Afterburner
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| A U.S. Navy F/A-18 Hornet being launched from the catapult on full afterburner |
An afterburner (or reheat) is an additional component added to some jet engines, primarily those on military supersonic aircraft. Its purpose is to provide a temporary increase in thrust, both for supersonic flight and for takeoff (as the high wing loading typical of supersonic aircraft designs means that take-off speed is very high). On military aircraft the extra thrust is also useful for combat situations. This is achieved by injecting additional fuel into the jet pipe downstream of (i.e. after) the turbine. The advantage of afterburning is significantly increased thrust; the 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.
Jet engines are referred to as operating wet when afterburning is being used and dry when the engine is used without afterburning.[1] An engine producing maximum thrust wet is at maximum power (this is the maximum power the engine can produce); an engine producing maximum thrust dry is at military power.
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Principle
Jet engine thrust is governed by the general principle of mass flow rate. Simply put, thrust depends on two things: first, the velocity of the exhaust gases; second, the mass of those gases. A jet engine can produce more thrust by either accelerating the gas to a higher velocity or by having a greater mass (quantity) of gas. In the case of a basic turbojet, focusing on the second principle produces the turbofan, which creates slower gas but more of it. Turbofans are efficient and can deliver high thrust for long periods of time but have large sizes for unit power. To create the same power in a compact engine for short periods of time, an engine requires an afterburner. The afterburner increases thrust primarily by the first method. While the mass of the fuel added to the exhaust does contribute to an increase in thrust, this effect is small compared to the increase in exhaust velocity.
The temperature of the gas in the engine is highest just before the turbine, known as the TIT (Turbine Inlet Temperature), one of the critical engine operating parameters. Then while the gas passes the turbine, it expands at a near constant entropy, thus losing temperature.[2] The afterburner subsequently injects fuel downstream of the turbine and reheats the gases. (Thus the more correct name from a thermodynamic standpoint is reheat.) In conjunction with the added heat, the pressure rises in the tailpipe and the gases are ejected through the nozzle at a higher velocity while the mass flow is only slightly increased (by the mass flow of the added fuel).
Design
A jet engine afterburner is an extended exhaust section containing extra fuel injectors, and since the jet engine upstream (i.e., before the turbine) will use little of the oxygen it ingests, the afterburner is, at its simplest, a type of ramjet. When the afterburner is turned on, fuel is injected, which ignites readily, owing to the relatively high temperature of the incoming gases. 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), 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).
To a first order, the gross thrust ratio (afterburning/dry) is directly proportional to the root of the stagnation temperature ratio across the afterburner (i.e. exit/entry).
Limitations
Due to their high fuel consumption, afterburners are not used for extended periods; 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 takeoffs from short runways (as on an aircraft carrier) and air combat situations.
Efficiency
Since the exhaust gas already has reduced oxygen due to previous combustion, and since the fuel is not burning in a highly compressed air column, the afterburner is generally inefficient compared with the main combustor. Afterburner efficiency also declines significantly if, as is usually the case, the tailpipe pressure decreases with increasing altitude.
However, as a counter-example, 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 effect.
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 being 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.
Influence on cycle choice
Afterburning has a significant influence upon engine cycle choice.
Lowering fan pressure ratio decreases specific thrust (both dry and when afterburning), but results in a lower temperature entering the afterburner. Since the afterburning exit temperature is effectively fixed, the temperature rise across the unit increases, raising the afterburner fuel flow. The total fuel flow tends to increase faster than the net thrust, resulting in a higher specific fuel consumption (SFC). However, the corresponding dry power SFC improves (i.e. lower specific thrust). The high temperature ratio across 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 relatively 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 favored. Such an engine has a good dry SFC, but a poor afterburning SFC at Combat/Take-off.
Often the engine designer is faced with a compromise between these two extremes.
Usage
As early as during the Second World War, the principle was in development for the British Power Jets W.2/700 with what was termed at the time a "a reheat jetpipe" for the Miles M.52 supersonic aircraft project.
Early US research on the concept was done by NACA, in Cleveland, OH, leading to the publication of the paper "THEORETICAL INVESTIGATION OF THRUST AUGMENTATION OF TURBOJET ENGINES BY TAIL-PIPE BURNING" in January 1947[3].
Post war, the McDonnell F3H Demon and the Douglas F4D Skyray were designed around the Westinghouse J40 turbojet engine, rated at 8,000 lbf (36 kN) thrust without afterburner. The new Pratt & Whitney J48 turbojet, at 8,000 lbf (36 kN) thrust with afterburner, would power the Grumman sweptwing fighter F9F-6, which was about to go into production. Other new Navy fighters with afterburners included the high-speed Chance Vought F7V-3 Cutlass, powered by two 6,000 lbf (27 kN) thrust Westinghouse J46 engines.
In the 1950s several large reheated engines were developed such as the de Havilland Gyron and Orenda Iroquois. In the United Kingdom, the Rolls-Royce Avon was made available with reheat and powered the English Electric Lightning, the first supersonic aircraft in RAF service. The Bristol-Siddeley Rolls-Royce Olympus was also given reheat for the TSR-2 and was fitted to Concorde in such a state (Bristol Siddeley had by then become part of Rolls-Royce and the nozzle and reheat system was developed by Snecma).
Afterburners are generally only used in military aircraft and are considered standard equipment for fighter aircraft. The handful of civilian planes that have used them include some NASA research aircraft, the Tupolev Tu-144 and Concorde, and the White Knight of Scaled Composites. Concorde and the Tu-144 had this capability and flew long distances at supersonic speeds. This would be impossible with the high fuel consumption of reheat, and these aircraft used afterburners at takeoff and to minimise time spent in the high drag transonic flight regime. Supersonic flight without afterburners is referred to as supercruise.
A turbojet engine equipped with an afterburner is called an "afterburning turbojet", whereas a turbofan engine similarly equipped is sometimes called an "augmented turbofan".
A "dump-and-burn" is a fuel dumping procedure where dumped fuel is intentionally ignited using the plane's afterburner. A spectacular flame combined with high speed makes this a popular display for airshows, or as a finale to fireworks, although the procedure itself is intended primarily to safely remove fuel from the aircraft before attempting a crash landing.
See also
- Ramjet
- Supercruise
- Bristol Siddeley BS100, an engine which was intended to use Plenum Chamber Burning, similar but not identical to an afterburner.
References
- ↑ Ronald D. Flack (2005). Fundamentals of jet propulsion with applications. Cambridge, UK: Cambridge University Press. ISBN 0-521-81983-0. http://books.google.com/books?id=MLlmJSRUY50C&pg=PA19&dq=afterburner+wet+dry&lr=lang_en&num=50&as_brr=3&ei=V55XSO-CM6OQjgGHq6yFDA&sig=-HazcCz3q21cDq-lYC-VBj62MGo=.
- ↑ Cengel YA and Boles MA,Thermodynamics - an engineering approach, McGraw Hill, 2006
- ↑ Theoretical investigation of thrust augmentation of turbojet engines by tail-pipe burning, Bohanon, H R; Wilcox, E C
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