Comparative Analysis of Forced Ignition Characteristics of Alcohol-to-Jet Sustainable Aviation Fuel and Jet-A Fuel in a Scramjet Combustor

The adoption of Sustainable Aviation Fuel (SAF) in future scramjet combustors requires an understanding of how alcohol-to-jet (ATJ) fuel ignites compared with conventional Jet-A fuel under supersonic combustion conditions. Using a Forced Ignition Model in Shear Layer (FIMS) coupled with detailed Hybrid Chemistry (HyChem) reaction mechanisms, the forced ignition limits of GEVO ATJ and Jet-A fuels were predicted using the Damköhler number over a wide range of freestream total temperatures, torch input energies, and cavity geometries. The ignition delay times and Damköhler numbers of ATJ fuel were consistently longer and lower than those of Jet-A fuel, respectively because the ignition of the former was governed by its fuel-pyrolysis pathway, which yields less reactive branched-olefin intermediates and suppresses radical growth, rather than by the oxidation of pyrolysis products, as confirmed by species evolution and reaction sensitivity analyses. Predictions based on the cavity shear-layer residence time alone substantially underestimated the ignition capability of both fuels. Introducing an effective residence time that includes the downstream constant-area section was essential to capture the continued reaction and flame-kernel growth, and showed that forced ignition of ATJ fuel is achievable under sufficiently high freestream temperature, torch input energy, and residence time. These findings clarify the chemical origin of the reduced ignitability of ATJ fuel and provide design guidelines for the ignition and flame-holding systems of future scramjet combustors employing SAFs.