Pulse Detonation Engine
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Joined: Dec 2008
20-09-2008, 10:58 PM
A pulse detonation engine is an unsteady propulsive device in which the combustion chamber is periodically filled with a reactive gas mixture, a detonation is initiated, the detonation propagates through the chamber, and the product gases are exhausted. The high pressures and resultant momentum flux out of the chamber generate thrust.
Quasi-steady thrust levels can be achieved by repeating this cycle at relatively high frequency and/or using more than one combustion chamber operating out of phase.A pulse detonation engine has a detonation chamber with a sidewall. At least two fuel ports are located in the sidewall, spaced longitudinally apart from each other. An oxygen fuel mixture is introduced into the forward port and detonated. This creates a detonation wave which propagates with an air fuel mixture introduced into the rearward fuel port.
After the detonation, purge air passes through the chamber before the next detonation. A rotating sleeve valve mounted around the detonation opens and closes the fuel ports as well the purge ports.One of the newest and most exciting areas of pulse-jet development is the Pulse Detonation Engine (PDE). While they work on similar principles to a regular pulsejet, the PDE has one very fundamental difference -- it detonates the air/fuel mixture rather than just allowing it to simply deflagrate (burn vigorously). The exact details on many of the PDE designs currently being developed are rather sketchy -- mainly because they have the potential to be extremely valuable so most of companies researching in this field are not about to tell us what they're doing. It seems that nobody yet has the PDE developed to the point of being a practical propulsion device (or at least if they have, they're not telling anyone). From what I've been able to gather, the main focus is currently being placed on researching and improving the detonation process.
The current generation of PDEs doesn?t seem capable of continuous running for any length of time -- they're more or less just single-shot devices requiring several seconds to recharge between detonations. Many developers have high hopes that the PDE will ultimately become the most cost-effective method of propelling supersonic sub-orbital craft. The ultra-high compressions obtained by detonation offer the potential for much better fuel-efficiency than even the best turbojet, and the fact that they are an air-breathing engine reduces the fuel-load and increases safety when compared to rocket motors.
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Joined: Feb 2012
13-03-2012, 02:49 PM
Pulse Detonation Engine
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This project and implimentation investigated the ability to split and utilize a propagating detonation wave as both an ignition source and a thrust producer. The resulting hardware could be directly employed in ignition system design. The research is aimed toward practical application, and therefore investigates using commercially available components rather than design optimization. Though system level effects were addressed in this work, the focus was on successful proof of concept.
The Air Force Research Laboratory Propulsion Directorate, Turbine Engine Division, Combustion Sciences Branch at Wright-Patterson AFB, Ohio, sponsored this research. All testing was conducted in the D-Bay test cell of Building 71 at Wright-Patterson AFB.
A Pulse Detonation Engine, PDE, is a tube, filled with a combustible mixture, closed at one end, and ignited. The high pressure behind the detonation wave against the closed end of the tube and the rapid expulsion of products out the open end produces thrust. Fig. 1.1 shows the test PDE located in Building 71 at Wright-Patterson AFB. Although the photographed configuration has four thrust tubes, testing for this project and implimentation used one or two thrust tubes. The expelled flames visible in Fig. 1.1 are a result of detonation combustion
Due to the high temperatures and harsh vibrations, the integration of components and systems into a PDE has posed new challenges. One example is the ignition system. Using spark plugs for ignition was convenient for small scale testing at low frequencies. Larger scale testing and practical systems could require frequencies on the order of 100 Hertz for long durations. These requirements and the relative complexity of a multi-tube engine required a sophisticated ignition system that could endure this punishing environment.
Zeldovich, von Neumann, Döring (ZND) wave model
The 1-D control volume analysis only incorporated part of the physical structure of a detonation wave. In the 1940’s Zeldovich, von Neumann, and Döring independently proposed modeling a detonation wave as a shock wave followed by combustion (Turns 2000:613). This simple structure was named the ZND detonation wave after these three individuals. Though this simplified the actual structure, it closely modeled the observed pressure trace produced as a detonation wave passed a pressure transducer.
Dimensional detonation wave structure
As noted throughout this discussion, the actual detonation mechanism and structure is quite complicated. A realistic understanding of results can only be discussed after considering detonation development and fully dimensioned structure. The actual structure involved a fully dimensioned process that included thermo-chemistry and multiple shock interaction. “According to Strehlow, the first evidence of multidimensional wave structure was obtained in 1926 by Campbell and Woodhead” (Kuo, 1986:263). They noted the non-steady and 3-dimensional nature of detonations. Denisov and Troshin in 1959 were the first to capture the visible cell pattern that defined detonation passage. They coated the interior wall with soot that collected the record of the passing detonation. Example photos of smoked-foil records are available in Kuo (Kuo 1986:264-265) The physics of this pattern is the intersection of Mach-stem, reflected, and incident shock waves. At this intersection, called the triple point, the heightened energy level prompts ignition.
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