A Modelling Study of Allene and Propyne Combustion in Flames A. Gazi, G. Vourliotakis, G. Skevis * and M.A. Founti Laboratory of Heterogeneous Mixtures and Combustion Systems Thermal Engineering Section, School of Mechanical Engineering National Technical University of Athens, Greece Abstract The combustion chemistry of allene and propyne is critical for the breakdown of higher hydrocarbon fuels and for molecular growth processes. Allene and propyne consumption reactions are primary sources of the key propargyl and allyl radicals which are closely linked to benzene, PAH and soot formation paths. Further, allene and propyne are the smallest pair of isomers encountered in combustion studies. However, uncertainties still exist concerning aspects of the C 3 H 4 isomers chemistry and only recently have the first set of neat allene and propyne flame data appeared in the literature. The paper is part of an on-going effort aiming towards the optimization of a comprehensive kinetic mechanism for the high temperature combustion of small (C 1 -C 6 ) hydrocarbon species. The mechanism has been validated against species data from stoichiometric and fuel-rich laminar premixed allene and propyne flames with considerable success. A critical evaluation of C 3 H 4 consumption pathways has been carried out and the dynamics of benzene formation and destruction are discussed. * Corresponding author: gskevis@central.ntua.gr Proceedings of the European Combustion Meeting 2011 Introduction The combustion chemistry of allene and propyne is critical for the breakdown of higher hydrocarbon fuels and in molecular growth processes. Allene and propyne constitute the smallest pair of isomers encountered in combustion studies and provide an early insight into the effects of structural differences in combustion performance and emissions. There is also substantial experimental and numerical evidence linking the above species to the formation of benzene, PAH and eventually soot. Allene and propyne are the major sources of propargyl radical, particularly in flames of higher hydrocarbons. Propargyl radical recombination is considered the major benzene formation path under most fuel and flame conditions [e.g. 1, 2]. Further, the reaction of C 3 H 4 isomers with phenyl radical is a potentially important step for molecular growth in flames [e.g. 3]. Early work on allene and propyne combustion in flames was almost exclusively focused on doped flames. Pauwels et al. [4] studied the oxidation of allene in a rich, low-pressure laminar premixed hydrogen flame and obtained stable species concentration profiles using mass spectrometry. OH radical levels were also quantified using LIF. In a related work Miller et al. [5] studied the effect of allene addition to a rich acetylene flame. Quantitative OH and CH concentration profiles were also reported. Speciation data from allene-doped ethylene [6] and methane [6] flames have also been reported. Gueniche et al. [7] also studied propyne addition to a rich methane flame and concluded that isomeric effects on reactivity and product formation were minor although propyne-doped flames exhibited lower benzene formation rates than allene-doped flames. Recently advances in combustion diagnostics have provide the opportunity to distinguish between structural isomers in flames with considerable accuracy. Hansen et al. [8, 9] used flame-sampling photoionization molecular beam mass spectrometry with tunable vacuum-ultraviolet synchrotron radiation to uniquely obtain stable species profiles (up to phenol) in stoichiometric and rich allene and propyne laminar premixed flames. Several detailed kinetic mechanisms for the combustion chemistry of allene and propyne have been developed over the last two decades partly based on the above flame data. These include work by Curran et al. [10], Fournet et al. [11], Davis et al. [12] and Faravelli et al. [13]. More recently, Hansen et al. [8, 9] updated an existing detailed kinetic mechanism in order to model their flame data with success. Despite the successes of the above models several unresolved issues remain, primarily related to the initial fuel breakdown, the allene to propyne isomerization, the chemistry of other C 3 H x isomeric forms (e.g. C 3 H 2 ) as well as the molecular growth processes resulting eventually to C 6 isomers formation. The objective of the present work is to address the above uncertainties through the further development and validation of an existing detailed kinetic mechanism for C 1 -C 6 hydrocarbons. The work also provides a critical evaluation of C 3 species thermochemistry and recent experimental and theoretical kinetic data involving C 3 H X species. The developed mechanism is validated against the low-pressure, stoichiometric (φ = 1.0) and rich (φ = 1.80) allene and propyne laminar premixed flames [8, 9]. Extensive reaction path and sensitivity analyses are employed in order to delineate the early parts of allene and propyne combustion, in particular the branching between C 1 , C 2 and C 3 chains and the importance of isomerization reactions, and to the formation of higher hydrocarbon including benzene.