• Version
• 154 Downloads
• 1.15 MB File Size
• 1 File Count
• November 2, 2016 Create Date
• November 2, 2016 Last Updated

National Advisory Committee for Aeronautics, Report - Relation of Turbojet and Ramjet Combustion Efficiency to Second Order Reaction Kinetics and Fundamental Flame Speed

One of the most serious problems encountered in the
operation of jet-propelled aircraft is the reduction in com-
bustion efliciency that occurs at high-altitude flight condi-
tions. Experimental investigations with both turbojet and
ramjet combustors have shown that combustion efficiency is
adversely affected by the high velocities at which these
combustors are required to operate and by the low pressures
and low inlet temperatures encountered at high altitudes
(ref. 1).

A theory of the jet—engine combustion procws is
therefore needed in order to explain these effects and to
indicate the design approaches that are most promising for
alleviating these adverse efiects. This report describes the
theoretical treatment of the combustion process in turbojet
and ramjet combustors and the correlation of experimental
data on combustion efficiency with the theoretical equations.

The NACA work in this area dates back to 1950 (refs. 2 to 7).
This work was subject to security classification until 1956,
at which time much of this material was presented in
reference 8.

If the fuel were properly mixed with air and the fuel—air
mixture were allowed adequate time in the combustor,
thermodynamic equilibrium would be achieved and the
combustion efficiency would be 100 percent. The occurrence
of combustion efficiencies below 100 percent indicates that
the conversion processes by which’the chemical energy of the
fuel is converted into sensible enthalpy of the exhaust
products are not rapid enough to proceed to completion dur—
ing the residence time allowed in high-velocity combustors.

A theoretical treatment of jet-engine combustion is diffi-
cult because of the many difierent conversion processes that
must occur. The fuel must be vaporized, mixed with air,
ignited, and oxidized to the final products of combustion.
These combustion products must then be mixed with dilution
air to reduce the combustor-exit temperature to the desired
value.

The combustion can be visualized as a competition
between the conversion processes (vaporization, mixing,
ignition, and oxidation) and the quenching that occurs when
the reacting mixture is cooled by the dilution air and when
the reacting mixture contacts the relatively cool walls of
the combustor. In a ramjet combustor operating at nearly
stoichiometric fuel-air ratios, dilution-air quenching ceases
to be important; the efliciency is then determined by the
extent to which the conversion processes proceed before the
reacting mixture is swept out of the exhaust nozzle. Because
of the complicated nature of the over-all process, no exact
theoretical treatment is possible.