Dominant reaction pathways for high-pressure soot formation

Significance 

Improving combustion efficiency is one promising way of reducing the consumption of fossil fuels and subsequent reduction of carbon footprint. Typically, engine combustors need to operate at high pressure to achieve high efficiencies, and further improvements in thermal efficiency require even higher pressure. However, high pressures increase the amount of soot yield, posing a significant research challenge. This phenomenon has been attributed to a higher temperature, increased gas density and steep soot concentration gradients. To date, numerous studies involving numerical simulations of pressurized flames have been conducted to investigate soot formation at elevated pressures. These numerical investigations produced different accounts explaining the underlying mechanisms behind the increase in soot formation with increased pressures.

For co-flow diffusion flames, changes in the residence time have negligible impacts on soot yield. However, the effects of chemical kinetics on soot precursor formation are reportedly significant due to the presence of several pressure-dependent reactions. Unfortunately, there are limited to no attempts to comprehensively investigate the effects of pressures and how changes in reaction pathways affect soot formation and evolution. This can be realized through a detailed numerical analysis which requires the use of systematic diagnostic tools to extract the required information. Global pathway analysis (GPA) is an effective technique for quantitative and automatic analysis of reacting systems and has been successfully used to study turbulent and laminar flames. Thus, it is a promising approach for studying the physics behind the formation of soot in high-pressure combustion.

On this account, Dr. Dezhi Zhou and Professor Suo Yang from the University of Minnesota–Twin Cities designed a new soot-based GPA (SGPA) for a systematic and detailed analysis of chemical kinetics underpinning the formation and evolution of soot at elevated pressures in co-flow diffusion flames. They performed a series of pressurized co-flow ethylene diffusion sooting flames simulations at pressure range 1 – 16 atm via molecular transport and finite-rate chemistry. Soot formation modeling was based on bivariate hybrid method of moments. Additionally, SGPA was used to determine the dominant reaction pathways at different pressures and the pressure effects on the Polycyclic Aromatic Hydrocarbon (PAH) formation, growth and evolution. Their research work is currently published in the research journal, Combustion and Flame.

The authors demonstrated the feasibility of the newly proposed SGPA in identifying the dominance and corresponding sensitivity of soot reaction pathways at elevated pressures and the global pathways responsible for controlling PAH kinetics. An increase in pressure shifts was characterized by the formation of the first PAH ring from the recombination of C3H3 and C2H2 reactions. While at low pressure (1 atm) the production of C2H2 was dominated by H-abstraction of C2H3 and C2H4, its production at elevated pressures (4 – 12 atm) was dependent on other reactions like third body reactions. On the other hand, condensation and nucleation at the fuel-rich regions were influenced by reactions related to A2R5, C9H8 and C9H7. Notably, the discrepancies in the predictions of PAH at elevated pressures were due to coefficient uncertainty of the C2H2 + A1CH2 = C9H8 + H reaction.

In summary, Dr. Dezhi Zhou and Professor Suo Yang conducted successfully an experimental simulation of ethylene-air co-flow diffusion flames to investigate the formation and evolution of soot at elevated pressures. The simulation data was consistent with the experimental data, even though a slight increase in discrepancy was observed with an increase in pressure. From the results, mechanism reduction without considering the effects of global carbon flux could delete important species like C9H8 and C9H7, resulting in incorrect soot distribution and prediction at elevated pressures. In contrast, the combined dominance of global pathways with heavy PAH species was of more significance than global pathways at flame wing regions, suggesting the critical role of PAH species in soot formation and evolution. In a statement to Advances in Engineering, Professor Suo Yang stated that the study proposes to modify the SGPA method into an autonomous tool for studying chemical mechanisms underlying soot formation.

Dominant reaction pathways for high-pressure soot formation - Advances in Engineering
Predictions of soot volume fraction at different pressures
Dominant reaction pathways for high-pressure soot formation - Advances in Engineering
The soot formation and evolution dominant chemical global pathways at different flame regions

About the author

Dr. Dezhi Zhou received his bachelor’s degree from Shanghai Jiao Tong University in 2013; Ph.D. from National University of Singapore in 2017. He then worked as a research fellow in National University of Singapore from 2017-2019, and subsequently as a research associate in University of Minnesota – Twin Cities, where he worked on this research. Dr. Zhou joined the University of Michigan – Shanghai Jiao Tong University (UM-SJTU) Joint Institute (JI) as an Assistant Professor in 2021. Dr Zhou’s research interests include high-fidelity multi-scale and multi-phase reacting flow simulations. His research covers both low-Mach and high-speed combustion, with the applications in internal combustion engines and gas turbines. He is experienced in aerosol modeling for understanding the formation of soot/nanoparticles in flames. He is also interested in developing high performance algorithms to accelerate the high-fidelity reacting flow simulations.

About the author

Dr. Suo Yang is a Richard & Barbara Nelson Assistant Professor of Mechanical Engineering at the University of Minnesota – Twin Cities. During 2017-2018, he was a Postdoctoral Research Associate in Mechanical & Aerospace Engineering at Princeton University. He received Ph.D. (2017) and M.S. (2014) degrees in Aerospace Engineering, and another M.S. degree in Computational Science & Engineering (2015), all from Georgia Institute of Technology. He received a B.Sc. degree in Mathematics & Applied Mathematics from Zhejiang University in 2011. His research focuses on the modeling and simulation of laminar and turbulent reacting flows, including combustion, plasma, particulate and multiphase flows. He has authored more than 40 papers in refereed journals and conferences, with Google h-index of 17 and i10-Index of 21. He is a recipient of the Defense Advanced Research Projects Agency (DARPA) Young Faculty Award (YFA).

He is a Senior Member of the American Institute of Aeronautics and Astronautics (AIAA), a Committee Member of the AIAA Propellants and Combustion Technical Committee, AIAA Plasmadynamics and Lasers Technical Committee, and AIAA High Speed Air Breathing Propulsion Technical Committee. His research is supported by National Science Foundation (NSF), Army Research Laboratory/Office (ARL/ARO), and DARPA.

Reference

Zhou, D., & Yang, S. (2021). Soot-based Global Pathway Analysis: Soot formation and evolution at elevated pressures in co-flow diffusion flamesCombustion and Flame, 227, 255-270.

Go To Combustion and Flame

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