European researchers are confident they have proven the feasibility of a hybrid blended-wing-body aircraft powered partially by cryogenic fuels, which they believe could enter commercial service before 2050.
The Advanced Hybrid Engines for Aircraft Development (AHEAD) project, sponsored by the EU’s seventh Framework Programme, kicked off in October 2011 with a mandate to assess the feasibility of developing a multifuel BWB aircraft, powered by a new hybrid propulsion system.
It was a tall order – the challenges of using cryogenic fuels, such as hydrogen and liquefied natural gas, are steep: they need to be kept cold and they require large, well-insulated cylindrical storage tanks.
But Arvind Rao, associate professor of flight performance and propulsion at Delft University of Technology in the Netherlands – a member of the AHEAD consortium – is confident that the BWB aircraft designed under the project can meet those challenges without incurring a significant drag penalty.
Powering an aircraft entirely with cryogenic fuels would result in too many aerodynamic disadvantages, as Rao explains: “We wanted to keep an open mind on whether to use hydrogen or LNG. The main problem is that hydrogen needs big fuel tanks, so the fuselage blows up a lot, thereby increasing drag and the required thrust. So even though carbon dioxide emissions are going down, the energy required per passenger is going up.”
The main idea, therefore, is to find an optimum level and design a hybrid, or multifuel, aircraft. “It’s a matter of optimisation. We’re looking at 70% cryogenic and 30% kerosene [or biofuel] but we don’t yet know if this is optimum,” says Rao.
Other AHEAD consortium members include German aerospace research centre DLR, the Technical University of Berlin, Poland’s WSK PZL- PZL Rzeszów, the Technion Israel Institute of Technology and Dutch aerospace consultancy AD Cuenta.
The researchers have made a preliminary design of a hybrid BWB aircraft, which is roughly the same size as a Boeing 777-200ER. The design includes space within the fuselage for the specialised tanks required for cryogenic fuel storage.
The group's concept includes space in the fuselage for specialised cryogenic fuel storage
AHEAD
The next big challenge was to develop a hybrid propulsion system with two combustion chambers, the second of which uses flameless combustion – a “new technique not yet applied to aero engines”, says Rao. The latest results from simulations show that this dual system can reduce nitrogen oxide emissions by “almost 90% over the year 2000 baseline”.
Tests on the flameless combustion chamber are being carried out at the IST Lisabon (Instituto Speror Technico) in Portugal, and Rao is optimistic about the outcome: “It seems to us we can achieve flameless combustion,” he says. The first combustion chamber has been designed for hydrogen and is undergoing testing in Berlin.
The characteristic low temperature of cryogenic fuels can have further benefits when it comes to engine efficiency. “Cryogenic fuel is very cold so we’re looking at how we can use the cold fuel to increase the thermal efficiency of the engine,” says Rao.
This involves using the fuel to cool the bleed air required for cooling the turbine vanes and blades, resulting in reduced fuel consumption. “In AHEAD, we take the bleed air and pass it through a heat exchanger which cools the air and heats the fuel. The amount of cooling air required is reduced, thereby increasing the thermodynamic efficiency of the engine.”
The current project comes to an end later this year but if the AHEAD consortium is offered a follow-on project, Rao says it would look at how the engine’s counter-rotating shrouded fans behave when flow is distorted as the engine ingests the boundary layer flow over the aircraft. This could lead to further efficiencies within the powerplant.
“Normal turbofans can’t do this because the air going to the fan has no distortions,” says Rao. The consortium members have put forward a proposal to the European Commission seeking a follow-on project and are waiting to receive the go-ahead.
If they get the green light, Rao envisions moving the project forward to technology readiness level 3 or TRL4, at which point it could enter the EU’s Clean Sky programme. Once a project reaches TRL6, the aviation industry takes over. Rao believes that by 2035, a prototype of the hybrid BWB aircraft could be ready and flight tests could begin around 2040.
“The results so far are quite encouraging – it could enter service before 2050,” says Rao, stressing the importance of making the project a reality as quickly as possible. “I don’t know how long we can afford to wait. By 2050, countries like Bangladesh and the Caribbean islands could be under water. The scenario is quite serious.”
The civil aviation industry consumes 1 billion litres of kerosene (264 million USgal) per day, which is expected to grow to 1.5 billion litres by 2027. “In such a scenario, you have an emissions problem,” says Rao. And he believes biofuels alone will be unable to address it: “People talk a lot about biofuel and, of course, it is possible to some extent. But it is very difficult to make biofuel for aviation in a sustainable way in the required volumes.”
Hydrogen, on the other hand, is a renewable fuel, while natural gas reserves can provide at least a century-long supply of fuel, according to AHEAD researchers.
From the work carried out so far under the AHEAD project, researchers believe the LNG fuel version of the multifuel BWB could reduce carbon dioxide emissions by around 65% and nitrogen oxide emissions by around 80% compared with a conventional Boeing 777-200ER. The hydrogen version would yield even greater emissions cuts.
However, the use of cryogenic fuels results in a higher water content and less soot in the engine exhaust, leading to a higher probability of contrail formation and a lower optical depth, both of which can affect global warming. This is all being investigated to determine at which altitude these effects can be minimised.
“If you use hydrogen, the exhaust is mainly water vapour. This causes no harm at ground level but it can become a significant contributor to greenhouse gases at higher levels,” says Rao. “Also, when you fly near the tropics the effect is different to northern latitudes. This is all still under investigation.”
Preliminary research outlined in an AHEAD presentation has found that contrail cirrus radiative forcing would strongly increase, particularly in southern latitudes, in a hydrogen-powered aircraft flying 6,500ft higher than a conventional aircraft. It would decrease if the same aircraft flew 6,500ft lower.
“We will see how the blended wing body can reduce this impact. It might have to fly lower which would increase fuel burn, but because it’s cryogenic fuel [this is less of a concern],” says Rao.
The AHEAD researchers say they will “define mission profiles for the new aircraft to optimise fuel efficiency and reduce the overall climate impact, comprising carbon dioxide, contrail cirrus, water vapour and nitrogen oxide impacts on ozone and methane”.
Rao describes AHEAD as “one of the flagship projects” for the Commission, which is sponsoring its work. “We are looking at a real step change, so it is a unique project.”
Source: Flight International
