A pulse detonation engine, or "PDE", is a type of propulsion system that has the potential to be both light and powerful and can operate from a standstill up to supersonic speeds. To date no practical PDE engine has been put into production, but several testbed engines have been built, proving the basic concept to some extent at least. In theory the design can produce an engine with an efficiency far surpassing more complex gas turbine Brayton cycle engines, but with almost no moving parts.
All regular jet engines and most rocket engines operate on the deflagration of fuel, that is, the rapid but subsonic combustion of fuel. The pulse detonation engine is a concept currently in active development to create a jet engine that operates on the supersonic detonation of fuel.
The basic operation of the PDE is similar to that of the pulse jet engine; air is mixed with fuel to create a flammable mixture that is then ignited. The resulting combustion greatly increases the pressure of the mixture to approximately 100 atmospheres, which then expands through a nozzle for thrust. To ensure that the mixture exits to the rear, thereby pushing the aircraft forward, a series of shutters are used with careful tuning of the inlet to force the air to travel in one direction only through the engine.
The main difference between a PDE and a traditional pulsejet is that the mixture does not undergo subsonic combustion but instead, supersonic detonation. In the PDE, the oxygen and fuel combination process is supersonic, effectively an explosion instead of burning. The other difference is that the shutters are replaced by more sophisticated valves.
Pulse detonation engine
Rocket engines that work much like an automobile engine are being developed at NASA’s Marshall Space Flight Center in Huntsville, Ala. Pulse detonation rocket engines offer a lightweight, low-cost alternative for space transportation. Pulse detonation rocket engine technology is being developed for upper stages that boost satellites to higher orbits. The advanced propulsion technology could also be used for lunar and planetary Landers and excursion vehicles that require throttle control for gentle landings.
The engine operates on pulses, so controllers could dial in the frequency of the detonation in the "digital" engine to determine thrust. Pulse detonation rocket engines operate by injecting propellants into long cylinders that are open on one end and closed on the other. When gas fills a cylinder, an igniter—such as a spark plug—is activated. Fuel begins to burn and rapidly transitions to a detonation, or powered shock. The shock wave travels through the cylinder at 10 times the speed of sound, so combustion is completed before the gas has time to expand. The explosive pressure of the detonation pushes the exhaust out the open end of the cylinder, providing thrust to the vehicle.
A major advantage is that pulse detonation rocket engines boost the fuel and oxidizer to extremely high pressure without a turbo pump—an expensive part of conventional rocket engines. In a typical rocket engine, complex turbo pumps must push fuel and oxidizer into the engine chamber at an extremely high pressure of about 2,000 pounds per square inch or the fuel is blown back out.
The pulse mode of pulse detonation rocket engines allows the fuel to be injected at a low pressure of about 200 pounds per square inch. Marshall Engineers and industry partners United Technology Research Corp. of Tullahoma, Tenn. and Adroit Systems Inc. of Seattle have built small-scale pulse detonation rocket engines for ground testing. During about two years of laboratory testing, researchers have demonstrated that hydrogen and oxygen can be injected into a chamber and detonated more than 100 times per second.
NASA and its industry partners have also proven that a pulse detonation rocket engine can provide thrust in the vacuum of space. Technology development now focuses on determining how to ignite the engine in space, proving that sufficient amounts of fuel can flow through the cylinder to provide superior engine performance, and developing computer code and standards to reliably design and predict performance of the new breed of engines.
A developmental, flight-like engine could be ready for demonstration by 2005 and a full-scale, operational engine could be finished about four years later. Manufacturing pulse detonation rocket engines is simple and inexpensive. Engine valves, for instance, would likely be a sophisticated version of automobile fuel injectors. Pulse detonation rocket engine technology is one of many propulsion alternatives being developed by the Marshall Center’s Advanced Space Transportation Program to dramatically reduce the cost of space transportation.
2. DIFFERENCES COMPARED TO OTHER ENGINE TYPES
The main differences between the PDE and the Otto engine is that in the PDE the combustion chamber is open and no piston is used to com- press the mixture prior to ignition (and also that no shaft work is extracted).
