Large Eddy Simulations of Alternative Jet Fuel Combustion

Sustainable Aviation Fuels (SAF) are the most promising strategy for reducing the carbon emissions of aviation in the near term, but their combustion behavior is not well understood compared to conventional fuels. This work investigates how different properties of SAF may lead to different emergent combustion behavior by targeting a small set of very distinct fuels. The investigation is carried out numerically using reactive Large Eddy Simulations (LES) and Finite Rate Chemistry (FRC), incorporating the effects of turbulence as well as crucial chemical differences between the fuels. Two cases are studied: a simple premixed bluff body flame and a generic aeroengine-like spray combustor operated at idle and cruise conditions. The targeted fuels include conventional Jet A, a pure alcohol-to-jet SAF called C1, and an aromatics-heavy test fuel called C5. A set of correlations is developed to capture the liquid properties of these fuels across all relevant temperatures. Combustion chemistry is modeled using two sets of pathway-centric reaction mechanisms that are detailed enough to differentiate between fuels. All simulations are validated against available experimental data and show good agreement.The results reveal several distinct fuel trends. In premixed flames, the size of the flame sheet is correlated with the flame temperature. Different fuels experience different levels of flame surface fluctuations, which are found to be correlated with Cetane number. A novel flame-dampening mechanism is proposed to explain this connection.In spray flames, the fuels behave similarly to each other at idle conditions, where premixed burning dominates. The spray length and emission profile of each fuel is demonstrably determined by its vaporizability and hydrogen/carbon ratio, respectively. At cruise conditions, where there is less premixing, new trends emerge. The more volatile fuels C1 and C5 have more compact flames than Jet A; this causes them to burn at a higher equivalence ratio, increasing NOx emissions. C1 and C5 also experience stronger thermoacoustic fluctuations, which is likely caused by their high vaporizability but may also be linked to their low Cetane numbers relative to Jet A.