Handbook of Turbomachinery Second Edition Revised - Ventech!

the greatest demands on turbopump performance of all the classic engine
cycles. The ‘‘stacking’’ of two or three relatively high-pressure drop items
between the pump discharge and the main chamber (throttling valve,
preburner injector, and turbine in this instance) gives rise to pump outlet
pressure demands that are two to three times the main combustion chamber
pressure. However, the mechanics of this cycle can also allow the turbine
designer a latitude of design that is lacking in the closed engine cycles. Due
to high pump outlet pressure requirements, high turbine blade speeds are
generally specified. Also, since the turbine mass flow rates are higher than
open-cycle engines (remember, most or all of the fuel is used for turbine
drive), the enthalpy drop per pound of working fluid is smaller than in open
cycles. The result of these factors is a stage velocity ratio that is generally the
highest of the conventional engine cycles. An alternate way of looking at this
is through the use of the speed-work parameter. This parameter is essentially
the ratio of the blade speed squared to the actual enthalpy drop of the stage.
As such, it constitutes a sort of energy ratio term, as opposed to the velocity
ratio term we have used here.
Regardless of which of these indices is used, the increased available
turbine working fluid and the reduced specific work required (compared to
an open cycle under the same conditions) result in the opportunity to unload
the turbine stage significantly. This manifests itself in higher attainable stage
velocity ratios. In the case of the Space Shuttle main engine, for instance, the
turbines for both high-pressure turbopumps operate in the velocity ratio
rangeof0.40–0.50.AsonecanseefromFig. 9,thisiswithintherangewhere
reaction-type blading can be used to good advantage. Also, this range of
operation results in turbine nozzle spouting velocities that generally are
subsonic through the machine’s operating range. This factor has the effect of
reducing the magnitude of the losses in the stage. These losses are due in
part to nozzle expansion, blade inlet incidence, and kinetic energy losses
traversing the blade row. These losses are proportional to the Mach number
of the flow relative to the inlet of the rotating blade row. Lowering the blade
inlet Mach number reduces these losses, contributing to higher stage
efficiency. Also, subsonic blade inlet conditions permit the use of relatively
generous radii on the blade leading edges. In supersonic stages, sharp inlets
are generally considered necessary to minimize the effects of shock wave
formation at the blade row inlet. These shock waves can restrict flow
through the blade row, and adversely affect the stage performance. Larger
radii are less sensitive than sharp-edged inlets to the effects of nonzero
incidence. As a result, incidence losses can be minimized over a fairly wide
flow range with a properly designed subsonic stage.
The turbine that powers the high-pressure fuel turbopump of the
SSME employs two reaction stages, with a degree of reaction of
Copyright © 2003 Marcel Dekker, Inc.