Handbook of Turbomachinery Second Edition Revised - Ventech!

stable operation is assured. In some aggressive engine system designs (i.e.,
those possessing low starting energy to minimize system weight), the effect
of the H–Q curve shape (as influenced by head coefficient) should be
evaluated for its effect on the system’s transient starting performance. Note
that while the exactitude of results of these types of analyses is questionable,
trends and relative margins can certainly be determined.
The number of blades used in a centrifugal stage impeller can be
readily approximated from a number of correlations that are in general use.
The number of impeller blades can be expected to vary from 5 or 6 to as
many as 24 or 25. However, the combination of high suction-performance
requirements and the desire to highly load the pump stage (for weight
reduction) leads to the use of impeller blades called ‘‘splitters’’ in many
propellant pumps for liquid rocket engine systems. If we refer to Fig. 15(c),
we can see that the required number of blades for a particular pump has
been determined as eight. However, if all eight blades extend into the
impeller eye, the amount of flow area that is ‘‘blocked’’ by the blades causes
flow to accelerate to the point where substantial cavitation is encountered.
This difficulty is dealt with by placing the leading edges of a number of the
blades (perhaps half of them, perhaps more) significantly downstream of the
impeller inlet. Of course, these blades extend all the way to the impeller exit,
and thus the solidity criterion at the impeller exit for achieving the design
head coefficient is met. It should be noted that a uniform criterion for
placement of the splitter leading edges does not exist to this author’s
knowledge. A technique often used to determine this is simply to place the
leading edges of the splitters at 50% of the meridonal distance through the
impeller flow passage, and then to simulate the resulting flow with any of a
number of computer programs available. The results of these simulations
can then be used to determine the point in the passage where loading of the
full-length blades becomes excessive, requiring the placement of the splitter
leading edges somewhat upstream of that point. For propellant pumps that
must operate over a significant flow range, these computations should
be carried out at the operating point consistent with the highest blade
loading.
The angle of the impeller vanes at the impeller inlet can be determined
easily from geometry and bulk flow considerations. Referring to Fig. 15(e),
flow is approaching the impeller inlet at a velocity C. The impeller blade is
moving as shown with a velocity U. The combination of these vectors
produces the vector W, or the fluid inlet velocity vector as seen from a point
on the impeller. The angle of the blade centerline is normally set to the
expected angle of the W vector, to minimize the losses associated with
negotiating the blade leading edge (referred to as incidence losses). It should
be noted that if the pump must operate over a significant range of inlet flow
Copyright © 2003 Marcel Dekker, Inc.