Afterburners or reheat module generally consists of repeating the combustion process in the gases coming out of the turbine. This increases the exit stagnation pressure in the nozzle and hence an increased outlet velocity is obtained. Since the aircraft works on the second law of Newton that is the force acting upon a body is proportional to the change in momentum of the body, the increased air velocity in turn increases the thrust of the engine. The thrust obtained by using an afterburner is known as "wet thrust" and that without it is known as "dry thrust". The following report gives a brief study of the afterburner, its technicalities and applications.
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. 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.
A modern turbine engine is extremely efficient, and there is still a lot of oxygen available in the exhaust stream. The idea behind an afterburner is to inject fuel directly into the exhaust stream and burn it using this remaining oxygen. This heats and expands the exhaust gases further, and can increase the thrust of a jet engine by 50% or more.
The big advantage of an afterburner is that you can significantly increase the thrust of the engine without adding much weight or complexity to the engine. An afterburner is nothing but a set of fuel injectors, a tube and flame holder that the fuel burns in, and an adjustable nozzle. A jet engine with an afterburner needs an adjustable nozzle so that it can work both with the afterburners on and off.
The disadvantage of an afterburner is that it uses a lot of fuel for the power it generates. Therefore most planes use afterburners sparingly. For example, a military jet would use its afterburners when taking off from the short runway on an aircraft carrier, or during a high-speed maneuver in a dogfight.
Fig 1.1 . Picture showing a turbojet engine with afterburner
This includes the compressor, combustion chamber and exhaust turbine. At the exhaust end of the engine, you can see a ring of injectors for the afterburner.
Chapter 2
Principle & Design
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: it accelerates the exhaust. The fuel added to the exhaust does also add to the total mass of flow, but this effect is small compared to the increased exhaust velocity, which also helps to increase thrust.
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. The afterburner subsequently injects fuel downstream of the turbine and reheats the gases. (Thus the more correct name from a thermodynamic standpoint is reheated.) 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).
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).
Chapter 3
Flame Stabilization
The two basic designs are the "straight through" which is similar to full sized aircraft afterburners , and the "dump" type .The "straight thru" type, when built in our small sizes will require a fair bit of work ,and will be rather awkward to make, and I'd doubt it would give any better performance than the simpler dump type . The "dump" type is probably the easiest to make, but will still need to conform to certain parameters to work.
Generally, whether "straight through" or "step", an afterburner needs to be able to do two things: provide a method that ensures proper mixing of the fuel and air and also provide a place where this fuel/air mixture can burn "safely" without flame-outs, i.e., a "quite zone". "Dump" style afterburners use a step up in diameter to create their quiet zone, straight through afterburners use more complicated "V" shaped flame holders to create theirs. For mixing, the "dump" style afterburner will use the space provided by shorter, smaller diameter connecting pipe from the turbo's exducer to the larger main body of the afterburner.
Fig 4.1 Diagram of V-shaped flame holder for afterburner.
As already mentioned, our "dump" style afterburner's size needs to be greater in size than a "standard" jet pipe to allow time and space for combustion and expansion of the
gases, whilst still maintaining gas velocities slow enough for combustion of the huge amounts of fuel required by the afterburner. The "step" in the afterburner as a result of it's
increased diameter also creates our "quiet corner", where there are aerodynamic "whirlpools" that provide a convenient place for the fuel/air mixture to maintain combustion in a localized relatively slow moving airstream.
The other requirement of our afterburner is to enable thorough mixing of the fuel and air. The "short" exducer diameter piece of pipe joining the A/B body to the turbo scroll outlet will need to be long enough to not only mount
TOT and jet pipe total pressure pitot gauges ( for tuning), but also long enough to allow good mixing of fuel and gases PRIOR to them reaching "the dump", because the dump needs to have a combustible mix to maintain a flame in the recirculation created at the dump. If the fuel is added too late in the transition tube it might not be able to create that
Flame at the dump, with a "difficult" A/B the result. The length of the small diametered
Pipe will depend on speed/quality of the mixing of fuel/air to create a combustible mix by the time it reaches the "dump" or V shaped flame holders, but could be anything from a couple of inches to several inches in length.
There are endless ways to introduce fuel into the 'Short' exducer pipe. Whatever will give a good, even and quick mix of fuel and air will do, if a single point distribution it'll need to be able to give reasonably fine atomization, something much easily achieved with multiple injection points equally spaced around the tube. Single point systems could be either fuel sprayed at low pressure into the centre drill hole of the exducers central boss, which will cause the fuel to be centrifuged off in a fine spray into the exhaust gases from the turbine, or a higher pressure "spray nozzle??" a bit further downstream where the spray can't contact the hot turbine blades, (thermal stresses, possible damage and destruction ), upstream or downstream orientations have been tried.
Chapter 4
Shock Diamond
Shock diamonds (also known as Mach diamonds, Mach disks or dancing diamonds) is a formation of stationary wave patterns that appears in the exhaust plume of an aerospace propulsion system, such as a supersonic jet engine, rocket, ramjet, or scramjet when it is operated in an atmosphere.
Shock diamonds are formed when the supersonic exhaust from a nozzle is slightly over or under-expanded, meaning that the pressure of the gases exiting the nozzle is different from the ambient air pressure. A complex flow field results as the shock wave is reflected back and forth between the free fluid jet boundary and a visible repeating diamond-shaped pattern is formed which gives the shock diamonds their name.
Fig 4.1 Picture showing the shock diamonds formed out of a jet nozzle
Conclusion
Afterburners were first used in the British power jets W 2/700 named as reheat jet pipes. It was then developed to be mostly used in military aircrafts for maneuvering and gaining speeds especially at the time of aerial combat.
The usage of afterburners in conventional aircrafts was not done due to its large fuel consumption which would make the aircraft uneconomical. Yet, it was installed in the Concorde and the Tu-144 supersonic passenger aircrafts to overcome the large amount of drag produced during the transonic regime.
Since then, a number of concepts are beings brought out in order to reduce the loses due to friction and kinetic energy and make the afterburner more efficient.
