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Handbook of Turbomachinery Second Edition Revised - Ventech!

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general indicator. Nss will give us only a ‘‘feel’’ for the severity <strong>of</strong> a suction<br />

performance problem, but only detailed analyses <strong>of</strong> candidate designs<br />

should be used for specific diagnostic work.<br />

In liquid rocket engine systems, the combination <strong>of</strong> low pump inlet<br />

pressure and cryogenic propellants (which are normally stored quite near<br />

their boiling points) generally drive the inlet Nss <strong>of</strong> a pump stage (or the first<br />

stage <strong>of</strong> a multistage pump) well above the 10,000 mark. In this event, a high<br />

suction-performance inducer is generally incorporated into a pump’s design<br />

from the beginning.<br />

The issue <strong>of</strong> pump inlet cavitation provokes much discussion, and<br />

some details are warranted before we explore centrifugal and axial-pump<br />

hardware in more detail. In our previous discussion <strong>of</strong> Fig. 15(c), we<br />

discussed the notion <strong>of</strong> behavior <strong>of</strong> the rotor’s blade cascade as hydr<strong>of</strong>oils,<br />

and the generation <strong>of</strong> a resistive torque that opposes the input torque, T.<br />

Under these conditions, the concave side (or ‘‘suction’’ side) <strong>of</strong> the rotor<br />

blade experiences a drop in local static pressure. This is followed (as we<br />

proceed along the blade camber line) by rotor-relative diffusion (as well as<br />

work input due to the radius change), which causes the suction surface static<br />

pressure to increase. If the rate <strong>of</strong> change <strong>of</strong> the local blade angle is<br />

sufficiently high near the leading edge, the region <strong>of</strong> minimum static<br />

pressure can drop below the local vapor pressure. At this point, phase<br />

change occurs, with the phenomenon being called ‘‘cavitation.’’ Several<br />

things should be pointed out relative to this phenomenon:<br />

A survey <strong>of</strong> the available introductory literature may lead the engineer<br />

to believe that cavitation is a ‘‘digital’’ phenomenon, i.e., either<br />

cavitation exists or it does not. This is far from the truth. Cavitation<br />

can exist to any degree <strong>of</strong> severity; from so low as to be unnoticeable<br />

to so great as to render a pump useless in its intended application.<br />

Most high-performance rocket system pumps operate with a<br />

significant degree <strong>of</strong> inlet cavitation. In fact, since most rocket<br />

engine system propellant pumps operate on systems that are<br />

expendable (a single use only), the required life is sufficiently short<br />

that degrees <strong>of</strong> cavitation are accepted that would be out <strong>of</strong> the<br />

question for a ground-based system.<br />

Although Fig. 15(c) shows the cavitative situation occurring in the<br />

context <strong>of</strong> a centrifugal rotor, the phenomenon occurs in both<br />

centrifugal and axial-pump stages.<br />

Although Fig. 15(c) shows the situation occurring due to the natural<br />

acceleration on the suction side <strong>of</strong> a blade, any situation that causes<br />

acceleration will produce the same result. For example, providing a<br />

Copyright © 2003 Marcel Dekker, Inc.

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