Evaluation of Current and Future Aviation Fuels at High-Pressure RQL-Type Combustor Conditions

De-carbonizing the aviation sector is one of the biggest challenges in order to minimize climate change effects associated with the emission of carbon dioxide. Sustainable aviation fuels (SAF) will play a major role in this context especially for long-distance flights where hydrogen or all-electric propulsion systems do not present a feasible alternative. In addition, aromatic-free aviation fuels offer a promising solution to lower soot emissions of aeroengines. Today, experience of SAF combustion under aeroengine-typical operation conditions is limited. In this paper five fuels samples, Jet A-1 as reference fuel, 2 blends and two neat synthesized paraffinic kerosenes from hydro-processed esters and fatty acids (HEFA-SPK) supplied by Neste were tested to characterize the combustion behaviour of drop-in / near drop-in fuels. These experiments were performed for the first time in an optically accessible high-pressure rich-quench-lean (RQL)-type combustor at typical aeroengine conditions. Optical diagnostics (OH* chemiluminescence, Mie scattering) as well as exhaust gas and particle sampling measurements were performed to delineate the fuel effects on spray, flame and emission characteristics. Measurements were performed at pressures up to 10 bar, air preheating temperatures up to 773 K and at primary zone equivalence ratios up to 1.66. The results showed that the liquid fuel distribution and fuel placement were sensitive to the fuel properties. By contrast, no pronounced influence of the fuel properties on flame position and shape were observed. As a consequence, NOx and CO emissions of the tested fuels differed only moderately. However, particulate matter measurements at the fuel richest operating condition showed a clear ranking of the five fuels with the lowest particle emissions evident for the two neat HEFA-SPK samples. Moreover, the particle volume concentration at the fuel richest condition followed the expected trend with fuel H-content.