All Navy aircraft and missiles use gasturbine engines for propulsion. Many ships are also dependent on gasturbine engines to generate both propulsive power and electricity. These engines are fundamentally similar to engines used to power commercial airplanes. Future ships moving to an “all electric” paradigm for the propulsion system will still require these gas-turbine engines to generate electricity for the propulsion system and also for other critical onboard systems. Because of the amount of power required by modern warfighting ships, and the prospect that this power requirement will only increase, there is a strong interest in improving the specific fuel consumption of these engines.

Example of a Rotating Detonation Engine (RDE, left), and simulation of the combustion chamber (right) for an RDE.
Gas-turbine engines are attractive because they scale nicely to large powers, are relatively small and self-contained, and are relatively easy to maintain. Current gas turbines are based on the Brayton thermodynamic cycle, in which air is compressed and mixed with fuel, combusted at a constant pressure, and expanded to do work for either generating electricity or for propulsion.

To make significant improvements to the performance of gas-turbine engines, different and possibly more innovative cycles must be investigated, rather than the Brayton cycle. An attractive possibility is to use the detonation cycle instead of the Brayton cycle for powering a gas turbine. NRL has been a major player in the development of pulse detonation engines (PDEs). The rotating detonation engine (RDE) is a different strategy for using the detonation cycle for obtaining better fuel efficiency. Like PDEs, RDEs have the potential to be a disruptive technology that can significantly alter the fuel efficiency of ships and planes; however, there are several challenges that must be overcome before their benefits are realized. Research over the last several decades on materials that are able to withstand the high pressures, temperatures, and heat fluxes associated with detonations, and on initiators that are efficient, fast, and reliable, have made detonation engines a possibility.

A Brayton cycle relies on a multistage compressor in order to increase the pressure of the air from atmospheric to a higher pressure. Without this compression, no work can be obtained from the gas-turbine engine. Typical compressor ratios vary from 10 to 30 and are easily the most complex machinery in a gas-turbine engine. Detonations, on the other hand, are close to a constant volume reaction process, and naturally generate high pressures that can then be expanded to do work without any compressor at all. The rotating detonation engine takes a different approach toward realizing the efficiency of the detonation cycle. By allowing the detonation to propagate azimuthally around an annular combustion chamber, the kinetic energy of the inflow can be held to a relatively low value, and thus the RDE can use most of the compression for gains in efficiency, while the flow field matches the steady detonation cycle closely.

A schematic of a rotating detonation engine is shown in the figure. Current basic studies done at the NRL are focused on a much simpler annular combustion chamber. The combustion chamber is an annular ring in which the mean direction of flow is from the injection end to the exit plane. A series of micro-nozzle injectors flows in a pre-mixture of fuel and air or oxygen axially from a high-pressure plenum, and a detonation propagates circumferentially around the combustion chamber, consuming the freshly injected mixture. The gas then expands azimuthally and axially, and can be either subsonic or supersonic (or both), depending on the back pressure at the outlet plane.

Rotating detonation engines, a form of continuous detonationwave engine, are shown to have the potential to further increase the performance of air-breathing propulsion devices above pulsed or intermittent detonation-wave engines. Simulation of pulse detonation engines shows many of the significant flow-field features of RDEs, and explains how the performance of these engines relates to the ideal thermodynamic detonation cycle.

This work was done by Douglas Schwer and Kazhidathra Kailasanath of the Naval Research Laboratory. NRL-0060

This Brief includes a Technical Support Package (TSP).
Rotating Detonation-Wave Engines

(reference NRL-0060) is currently available for download from the TSP library.

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This article first appeared in the February, 2013 issue of Defense Tech Briefs Magazine.

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