Large Eddy Simulations of Conventional and Alternative Aviation Fuel Spray Breakup

Mid-term climate neutrality cornerstone policies by international governments and institutions target a climate-neutral aviation system by 2050. In this context, sustainable aviation fuels (SAFs) represent a drop-in key enabling technology to foster an effective transition of the aviation sector towards net zero carbon. Nonetheless, peculiar and unexplored properties of unconventional fuel blends may profoundly impact the performance and safe operability of jet engines in terms of altitude relight, lean blow-out, and cold-start ignition, as well as emission levels of specific pollutants. In this regard, computational fluid dynamics (CFD) offers a pivotal active support tool, partially or entirely replacing vast, expensive, and practically difficult experimental campaigns. In the present research study, we first formulate a four-component and a five-component surrogate mixture describing the thermophysical properties of a petroleum-derived conventional aviation fuel, Jet A-2 POSF-10325, and a synthetic alcohol-to-jet (ATJ) fuel, Jet C-1 POSF-11498, leveraging Bayesian inference techniques provided by the BayeSAF algorithm. As a preliminary analysis, we investigate the effects of chemical composition on the fuel vaporization process by addressing zero-dimensional isolated droplet evaporation in a low-vaporization rate quiescent environment for Jet A-2 and Jet C-1 through an in-house MATLAB code. The outcome of this analysis highlights non-negligible differences in droplet diameter evolution due to preferential vaporization effects, remarking on the fundamental role of surrogate mixture formulation in the numerical characterization of both conventional and alternative jet fuels. Thereafter, we carry out non-reacting Eulerian-Lagrangian large eddy simulations (LES) of Jet A-2 and Jet C-1 spray breakup within a referee combustor test rig equipped with a realistic gas turbine injection system. Numerical simulations show that the liquid spray topology remains unaltered from a qualitative standpoint regardless of the jet fuel category, with Lagrangian parcels following a conical pattern downstream of the swirler exit plane as a result of the hollow-cone spray injection strategy and the radial advection due to the high centrifugal forces induced by the establishment of a conical-type vortex breakdown regime. As a result, both Jet A-2 and Jet C-1 test case configurations exhibit almost negligible liquid volume fraction levels within the breakdown-induced recirculation region. Nonetheless, relevant differences arise in the radial distribution of the droplet axial velocity and Sauter mean diameter within the near-injector region. Notably, liquid parcels describing Jet C-1 evolution in the computational domain exhibit overall lower values of the Sauter mean diameter compared with Jet A-2, indicating a more efficient spray breakup process.