Abstract
Ignition delay times of small alkenes are a valuable constraint for the refinement of the core kinetic mechanism of hydrocarbons used in representing combustion properties of real fuels. Moreover, the chemical reactivity comparison of those small alkenes provides a reference in object-oriented fuel design and logical combustion utilization. In this study, the ignition delay times of C2–C4 alkenes (ethylene, propene and 1-butene) were measured behind reflected shock waves first, with a fixed oxygen concentration (XO2 = 6%) and equivalence ratio (φ = 1.0) at various pressures of 1.2, 4.0 and 16.0 atm, in order to facilitate the comparison. Three chemical-based-Arrhenius-type correlations covering a wide range of temperature, pressure, equivalence ratio, and dilution were proposed. The simplified reaction network for pyrolysis and oxidation of 1-alkenes was depicted relying on the reaction classes of alkenes. Nine generally accepted mechanisms were used to simulate the ignition delay times measured by this study as well as literature. All the kinetic models show reasonable structure-reactivity trends for all of the three alkenes, but only NUIGMech 1.1 is capable of representing quantificationally the chemical reactivity at all tested conditions. Generally, ethylene exhibits the highest reactivity while propene presents the lowest at high temperatures. Analyses of sensitivity and flux indicate that the main oxidation pathway of ethylene is chain-branching, which accelerates the accumulation of free radical pools, especially for the Ḣ atom, Ȯ atom and ȮH radical, which results in the highest reactivity of ethylene. For propene and 1-butene, due to the presence of the allylic site, consumption of allylic radicals becomes the decisive step of oxidation and allylic radicals are mostly consumed by the HȮ2 radical. However, there are no such efficient reaction pathways for the formation of HȮ2 radicals during the propene oxidation process, while reaction pathways for HȮ2 formation in 1-butene are efficient. Thus, 1-butene presents higher reactivity compared to propene.
Highlights
tTim[Ke-s1]of small alkenes are a valuable constraint for the10r0ef0i/nTem[Ke-1n]t
with stable allyl radicals forming allyl-hydroperoxide radicals
the rise in reactant concentration is paralleled by the increases of series reactions involving
Summary
Dominant unimolecular reactions include simple C–C and C–H bond fission reactions, which are crucial to accurately describe high-temperature pyrolysis and oxidation of 1alkenes. Vinylic C–H bond fission reaction is the preferred channel. The allylic C–H bond fission reaction channel is more competitive. When the carbon number is greater than three, the allylic C–C bong fission becomes more favorable due to the lowest BDE. The collected experimental data of C2–C4 1-alkenes do not show negative temperature coefficient (NTC) behavior, as shown in Supplementary Materials. The above chemical kinetic analysis indicates that possible reasons for 1-alkenes NTC behavior are generally originated from reaction pathways flowing into the alcohol low-temperature mechanism. At lower temperatures, the fuel-OH reaction branch shifts to addition reactions with hydroxyalkyl radicals producing. 722–1108 941–1220 1109–1238 898–1008 988–1129 781–957 859–1009 813–1062 892–1178 912–1021 p (atm) 40
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