The Role of Diffusive Mixing in Current and Future Aviation Fuels at Relevant Operating Conditions

Abstract With aviation's dependence on the high volumetric energy density offered by liquid fuels, Sustainable Aviation Fuels (SAFs) could offer the fastest path towards the decarbonization of aircrafts. However, the chemical properties of SAFs present new challenges, and research is needed to better understand their injection, combustion and emission processes. One of these processes in particular is about droplet evaporation at relevant pressures and temperatures, and this represents the focus of the present manuscript. To address this gap we characterized the evaporation and mixing of spray droplets at conditions relevant to modern and next generation aero-engine combustors. We tested three fuels from the National Jet Fuel Combustion Program, namely an average Jet A fuel (A-2), an alcohol-to-jet fuel (C-1), and a blend made of 40 % C-1 and 60 % iso-paraffins (C-4). We also tested a single component normal alkane: n-dodecane, as well as an advanced bio-derived cyclo-alkane fuel: bicyclohexyl. The time evolution of fuel droplets was monitored using high-speed long-distance microscopy. The collected images were processed using a purposely-developed and trained machine learning (ML) algorithm to detect and characterize the droplets' evaporation regime. The results revealed different evaporation regimes, such as classical and diffusive. In agreement with previous studies, evaporation regimes appear to be controlled by ambient pressure, temperature, and fuel type. The measurements demonstrate that diffusive evaporation is relevant at high-pressure conditions, such as take-off combustor pressures for modern commercial aircraft engines.