Every one of the modern US Navy’s 129 ships, and its entire fleet of aircraft, relies on gas turbines for either basic propulsion or to generate electricity for their critical systems — typically both. But as fuel costs continue to rise, these turbines now burn through nearly $US2 billion of fuel annually.
That’s why the US Naval Research Lab is developing a revolutionary new type of engine that could reduce our armada’s energy consumption by as much as 25 per cent (and save $US400 million a year) even as the navy transitions to “all electric” propulsion systems.
Gas turbines, the same sort that power commercial aircraft, boast significant advantages over the diesel-fed internal combustion engines that they replaced; they’re more readily scaled to higher power outputs, smaller, lighter and easier to service. Operating on the Brayton cycle, whereby fuel and air are mixed then compressed before combustion, these engines are already a mature, optimised technology. As such, there unfortunately isn’t much that can be tweaked to improve power output or efficiency by more than a few percentage points — we’ve effectively wrung every last ounce of power out of the Brayton cycle.
That’s why the Naval Research Lab in Washington DC is looking beyond Brayton for its next power source, and has been exploring the potential use of the Rotating Detonation Wave Engine (RDWE) for nearly a decade. RDWEs are a variation of Pulse Detonation Engines, which rely on continuous detonation waves to combust the fuel/oxidiser mix. These propulsion systems can theoretically operate at speeds of up to Mach 5, and lack many of the moving internal parts that current turbines do, resulting in significant weight and space savings.
As the NRL explains, a RDWE operates like so:
The combustion chamber is an annular ring, in which the mean direction of flow is from the injection end (bottom in figure) to the exit plane (top). A series of micro-nozzle injectors flow in a premixture 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. The flow has a very strong circumferential aspect due to the detonation wave propagation. Because the radial dimension is typically small compared to the azimuthal and axial dimensions, there is generally little variation radially within the flow. Because of this, the RDE is usually “unrolled” into two dimensions, and we do this for many of our simulations with small thickness-to-diameter ratios.
Basically, the fuel/oxidiser mix is injected into a cylinder and exploded by a detonation wave (aka a shock wave) travelling around the inner surface of the tube. As the wave hits the fuel, it creates another detonation, propagating the wave as well as generating forward thrust. While PDW engines have yet to be practically implemented 70 years after their inception, researchers at the NRL hope that developing this technology will yield a 10 per cent increase in power while simultaneously improving fuel efficiency by 25 per cent. [Defence Talk – UTA – NRL – Wikipedia]