Session: 04-40: Emissions III

Paper Number: 153057

Combustion and Soot Formation From Sustainable Aviation Fuels Near the Lean Sooting Limit 

With the increasing demand for aviation transportation, the aviation-related emissions are being scrutinized more seriously regarding climate change. Sustainable aviation fuels (SAFs) present an opportunity to lower the carbon footprint while meeting current criteria and standards for aircraft fleets. Beyond decarbonizing the aviation industry from a fuel production’s perspective, another advantage of SAFs is the anticipated lower soot emissions. One caveat is that current fuel standards require a minimum amount of aromatic, to ensure proper seal swelling, a characteristic supported by aromatics, but not by currently certified SAFs. Cyclo-alkanes are seen as an option to both lower soot emissions, while maintaining seal swell properties.

 

This study investigates the spray combustion of SAFs in an optically-accessible high-pressure and high-temperature chamber, where conditions representative of modern and next generation aero-engine are emulated. Seven fuels were tested: Three advanced cyclo-alkanes that were blended with hydroprocessed esters and fatty acids synthetic paraffinic kerosine (HEFA-SPK), were compared to neat HEFA-SPK, n-dodecane, n-butylcyclohexane and low-sulfur Jet A, in the role of our reference fuel. The fuels were injected via a single-hole injector into quiescent environments with the aim to target mixtures in the combustion region that align with equivalence ratio conditions in modern aero-engine combustors. Three advanced high-speed optical diagnostics were applied to understand the combustion processes of SAFs: Diffuse back illumination extinction imaging (DBI-EI) was used to measure liquid penetration and soot concentration/mass via optical extinction. Planar laser-induced fluorescence (PLIF) with a 355-nm excitation was applied to characterize low-temperature reactions by monitoring the formation and consumption of CH₂O formation as well as the production of polycyclic aromatic hydrocarbons (PAHs) as soot precursors, while OH* chemiluminescence was utilized to capture ignition and flame stabilization during high-temperature reactions.

 

The results showed that the liquid phase behaved similarly across fuels, with low-sulfur Jet A showing slightly longer liquid penetration. The formation of CH₂O is fastest in dodecane (n-alkane), followed by HEFA-SPK (iso-alkane), and then n-Butylcyclohexane (cyclo-alkanes). Ignition and flame stabilization measurements showed that low-sulfur Jet A featured the lowest reactivity across all conditions, while the advanced cyclo-alkane blends demonstrated good combustion robustness. While the results confirmed the fair correlation between ignition delay and derived cetane number under some conditions, the cetane number metric failed to capture reactivity across the board and proved not to be an appropriate metric for flame stabilization. A metric to provide an estimate of a fuel’s ability to sustain combustion under relevant high-pressure conditions was proposed. The soot measurements followed the trends expected based on the literature regarding the classification of fuel type from a soot production level from low to high sooting propensity: n-alkanes, iso-alkanes, cyclo-alkanes, and aromatics, when accounting for differences in fuel reactivity, with the introduction of the soot production index. This index showed that the advanced cyclo-alkane blends produced equivalent or lower soot levels compared to neat HEFA-SPK while presumably offering seal swelling characteristics in line with jet fuel requirements.

Presenting Author: Kevin Wan Sandia National Laboratories

Presenting Author Biography: Kevin Wan is a researcher at Sandia National Laboratories at the Combustion Research Facility, focusing on advanced spray, combustion, and emission studies under engine relevant conditions. He received his PhD in mechanical engineering from Stanford University in 2020. His research explores the intricacies of fuel mixing, combustion, and soot formation in the context of renewable and sustainable fuel technologies, contributing to cutting-edge research in energy and sustainability.

Authors:

Kevin Wan Sandia National Laboratories
Junghwa Yi Sandia National Laboratories
Julien Manin Sandia National Laboratories