Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 8 Staged combustion system, turbine power available versus pump power
required (pump/turb efficiency ¼ 85%).
and additional turbine performance is needed, other design refinements are
employed. These are mentioned later. The mid-1950s, generation of geardriven
turbopumps probably allowed the turbine designer the greatest
freedom in setting blade speed, since turbine speed was not directly related
to pump speed. The relatively high turbine shaft angular velocities permitted
by the gear-driven configuration resulted in a rather compact turbine
package delivering relatively high efficiency (somewhat in excess of 60%).
The Thor engine turbopump and its descendants are an example of these. It
should be noted, however, that this ‘‘decoupling’’ of the turbine from the
pump is obtained only at the expense of a large, heavy gearbox. As research
efforts increased the suction performance capability of pumps, the increase
in attainable pump shaft angular velocity was such that the gear reduction
system could be eliminated.
In the engine definition and preliminary design phases, the turbine
velocity ratio probably exerts more influence on turbine configuration than
any other single quantity. Velocity ratio is a term that can be defined either
for a single turbine stage or for a group of multiple turbine stages. The usual
definition of velocity ratio is U=C0, where U is the turbine blade tangential
velocity (usually taken at the blade midspan for our purposes) when
referring to a single stage. When referring to a multistage configuration, U
represents the square root of the sum of squares of the tangential velocity of
Copyright © 2003 Marcel Dekker, Inc.