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Fuel efficiency and powerplant cost reduction drive engine-makers' tests
Pulse detonation technology being pursued by General Electric, Pratt & Whitney and Rolls-Royce offers engine-makers the potential for improving gas turbine specific fuel consumption by double digits while reducing powerplant complexity and cost.
 | | Five-tube integrated test rig was run at the Navy's China Lake, Calif., facility. The PDE, reflecting a flight architecture configuration, was used to measure performance. |
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If pulse detonation technology were added to a gas turbine, it would result in a hybrid engine in which the traditional core-high pressure compressor, combustor and high pressure turbine is replaced by a pulse detonation system. Besides simplifying systems, pulse detonation technology also could lead to reductions in emissions because of its characteristic short bum times, said Tom Bussing, general manager at Pratt & Whitney Seattle AeroSciences Center, the company's lead site for pulse detonation work.
Pulse detonation is simple in concept. Admit air and fuel into the head of a tube and detonate it, creating a high-pressure detonation wave that travels at supersonic speeds down the length of the tube, promoting burning at thousands of meters per second. Exhaust gases are expelled at the far end of the tube and, as the exhaust gases clear out, a new cycle begins.
"PROPULSION BASED on the constant-pressure Brayton cycle, which defines the work cycle of traditional gas turbines, is one of the least efficient we have, but we've been able to offset this by successes in materials, three-dimensional aerodynamics, etc.," said Anthony Dean, leader of the advanced technology program in advanced propulsion at General Electric Global Research. "Unfortunately, we're starting to plateau in gas turbine system improvements, so how do we get more out of them?" Dean queried rhetorically. The answer, he said, may be in the Humphrey cycle's constant volume detonation, or pulse detonation.
Rolls-Royce's Allison Advanced Development Co. has explored pulse detonation in stationary tubes and, as a next step, plans to combine pulse detonation with multiple tubes rotating on a cylinder, forming an engine that resembles a revolver in operation. According to AADC officials, detonation occurs in the rotating framework and flow is steady-state upstream and down-stream of the tubes. The rotating component, known as a wave rotor, was developed by Rolls-Royce more than 20 years ago as a potential supercharger, said Lynn Snyder, director of advanced systems at AADC.
| "The technology could be a real game-changer" |
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According to Rolls officials, combining the wave rotor technique with pulse detonation should make the gas flow as steady as it is in a traditional gas turbine.
Cooling demands, though, would be lessened. Company officials explained that air entering the rotating tubes for combustion will help cool them, particularly since detonation would not repeatedly be carried out at the same place in the same tube. In a traditional PDE, detonation always occurs in the same place in the tube, said Steve Heister, a professor of aeronautics and astronautics at Purdue University.
Under a proposal made to the state of Indiana, which funded AADC's earlier PDE work, a two-year wave rotor PDE program will begin this month. Indiana is contributing $510,000 in funds from its 21st Century Research and Technology Fund, and rig tests associated with the PDE effort should be performed in the last half of 2005, officials said. The trials would be run at Purdue University's West Lafayette campus, site of Indiana's Propulsion and Power Center of Excellence and also the location of the company's earlier PDE work.
AS PART of the proposed tests, researchers will examine the quality of the detonations (including the effect of fuel-air mixtures and tube geometries), explore wave formation, and study the properties of ignition timing and air injection.
Fuel used in the trials will probably be pre-vaporized or direct-injected Jet A. Earlier AADC tests with stationary pulse detonation tubes used both ethylene and propane to simplify the demands placed on the test facilities. "Propane is not that far off from Jet A in detonation," noted Heister. "And in our previous tests, we were able to reliably and repeatedly get detonations to occur, with some tests achieving a rate as high as 50 Hz."
General Electric's approach to pulse detonation grows out of work it has conducted for about four years with the Defense Advanced Research Projects Agency. That effort initially was focused on a pulse detonation system for micro-unmanned aerial vehicles. However, that eventually morphed into a program to examine pulse detonation in an engine capable of generating 10-20 Ib. of thrust, officials said.
During the first two years of the Defense Advanced Research Projects Agency (Darpa) effort, GE demonstrated hardware and, using gaseous hydrogen, ethylene and propane, the company verified that detonations were possible in an engine of that size.
