Emissions estimation for a liquid hydrogen powered hypersonic aircraft in the conceptual design phase

The effects of the climate crisis needs to be addressed in order to safeguard our Planet and human life. For this purpose, the H2020 STRATOFLY project aims at developing a hypersonic hydrogen-fueled vehicle that will be able to fly antipodal routes without emitting carbon oxides. The reference vehicle is STRATOFLY MR3, which is powered by six Air-Turbo Rocket (ATR) engines and a single Dual-Mode Ramjet (DMR) engine. The ATR engines are operational from take-off up to Mach 4, after which they are gradually shut down while the DMR is activated to reach Mach 8. The cruise takes place at Mach 8 and at an altitude between 32 and 36 km. The focus of the thesis consists of developing parametric models able to predict nitrogen oxides (NOx) emissions of the STRATOFLY MR3 vehicle since the conceptual design phase. The work was conducted in close cooperation with the Italian Aerospace Research Centre (CIRA). A propulsive and emissive database built with the CIRA in-house SPREAD code has been provided and it has been further enriched thanks to 0D simulations by the Cantera software. After an examination of the prediction methods available in the literature, the “P3-T3” method was selected to assess the NOx emissions. However, since this method has been specifically tailored for subsonic kerosene-fueled turbofans, it must be adapted to a high-speed hydrogen-fueled aircraft. Therefore, new formulations were sought in order to decrease the estimation errors with respect to the original formulation. After applying the original formulation of the P3-T3 method to the case study, a first variation was accomplished by optimizing its exponents. The following modifications consist of including first the flight level Mach number and then the Damköhler number, i.e. the ratio between the flow residence time and the ignition delay time of the hydrogen/air mixture. This workflow has been applied to both the ATR engines and the DMR engine, selecting for each case the most accurate formulation. Concerning the ATR, the sea level conditions have been assumed as a reference, while for the DMR a new suitable reference has been selected since this engine is switched-off at the ground level. Excluding the mission segment between Mach 4 and 5 where ATR and DMR are operating jointly, the Mach 6 condition has been selected as the 100% thrust value from which the additional four throttle settings conditions have been obtained. For both types of engines, the formulation providing the most accurate prediction is the one that calculates EINO as a function of the combustor inlet pressure (p3), the fuel-to-air ratio (FAR), the Mach number (M) and the Damköhler number (Da). In order to verify the independence of the "P3–T3” method from the database, the DLR–Stöppler method was considered and upgraded following the same methodology described for the "P3–T3" method. Furthermore, an experimental validation database for the Z22 kinetic scheme employed in this work was sought out. The work carried out has shown promising results. The novel formulations of the "P3–T3" method obtained both for the ATR and the DMR provide a very accurate EINO estimation with respect to the STRATOFLY MR3 database. However, the DMR could benefit from further study, especially regarding the selection of the reference conditions. The DLR–Stöppler method also offers an acceptable prediction of the EINO of the ATR and the DMR, but this method is not the preferred one for the conceptual design as it requires numerous input variables.