For more information on the project please contact the coordinator: Arvind Gangoli Rao A.GangoliRao@tudelft.nl

This project is made possible by

FP7

the EU

Project description

Motivation: Challenges Faced by Aviation

Commercial aviation has made substantial progress since its inception and is now the backbone of a modern society. In the past ten years, passenger numbers have grown by 45%. Freight traffic has increased by more than 80% on a tonne-kilometre basis [1]. Moreover, aircraft emissions have reduced significantly over the last 40 years, for example, noise has reduced by 20 decibels, fuel consumption by 70%, carbon monoxide emissions by 50% and unburned hydrocarbon and smoke by 90% [1]. Although all these positive developments, some serious challenges toward the environment, the community and the availability of fuel resources are encountered by aviation. The total emission from the aviation sector is still increasing rapidly and now the sector is an active contributor to global warming. Therefore, the Advisory Committee for Research in Aeronautics (ACARE) has targeted to reduce various fronts noise, air pollution and fuel consumption. The ACARE goal is shown in Figure 1.It aims to reduce CO2 emission by 75%, NOx emission by 90% and the perceived noise levels by half in the year 2050, compared to the baseline year 2000 [2]. To achieve this objective, a combined improvement in the aircraft, power-plant and the air traffic management system is required.

                                                    Figure 1. ACARE vision for Europe.

As is known, the reduction of NOx emission mostly depends on the evolvement of the combustion technology. However, CO2 emission is the product of chemical reactions between the carbon content of the fuel and air, as long as the conventional fuel is in use, the ACARE goal to reduce CO2 emission remains different. To achieve this target, alternative fuels have been put forward. A scenario of future aviation fuels are indicated in Figure 2. It is anticipated that aviation will see a significant use of alternative fuels, starting with synthetic fuels such as Gas To Liquid (GTL), Coal To Liquid  (CTL) and biofuels. In a few decades hydrogen or hydrogen rich fuels, such as Liquid Natural Gas (LNG) can probably be used to reduce the carbon foot print of aviation for long range aircraft. By the end of this century, we might have the technologies in place for powering an aircraft electrically.

                                          Figure 2: Primary Energy source for a long range aircraft.

Due to the much higher energy density than kerosene, using hydrogen can reduce the amount of fuel which needs to be carried onboard. Moreover, as a type of non-carbon fuels, burning hydrogen will be able to reduce the CO2 emission significantly, which makes it very attractive. Some efforts have been made to prove the feasibility of implementing the hydrogen in aviation. The “Cryoplane” concept under the 5th Framework Program of the European Commission is one of the examples. Conventionally, fuel is stored in the wings of an aircraft. However, the LH2 has to be stored in pressurized cryogenic cylindrical tanks, which doesn’t fit in the wings. Therefore, the fuselage was considered to accommodate the fuel tanks in the “Cryoplane”, as indicated in Figure 3 [3]. Although the fuel tanks are accommodated, this configuration is not so suitable from a passenger comfort, safety and aerodynamics point of view.

                               Figure 3. LH2 tanks storage scheme of “Cryoplane” project.

The multi-fuel Blended Wing Body Aircraft

To summarize, according to the current research efforts based on current technologies, there seems to be a very little chance to meet the ACARE goals, hence the breakthrough technology is needed. During the past years, an innovative Blended Wing Body (BWB) configuration has been studied by many researchers around the world including the “CleanEra” group from TUDelft, and it seems to be a promising candidate to replace the existing aircrafts. Instead of a separate fuselage with wings, an integration of body and wing is used for the BWB [4]. A larger amount of space available within the aircraft, thus making it possible to carry cylindrical fuel tanks to store the cryogenic fuel.
A novel way to overcome the storage problems of the hydrogen is a multi-fuel BWB aircraft presented in Figure 4. The wings of a BWB have sufficient room for storing LH2 tanks, without interfering with the passenger section. Further away from the central line where wing thickness is reduced, liquid biofuel can be stored. Thus, a multi-fuel BWB concept with a combination of biofuel and cryogenic fuel is proposed and investigated by the “AHEAD” project sponsored under the 7th Framework of EU.

                           Figure 4. A futuristic BWB aircraft layout with LH2 tanks& biofuels.

The Hybrid Engine

To power the multi-fuel BWB aircraft, a new type of propulsion system—called the hybrid engine—has been conceived, which is able to meet the requirements of the multi-fuel BWB aircraft. The novel features of this engine and its schematic is shown in Figure 5.

