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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Pumping elements (i.e., inducers, centrifugal stages, and axial stages)<br />

are also generators <strong>of</strong> axial forces. The fundamental processes that produce<br />

axial loads in these elements are exactly the same as those in turbine stages.<br />

Consequently, the same analytical techniques used for turbine elements<br />

apply also to pumping elements. We discuss an example <strong>of</strong> each.<br />

Inducers (an example <strong>of</strong> which is shown in Fig. 25) generate axial<br />

loads due to both blade-row pressure forces and fluid momentum thrust.<br />

Euler’s equation may be written across the blade midspan and the static<br />

pressure rise across the blade calculated. This estimate may be functional as<br />

a ‘‘first cut’’ for the inducer pressure force. If the inducer has ‘‘tall’’ blades,<br />

and if greater accuracy is justified, the blade section may be divided into a<br />

series <strong>of</strong> radial stations. Odd numbers <strong>of</strong> stations are typically used so that<br />

the blade midspan falls inside one <strong>of</strong> the sections. The blade-row pressure<br />

force is then obtained integrating radially across the blade section.<br />

It should be mentioned that thrust analyses <strong>of</strong> preexisting inducers<br />

should be done carefully. The nature <strong>of</strong> the vortex flow through a typical<br />

inducer (particularly a high-performance one) is normally not that <strong>of</strong> a free<br />

vortex, as might be the case with an axial pump rotor row. An inducer blade<br />

cross section designed to produce free-vortex flow can easily result in<br />

excessive turning and diffusion near the blade trailing edge at the hub. The<br />

result <strong>of</strong> this can be large-scale flow separation in this region. For this<br />

reason, high-performance inducers are not usually designed around freevortex<br />

flow.<br />

The other major pressure force term associated with contemporary<br />

inducers is that which exists in the cavity inboard <strong>of</strong> the inducer hub. In<br />

many current designs, this cavity is also home to a set <strong>of</strong> rolling element<br />

bearings that support the turbopump rotor. A similar set can usually be<br />

found near the turbine end <strong>of</strong> the machine. This design strategy is shown in<br />

Figure 25 Inducer axial thrust components: F 1, inducer inlet pressure force; F 2,<br />

inducer outlet pressure force; F3, inducer momentum force; F4 inducer hub cavity<br />

pressure force (nonconstant over cavity area).<br />

Copyright © 2003 Marcel Dekker, Inc.

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