THIS SLIDE PROVIDE THE INFORMATION ABOUT " AFTERBURNEFR" which is used in jet this slide is useful for the engineering student ; and i hope that you enjoy this slide

1. Afterburner
An afterburner (or a reheat) is an additional component present on some jet engines, mostly
military supersonic aircraft. Its purpose is to provide an increase in thrust, usually for
supersonic flight, takeoff and for combat situations. Afterburning is achieved by injecting
additional fuel into the jet pipe downstream of (i.e. after) the turbine.
Advantage of afterburning is significantly increased thrust
disadvantage is its very high fuel consumption
and inefficiency, though this is often regarded as acceptable for the short periods during which
it is usually used. Pilots can activate and deactivate afterburners in-
flight, and jet engines are referred to as operating wet when afterburning is being used
and dry when not.[1] An engine producing maximum thrust wet is at maximum power, while an
engine producing maximum thrust dry is at military power.
Principle
Jet-engine thrust is governed by the general principle of mass flow rate. Thrust depends on two
things: the
velocity of the exhaust gas and the mass of that gas. A jet engine can produce more thrust by eit
her
accelerating the gas to a higher velocity or by having a greater mass of gas exit the engine. Desig
ning a
basic turbojet engine around the second principle produces the turbofan engine, which creates
slower gas
but more of it. Turbofans are highly fuel efficient and can deliver high thrust for long periods, bu
t the design trade-
off is a large size relative to the power output. To generate increased power with a more
compact engine for short periods, an engine requires an afterburner. The afterburner increases
thrust
primarily by accelerating the exhaust gas to a higher velocity. While the mass of the fuel added t
o the
exhaust does contribute to an increase in exhaust mass, this effect is small compared to the incr
ease in exhaust velocity. The temperature of the gas in the engine is highest just before the
turbine, and the ability for the turbine to withstand these
temperatures is one of the primary restrictions on total dry engine
thrust. This temperature is known as the Turbine Entry Temperature
(TET), one of the critical engine operating parameters. Because a
combustion rate high enough to consume all the intaken oxygen
would create temperatures high enough to overheat the turbine, the

2. flow of fuel must be restricted to an extent that fuel rather than
oxygen becomes the limiting factor in the reaction, leaving some
oxygen to flow past the turbine. After passing the turbine, the gas
expands at a near constant entropy, thus losing temperature.[2] The
afterburner then injects fuel downstream of the turbine and reheats
the gas. In conjunction with the added heat, the pressure rises in the
tailpipe and the gas is ejected through the nozzle at a higher
velocity. The mass flow is also slightly increased by the addition of the fuel.
Afterburners do produce markedly enhanced thrust as well as
(typically) a very large flame at the back of the engine. This exhaust
flame may show shock diamonds, which are caused by shock waves
formed due to slight differences between ambient pressure and the
exhaust pressure. These imbalances cause oscillations in the exhaust
jet diameter over distance and cause the visible banding where the
pressure and temperature is highest.
Plenum chamber burning
A similar type of thrust augmentation but using additional fuel burnt
in a turbofan's cold bypass air only, instead of the combined cold
and hot gas flows as in a conventional afterburning engine, is
Plenum chamber burning (PCB), developed for the vectored thrust
Bristol Siddeley BS100 engine for the Hawker Siddeley P.1154. In
this engine, where the cold bypass and hot core turbine airflows are
split between two sets of nozzles, front and rear, in the same manner as the Rolls-
Royce Pegasus, additional fuel and afterburning was
applied to the front cold air nozzles only. This technique was
developed to give greater thrust for takeoff and supersonic
performance in an aircraft similar to, but of higher weight, than the Hawker Siddeley Harrier.
Design
A jet engine afterburner is an extended exhaust section containing
extra fuel injectors. Since the jet engine upstream (i.e., before the
turbine) will use little of the oxygen it ingests, additional fuel can be
burned after the gas flow has left the turbines. When the afterburner
is turned on, fuel is injected and igniters are fired. The resulting
combustion process increases the afterburner exit (nozzle entry)
temperature significantly, resulting in a steep increase in engine net
thrust. In addition to the increase in afterburner exit stagnation
temperature, there is also an increase in nozzle mass flow (i.e.
afterburner entry mass flow plus the effective afterburner fuel flow),

3. but a decrease in afterburner exit stagnation pressure (owing to a
fundamental loss due to heating plus friction and turbulence losses).
The resulting increase in afterburner exit volume flow is
accommodated by increasing the throat area of the propulsion
nozzle. Otherwise, the upstream turbomachinery rematches
(probably causing a compressor stall or fan surge in a turbofan
application). The first designs, e.g. Solar afterburners used on the F7U Cutlass, F-
94 Starfire and F89 Scorpion, had 2position eyelid nozzles.
[4] Modern designs incorporate not only VG nozzles but multiple stages of augmentation via
separate spray bars.
To a first order, the gross thrust ratio (afterburning/dry) is directly proportional to the root of th
e stagnation temperature ratio across the afterburner (i.e. exit/entry)
Limitations
Due to their high fuel consumption, afterburners are usually used as little as possible; a notable
exception is the Pratt & Whitney J58 engine used in the SR-
71 Blackbird. Afterburners are generally used only when it
is important to have as much thrust as possible. This includes during takeoffs from short runway
s, assisting catapult launches from aircraft carriers and during air combat situations. Efficiency
Main article: Propulsive efficiency
In heat engines such as jet engines, efficiency is best when combustion is done at the highest pr
essure and temperature possible, and expanded down to ambient pressure (see Carnot cycle).
Since the exhaust gas already has reduced oxygen due to previous combustion, and since the fu
el is not
burning in a highly compressed air column, the afterburner is generally inefficient compared wit
h the main
combustor. Afterburner efficiency also declines significantly if, as is usually the case, the inlet an
d tailpipe pressure decreases with increasing altitude.
This limitation only applies to turbojets. However, in a military turbofan combat engine the bypa
ss air
serves to cool the turbine blades and is added into the exhaust, hence, increasing the core and a
fterburner
efficiency. For turbojets the gain is limited to 50%, while it depends on the bypass ratio in a turb
ofan and can be as much as 70%.[5] However, as a counterexample, the SR-
71 had reasonable efficiency at high altitude in afterburning mode
("wet") due to its high speed (mach 3.2) and hence high pressure due to ram intake
Influence on cycle choice
Afterburning has a significant influence upon engine cycle choice.

4. Lowering fan pressure ratio decreases specific thrust (both dry and wet afterburning), but result
s in a lower
temperature entering the afterburner. Since the afterburning exit temperature is effectively fixe
d, the
temperature rise across the unit increases, raising the afterburner fuel flow. The total fuel flow t
ends to
increase faster than the net thrust, resulting in a higher specific fuel consumption (SFC). Howeve
r, the
corresponding dry power SFC improves (i.e. lower specific thrust). The high temperature ratio ac
ross the afterburner results in a good thrust boost.
If the aircraft burns a large percentage of its fuel with the afterburner alight, it pays to select an
engine cycle
with a high specific thrust (i.e. high fan pressure ratio/low bypass ratio). The resulting engine is r
elatively fuel efficient with afterburning (i.e. Combat/Take-
off), but thirsty in dry power. If, however, the afterburner
is to be hardly used, a low specific thrust (low fan pressure ratio/high bypass ratio) cycle will be f
avored. Such an engine has a good dry SFC, but a poor afterburning SFC at Combat/Takeoff.
Often the engine designer is faced with a compromise between these two extremes.