Handbook of Turbomachinery Second Edition Revised - Ventech!

As the name implies, axial thrust refers to forces acting on the rotor in
a direction parallel to the axis of rotation. There are two main contexts for
discussion of these forces. In the context of overall rotor design, the term
‘‘axial thrust’’ is generally used to refer to the resultant of all axial forces
acting on the rotating assembly. This resultant force is used to determine
what type (if any) of axial thrust control devices are necessary. The type of
thrust control system used can also depend on the perceived uncertainty of
the axial thrust predictions. In some cases, the aerodynamic or hydraulic
design of certain turbopump components may have to be altered due to
axial thrust considerations. We discuss this later.
The second context for discussion of axial forces concerns the
structural design of rotor components when these components are
considered as deformable bodies. For example, a turbopump drive shaft
may be subjected to a net compressive load due to thrust forces from two
opposing rotor components. The structural integrity and/or the stiffness of
the drive shaft may be questionable under these conditions. In the case of a
built-up rotor (a configuration without a shaft proper and axially retained
by a central draw bolt), the distribution of axial loads along the rotor may
decide the fundamental orientation of turbopump components. An example
of this might be orienting the turbine of a common shaft turbopump such
that the tie bolt is subjected to tensile force rather than a compressive force
that would be sufficient to relieve the draw bolt preload. Although most of
our discussion is oriented toward the first of these contexts, we attempt to
carefully distinguish between them when necessary.
The primary generators of axial loads on a turbopump rotor are the
power-generating and absorbing components, namely turbine stages, axial
and centrifugal pump stages, and pump inducers. We try to provide
examples of each.
Refer to Fig. 22. Depicted here is an example of asingle-stage turbine
rotor. In this example, the turbine rotor is assumed to be supported on its
own bearings, independent of any other rotating elements. The rotor shown
in the figure represents a classic single-stage impulse turbine. Such an
assembly could have been typical of small rocket turbopumps of the 1950s
and early 1960s and might be found today in numerous small auxiliary
power units. Note the stated flow conditions at various points in the turbine.
Since the stage is an impulse stage, we may assume that there is no static
pressure drop across the rotor. Therefore, Protorin ¼ Protorout (actually, a
small static pressure drop would exist across the rotor, but we ignore this).
Also, note the velocity diagrams showing the absolute and rotor-relative
velocity components at the rotor inlet and exit. In a ‘‘perfect’’ rotor, the
rotor-relative velocity at the stage outlet ðV2Þ would equal that at the rotor
inlet ðV1Þ. However, in reality, the magnitude of V2 will be less than V1. This
Copyright © 2003 Marcel Dekker, Inc.