Modeling Fuel Effects on Aero Engine Out Gas Phase and Soot Emissions

Abstract

Implementing synthetic aviation turbine fuels (SATFs) in commercial jet engines requires an extensive understanding of fuel effects on the emissions, performance, stability and reliability of the engine. Detailed chemical kinetics analysis is a powerful tool to study fuel effects and fuel-engine interactions. However, using detailed chemistry in 3-dimensional computational fluid dynamics simulations is prohibitive. 1-dimensional engine models combined with detailed chemical mechanisms show a promising approach for analyzing fuel effects with reasonable computational cost.Using, a previously developed and validated 0D-1D combustor model of a CFM56-7B turbofan jet engine has been extended to include the turbomachinery and nozzle components, allowing for an analysis of engine-out emissions. An optimized gas-phase chemical kinetic mechanism, which includes nitrogen and sulfur oxides chemistry, was developed by reducing a comprehensive mechanism from 8365 to 896 species. A sectional method approach was combined with detailed soot chemistry to model soot emissions.  Multiple soot chemistry models were evaluated, leading to orders of magnitude different particulate mass results, showing the complexity of accurately simulating soot. The final model, validated against experimental measurements of Jet-A’s combustor exhaust temperature, nitrogen oxides (NOx), carbon monoxide (CO), and soot. After validation, three previously developed SATF surrogate fuels for ATJ (Alcohol-to-Jet), IPK (Iso-Paraffinic Kerosene), and a blend of ATJ and IPK were compared with a Jet-A surrogate across the operating conditions of the Landing and Take-Off (LTO) cycle.  Across all cases, the SATFs preformed similarly to each other. SATF showed slightly lower NOx and CO than Jet-A at take-off, but the differences are marginal. Despite showing a 15x decrease in 4-ring polycyclic aromatic hydrocarbons (PAH) at take-off conditions, which led to 60x lower soot nucleation rate, the particulate mass of SATF remained only 4.2 times lower than that of Jet-A. This is because soot emissions were mainly controlled by growth and PAH condensation rather than nucleation at these conditions, and the contribution of mono-aromatics to soot growth via condensation of benzene and phenylacetylene was similar in both SATF and Jet-A cases. Sulfur oxides (SOx) chemistry was analyzed and results showed that modeling the turbomachinery and nozzle was required to capture sulfuric acid formation.

Keywords: Synthetic Aviation Turbine Fuels, Jet A, Surface chemistry, Soot, emissions, Sulfuric Acid