Flying high

Flying high
Flying high

Emerging research in aerospace engines – on the cusp of commercialisation – could cut fuel consumption, extend lifespan and protect against high temperatures. Lou Reade reports.

Emerging research in aerospace engines – on the cusp of commercialisation – could cut fuel consumption, extend lifespan and protect against high temperatures. Lou Reade reports. Ongoing research into aerospace engines – some of which is likely to hit the market within two years – could dramatically boost fuel efficiency and extend engine lifetimes.

Researchers in Sweden, for instance, have looked into two new engine concepts to assess their fuel-saving potential and found that the open rotor engine could cut fuel consumption by 15% if it were commercialised.
Linda Larsson, a researcher at Chalmers University, says that open rotor has great potential as a way of boosting fuel efficiency.

“It has propulsive efficiency – which means that the energy generated by the core engine can be efficiently converted into thrust,” she says.

An open rotor engine generates most of its thrust from two counter-rotating propellers, rather than a ducted fan. This allows engine diameter to be increased, allowing higher propulsive efficiency but without an overly large or heavy engine nacelle.

The concept was widely studied in the 1980s, when the oil crisis caused huge price hikes. Both GE and Pratt & Whitney tested the concept. But when fuel prices dropped again, the technology lost its appeal, she says.

“Now it is starting to get noticed again. Plenty of research is being conducted on the concept. It could be in place around 2030.”

One project that is actively looking into open rotor technology is the European Clean Sky programme.
Larsson's study was carried out using a small aircraft – which, she says, would be a potential likely user of this type of engine in future for regional flights.

She also studied a second engine concept, called geared turbofan. It differs from a regular turbofan in that the large fan in front of the engine operates at a lower speed than the turbine that drives it, because a gearbox between the two reduces the number of revolutions. This enables a lighter turbine and leads to greater turbine efficiency.

Larsson says the design could cut fuel consumption by a more modest 4% – and is likely to be available by next year.
Pratt & Whitney recently announced that five commercial operators – Airbus, Bombardier, Embraer, Irkut and Mitsubishi – have between them ordered 5,500 of its new PurePower geared turbofan engines – into which it has sunk more than $1 billion over the last 20 years. The aircraft are expected to enter service in the next few years, it says.

Nano protection

Also in Sweden, researchers at University West have used nanoparticles to improve the insulating coating that protects engines from the effects of high temperatures.

Aircraft engine surfaces are protected by thermal barrier coatings using a method called thermal spray application, in which a ceramic powder is sprayed onto the surface at 7,000-8,000°C using a plasma stream. The ceramic particles melt and strike the surface, where they form a protective layer about 0.5mm thick.

By adding nanoparticles to the mix, the researchers extended the service life of the coating by 300%. As well boosting the service life of the engine, the coating will allow it to run at higher temperatures – and so burn fuel more efficiently.
“The base is a ceramic powder, but we have also tested adding plastic to generate pores that make the material more elastic,” says researcher Nicholas Curry.

Increasing the elasticity of the layer is crucial, as this helps it to absorb the stress caused by expansion and contraction.
“We've tested the layer formed from nanoparticles,” adds Curry. “The particles are so fine that we can't spray the powder directly onto a surface.”

Instead, the powder must first be mixed with a liquid before being applied using a process called suspension plasma spray application. When Curry and his colleagues tested the layer in thermal shock tests – to simulate the temperature changes in an aircraft engine – they found the threefold increase in service life. He adds that the new method is far cheaper than conventional technology.

The next step is to find ways to monitor what happens to the structure of the coating over time, and understand how the microstructure in the layer works.

“A conventional surface layer looks like a sandwich, with layer upon layer,” says Curry. “Ours is more like standing columns.”

This makes the layer more flexible and easier to monitor, he says. And it sticks to the metal, regardless of whether the surface is completely smooth or not.

“The most important thing is not the material itself, but how porous it is,” explains Curry.

University West works closely with aircraft engine manufacturer GKN Aerospace, and is confident that the nano-coating could be used commercially within two years.

From coal to coating

A similar concept – to develop nanostructured coatings that can withstand high temperatures – is also being investigated by researchers at the Centre for Research in Advanced Materials (Cimav) in Mexico. In this instance they are using a commonly available industrial waste material – fly ash – as the basis for the coating.

The team, led by Ana Maria Arizmendi Morquecho, is developing the coating in order to prevent microstructural degradation of the superalloy from which the turbine is made.

“The nickel superalloy components of the blade and nozzle in the hot zone of the turbines are exposed to temperatures above 1,000°C,” she says. “This causes degradation of the substrates and affects thermal and mechanical properties by decreasing the energy efficiency of the turbines.”

Fly ash – a waste product of coal-fired power stations – is used as the base of a ceramic matrix that incorporates a variety of nanoparticles. The researchers are interested in a particular element within the fly ash.

“We found that mullite, a chemically and thermally stable compound in the fly ash, can be used as a ceramic matrix,” she explains. “By adding different particles, we have obtained novel nanocomposites that can be used as coatings for superalloys.”

Mullite is a silicate material that is known to withstand very high temperatures. It constitutes around 10% of the weight of fly ash.

After five years of research, the team is moving towards final validation of the materials obtained in the laboratory, before scaling up the process for potential commercialisation, concludes Arizmendi Morquecho.

www.chalmers.se
www.cimav.edu.mx

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