Handbook of Turbomachinery Second Edition Revised - Ventech!

U ¼ rotor tangential velocity.
By inspection, we can see that Eq. (14) relates the square of the rotor speed
(and thus, the rotor’s specific kinetic energy) to a change in fluid energy. In
this way, C can be thought of as an energy ratio—that is, the fraction of
rotor kinetic energy that is transferred as energy to the pumped fluid.
As with numerous other design parameters, care and consistency in
usage is of great importance. For example, we could refer to the head
coefficient of a rotor alone, or of an entire pump stage. While these values
would probably be not too different, energy losses between the outlet of a
rotor and the outlet of a stage would result in C pump being less than C
rotor. Also, when applied to a multistage pumping machine, specificity is
required to understand whether C is specified for an entire pump or just one
(or possibly more than one) stage.
ROCKET PROPULSION SYSTEMS—CENTRIFUGAL PUMPS
Centrifugal pumps (both single and multistage) dominate the design of
propellant pumping elements for liquid rocket engine systems. There are
several reasons for this.
First, centrifugal machines are normally fairly simple (both hydrodynamically
and otherwise) and rugged. Because of the change in radius
through which the pumped fluid is processed, centrifugal stages possess
relatively high head coefficients. This normally results in only a single pump
stage being required for most cases (with the exception of very high head
liquid hydrogen pumps).
The relative hydrodynamic simplicity of centrifugal stages results in a
decreased requirement for diffusion within the pump. This makes the
centrifugal machine relatively forgiving of changes in the operating point at
which it is forced to function. This consideration is particularly important
for an engine system that must operate over a range of thrust levels. Since
the H–Q characteristic for the rocket engine system and that for the
centrifugal pump are not the same shape (see Fig. 19), the pump is forced to
operate ‘‘off-design’’ at all operating points except the ‘‘design point.’’
Stability of the pump’s operation over this range (and often a somewhat
larger range) is mandatory for these variable thrust engine systems.
Since the centrifugal pump is capable of delivering relatively high head
per stage, these machines generally result in a minimum length of the
turbopump rotor assembly. This is important, since long bearing spans
often contribute to rotor dynamic difficulties, especially at high operating
speeds.
Copyright © 2003 Marcel Dekker, Inc.