SCI Publications

The approach of mechanism generation is the accepted one of assigning generic rates to reactions in the same class. The procedure has been successfully applied to higher paraffins that include detailed sub-models of n-hexane, cyclohexane, n-heptane, n-decane, n-dodecane, and n-hexadecane and semi-detailed sub-models of iso-octane and methyl cyclohexane, in addition to reactions of aromatic formation and oxidation. Comparison between predictions and experimental data were found to be satisfactory for n-heptane, iso-octane, n-decane and gasoline premixed flames. The mechanism was also able to reproduce the measured concentrations for a n-hexadecane experiment in a jet stirred reactor. The numerical accuracy in predicting the flame structures of soot precursors, including acetylene and benzene, is one of the major foci of this study. The predicted maximum concentrations of acetylene and benzene are within 20% for most flames in this study.

Three different n-heptane mechanisms were used to simulate a fuel-rich normal heptane premixed flame in order to identify major reaction pathways for olefin formation and consumption and areas of uncertainties of these reactions. Olefins are formed mainly via β-scission and hydrogen abstraction, and smaller olefins are sometimes formed by combination of allylic radicals and H/CH₃ radicals. Olefins are consumed by direct decomposition, radical-addition, and hydrogen-abstraction reactions. Isomerization between alkyl radicals plays an important role in olefin formation and in determining olefin species distribution. Peroxy radicals contribute to the olefin formation in the low-temperature region, but further studies are needed to resolve many uncertainties. Simulation results using the Pitsch, LLNL, and Utah heptane mechanisms were compared to experimental concentration profiles of selected species, and the uncertainties in the olefin chemistry thus identified are discussed. The discrepancies in the computed concentrations of most olefin species are usually due to the combined effects of uncertainties in the kinetics of β-scission and isomerization reactions. Resolving these uncertainties in n-heptane combustion chemistry is critical for building practical mechanisms for the larger paraffins that are major components of liquid aviation and diesel transportation fuels. In addition, olefin chemistry is critical to any combustion mechanisms that focus on the soot formation, because products of olefin decomposition such as C₃H_x and C₄H_x species are well-known precursors for the formation of the first aromatic ring.

Reaction pathway analyses were conducted for three mechanisms (designated as the Pitsch, Utah, and Lawrence Livermore National Lab) for a normal heptane premixed flame (Φ = 1.9) and a normal heptane opposed diffusion flame, in order to identify the relative importance of the major fuel consumption pathways in the two flame classes. In premixed flames, hydrogen abstraction is found to be the major fuel consumption route although it is surpassed by thermal decomposition when the flame temperature exceeds 1400 - 1500 K. At the higher temperatures, however, little fuel remains in a premixed flame so that thermal decomposition provides a minor pathway for overall fuel decomposition. The principal abstractor is the hydrogen radical in all three mechanisms with the hydroxyl radical having a secondary role. In opposed diffusion flames, thermal decomposition competes with hydrogen abstraction in providing the major pathway for fuel consumption. Thermal decomposition becomes important when a large fraction of the fuel reaches the high-temperature zone in a flame. By understanding the relative importance of competing fuel consumption pathways, mechanisms can be tailored to each specific application by eliminating or lumping insignificant reactions. The results obtained in this study for n-heptane may be used to guide the reduction of existing mechanisms for a particular application or the generation of mechanisms for the combustion of larger paraffins that are major components of liquid aviation and transportation fuels.

