naca-rm-e50i07
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National Advisory Committee for Aeronautics, Research Memorandum - Turbojet Combustor Efficiency at High Altitudes
NACA research on the single problem of combustion efficiency of
turbojet engines at high altitudes is discussed. Representative results
of investigations with turbojet combustors are presented to illustrate
the trends obtained with the following categories of variables: (1)
combustor operating variables, (2) combustor—design variables, and (5)
fuel variables.
The data indicated that as the environment of the combustor becomes
one of low pressure and low temperature at high altitude, low combustion
efficiencies and limited values of obtainable temperature rise were
encountered. Increased cross-sectional area of combustor for a given
weight flow of air decreased velocities and facilitated high combustion
efficiency at altitude. For the design of the liner, increased volume
in the flame zone and gradual admission of the air into the combustion
space were shown to aid-combustion by helping to provide localized fuel-
air mixtures of correct composition.that exist sufficiently long for
ignition and combustion to occur. For high combustion efficiency to
occur, all the fuel must be involved in this manner, and it was shown
that the combustor design, the fuel injection, and the fuel Volatility
must be matched if optimum combustion efficiency is to be achieved; in
general, combustion efficiency decreased with increased fuel boiling
temperatures. Further, it was shown that fuels of higher flame speed
and/or lower ignition.temperatures gave higher combustion efficiency
when other properties were similar.
The value of an aircraft propulsion system is measured in terms
of the reliable delivery of required thrust for a minimum fuel-cons
sumption rate, engine weight, engine frontal area, and engine cost
for a range of speeds and operating altitudes. For turbojet engines,
these factors impose many requirements on the combustion chamber; the
principal factors are stability and reliability of operation, high
combustion efficiency, small volume or size, minimum pressure drop,
minimum weight, a preferred pattern of outlet—temperature distribu-
tion, strength and durability, ease of ignition, ability to utilize a
variety of fuels, and ease of manufacture.
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