Instead the compression is an integral part of the detonation, and two of the main advantages of the PDE - the efficiency and simplicity - can be explained by the fact that the combustion occurs in detonative mode. The efficiency of the cycle can be explained by the high level of precompression due to the strong shock wave in the detonation.
Also, the simplicity of the device is a result of the fact that the shock wave - responsible for this compression – is an integrated part of the detonation. Therefore, pre-compression through mechanical devices (e.g., a piston) is not necessary. In this sense the PDE is similar to both the pulse-jet (e.g., the engine used for propulsion of the V-1) and the ram jet engine. But in those two cases the mechanism behind the pre-compression is completely different:
For the pulse-jet the pre-compression is a result of momentum effects of the gases, and is a part of the resonance effects of the engine. The resonance effects are influenced strongly by the external conditions of the engine, and the thrust is drastically reduced at higher speeds (approaching speed of sound). Furthermore, both the specific impulse and the specific thrust are significantly lower than for turbo-jet or turbo-fan engines. This is due to the fact that the levels of preconditioning that can be obtained through the resonance effects are rather low.
In the ramjet, pre-compression is obtained through the ram effects as the air is decelerated from supersonic to subsonic. The major drawback with this concept is that the engine is ineffective for speeds lower than around Ma=2. Spiral (used to enhance the transition from flame to detonation), spark plug and central body
2.1 EXPERIMENTAL SET-UP
One example of a PDE is shown in Fig 1. This particular engine - which was assembled at one of FOI's (the Swedish Defence Research Agency) departments, Warheads and Propulsion - runs on hydrogen and air and is capable of reaching frequencies up to 40~Hz. The experimental set up is rather simple, basically consisting of a straight tube (in this case with a length of about one metre) in which
hydrogen and air is injected, and ignited by an ordinary spark plug. In this experimental engine, the pressure transducers are only used to find out whether the engine operates successfully in detonative mode.
This can be seen both by the level of pressure and the speed of propagation of the wave (a detonation in hydrogen air reaches pressures over 20 bar and propagates at around 2,000 m/s). That is, the pressure transducers are used just for the experiments and are not necessary for the operation of the engine.
Also shown is a spiral, which, since it helps to induce turbulence in the flow field is known to speed up the transition from flame to detonation. The hydrogen enters the engine through twelve holes of 1 mm. diameter at the edge of a 72 mm. diameter disk at the right end of the engine. The air enters between the central body through which the hydrogen is emerging and the interior walls of the tube.
3 PRE-COMPRESSION AND DETONATION
In the PDE the pre-compression is instead a result of interactions between the combustion and gas dynamic effects, i.e. the combustion is driving the shock wave, and the shock wave (through the increase in temperature across it) is necessary for the fast combustion to occur. In general, detonations are extremely complex phenomena, involving forward propagating as well as transversal shock waves, connected more or less tightly to the combustion complex during the propagation of the entity.
The biggest obstacles involved in the realization of an air breathing PDE are the initiation of the detonation and the high frequency by which the detonations have to be repeated. Of these two obstacles the initiation of the detonation is believed to be of a more fundamental character, since all physical events involved regarding the initiation are not thorough- ly understood. The detonation can be initiated in two ways; as a direct initiation where the detonation is initiated by a very powerful ignitor more or less immediately or as a Deflagration to Detonation Transition (DDT) where an ordinary flame (i.e. a deflagration) accelerates to a detonation in a much longer time span.
Basic Features of PDE
Application of the detonation into jet propulsion combustion chamber offers significant advantages over the conventional solution with deflagrative combustion mode.
In the detonation chamber, combustion occurs at a very small distance and the combustion chamber is compact. Since the pressure in detonation is increasing, the thermal efficiency of the system is also increasing.
At the present stage, the development of the PDE is carried out in many countries including:, China, France, Japan, Poland, Russia, USA and other. The experiments were conducted for all kind of possible configuration of the PDE . It is just only matter of time that the PDE will be utilized in the majority of jet propulsion systems.
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