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| Fig. 1: Pulse Detonation Engine Cycle. (Source: S. Cassady, after Kailasanath. [1]) |
The Pulse Detonation Engine (PDE) is an experimental propulsive device that uses supersonic detonation waves as a combustion mechanism. In theory, the PDE design offers numerous advantages over traditional gas turbine engines, including improved efficiency and reduced mechanical complexity. [1] However, PDE designs must overcome significant hurdles in order to become a viable and efficient form of propulsion, and research into PDE designs, engineering properties and potential is ongoing.
Pulse Detonation Engines are the supersonic relatives of pulse-jet engines. Pulse- jets rely on intermittent, subsonic deflagration flames in a long tube to burn an injected fuel- oxidizer mixture. [2] Pulse-jets were thrust on the world stage during World War II, as the propulsion system of the Nazi V-1 bomb. [2] Deflagration flames propagate rather slowly, and their combustion can be modeled as a constant pressure process. [2] As a result, the performance of pulse- jet engines is limited by the slow flame speed. [2] Detonation waves, a supersonic phenomenon, propagate at speeds in the thousands of meters per second, and can therefore be modeled as a constant volume process. [2] Serious research into detonation propulsion systems began in the 1950s, when researchers at the University of Michigan published a series of papers on detonation waves. [1,3] The novel idea of intermittent detonation gained traction in the 1980s as the Naval Postgraduate School investigated the design further. [4] However, experimental work encountered a number of challenges, namely the difficulty with transitioning a subsonic deflagration wave (a flame) into a supersonic detonation wave, as well as properly mixing the fuel and oxidizer to produce a uniform detonation. [1] More recently, the PDE concept has continued to garner academic research interest, and investigators approach PDE research from a variety of backgrounds, including computational fluid dynamics, experimental thermodynamics, as well as laser diagnostics. [1,4]
The fundamental physics behind PDEs is rather simple. Combustion occurs in a shaft with valves or carefully crafted openings at each end, so that gas can only pass one way through the device. [1] The fuel mixture in the chamber is ignited in such a way that it combusts and expands supersonically (detonation), sending a shock wave down the length of the chamber. Because the shock wave moves so rapidly, the rest of the fuel in the engine combusts before it has time to expand; thus, combustion occurs at approximately constant volume. [1] A constant-volume combustion process releases more chemical potential energy as heat than the constant-pressure process found in conventional turbine engines. [1] Theoretically, at constant volume, all of the chemical potential energy stored in the fuel is converted into the internal energy (U) of the gas. If the gas were to expand, some of that chemical energy (PV) would have to be expended as work against the atmosphere. Propulsion is generated by a nozzle at the back of the engine, which allows the hot gas to expand as it exits the shaft. [1] As the exhaust gas blows out of the back of the engine, air rushes into the front to fill the vacuum, where it mixes with fuel, ignites, and restarts the process with a new detonation wave (see Fig. 1). [1]
However promising they may appear in theory, PDEs must overcome significant challenges before they can practically be implemented. For example, providing the proper conditions for detonation to occur can be quite difficult. In order to achieve detonation, either the combustion event must be sufficiently powerful, or the downstream deflagration flame must be converted into a supersonic wave in a process known as deflagration-detonation transition (DDT). [1] One method of inducing DDT involves the placement of internal obstacles along the flow path of the combustion wave to increase the turbulence of the flow. [5] Current research into DDTs emphasizes minimizing the DDT transition period and improving the materials chosen for interference. [5] Moreover, the PDE produces an extremely high heat per unit fuel burned. [1] As a result, the materials needed and test times available of experimental PDEs are limited. Such challenges have fueled ongoing research into PDE nozzles, flow properties, and cooling mechanisms.
Theoretically, PDEs offer numerous advantages over current jet and rocket propulsion systems. However, their practical development has met with numerous challenges, many of which remain unsolved today. Even if PDEs never become a viable means of propulsion outside the laboratory, their study will not have been in vain. PDE research has pushed the boundaries of engineering knowledge by fueling the development of improved gas dynamic models and diagnostics, while improving understanding of combustion science and fluid dynamics.
© Sean Cassady. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] K. Kailasanath. "Review of Propulsion Applications of Detonation Waves," AIAA J. 38. 1698 (2000).
[2] T. Bussing and G. Pappas, "Introduction to Pulse Detonation Engines," AIAA 94-0263, 1994.
[3] J. A. Nicholls, H. R. Wilkinson, and R. B. Morrison. "Intermittent Detonation as a Thrust-Producing Mechanism." Jet Propulsion 27, 534 (1957).
[4] C. S. Goldenstein et al., "Diode Laser Measurements of Temperature and H2O for Monitoring Pulse Detonation Combustor Performance," Stanford University, 28 Jul 13.
[5] S. Y. Lee, et al., "Deflagration to Detonation Transition Processes by Turbulence-Generating Obstacles in Pulse Detonation Engines," J. Propul. Power 20, 1026 (2004).