GE Pushes Frontiers of Pulse Detonation Engine Test Work
Aviation Week & Space Technology, 09/24/2007, page 78
Guy Norrris, Niskayuna, N.Y.
Dramatic fuel-saving potential sparking new interest in GEs pulse-detonation engine work
General Electric is ramping up studies into how to make production-standard pulse-detonation engines as it prepares to run the first-ever valve-controlled multi-tube PDE with a power turbine and gear system at its Global Research facility here.
The drive to "productionize" what GE increasingly believes is a game-changing propulsion concept is being propelled by the surge in fuel prices and the recognition that PDEs offer potential fuel savings of up to 10%, and possibly beyond, over current-technology engines. "Its all about fuel burn," says propulsion systems laboratory manager Tony Dean.
GE estimates that even if the technology results in only a 5% efficiency improvement, its worldwide use in place of conventional aircraft engine combustors would have the same effect as removing roughly 5.5 million cars from the road, or approximately the same amount of carbon dioxide that would require 8.5 million acres of forest to absorb. Based on current fuel prices and the worldwide annual jet fuel consumption of around 65 billion gal., GE estimates the airline fuel bill could be slashed by almost $7 billion per year if the technology were widely adopted.
PDEs are potentially thermodynamically more efficient because they rely on a pressure rise from a supersonic, shock-induced combustion wave, rather than the constant-pressure deflagration process in a standard combustor. The flame speed in a PDE detonation, for example, travels at around 6,000 fps. compared with 20-70 fps. in a conventional combustor. Among the key challenges PDE researchers face, however, are how to consistently harness that energy, and how to integrate an unsteady PDE-based core (also known as PDC or pulse-detonation combustor/chambers) into a gas turbine engine.
"We feel theres more urgency and were looking for resources to drive the research forward," says PDE advanced technologies leader Narendra Joshi. "The question has been, Can you make an engine combustor work like a diesel engine, and do it with common fuels and air? Nobody had demonstrated any of that, and after a lot of work, were starting to feel really good about this technology."
The newest test rig will take GE one step closer to developing a workable hybrid PDE-turbine engine by integrating a three-tube PDC with a small single-stage power turbine. The work builds on a recently completed, two-year joint research effort with NASA dubbed the PDE Turbine Interaction program, in which a PDC with eight tubes was integrated with a single-stage axial turbine. This configuration was valveless and used common inlet and exit plenums at the inlet and exhaust end, respectively. GE says results indicated "significant downstream tube-to-tube interactions affecting operability" and allowed upstream propagation of pressure waves as well as backflow of detonation products with further knock-on operability effects.
"This time we will have a pretty sophisticated valve at the front end of the tubes and a different outlet plenum," says Joshi, who hopes the new configuration will prevent the operability issues seen with the tube-to-tube interactions of the former test. Although reluctant to detail whats thought to be a rotating valve design or the revised plenum, Joshi says the technology is "subject to a bunch of patent applications."
Included in a spate of recently published GE PDE/PDC applications are various integrated upstream inlet plenums and downstream chambers that feed the exhaust gases into the turbine. Several of these designs are due to be tested in the new rig, which is expected to start running by year-end. At least one of the recent patent applications also includes a design for a PDC thats integrated with fuel-injection and ignition systems and incorporates an upstream chamber that also acts as an inlet plenum.
The small PDE, which is expected to generate in the region of 100 shp., will work with a Honeywell (formerly AlliedSignal) JFS-100-13A power turbine taken from a Fairchild A-10 starter motor and which was selected for its small blade diameter and low power rating. The turbine has an axial-flow free-turbine wheel attached to a main gear train pinion and reduction gear, which will be connected to a water brake for power measurements.
The three PDC tubes, each 2 in. in diameter and 40 in. long, have already been tested as a valveless unit firing at a frequency of 20 Hz. Fuel used for these tests was ethylene, which "is a good surrogate for vaporized fuel," says Dean. Future tests will see "kerosene, JP and diesel fuel," he adds. "I believe it will set up perfectly well for alternative fuels," says Dean.
Adam Rasheed, an aerospace research scientist and PDE specialist at GE, says, "With the multi-tube hybrid, we learned quite a lot over the two years, and so we decided to do a smaller three-tube rig as a technology demonstrator that would be easier to swap out different components. This will also enable us to look at things like different valve technologies."
Beyond the next test, GE is openly courting involvement in other U.S. Defense Dept. or NASA research programs to help further development. "Frankly, were looking for government programs," says Dean, who adds that with sufficient funds a prototype could be running in 510 years. First practical applications, he believes, will be the generation beyond projects such as Advent (Adaptive Versatile Engine Technology).
