An extended methodology for automated calculations of non-Boltzmann kinetic sequences: H + C₂H₂ + X and combustion impact

Journal Article · Thu Oct 08 00:00:00 EDT 2020 · Proceedings of the Combustion Institute

DOI:https://doi.org/10.1016/j.proci.2020.06.385· OSTI ID:1853341

Lei, Lei ^[1]; Burke, Michael P. ^[2]

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1. Columbia University, New York, NY (United States); OSTI
2. Columbia University, New York, NY (United States)

It is generally assumed in phenomenological kinetic models that bimolecular reactions only occur between species whose rovibrational energy follows a Boltzmann (thermal) distribution. That is, any complexes initially formed in non-Boltzmann distributions are assumed to be thermalized by energy-transferring collisions prior to bimolecular reactions. Given the high mole fractions of reactive species, X, in combustion environments, reactive collisions of the complexes with X often occur on the same timescale as energy-transferring collisions – yielding sequences proceeding through non-Boltzmann intermediates across multiple potential energy surfaces. Recent studies have shown that such non-Boltzmann kinetic sequences can have substantial impact on the global reactivity in combustion systems. Simulations of these non-Boltzmann reaction sequences, which can be described in phenomenological kinetic models via chemically termolecular reactions, require that rovibrational excitation from one potential energy surface be carried over to the next. Here this paper presents an extended theoretical and computational methodology that couples multiple master equations and derives rate constants for phenomenological reactions describing the conversion of thermal reactants to thermal products for use in phenomenological kinetic schemes. The methodology is then implemented using in-house scripts for non-Boltzmann sequences involving C2H$$^*_3$$ + X (with X = O₂, H, and OH) where C₂H$$^*_3$$ is formed via H + C₂H₂ association – which were identified as having strong potential for influencing combustion predictions in a recent study. The results reveal that non-Boltzmann reaction sequences for X = O₂ (the primary focus of this paper) significantly alters the total conversion rate from H + C₂H₂ to products and product branching fractions from those of thermal sequential pathways. Furthermore, the present results demonstrate that non-Boltzmann reaction sequences have significant impact – as high as an order of magnitude – on predicted ignition delay times. Similarly, they yield significantly different dependence of ignition delay times with temperature and O₂ mole fraction – yielding signatures that are likely observable experimentally.

• Research Organization: Columbia University, New York, NY (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Science (SC); USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0019487
• OSTI ID: 1853341
• Alternate ID(s): OSTI ID: 1809657
• Journal Information: Proceedings of the Combustion Institute, Journal Name: Proceedings of the Combustion Institute Journal Issue: 1 Vol. 38; ISSN 1540-7489
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
chemically termolecular reactions
non-Boltzmann
rovibrationally excited states