To consume fuel more efficiently, a gas turbine engine needs to squeeze air harder before it enters the combustion chamber and turn up the heat.
If only it were so easy.
As the temperature of exhaust gases creeps higher, the next trick is to prevent the high-pressure turbine blades immediately aft of the combustor from melting. Modern turbofans are reaching the limits of metallic survival. Even with blades fashioned from the most robust alloys and systems parasitically diverting airflow into intricate cooling systems, the heat eventually wins.
That is why a 10 November test of a modified General Electric F414 engine is so critical. For the first time, GE has tested a ceramic matrix composite (CMC) turbine blade in a working engine.
Although the 4h-test is only the beginning in a long series, it means CMC receives credit within GE's processes for a technology readiness level of six, meaning that hardware has been tested in an operational environment. It is now eligible to be incorporated in GE's next generation of commercial and military engines.
"As we look to our future engines late this decade or early next decade, we'll be able to get that [CMC] material to a prime, reliable application state," says Dale Carlson, manager of advanced programmes at GE Aviation.
CMC materials have been used in various aerospace applications before. GE incorporates CMC in static parts of the GE/Rolls-Royce Fighter Engine Team F136, the alternate engine for the Lockheed Martin F-35 Joint Strike Fighter.
The recent F414 test, however, represents the first application of CMC material in a rotating engine part, such as a turbine blade. Although part of GE's overall technology roadmap, the test was funded by the US Navy's energy task force. It is being considered for a turbine blade upgrade in the F414, which powers the Boeing F/A-18E/F Super Hornet and the Hindustan Tejas light combat aircraft.
Meanwhile, GE is facing competitive pressure to dramatically improve fuel efficiency rates in future generation of engines. NASA's N+3 concept study, for example, has established a goal to reduce fuel burn by more than 70%.
"You're not going to get there by working coatings [on metallic blades]," says John Kinney, GE's director of advanced programmes.
In the near term, GE is considering CMC materials for its next wave of commercial engines programmes. GE is partnered with Snecma on the CFM International CFM56 turbofan. The replacement Leap-X concept is considering CMC materials for turbine nozzles, but not blades.
On turbine blades, CMC materials are likely to prove attractive on the next generation of widebody engines, such as the powerplant that replaces the Boeing 777's GE90.
The key potential benefit is weight savings. CMC material is lighter than metallic turbine alloys, but it also reduces the weight of the cooling systems required.
"Taking two-thirds out of the weight of a set of turbine blades means you have a lot less turbine structure," Carlson says. "That translates into smaller shafts, smaller bearings. It allows the design to improve all areas of the engine."
GE has estimated that incorporating CMC turbine blades on a GE90-sized engine could reduce the overall weight by about 455kg (1,000lb), which represents about 6% of 7,550kg dry weight of the full-sized GE90-115.
Advanced materials has been identified as one of the key technology investments by NASA and the US military. The latter has launched two programmes - the advanced versatile engine technology and the highly efficient embedded turbine engine - to dramatically improve performance and reduce fuel burn in future engines.
"These are incredible targets," Kinney says. But "you see a pathway to get there. We're marching our way towards those goals."
Source: Flight International
