The growing need to drastically reduce aircrafts CO2 emissions has led engineers and scientists in the last years to develop a clean, renewable and sustainable energy system. Hydrogen as a fuel in aviation has shown to be a good choice, since it has an energy release much higher than common and long chain hydrocarbons (119.96 MJ/kg vs ∽42.8 MJ/kg, respectively), a wide flammability limits, a high diffusivity and a very short ignition time. Its thermal conductivity, the highest among all gases, its high heat capacity and its very low dynamic viscosity provide superior cooling properties for operation at high flight speeds and at combustor high temperatures. Furthermore, its low molecular weight makes it the fuel with the higher specific impulse (ISP), ∼450 s: this means that burning 1 kg/s of hydrogen with oxygen produces a thrust of 450 kg-force.
However, for its high combustion temperature, it has been demonstrated to be disadvantageous in terms of NOx production. Although NOx pollution is a relatively small part of global human pollution (less than 4%), a particular feature of air transport is that pollutants from air traffic are emitted at high altitudes, in the upper troposphere/lower stratosphere (8 to 12 km), where they are of greater influence than those emitted at ground level. Moreover, further emission reductions need to be achieved by the air transport community, since air traffic has a growth (3% to 5% per year) which exceeds the technology improvement rate.
Emissions may be controlled by operating at lean or very lean equivalence ratios (thanks to the wider flammability limits of the hydrogen-air flames compared to kerosene-air flames), or reducing the combustor length (thanks to the higher flame speed of hydrogen compared to other fuels), or via innovative strategies. In this paper, the RQL (Rich-Quench-Lean) strategy for the NOx abatement will be proposed for a high speed hydrogen fuelled vehicle.