On PDEs

What is a Pulsed Detonation Engine?

A pulsed detonation engine (PDE) is an internal combustion reaction engine that works in a pulsed cyclic fashion, utilizing the constant volume detonation combustion process. The PDE is similar to the pulsejet in that they both make use of the pulsed mode of operation. However, the pulsejet uses deflagrative combustion and has traditionally been used for subsonic applications. PDEs, on the other hand, can theoretically operate up to nearly Mach 5, when the total temperature at the inlet becomes higher than the auto-ignition temperature of most fuels. However, with proper inlet design and flow control techniques, PDEs can be operated up to about Mach 8. At high Mach numbers, a hybrid engine is recommended such that the PDE transitions into a continuous detonation engine (CDE).

detonation (which is what most explosions are) is a supersonic combustion process. A detonation travels as a wave front at supersonic speeds. A DW can be modeled as a thin shock wave followed by a chemical reaction front traveling through a reactive medium (fuel-oxidizer mixture) generating large amounts of heat and pressure. Detonations are constant volume process (because the DW goes through the medium at supersonic speeds before the fluid has time to expand) and produces much higher temperatures and pressures than the constant pressure deflagration process. All burning or combustion we encounter normally (e.g. candle flames, fires, auto-engines, rockets,  etc.) are deflagrations, which are chemical reactions occurring at subsonic speeds.

A DW vs. deflagration

Although, the energy content of a fuel-oxidizer mixture is finite and constant for both detonations and deflagrations, detonations produce much higher pressures and temperatures than deflagrations and at higher speeds (therefore higher power output) and a higher thermodynamic efficiency. In addition, with hydrocarbon fuels, PDEs can theoretically deliver a higher specific impulse (~3000 to 4000), much higher than turbo-jets (~1400 to 3500), ramjets (~1000 to 2200), scramjets (~800 to 1200) and rockets (~400). The promise of higher fuel efficiencies and power output are reason enough for driving the growing efforts worldwide to harness detonation waves for propulsion applications.

The basic PDE has a very simple structure consisting essentially of a constant area tube, with valving to control the supply of fuel and oxidizer, an ignition system, and a nozzle for accelerating the flow if the engine is to be applied for vehicular propulsion. A practical PDE may also have one or more devices to help bring about deflagration to detonation transition (DDT), such as a Shchelkin spiral, which is a helical coil usually made of steel wire, named after its inventor, K.I. Shchelkin, a Russian physicist, who used it in his detonation tube studies in the early 1960s.

The PDE cycle has four stages, namely, the fill stage, combustion stage, blow down stage(exhaust) and purge stage. The PDE combustion chamber is filled with fuel and oxidizer during the fill stage. After the fuel-oxidizer mixture is allowed to fill the combustion chamber to the required volume, the combustion stage is initiated by firing a spark (arc or any other ignition initiator). A detonation wave is soon created that moves through the mixture and causes the pressure and temperature behind it to rapidly shoot up. The next stage is the blow down stage, when a series of rarefaction waves travel upstream into the combustion chamber and reflects off the end wall, causing the high pressure burnt gases to exit the combustion chamber at a high speed. This is then followed by the purge stage, when fresh air is blown through to clean and cool the engine before the fill stage starts again. The purging process is very important as this cools the tube and prevents the fresh fuel-oxidizer mixture from igniting due to residual heat on entry into the combustion chamber. It also protects the structure of the tube from heat buildup.

A PDE can be scaled to match required power or thrust output by increasing the area of the combustor or the number of combustors, wherein the combustors work in a particular sequence, as in the case of an automobile engine with four, six or eight cylinders. The PDE can also be matched with compressors and turbines to form a hybrid PDE-gas turbine engine.

The PDE is still in the developmental stages because there are many obstacles to be cleared before the PDE can be made commercially available. There is much research underway since the 1980s including experimental and computational modeling of PDEs. In the US, experimental test rigs of multi-cycle PDEs have been developed by researchers at General Electric, Pratt and Whitney and several universities, such as the University of Texas at Arlington which has had a PDE program since the 1990s. Researchers in Canada, Russia, China, Japan and Singapore have also demonstrated experimental PDE test rigs. At present, all experimental PDEs can only achieve a few minutes of run time at between 1 and 25 Hz.

The first PDE powered flight was conducted on January 31st, 2008 by a team of researchers from the Air Force Research Laboratory (AFRL). More information on this flight can be found on the PDE Wikipedia page. In addition to experimental research, there is a burgeoning interest in computational modeling of PDEs around the world. An evidence of the growth of PDE research is clearly seen in the large number of presenters and visitors at AIAA conferences in sessions dedicated towards PDEs alone. Continuous detonation engines have also been receiving a rising interest since 2008. AIAA also offers short-courses on PDEs at major conferences, such as the Joint Propulsion Conference, usually held in July.

Source: Panicker, Philip K., The Development and Testing of Pulsed Detonation Engine Ground Demonstrators,” Doctoral Dissertation, Department of Mechanical and Aerospace Engineering, The University of Texas at Arlington, Arlington, TX, 2008.

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One Response to On PDEs

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