Pratt & Whitney’s successful PDE combustor trials could lead to demonstrator getting airborne by 2008
Pratt & Whitney plans to launch a new phase of collaborative research work with NASA and the US Air Force aimed at fielding a pulse detonation engine (PDE) flight demonstrator as early as 2008, following positive results from recently conducted combustor tests.
Conducted at the US Naval Postgraduate School propulsion test site in Monterey, California, the tests began in 2004 as part of NASA’s now abandoned low-emissions alternate propulsion research programme, and were completed last December. The work “ended up being a co-operative programme between NASA and the Office of Naval Research, and involved a lot of technical help from the Air Force Research Laboratory,” says Gary Lidstone, divisional manager of P&W’s Seattle AeroSciences Center.
Tests were made using a single-chambered pulse detonation combustor running on conventional jet A fuel, rather than gaseous hydrogen, ethylene and propane. Just as importantly, the fuel was premixed with air rather than oxygen, and the combustor operated with “inlet pressures and temperatures at what a typical turbine would see,” says P&W advanced engines programme manager Steve Spangler. “We demonstrated detonative combustion at frequencies that were very product representative,” he adds.
The combustor internal diameter was just over 50mm (2in) and was “run at the appropriate backpressures, representative of the product conditions we were simulating”, says Lidstone. This “was accomplished using a back-pressure tank downstream of the combustor section”, he says.
P&W, a long time PDE pioneer, believes the simple operation and potential efficiencies of the concept could offer long-term advantages in future engines for both commercial and military uses. Fuel consumption, for example, is expected to be reduced by an estimated 30%, while the design also offers greater thermodynamic efficiency by operating on the principle of constant-volume, rather than the constant-pressure cycle of a conventional turbine engine.
Another appeal is the concept’s simplicity. A PDE has few, if any, moving parts and is basically a tube into which fuel and air is admitted at one end and then detonated, creating a high-pressure wave that travels at supersonic speed down the length of the tube.
This causes a burning rate in excess of 1,500m/s (295,000ft/min) and a pressure rise of 30-100 atmospheres. Exhaust gases are expelled at the far end of the tube and the process repeats as pressure in the tube drops.
Although initial results do not indicate the 9-10% levels of efficiency improvement originally predicted, the numbers do show potential savings of around 5%.
GUY NORRIS / LOS ANGELES
Source: Flight International