Eventually, the scope of the project grew to examine pulse detonation for larger engines. That work, which concluded late last year, had two components. One was aimed at replacing military engine augmentors with pulsedetonation systems. The other focused on replacing conventional core engine technology, in both military and commercial engines, with pulse detonation technology.
According to GE officials, their approach to pulse detonation technology is significantly different from that of their competitors. "Other companies are using mechanical valving systems which limit the frequency of their detonations. We're relying on an aerodynamic valving system," said Harvey Maclin, GE's manager, advanced technology marketing and government programs.
The company's other differentiator is that its pulse detonation systems use liquid fuels like Jet A instead of alternate gaseous fuels. "It's a significant break-through, one that allows potential users to rely on the current [fueling] infra-structure," Maclin said.
Under the Darpa project, GE achieved what Maclin has termed "significant advances." According to Maclin, GE was able to get "a 'reasonable-sized' rig operating on liquid Jet A fuel to develop over 100 Ib. of thrust and the approach seems scalable." He also noted that there were no valves on the air side of the PD tubes, "so we achieved that objective, too."
Next on the horizon for the company is the transition from components to systems, which would be more application-specific. GE's Global Research arm is leading the company's
pulse detonation effort, because the technology has non-aircraft applications such as ground-based power systems.
High on the agenda, too, is participation in NASA's Pulse Detonation Engine Turbine Interaction Program, or PDE TIP. It's a two-year project with $500,000 in NASA funding directed at taking small-scale PDEs and running them in existing turbine facilities to study system behavior. NASA has done a number of simulations determining how PDEs will affect turbines and they have gotten a range of answers, GE officials said. PDE TIP will enable researchers to capture component performance data that can be used to verify which of NASA's answers are correct.
Also underway at all three engine-makers is the CVCCE project, or constant volume combustion cycle engine effort, which itself is part of NASA's Low Emissions Alternative Power, or LEAP, program. An ambitious six-year project aimed at demonstrating a hybrid engine, CVCCE's first phase is being worked on by Rolls, Pratt and GE. However, two years along, a downselect will be made and one company will be allowed to continue, working on a hybrid system that would probably be available for tests in 2009. CVCCE began late last year.
Pratt & Whitney recognized the potential advantages of pulse detonation propulsion as early as 2000, and in 2001 bought Adroit Aeronautical Systems' Seattle Aerosciences Center, which had been immersed in PDE work since 1992, according to Pratt & Whitney's director of advanced programs, Sim Austin. Pratt is so bullish on the technology, that it has laid out a development roadmap that begins with component technology and risk reduction and ends with flowpath demonstrators. "The technology could be a real game-changer," said Pratt's program manager for PDE activity, Steve Spangler.
In addition to participating in NASA's LEAP effort, Pratt is currently working with Boeing on a U.S. Navy program aimed at developing a pure PDE for a Mach 2-4 standoff weapon that has a range of several hundred miles.
Under that effort, the engine-maker tested a full-scale, flight frequency flow-path demonstrator on a teststand at the Navy's China Lake Naval Air Warfare Center (Calif.). The year-long tests, which concluded late in 2003, examined a five-tube PDE where each tube fired at 80 Hz. The tubes for this engine were 4 in. in dia. and 30 in. long, and the en-gine was capable of generating about 1,500 Ib. thrust at 50,000 ft.
Pratt also recently completed rig tests of a new PDE nozzle design called a compound choke nozzle. The component efficiently expands combustion gas-es to atmospheric pressure, officials said. "Expansion is very important in this technology because you're dealing with an unsteady process. To get the full benefit of a constant volume system, expansion must be as efficient as possible," Bussing explained.
This year, the company will continue its work to optimize the nozzle, using the same rig at China Lake that was used in previous testing. Other work over the next couple of years will focus on system risk reduction.
Pratt officials believe that the upper speed limit of a pure PDE is about Mach 4 when using a hydrocarbon fuel. But that could be raised if hydrogen were used. Company officials also point out that many of the technologies needed for the pure PDE are exactly the same as those required by a potential hybrid system.
"Both need efficient ways of initiating detonation, both use multi-tube configurations, and the valving, seals, nozzles and thermal management aspects are also common between the two," Pratt officials said.
Bussing believes that a pure PDE for a Navy missile project could be ready for flight as early as 2010, but he's quick to point out that timing depends on the Navy's needs and budgets. This article has been republished with the permission of Aviation Week.
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