This paper discusses NACA research on the single problem of combustion efficiency of turbojet engines at high altitudes. Representative results of investigations with turbojet combustors are presented to illustrate the trends obtained with the following categories of variables: (a) Combustor operating variables, (b) combustor design variables, and (c) fuel variables. The data indicate 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 are 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 that localized fuel-air mixtures of correct composition exist sufficiently long for ignition and combustion to occur. For high combustion efficiency to occur, all of 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. Further, it was shown that, for a given combustor, fuel injector, and fuel volatility, fuels of higher flame speed and/or lower ignition temperatures gave higher combustion efficiency at high-altitude operating conditions.

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