Reducing soot emissions from jet engines is a major challenge to engine designers as new environmental regulations for aircraft and power turbines will require substantial reductions in both soot and NOx emissions. Adding another challenge to the work of the engine designer is the widening range of combustion fuel options and increased fuel composition variability that engines must tolerate.
Combustion simulation has the potential to help engine manufacturers design better engines in less time with less cost. Until now, however, combustion simulation has yet to be proven as a reliable design tool for the prediction of soot emissions. An increasing number of innovative engine designers are improving soot simulation accuracy by developing accurate chemical models and applying them in computational tools that can determine impacts of fuel composition and operating conditions.
Today’s advanced gas turbine designs target reduced pollutant emissions through combustion staging techniques such as the rich burn-quick mix-lean burn (RQL) combustion approach. But significant amounts of soot can be formed in the rich section of RQL combustors due to poor fuel-air mixing.
Soot formation is a difficult problem that involves complex chemistry combined with the complex physics of the flow in a combustor. As fuel molecules begin to decompose, certain precursor species are formed in rich regions that can combine together to form a soot particle that agglomerates with other soot particles and oxidizes. Fuels such as Jet-A and diesel are a blend of many different hydrocarbon species, and some of these species contribute more to soot formation than others.
Traditional combustion CFD tools are limited in their ability to make use of accurate fuel chemistry models and produce reliable results within an acceptable time-to-solution window. In an attempt to mitigate the chemistry limitations of traditional CFD tools, engineers use severely reduced chemistry models and then employ empirical correlations to predict soot emissions such as with the two-step particle model. This simple model employs only a single soot precursor (acetylene-C2H2).
In reality, acetylene is only one of many precursors that contribute to carbon growth and the formation of the single-ring molecule benzene (C6H6), which grows further to form polycyclic aromatic hydrocarbons (PAH) such as pyrene (C16H10), contributing to particle nucleation as well as to particle growth. Using a simplified model requires a substantial amount of experimental data and expert calibration to reproduce trends over a small range of operating conditions for each fuel of interest.
Accurate simulation of soot in engines requires both a validated fuel chemistry model for soot particle nucleation and a combustion simulation tool with a particle model that tracks soot particle size information. Validated chemistry models have been developed through efforts such as Reaction Design’s Model Fuels Consortium (MFC) that can be used with predictive simulation tools such as the ENERGICO software package.
A key premise of the MFC effort is that predictive combustion simulations require both accurate fuel chemistry models and software tools that can handle large mathematical calculations in a practical time frame. This approach has been successful for combustion predictions of liquid and gaseous fuels and has recently been extended to soot chemistry.
Detailed fuel chemistry models are validated with fundamental combustion experiments to ensure accuracy of soot precursors and soot size distribution with changes in fuel type and operating conditions. Reaction Design has applied the MFC soot chemical model on various gas turbine designs yielding reliable results.
The combustor flow field is modeled in CFD and mapped into a chemical reactor network in ENERGICO where the validated chemistry model can be applied to calculate soot emissions and identify regions of high soot formation. Designers then use this information to make design changes to reduce soot formation.
As a result of such developments, gas turbine engine designers now have the ability to apply a detailed understanding of fundamental soot chemistry to their combustion simulations. Validated fundamental soot chemistry models, such as those developed and validated by the MFC, are being used in engine simulations to predict soot emissions from contemporary engine designs that use a variety of fuels.
Engineers are also able to investigate soot formation in engines using an approach that encompasses multiple chemical reaction pathways to soot formation with acceptable time-to-solution. While the understanding of where soot is formed in an engine is a substantial advancement over previous numerical or experimental approaches, the ability to understand the impact of fuel effects on soot formation elevates combustion simulation to a must-have in modern engine design processes.
This article was written for Aerospace Engineering by Ellen Meeks, Reaction Design.