Handbook of Turbomachinery Second Edition Revised - Ventech!

using hydrocarbon fuel combustion products have suffered progressive
coking of the turbine nozzles. This gradual reduction in nozzle flow area
resulted in decreasing turbine performance. To avoid this, the turbine inlet
temperature was adjusted to move out of the range that accentuated the
coking problem. Ironically, one production turbine had its inlet temperature
lowered, while another responded favorably only with increased inlet
temperature. Finally, a brief mention of the working fluid chemical
properties is worthwhile. In the range of turbine inlet temperatures typically
specified (800–1,700 8F is representative), the mixture ratio of oxidizer to
fuel required is quite heavily biased by combustion system standards. This
could, of course, be accomplished by biasing toward either excess fuel or
excess oxidizer. Almost without exception, fuel-rich combustion products
are specified for the working fluid of rocket turbines. There are two main
reasons for this: first, the chemistry of fuel-rich gases, is typically favorable
while that of oxidizer-rich gases can be quite adverse; and second, turbine
mass flow requirements are normally lower with fuel-rich gases since they
tend to have lower molecular weight than equivalent oxidizer-rich gas. It
should be noted, however, that these conditions scarcely prohibit the use of
oxidizer-rich gases for turbine drive with proper attention to detail. Several
engines operating with this type of turbine working fluid have been in use in
other countries for many years. These engines have been developed to a high
degree of refinement.
Engines that operate on the expander cycle typically use the engine’s
fuel (and/or oxidizer) heated in the combustion chamber cooling circuit for
turbine drive. The success of this arrangement depends on the combustion
chamber coolant fluid undergoing an adequate enthalpy increase in the
chamber cooling process to satisfy turbine energy requirements. For this
reason, the successful operation of this cycle can be thought of as a
significant technical accomplishment. Fluids used in this application to date
include supercritical hydrogen and oxygen.
Given the turbine working fluid and its properties, we next discuss
specification of the turbine inlet and exit pressures. Monopropellant turbine
drive cycles probably have the greatest flexibility in this area, since the
turbine working fluid is contained in an independent system. In these
applications, turbine inlet pressure is determined by the allowable
monopropellant tank pressure, while accounting for the losses between
the monopropellant tank and the turbine inlet. Also, since the turbine fluid
is exhausted to atmosphere in this cycle, high turbine pressure ratios are
possible. The most recent example of such a system in a production rocket
engine in the United States is the Redstone engine, built by the Rocketdyne
Division of Rockwell International (then North American Aviation Inc.).
This system employed a two-row velocity-compounded impulse turbine
Copyright © 2003 Marcel Dekker, Inc.