The Utah Surrogate Mechanism was extended in order to model a stoichiometric premixed cyclohexane flame (P = 30 Torr). Generic rates were assigned to reaction classes of hydrogen abstraction, beta scission, and isomerization, and the resulting mechanism was found to be adequate in describing the combustion chemistry of cyclohexane. Satisfactory results were obtained in comparison with the experimental data of oxygen, major products and important intermediates, which include major soot precursors of C2-C5 unsaturated species. Measured concentrations of immediate products of fuel decomposition were also successfully reproduced. For example, the maximum concentrations of benzene and 1,3-butadiene, two major fuel decomposition products via competing pathways, were predicted within 10% of the measured values. Ring-opening reactions compete with those of cascading dehydrogenation for the decomposition of the conjugate cyclohexyl radical. The major ring-opening pathways produce 1-buten-4-yl radical, molecular ethylene, and 1,3-butadiene. The butadiene species is formed via beta scission after a 1-4 internal hydrogen migration of 1-hexen-6-yl radical. Cascading dehydrogenation also makes an important contribution to the fuel decomposition and provides the exclusive formation pathway of benzene. Benzene formation routes via combination of C2-C4 hydrocarbon fragments were found to be insignificant under current flame conditions, inferred by the later concentration peak of fulvene, in comparison with benzene, because the analogous species series for benzene formation via dehydrogenation was found to be precursors with regard to parent species of fulvene.

Normal heptane, isooctane and cyclohexane have been the most interested surrogate components for liquid transportation and aviation fuels, due to their roles as indicative fuels for octane number and the representative compounds for normal, iso and cyclo-paraffins. Methodologies of mechanism generation for these representative fuel fractions have been discussed in detail in literature. The basics of fuel consumption in flames have been discussed by Vovelle1, Ranzi2, Zhang3 and coworkers, among others. Ranzi et al.2 presented a lumping technique that was also discussed in detail in an earlier study3 and used for generation of reaction mechanisms that can be used to model flames of liquid fuels. The lumping approach is an effective reduction technique for models of large aliphatic fuels. Reaction pathway analysis presents another reduction technique that was used to reduce a complete kinetic set to smaller models. Doute et al.4 reduced a n-decane model by removing less important reaction routes systematically and still obtained satisfactory agreement between the experimental data and predicted results. Bollig et al.5 proposed a reduced n-heptane mechanism and modeled a diffusion flame with the emphasis on pollutant-related intermediates. The mechanism was further reduced using another technique with the assumption of partial equilibrium for intermediates. There are many important applications that need reduced kinetic mechanism, especially in those that require expensive computations but are less demanding in kinetic details. For example, only a few dozen reactions can be comfortably acquired in aerodynamic applications. In this study, the detailed Utah Surrogate Mechanisms of about 1200 reactions and 210 species3 will be reduced by a combined technique. The resultant mechanism will be used to simulate premixed and counter-flow diffusion flames of normal heptane, iso-octane and cyclo-hexane fuels. And the pollutant formation of soot precursors, e.g. benzene and acetylene, will be investigated for the three common surrogate components.

The present paper addressed the production of soot precursors, acetylene, benzene and higher aromatics, by the paraffinic (n-, iso-, and cyclo-) and aromatic components in fuels. To this end, a normal heptane mechanism compiled from sub-models in the literature was extended to large normal-, iso-, and cyclo-paraffins by assigning generic rates to reactions involving paraffins, olefins, and alkyl radicals in the same reaction class. Lumping was used to develop other semi-detailed sub-models. The resulting mechanism for components of complex fuels (named the Utah Surrogate Mechanism) includes detailed sub-models of n-butane, n-hexane, n-heptane, n-decane, n-dodecane, n-tetradecane and n-hexadecane, and semi-detailed sub-models of i-butane, i-pentane, n-pentane, 2,4-dimethyl pentane, i-octane, 2,2,3,3-tetramethyl butane, cyclohexane, methyl cyclohexane, tetralin, 2-methyl 1-butene, 3-methyl 2-pentene and aromatics. Generic rates of reaction classes were found adequate to generate reaction mechanisms of large paraffinic components. The predicted maximum concentrations of the fuel, oxidizer, and inert species, major products and important combustion intermediates, which include critical radicals and soot precursors, were in good agreement with the experimental data of three premixed flames of composite fuels under various conditions. The relative importance in benzene formation of each component in the kerosene surrogate was found to follow the trend aromatics > cyclo-paraffins > iso-paraffins > normal-paraffins. In contrast, acetylene formation is not that sensitive to the fuel chemical structure. Therefore, in formulation of surrogate fuels, attention should be focused on selecting components that will yield benzene concentrations comparable to those produced by the fuel, with the assurance that the acetylene concentration will also be well approximated.