Civil air transportation has undergone significant expansion over the past decades and is continuing to grow. Nevertheless, the tendency of energy depletion and the severe environmental problems yield challenges in its further development. To mitigate the climate impact of civil aviation, the Advisory Council for Aeronautics Research in Europe (ACARE) has set ambitious objectives for the year 2050 to reduce the CO2 emission by 75% per passenger kilometre, the NOx emissions by 90% and the perceived noise emission by 65% relative to the capacities of aircraft operating in the year 2000.
The conventional approach of increasing Bypass Ratio (BPR), Overall Pressure Ratio (OPR), and Turbine Inlet Temperature (TIT) to improve the cycle efficiency, and thereby reducing the fossil fuel consumption and the associated emissions is unlikely to meet the ACARE goals. Moreover, the high OPR and TIT aggravate the NOx emissions for a given combustion technique. A novel multi-fuel hybrid engine for a Multi-Fuel Blended Wing Body (MFBWB) aircraft conceived in the “Advanced Hybrid Engine for Aircraft Development (AHEAD)” project brings to light promising solutions in this regard.
The multi-fuel hybrid engine is a turbofan engine with the following added components: a Contra-Rotating Fans (CRF) system, two sequential combustors burning different fuels simultaneously, and a Cryogenic Bleed Air Cooling System (CBACS). The CRF can sustain the non-uniform flow ingested from the boundary layer of the airframe. The first combustor is the main combustor, where the Liquid Hydrogen (LH2) or the Liquid Natural Gas (LNG) is burnt to reduce CO2. The second combustor, Interstage Turbine Burner (ITB), is located between the high pressure and the low pressure turbine burning kerosene or biofuel in a Flameless Combustion (FC) mode. With the thermal energy provided by different fuel sources, the volume required to store cryogenic fuels is less; meanwhile, the FC technique is beneficial to reduce NOx. By introducing the CBACS, LH2 or LNG is used as a coolant to cool down the bleed air.
According to fuel combinations, the hybrid engine is classified as LNG-kerosene version and LH2-kerosene version, where kerosene might be replaced by biofuel. By defining an “ITB energy fraction” as the ratio of the energy input of the ITB to the overall energy consumed, the fuel flow rates of two combustors are controlled. Using the developed model framework, the characteristics of the hybrid engine are studied and summarized in the following three aspects:
Potentials of the ITB engine cycle:
The sequential combustor configuration of the hybrid engine forms a reheat cycle. By distributing the energy into two combustors, the heat addition to each combustor decreases; therefore, the TIT is lower. Consequently, the turbine cooling air and the associated loss in the turbine efficiency reduces. Moreover, the NOx produced from the upstream combustor dissociates again in the ITB, which helps to lower the overall NOx emissions. These remarkable features are appreciable when the OPR and BPR are forced to continuously increase, which causes a substantial increase in the TIT of a classical engine. A turbine with very high inlet temperature has to be cooled substantially. Eventually, the gain in cycle efficiency might be canceled by the loss in the turbine efficiency. Moreover, when the TIT is increased beyond 1800 K, the NOx exhibits an exponential increase. Hence following the evolution of the engine technology, the reheat cycle would be an option for the next step.
Characteristics of the multi-fuel hybrid engine:
The features of the hybrid engine have been explored from various aspects. The isobaric heat capacity of the combustion products from LNG and LH2 is higher than that from kerosene, which is beneficial to the thermal efficiency. Using LNG and LH2 as a coolant, the bleed air temperature reduces substantially (maximum by more than 500 K), thereby, the turbine cooling air mass flow rate decreases by half. Moreover, the increase in fuel temperature is favourable to enhance the thermal efficiency. The hybrid engine has been optimized at cruise considering various ITB energy fractions. The optimized engine cycle is verified at critical operating points. The assessment of the standalone engine performance with baseline engines shows that the LH2-kerosene hybrid engine is superior to the LNG-kerosene hybrid engine in terms of the cycle efficiency and the CO2 reduction. However, the mission analysis shows conflicting results. Due to the stronger installation effect, the MFBWB together with the LH2-kerosene hybrid engine scores lower, implying that the LNG-kerosene BWB would have the least climate impact.
Operating strategy of the multi-fuel hybrid engine:
The operating strategy of the hybrid combustion system has been developed to enhance the steady state performance of the hybrid engine. The analysis exhibits that using an ITB is beneficial for the high pressure spool speed, the HPC exit temperature, and the HPT inlet temperature. However, the LPC surge margin and LPT inlet temperature conflict their limits as the ITB energy fraction increases. For the various thrust requirements at Sea Level Static (SLS) standard condition, a fuel control schedule together with a variable bleed valve schedule is proposed. Moreover, another fuel control strategy is suggested for the flat rating at SLS.