Analytical Formulations for Nitrogen Oxides Emissions Estimation of an Air Turbo-Rocket Engine Using Hydrogen

According to the latest report of the Intergovernmental Panel on climate change, aviation contributes to only about 2% to anthropogenic global greenhouse gas (GHG) emissions. However, in view of the growing market demand and the dramatic reductions in other transport sectors, including maritime and automotive, the aviation sector’s percentage impact on global GHG emissions is expected to reach 50% of the transport share by 2040. High-speed aviation exploiting liquid hydrogen as the propellant can represent a valuable solution toward the decarbonization of the sector. However, to avoid jeopardizing the dream of a new generation of high-speed aircraft, it will be necessary to introduce non-CO2 emissions estimations beginning with the design process. To unlock the possibility of anticipating the nitrogen oxides emissions estimation, the authors developed the hydrogen and high-speed P3-T3 methodology (H2-P3T3), an evolution of the widely used P3-T3 method, properly conceived to support (i) innovative air-breathing propulsive systems for supersonic and hypersonic flights and (ii) greener fuels, such as hydrogen. This paper presents a step-by-step approach to developing novel analytical formulations customized for an Air Turbo-Rocket engine and discusses the discovered correlation of nitrogen oxides production with the fuel-to-air ratio (FAR), the Mach number, and the Damköhler number (Da), the last being a nondimensional variable directly related to hydrogen/air combustion, considering the matching between the residence time and the ignition delay times. The most complete formulation allows for reduction in the prediction errors below 5%.