Technology could spread to civil aircraft applications as need to find cleaner and cheaper power spurs researchers
Continuing high fuel costs and growing environmental pressures could inject new urgency into development of pulse-detonation engine (PDE) technology for wider commercial applications, rather than just military use, says a leading propulsion researcher.
Although missiles are widely expected to be the first practical PDE application, the potential for 5-10% specific fuel consumption (sfc) savings is attracting wider interest, says Anthony Dean, propulsion systems laboratory manager at General Electric's Global Research site in Niskayuna, New York.
The US military spent $13 billion on petroleum in 2006, up from $9 billion in 2005, he says. "You're talking about $6.7 billion annual savings for a 5% sfc improvement at $2.09 per US gallon, so there's a huge economic driver," says Dean.
"The benefits of PDE constant-volume combustion are estimated to be up to 10% [lower sfc], depending on the base Brayton [constant-pressure] cycle it is compared against. This is the carrot we want to go after," says Dean, adding that a 5% improvement in fuel efficiency is roughly equivalent to "removing 5.5 million passenger cars from the road". But the practical issues of perfecting PDE technology remain. "This is a hard problem, but there are a lot of reasons why we should be doing this work," he says.
PDE's have few or no moving parts and are thermodynamically more efficient than conventional piston, turbine and ramjet powerplants, because the pressure rise inside the engine is caused by a supersonic detonation rather than combustion or deflagration at constant pressure.
Once triggered, the detonation travels like a wave through a reactive mixture, and the heat released provides the energy to sustain the shockwave. The engine purges itself as the reactive gases are expelled from the detonation tube, allowing fresh fuel and air to be drawn in for the cycle to continue.
An early possibility being investigated by GE, as well as Pratt & Whitney and other manufacturers, is a PDE/gas turbine hybrid in which a pulse-detonation chamber (PDC) replaces the conventional combustor. GE has tested a large-scale, multi-tube PDC with eight tubes arranged in a can-annular configuration and a single-stage axial turbine. The engine was tested with ethylene-air mixtures, with each tube firing at 20Hz. "We operated it for 96 minutes, and more than 900,000 turbine pulses, 110,000 tube pulses and with six different firing patterns. A lot of things were learned," says Dean.
"Noise is a key challenge, but we did show this is dampened by the turbomachinery downstream," says Dean, citing "approximately a 10dB reduction in peak pressure across the turbine stators and rotor, with further reductions as we went to higher speeds. So this is very exciting and a big deal in terms of whether you can apply this technology in a turbine engine."
Future technology challenges and key research areas include shorter and more practical detonation tubes, alternative means of ignition such as using plasma to reduce oxygen/air requirements, and more work with practical jet fuels as well as at higher pressures and temperatures.
Source: Flight International
