Handbook of Turbomachinery Second Edition Revised - Ventech!

solutions of Eq. (7) for both stages will yield Euler heads for a centrifugal
stage in the range of 2 to 5 times that of an axial stage. This result illustrates
the contribution of radius change to head generation in a centrifugal stage
versus the axial stage.
For the reasons listed above, design point head coefficients available
from axial-flow stages are limited in comparison with typical centrifugal
stages. For example, whereas a heavily loaded centrifugal rotor may be
capable of head coefficients as high as 0.7, an analogous axial-flow stage
may have a peak head coefficient of on the order of 0.4. It should be noted
that a stage such as this would have a relatively poor operating range due to
the low rotor-relative outlet velocities implied by the vector diagram of such
a stage. Achieving this low rotor-relative outlet velocity would require large
amounts of diffusion within the rotor, with the attendant problems at flow
rates less than design.
In addition to the relatively limited total head per stage in a typical
axial pump, it should be understood that kinetic energy constitutes a larger
fraction of the total rotor work input than in a centrifugal machine. For this
reason, efficient diffusion between the stages of a multistage axial-flow
pump is crucial to efficient operation at the design point. In addition, the
ability of the interstage diffusion system to perform at off-design flow
coefficients may in itself determine the range of the pump’s H-Q
characteristic over which stable operation is possible.
The characteristics of large inlet flow area (larger than a typical
centrifugal stage) combined with relatively low head per stage combine to
place axial-flow pump stages at the high end of the specific speed spectrum.
ItcanbeseenfromFig. 14that axialpumpstagesaregenerally applicableat
stage specific speed values of between 3,000 and 10,000 (rpm-gpm-ft units).
It is important to remember that this reference to applicable Ns ranges is for
single-pump stages only. As was mentioned previously, differentiating
between the Ns-value of an entire pump and that of a single stage is required
to prevent nonsensical interpretation of results. For example, an LH 2 pump
for a certain large engine operated at an overall pump Ns of approximately
900. The high head implied by this low Ns was split between sufficient axialflow
stages that each pump stage operated at an Ns-value of approximately
4,500.
Figure 15(j) shows anotional rendition of several axial-flow pump
stages separated by specific speed. The variation of stage geometry with
specific speed is apparent from the figure and is analogous to that of
centrifugal stages. In particular, the rotor’s radius ratio (hub radius divided
by tip radius) varies inversely with specific speed, much as the b 2 dimension
of a centrifugal stage varies with specific speed. In addition, the optimum
head coefficient varies inversely with specific speed, with the implication
Copyright © 2003 Marcel Dekker, Inc.