Afterburning is a method of increasing the thrust of a jet engine for short periods of time in order to improve the aircraft take-off, climb, or combat performance.
An increase in power for jet turbines could be accomplished by using a larger engine, but due to an increase in weight, frontal area, and overall fuel consumption, afterburning provides the best method of thrust augmentation for short periods of time.
The fuel in a turbine engine burns in an excess amount of air, providing sufficient amounts of oxygen to support further combustion. Because of this, it is possible to inject additional fuel for burning downstream of the turbine, increasing the overall thrust of the jet engine.
Since the temperature of an afterburner can reach 1700 deg. C, the flame is usually concentrated around the jet pipe axis, allowing a portion of the discharge gas to flow along the wall of the jet pipe and therefore maintain a safe wall temperature.
An afterburning jet pipe is larger than a normal jet pipe would be so as to obtain a reduced velocity gas stream. Therefore, an afterburning jet pipe is equipped with either a two-position or a variable nozzle to provide for operation under all conditions. The nozzle will open or close to give an exit area suitable for the volume of the gas stream, which prevents any pressure increase from occuring in the jet pipe.
Afterburning is acheived on low by-pass engines by mixing the by-pass and turbine streams before the injection of the afterburner fuel. With this, combustion takes place in the mixed exhaust stream. Another method is to inject the fuel and stabilize the flame individually in the by-pass and turbine streams, burning the available gases to a common exit temperature at the nozzle's exit.
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