (known as Francis impellers) is
commonly used in rocket turbomachines. These impellers expel fluid with a
velocity that contains an axial component, and this must be accounted for if
axial thrust computations are to be done accurately. The forces shown as F3
and F4 are pressure forces exerted on the front and rear shrouds of the
impeller, respectively. These pressure forces vary with radius and are due to
the recirculating flow that exists between the impeller discharge and its inlet
Figure 26 Centrifugal pump stage axial thrust components: F 1, impeller inlet
pressure force; F2, impeller momentum thrust force; F3, impeller leakage pressure
force; F4, impeller rear face (hub) pressure force.
Copyright © 2003 Marcel Dekker, Inc.