3. Introduction:
Afterburning or reheating is one of the method of
augmenting the basic thrust of the turbojet engine or the
turbofan engine, when required, without any increase in
frontal area and weight.
As fuel-air ratio is restricted to 0.02 ( due to material
consideration of the turbine blades), the exhaust gases
leaving the turbine are still rich in oxygen to support the
further combustion of fuel.
Additional fuel can be burn in tailpipe with suitable burners
before entering in to the nozzle.
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4. Introduction
50% increase in thrust leads to three fold increase in fuel consumption, that is why
afterburners are used only for limited duration during climb, takeoff and maximum
bursts of speed.
For turbofan engine, augmentation can be used in both fan and core streams.
Afterburning in a separate fan stream is normally referred to as duct burning, and
this alone or in combination with afterburning in the core stream may be
advantageous for certain flight conditions.
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5. Afterburner components
An afterburner will be having following components:-
Diffuser
Fuel spray bars (tubes) or rings
Igniter
Flame holders
Flame holders
Liners ( cooling liners and screech antihowl liners)
Afterburner fuel pump
Afterburner fuel control
Pressurizing valve, if multistage operation is required.
Variable-area throat exhaust nozzle.
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7. Afterburner components
Diffuser:- Flow entering the afterburner is fist slowed to a mach
number that provides a balance between the total pressure loss
and afterburner cross sectional area.
A short diffuser length is desired without producing flow
separation to reduce engine weight and length.
Fuel injection:- The goal of fuel injection system is to produce a
specified distribution of fuel vapor in the gas stream entering
the afterburner.
In most engines, fuel is introduced in a staged manner so that
heat addition rate can be increased gradually from zero to the
desired value.
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9. Ignition
Ignition of the fuel/air mixture in afterburner is usually
accomplished by using a spark or arc igniter or pilot burner.
Spark or arc igniter uses a high- energy electric arc to initiate
the combustion of the primary stream tube.
The igniter is usually placed in the wake of a sheltered flame
stabilizer having its own fuel supply.
stabilizer having its own fuel supply.
The pilot burner consists of pilot zone where a small portion of
the inlet air( usually 10% or less) is burned to stoichiometric in
an enclosed protected region.
Hot gases generated by pilot burner are used as ignition and
stabilizing source for the main fuel injection system.
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11. Flame stabilization
Two types of flame stabilizing devices that have been
used in the afterburners are Bluff body vee-gutter
flame holders and pilot burners where a small piloting
heat source is used to ignite the main Fuel flow.
The bluff body vee-gutter flame holders have the
advantage of the low flow blockage and low total
pressure loss. They are simple, light weight and have a
good development history.
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12. Afterburner liners
The afterburner liner is used as the cooling liner and to improve
the combustion stability.
A film of cooler air is distributed along the length of the cooling
liner, which reduces the metal temperature.
The liner is also used as screech or antihowl liner to prevent
The liner is also used as screech or antihowl liner to prevent
extreme high frequency and amplitude pressure fluctuations
resulting from combustion instability.
This function is accomplished by use of multiple holes along the
initial length of liner. Selective frequencies can be dampened by
the selection of the proper size hole.
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14. Variable-area exhaust nozzle
in order to get high
supersonic jet speed a
variable-area nozzle is
needed.
Usually Convergent
Usually Convergent
divergent nozzle is
preferred.
on some high
performance fighter
aircrafts we use movable
surface nozzle.
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15. Specific design requirements for an afterburner
Large temperature rise.( not limited by turbine
material)
Low dry loss.
Wide temperature modulation. ( better control over
Wide temperature modulation. ( better control over
thrust).
High combustion efficiency.
Short length: light in weight.
No acoustic combustion instabilities.
Long life, low cost and easy repair.
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