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| Pulsed Detonation Engines |
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The PDE is a propulsion system that has been receiving considerable interest in the last decade, due to the numerous advantages that it offers over traditional jet engines. PDEs operate in an intermittent cyclical manner, by giving rise to detonation waves that combust the fuel-oxidizer mixture within the engine, release vast amounts of energy and develop much higher pressures than a deflagration process. |
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| Figure 1: Schematic of a Turbojet Engine |
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In conventional jet engines, air is compressed and slowed down by means of a compressor and then mixed with fuel before the combustion stage, where the combustion is also a slow subsonic process. The hot products of the reaction then drive a turbine, which also drives the compressor, before being accelerated through a nozzle thereby producing thrust. The fact that the turbine and compressor are coupled means that the engine cannot start from rest by itself and requires the use of a starter to get the compressor spooled up to speed before the engine can sustain itself. Jet engines follow the Brayton cycle, which require the compression of air to high pressures before heat release is possible, thereby requiring the heavy compressor and turbine machinery.
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PDEs, on the other hand, can be operated theoretically from a stand still up to a Mach number of 5. PDEs do not require the heavy rotor equipment to compress air before combustion, thereby reducing the overall weight and complexity of the engine. Moreover, the geometry of PDEs is very simple, consisting essentially of a tube with control valves for the fluid delivery. The detonation process also delivers higher pressures and temperatures from the reaction and offer better efficiencies. PDEs bridge the gap between the subsonic regime and the hypersonic regime, when scram jets and rockets take over. As can be seen in Fig. 2, PDEs offer higher specific impulses than rockets and conventional air-breathing engines at all Mach numbers. Therefore, there are studies underway attempting to integrate pulsed detonation mode of combustion within rockets and scram jet engines, which takes advantage of the boost in performance achieved from the detonation process over the deflagration process. All of the above account for the explosion in the field of detonation and PDE research recently. This has led to the launch of several competing research programs with the aim of developing a working PDE system. |
Figure 2: Mach Number versus Specific Impulse for Various Propulsion Systems
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Figure 3: The Various Stages of the PDE Cycle are Shown Above
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Figure 4: T-S Diagrams and Pressure versus Specific Volume Charts for Various Engine Cycles, Brayton Turbojet Engine Cycle Shown in the Lower Right Corner.
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The difference between Detonation and Deflagration
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Detonation is a supersonic combustion process whereas deflagration is a subsonic combustion process. Almost all engines that burn fuel employ deflagration to release the energy contained within the fuels. In detonation, a shock wave compresses the gas which is then followed by rapid release of heat and a sudden rise in pressure. In the Chapman-Jouguet theory, the detonation wave consists of a shock wave and a flame front. As the wave front passes through the gas, the gas is compressed and the chemical reaction is completed at the rear of the wave front. Another theory, known as the Zeldovich-von Neumann-Doering (ZND) theory, uses finite rate chemistry to describe the model. In the ZND model, the detonation wave is depicted as a shock wave followed closely by a reaction front, with the induction zone separating the two. In reality, the detonation wave is not a 2-D wave front, but composed of smaller wavelets, that creates diamond shaped cellular structures behind it.
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One of the factors affecting the practical implementation of PDEs is the difficulty in achieving consistent detonations within the combustion chamber, within a short tube length. Detonation is often difficult to initiate within fuel and air mixtures in shorter tubes, requiring the addition of large amounts of energy. A more useful method is to start a deflagrative combustion and then to drive the reaction to a detonation by placing obstacles within the path that will create turbulent mixing and also speed up the flow.The process of accelerating the pressure wave into a detonation wave is known as Deflagration to Detonation Transition (DDT). The most effective DDT inducing object is the Shchelkin spiral, which is similar to a helical spring. Other DDT devices include orifice plates and converging-diverging nozzles. |
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