Comparison of Brayton and detonation cycles on a P-v diagram, with an operating pressure ratio (OPR) of 2 for the detonation cycle, and 10 for the Brayton cycle. Schwer and Kailasanath 2011c. Click to enlarge.

The gas-turbine engines the Navy uses today 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 significantly improve the performance of gas-turbine engines, researchers need to look beyond the Brayton cycle to explore alternative and possibly more innovative cycles.

NRL researchers believe that one attractive possibility is to use the detonation cycle instead of the Brayton cycle for powering a gas-turbine. NRL has been on the forefront of this research for the last decade and has been a major player in developing Pulse Detonation Engines (PDEs).

In a 2011 paper in the NRL Review Dr. Kazhikathra Kailasanath, who heads NRL’s Laboratories for Computational Physics and Fluid Dynamics and his colleague Douglas Schwer noted:

Detonations have long been associated with explosions (and explosives), not with engines. There are many reasons for this; however, the most important is that detonations produce extremely high pressures, shock waves, and high velocities. Another difficulty for engine applications is repeatedly generating detonations consistently and efficiently. 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.

...There are quite a few interesting aspects to the detonation cycle that make it an attractive alternative to the typical Brayton cycle. 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. Without a compressor, an engine based on the detonation cycle provides a cycle efficiency of about 30% (compared to 0% for the Brayton cycle). This means that much simpler compressors can be used to generate the equivalent efficiency. Adding a compressor to a detonation engine increases the efficiency further, and so technology developed for Brayton cycle engines can still be used for detonation engines.

The challenge with detonation engines is realizing the efficiency of the detonation cycle. Concepts such as oblique detonation-wave engines have failed to be able to recover the efficiency of this detonation cycle, because much of the energy of the inflow is bound up in kinetic energy, which does not increase the pressure and thus does not improve the efficiency. Pulse detonation engines have taken a different approach by creating an unsteady process that removes the requirement of having high velocity inflow. This creates a whole new set of issues, such as rapid initiation of detonations and the requirement of efficient detonators.

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.

Schematic of an RDE. Source: Schwer and Kailasanath, 2011c. Click to enlarge.

The PDE is an intermittent combustion engine that relies on unsteady detonation wave propagation for combustion and compression elements. Rotating wave-detonation engines (RDEs) thus represent a logical step from the intermittent, pulsed wave-detonation engine concepts to a continuous detonation engine concept for obtaining propulsion from the high efficiency detonation cycle, Kailasanath and Schwer note in a new paper published in the Proceedings of the Combustion Institute. (In that paper, they conclude that hydrocarbon RDE’s are viable and that basic flow-field patterns and behaviors are very similar to the hydrogen/air cases detailed previously.)

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 the benefits are realized, explains Dr. Kailasanath. NRL scientists are now focusing their current research efforts on getting a better understanding of how the RDE works and the type of performance that can be actually realized in practice.

Resources

• Douglas Schwer, K. Kailasanath (2012) Fluid dynamics of rotating detonation engines with hydrogen and hydrocarbon fuels. Proceedings of the Combustion Institute doi: 10.1016/j.proci.2012.05.046

• Douglas A. Schwer and Kailas Kailasanath (2011a) Fuel Effects on Rotating Detonation Engines. Paper 106, 23^rd International Colloquium on the Dynamics of Explosions and Reactive Systems (ICDERS), UC Irvine

• K. Kailasanath (2011) The Rotating-Detonation-Wave Engine Concept: A Brief Status report. 49^th AIAA Aerospace Sciences Meeting (AIAA-2011-0580)

• D. Schwer, K. Kailasanath (2011b) Numerical Study of Engine Size Effects on Rotating Detonation Engines (AIAA-2011-0581)

• D.A. Schwer and K. Kailasanath (2011c) Rotating Detonation-Wave Engines, 2011 NRL Review