Chalmers University of Technology investigated the concept of pre-cooled cores with pulsed detonation combustion (PDC). The engine core is developed to power a counter-rotating open-rotor propulsor that includes a novel Boxprop design in the front rotor. Pre-cooling the core flow before detonation combustion, improves the volumetric efficiency, allows for increased combustion pressure ratios, reduces the risk of pre-ignition and reduces the engine cooling requirements. Such synergies outline a promising engine core concept that, combined with the innovative counter-rotating Boxprop design, will result in a quieter propulsion unit delivering significant improvements in performance and fuel burn. The engine concept, featuring year 2050 technology, is optimized for a typical intra-European missions and matured to TRL2.

Chalmers University investigated the combination of intercooling with pulsed detonation combustion (PDC) in turbofan engines. The flow is pre-cooled using a compact two-pass cross-flow heat-exchanger design located in a secondary bypass duct. The spent cooling air is ejected through a variable area nozzle and is recovered as thrust. The variable area nozzle allows for a greater control of the amount of rejected heat as well as a reduction of the intercooler external Mach number and its associated pressure losses. Intercooling before PDC, improves the volumetric efficiency, allows for increased combustion pressure ratios, reduces the risk of pre-ignition and reduces the engine cooling requirements.  The engine concept is matured to TRL2 and optimized for a typical long-range mission. This study allowed to estimate CO2 and NOx emissions associated with intercooled PDC technology, operating at relevant pressures and temperatures expected to occur in state of the art 2050 gas turbine applications for long-range civil aircraft. 

The schematic diagram shows the first of three advanced engine concepts researched in ULTIMATE at Cranfield University. Open rotor powerplants offer very high propulsive efficiency, but their small turbomachines suffer from relatively poor component efficiencies. This limits engine overall pressure ratio and thermal efficiency. Topping cycles improve engine thermal efficiency by adding pressure-rise combustion to increase overall pressure ratio and core specific power. Pressure-rise combustion could be realized using pulse detonation, wave rotors or piston engines, but this design concept uses compact ‘nutating disc’ machines. Six nutating disc modules are arranged between the core compressor and turbine stages. Colours in the diagram are indicative of component and fluid temperatures, with blue being cold and orange much hotter. However, the highest cycle temperatures are found in the nutating-disc module, shown here as a ‘black box’. Variant cycles have been analysed adding intercooling ahead of the nutating-disc modules and/or secondary combustion afterwards, to increase core specific power and reduce powerplant weight.

The schematic diagram shows the second advanced engine concept researched at Cranfield University in ULTIMATE. Bottoming cycles can improve engine efficiency by extracting extra energy from the engine’s exhaust heat. Open-circuit bottoming cycles use this heat to raise the temperature of compressed air and then generate more power by expanding this hot air through a turbine. Closed-circuit systems can use different working fluids, but need to cool the fluids before compressing and recirculating them. The schematic arrangement adds a supercritical carbon dioxide bottoming cycle to a turbofan engine. Supercritical CO2 gives more compact and efficient bottoming cycles than air or steam turbines, but the weight and drag of an air-cooled pre-cooler partly offsets this performance benefit. In the diagram the colours indicate component and fluid temperatures, with blue the coldest and red the hottest. A variant cycle increases core specific power by adding inter-turbine-reheat combustion, which increases the turbine exhaust gas temperature to generate more power from the bottoming cycle.

The schematic diagram shows the third ULTIMATE engine concept studied at Cranfield University. It combines an open-circuit air bottoming cycle with a ‘nutating disc’ topping cycle, intercooling and secondary combustion, exploiting potential synergies between these technologies. Intercooling and secondary combustion increase core specific power and reduce weight, while the topping and bottoming cycles improve thermal efficiency. The air bottoming cycle extracts extra power from the turbine exhaust heat. The six topping cycle modules each contain a disc compressor and a disc expander, together with an air accumulator and four pre-combustors. Machines like these, or piston engines, may replace existing core components in future turbofan or open rotor engines. The colours in the diagram indicate component and fluid temperatures, with blue being cold and red much hotter, though the highest temperatures are found within the nutating disc module, shown here as a ‘black box’. The reverse-flow turbine arrangement avoids compromising the high-speed turbine design and enables a quiet mixed exhaust arrangement.

ISAE has proposed a slotted inlet concept as a potential solution for enhancing the operability margins of a UHBR turbofan engine equipped with an ultra-thin and ultra-short nacelle.

Such a nacelle has low inlet contraction ratio and low highlight to maximum diameter ratio. This results in lower flow acceleration on the external side of the nacelle, which leads to lower wave drag, and lower flow acceleration along the inlet, which mitigates the speed overshoot before the fan face and thus prevents efficiency losses.

At low speed and high incidence conditions, however, the reduced inlet bluntness leads to excessive flow accelerations and possible flow separation, which damage engine performance and can compromise the stability margins of the fan.

The opening of the slot creates a convergent channel that provides an artificial increase in the aerodynamic contraction ratio, modifying the flow structure in a way that helps prevent separation and over-accelerations at the fan face region.

In the Secondary Fluid Recuperator (SFR) concept, two heat exchangers are installed inside the aero engine core. The first-one (cold-SFR, blue colour) is installed right before the combustor and the second one (hot-SFR, red colour) is installed downstream the low pressure turbine (LPT), inside the hot-gas exhaust nozzle. The two SFR heat exchangers are linked together by a separate internal closed circuit which is occupied by a secondary working fluid (i.e. liquid metal), that flows independently from the core engine air and gas flow, and combines favourable heat transfer and pressure loss characteristics. The design of SFRs is based on circular tubes placed in a counter-flow staggered arrangement. Each SFR is optimized independently based on available space and operational conditions for achieving high effectiveness and low pressure losses. Heat is transferred with the SFRs from the exhaust hot-gas region after LPT, to preheat the cold air before the combustor, resulting to significant reduction of fuel consumption and pollutant emissions.

The CCE combines the gas turbine and a piston engine into one engine concept. Instead of one combustion chamber followed by a High Pressure Turbine, the CCE features two banks of V-10 motors that drive the High Pressure Compressor (HPC). For compactness, the HPC has three axial and one radial stage. A high-speed turbine drives the low-pressure system. The extremely high peak pressure (300 bar at take-off) and temperature within the piston cylinders increase the overall engine´s thermal efficiency. An intercooler (not shown in the Figure) between the Intermediate Pressure Compressor (IPC) and the HPC helps to reduce the propulsion system weight by 18.5 % from 7400 kg to 6000 kg and makes the intercooled CCE the best performing option. It has a Thrust Specific Fuel Consumption (TSFC) of 11.5 g/kN/s in mid cruise and reduces design mission fuel burn by 52 % compared to a year 2000 engine. It outperforms a reference Geared Turbofan engine of the year 2050 by 12.5 % in fuel burn.