Analytical Investigation of Fuel-Cooled Turbine Blades With Return-Flow Type of Finned Coolant Passages Page: 3 of 38
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2 CONFIDENTIAL NACA RM E57D03
turbine blade cooling because the cooling-air temperature is directly
dependent on the flight speed and altitude. It is desirable, therefore,
to find suitable methods of reducing the cooling-air temperature. This
temperature can be reduced by (1) rejecting heat to a colder fluid by
means of a heat exchanger, (2) using refrigeration systems such as those
discussed in reference 3, or (3) spraying water into cooling air to make
use of the latent heat of vaporization of the water. The specific liq-
uid consumption required for the engine is increased, of course, if
water is sprayed into the cooling air. Also, it may be difficult to
provide a refrigeration cycle that is much better than using a heat ex-
changer alone (ref. 3). Fuels such as liquid hydrogen or liquid methane
provide an excellent heat sink, so that a heat exchanger could be used
with the fuel as the receiver. This system, as well as the other sys-
tems mentioned, however, results in added engine weight. Therefore,.
the use of the fuel as a coolant directly would be more desirable and
would thus eliminate the weight and bulkiness of a heat exchanger.
There are two other important advantages to using fuel as a coolant.
First, this type of turbine-cooling system is free from flight Mach
number limitations because the fuel-coolant inlet temperature is largely
independent of flight speed and altitude if the fuel tanks are insulated.
Second, with regard to engine performance, rejecting the heat absorbed
by the fuel coolant into the engine compressor-discharge air in the com-
bustor is probably the most efficient turbine-cooling method known.
The effects on engine performance are shown in reference 4 for a simi-
lar case where heat is rejected at the compressor exit for liquid-
cooled turbines.
The use of fuel as a turbine blade coolant requires a blade that
does not discharge the coolant into the gas stream but, instead, has a
return-flow path in the blade so that the coolant (fuel) can be ducted
to the engine burners after it cools the turbine blades. The feasibil-
ity of such a blade was investigated analytically with hydrogen as the
coolant and is reported in reference 5. The blade configuration used
(ref. 5) was about the simplest configuration that could be conceived
for a return-flow blade. Other configurations could undoubtedly make
more-effective use of the coolant.
Other factors that must be considered when fuel is used as a cool-
ant are the stability of the fuel and any effects that the fuel may
have on the turbine blade materials. Methane is one of the most stable
of all the hydrocarbons; however, it does start to decompose at a tem-
perature of 15300 R (8500 K) (ref. 6). The bulk temperature of the
methane would probably never reach this temperature, and the residence
times would be very small for flow through a turbine blade. Fuel sta-
bility, therefore, will probably not be a problem, but experimental
evaluation of the problem would undoubtedly be required. Using hydro-
gen as a coolant could possibly cause hydrogen embrittlement in turbine
blades of some materials. With the proper material selection, this needCONFIDENTIAL
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Nachtigall, Alfred J. & Slone, Henry O. Analytical Investigation of Fuel-Cooled Turbine Blades With Return-Flow Type of Finned Coolant Passages, report, June 26, 1957; (https://digital.library.unt.edu/ark:/67531/metadc52841/m1/3/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.