                                               Figure 5. Schematic of the Hybrid Engine.

The novel engine proposed is quite different than a conventional turbofan and includes many breakthrough technologies. The various novel technologies involved in the proposed engine configuration are described as follows.
 
Boundary Layer Ingestion (BLI): this is a method of increasing the propulsive efficiency of the engine by embedding the engine within the airframe such that the engine can ingest the low velocity boundary layer flow of the aircraft, reducing the engine ram drag. Also, the jet of the engine contributes to aircraft “wake filling”, thus reducing the overall dissipation.

Counter Rotating Fans (CRF): The aircraft-engine integration of future BWB aircraft presents unique challenges due to BLI. Such configurations also require that engines be smaller in diameter to reduce the nacelle-wetted area. Thus, it can be seen that the current trend of increasing bypass ratio and diameter of engines will not be able to meet the requirements of future BWB class of aircraft. The proposed hybrid engine with counter rotating fans has a smaller diameter and higher propulsive efficiency for the same bypass ratio. Furthermore, since each stage of the fan is less loaded than a single stage fan, a CRF can sustain more non-uniformities in the flow generated due to BLI compared to a conventional architecture. 

Bleed Cooling: With increasing pressure ratio, the temperature of bleed air (the air that is used for cooling the hot section components like the turbine blades and vanes) increases leading to the increase of the amount of bleed air required for the hot components cooling. This increase has an adverse effect on the thermodynamics of the gas turbine engine, reducing the efficiency of the cycle. The cryogenic fuel used in the proposed hybrid engine is an excellent heat sink which can be used for cooling the bleed air, therefore, reducing the amount of bleed air required. Meanwhile, the temperature of the cryogenic fuels is increased which reduces the use of combustion heat to increase its temperature, thus resulting in less fuel consumption for a given temperature within combustion chamber.

The Hybrid Dual Combustion System: The proposed innovative hybrid engine uses two combustion chambers as shown in Figure 5. The main combustor operates on LH2/LNG while the second combustor (between HPT and LPT) uses biofuel in the flameless combustion mode. Such a novel combustion system has never been used before for aero-engines. There are several advantages of this unique. Firstly, since the flammability limits of H2/Methane are wider than kerosene, the combustion can take place at lean conditions, thus reducing NOx emissions significantly compared to a conventional kerosene combustor. Secondly, the LH2 used for the first combustor can be used for cooling the bleed air (the features has been mentioned in the previous session). Moreover, using LH2 in the first combustion chamber will increase the concentration of water vapor and reduce the concentration of O2 in the second combustion chamber, thus creating a vitiated environment in which Flameless Combustion can be sustained. The implementation of the flameless combustion can minimize the emission of CO, NOx, UHC and soot. Additionally, the reduced emission of soot and UHC also reduces the amount of nucleation centers available for condensation of water vapor in the plume, thus reducing the contrail formation.

Results and conclusions

The “AHEAD” BWB aircraft is an environmentally friendly aircraft burning cryogenic fuels (like LNG\LH2) and biofuels. It is preliminarily designed for carrying around 300 passengers and flying 14000 km range. The comparison of the layout of the BWB to the Boeing 777-200ER is provided in Figure 6. The shorter and wider body of the aircraft makes it aerodynamically more efficient than a conventional cylindrical body aircraft. Combined with the advanced hybrid engine, the multi-fuel BWB is able to reduce CO2 emission by around 65% than a conventional Boeing 777-200ER aircraft.

                             Figure 6.Comparison of the BWB with the Boeing777-200ER.

References

1 Air transport action group (ATAG), “The economic & social benefits of air transport”, 2005.
2 Advisory Council for Aeronautics Research in Europe(ACARE), “Flight path 2050 Europe’s vision for aviation”, Report of the High Level Group on Aviation Research, Publication Office of European Union, 2011, ISBN 9789279197246.
3 Klug, H.G., and Reinhard, F., “CRYOPLANE: hydrogen fuelled aircraft — status and challenges”, Air & Space Europe, Vol. 3, No.3-4, May-August 2001, Pages 252-254
4 Liebeck R.H., “Design of the Blended Wing Body Subsonic Transport”, Journal of Aircraft, Vol. 41, No. 1, January-February 2004.