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Handbook of Turbomachinery Second Edition Revised - Ventech!

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worth mentioning. First, both the hub and tip are <strong>of</strong> constant diameter,<br />

referred to as a cylindrical hub and tip. For this reason, this inducer is truly<br />

an axial-flow pumping element. In addition (although this cannot be<br />

discerned from the figure), the blading <strong>of</strong> this inducer is noncambered, or<br />

flat-plate blading. This implies that the blade exit angle is equal to the blade<br />

inlet angle. All flow turning is then done as a result <strong>of</strong> traversing the leadingedge<br />

incidence angle. Attempts to generate increasing head with the flatplate<br />

blading will result in ever-increasing inlet incidence angles, with high<br />

losses and large-scale cavitation resulting when inlet incidence becomes<br />

excessive. For this reason, head coefficients <strong>of</strong> these flat-plate inducers are<br />

generally limited to values <strong>of</strong> around 0.08 or less, and best performance is<br />

achieved at inlet flow coefficients in the neighborhood <strong>of</strong> 0.1. Of course, for<br />

any given blade inlet and exit angle and inducer speed, the ideal total<br />

developed head can still be computed from Euler’s equation. The somewhat<br />

inflexible nature <strong>of</strong> the flat-plate blading makes this inducer rather primitive<br />

by today’s standards. However, the incorporation <strong>of</strong> this inducer<br />

constituted a significant breakthrough in performance and packaging<br />

potential. It allowed substantial increases in the pump operating speed at<br />

low inlet NPSH. This speed increase allowed reduction in outside diameters<br />

<strong>of</strong> rotating components such that a turbopump <strong>of</strong> a given physical size could<br />

generate over twice the power <strong>of</strong> that previously possible.<br />

Attempts to generate higher head rises with the inducer in Fig. 15(l)(a)<br />

proved to be disappointing. A change from flat-plate to cambered blading<br />

enabled some increase in head coefficient capability; however, these<br />

attempts were hampered by the need for excessive turning at the inducer<br />

hub [blade speeds are slow here, so large hub cambers are required to satisfy<br />

Eq. (7) if developed head is to be constant at all radii]. The low rotor-relative<br />

outlet velocity that comes about requires very large amounts <strong>of</strong> diffusion,<br />

which result in flow separation at the hub. This phenomenon strictly limits<br />

the inducer head coefficients that are attainable with the cylindrical hub<br />

design.<br />

The need to provide inducers with head coefficients substantially in<br />

excess <strong>of</strong> 0.1 was answered by the design change shown in Fig. 15(l)(b).<br />

While the tip <strong>of</strong> this inducer is still a cylinder, the inducer hub is tapered.<br />

The result is that the flow streamlines adjacent to the hub change radius<br />

during the pumping process, reminiscent <strong>of</strong> a centrifugal pump stage. This<br />

feature greatly increases the head rise capability <strong>of</strong> the inducer. The<br />

mechanism behind this increased head capability can be interpreted in either<br />

<strong>of</strong> two ways: if we refer to Eq. (7), and station 2 is taken to be the inducer<br />

exit with station 1 at the inducer inlet, we can see by inspection that U 2 is<br />

substantially greater than U1 along the inducer hub. For a given head rise,<br />

the required change in fluid tangential velocity (i.e., the amount <strong>of</strong> blade<br />

Copyright © 2003 Marcel Dekker, Inc.

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