In the coming decades the ubiquitous turbofan powered tube and wing aircraft configuration will be challenged by diminishing returns on investment with regards to fuel efficiency. From the engine perspective two routes to radically improved fuel efficiency are being explored; ultra-efficient low pressure systems and ultra-efficient core concepts.

The first route is characterized by the development of geared and open rotor engine architectures but also configurations where potential synergies between engine and aircraft installations are exploited. For the second route, disruptive technologies such as intercooling, intercooling and recuperation, constant volume combustion as well as novel high temperature materials for ultra-high pressure ratio engines are being considered.

—Grönstedt et al.

The goal of the ULTIMATE project is to explore and develop synergistic combinations of such radical technologies as described above to TRL2. The combinations are integrated into optimized engine concepts promising to deliver ultra-low emission engines.

Europe’s Strategic Research and Innovation Agenda (SRIA 2050) outlines a 75% reduction in aviation CO₂emissions to year 2050 relative to a year 2000 reference. This revolution in CO₂ emissions needs to be achieved while fulfilling a 90% NO_x and a 65% perceived noise reduction.

SRIA2050 does not specify how the CO₂ reduction contributions will be distributed between engine and airframe, only that a 68% total efficiency is targeted. The 2050 scenario explored by ULTIMATE envisions an ultra-efficient engine with a revolutionary core installed on an advanced tube and wing aircraft.

The EU is targeting a 75% reduction in carbon dioxide emissions per passenger kilometer between the years 2000 and 2050. Decades of investment in advanced technology, supported by EU programs such as Clean Sky, have made such ambitious goals feasible. To reach the 75% reduction target, we estimate that the last 18% will have to come from radical technology developed within our project.

—Tomas Grönstedt, Professor in Turbomachinery at Chalmers and coordinator of the project

ULTIMATE route to realizing the SRIA 2050 targets. Grönstedt et al. Click to enlarge.

The ULTIMATE technologies will need to address the three main sources of lost work potential in a current state-of-the-art turbofan:

1. Combustor loss. Adding heat through internal combustion generates considerable entropy. However, the state-of-the-art constant pressure type of combustion process (associated with 3-4% pressure drop) introduces unnecessarily high levels of irreversibility. The alternative of a constant volume combustion process gives a pressure rise, with the potential to reduce substantially the entropy increase needed for the temperature rise.

2. Core exhaust loss. The “Core exhaust” loss is primarily due to thermal energy lost to the surroundings, although some excess kinetic energy and fluid friction associated pressure losses contribute as well.

   A modern turbofan engine may run with a cruise core exhaust of 800 K (527 ˚C), which gives significant potential for energy recovery considering that the ambient temperature is about 600 K lower. Recovered heat may either be recuperated back into the cycle or captured by a secondary cycle.

   Recuperation would incur some additional irreversibility through pressure losses and finite temperature difference heat transfer. For a secondary cycle thermodynamic limitations, pressure and heat transfer associated losses will reduce the potential to convert the exhaust heat into useful power. Nevertheless, the ULTIMATE team believes it is worth exploring solutions that have the potential to recover large parts of the core exhaust losses through a dedicated technology.

3. Bypass flow loss. The “Bypass flow” loss is associated primarily with excess kinetic energy lost in the bypass jet, but also with fluid friction losses in the fan, bypass duct and the bypass nozzle.

   Radically reducing the excess kinetic energy is possible by increasing engine mass flow and reducing exhaust jet velocities (reducing specific thrust). Considerable research and development effort is being spent to introduce advanced geared and open rotor propulsors promising to recover a large part of these losses.

   Still, the team notes, further research is needed to provide breakthrough technologies to enable capturing as large part of the loss source as possible. To maximize the benefit for the radical technologies targeting the combustor and core exhaust losses, a greater degree of flexibility with respect to operability and variability is also expected to be needed, compared to propulsor technologies integrated on a conventional aero engine.

In the ASME paper, the researchers briefly discuss technologies that might address the three core sources of loss:

1. Combustor loss. A breakthrough reduction in the combustor component loss can be achieved by exploiting a constant volume type of process rather than the constant pressure process used in a state-of-the- art turbofan. Three technologies that might provide this benefit are: piston engine technology; nutating disc technology; and pulse detonation technology.

2. Core exhaust loss. A breakthrough reduction in core exhaust component losses can be achieved by technologies that substantially reduce the exit temperature in comparison with a state-of-the-art turbofan. Three technologies that could achieve this are recuperation; Rankine bottoming; and intercooling.

3. <p>Bypass flow loss. A large part of the reduction of this loss source is expected from the use of advanced powerplant architectures targeting ultra-high propulsive efficiency. In the ULTIMATE scenarios, this comprises an advanced geared turbofan engine for long-range missions and an open rotor concept primarily targeting short- and medium-range missions.

The ULTIMATE engine configurations will be integrated and evaluated on an advanced tube and wing (ATW) year 2050 aircraft platform. The long range intercontinental and the short range intra-European ATW concepts will be defined by exploring:

1. Aerodynamics: advanced very flexible slender in-plane wing; exploitation of passive or hybrid laminar flow on wing, empennage, forward fuselage and nacelles, riblets on the fuselage surface and shock contour bumps on wing upper surface.

2. Structures: Omnidirectional ply orientation according to the primary stress distribution; Nano-technologies with greatly reduced density and superior strength properties; geodesic fuselage design; advanced bonding; variable camber and cant control on wing; foldable wing concepts and adaptive structures applied to the engine cowl for optimizing propulsion system performance within the operating envelope.

3. Systems: introduction of a fuel-cell to serve as an auxiliary power generation device; and, wholesale application of a solely Direct Current (DC) power transmission architecture.

ULTIMATE technology screening and development process. Grönstedt et al. Click to enlarge.

Feasibility of the innovations is ensured through close collaboration with industry, so that concepts that would be unrealistic to put into practice can be discarded at an early stage. The engines to be developed must also be capable of meeting very stringent noise and NO_x emissions targets.

Results from the project will be used in industry roadmapping to plan technology acquisition for future aero engines. It is anticipated that these sophisticated technological solutions would bring many high technology jobs to Europe.

Resources

• Grönstedt, T. et al. “Ultra Low Emission Technology Innovations For Mid-Century Aircraft Turbine Engines” (2016) GT2016-56123