Isobutene is an important intermediate in the pyrolysis and oxidation of higher-order branched alkanes, and it is also a component of commercial gasolines. To better understand its combustion characteristics, a series of ignition delay time (IDT) and laminar flame speed (LFS) measurements have been performed. In addition, flow reactor speciation data recorded for the pyrolysis and oxidation of isobutene is also reported. Predictions of an updated kinetic model described herein are compared with each of these data sets, as well as with existing jet-stirred reactor (JSR) species measurements.
IDTs of isobutene oxidation were measured in four different shock tubes and in two rapid compression machines (RCMs) under conditions of relevance to practical combustors. The combination of shock tube and RCM data greatly expands the range of available validation data for isobutene oxidation models to pressures of 50 atm and temperatures in the range 666–1715 K. Isobutene flame speeds were measured experimentally at 1 atm and at unburned gas temperatures of 298–398 K over a wide range of equivalence ratios. For the flame speed results, there was good agreement between different facilities and the current model in the fuel-rich region. Ab initio chemical kinetics calculations were carried out to calculate rate constants for important reactions such as H-atom abstraction by hydroxyl and hydroperoxyl radicals and the decomposition of 2-methylallyl radicals.
A comprehensive chemical kinetic mechanism has been developed to describe the combustion of isobutene and is validated by comparison to the presently considered experimental measurements. Important reactions, highlighted via flux and sensitivity analyses, include: (a) hydrogen atom abstraction from isobutene by hydroxyl and hydroperoxyl radicals, and molecular oxygen; (b) radical–radical recombination reactions, including 2-methylallyl radical self-recombination, the recombination of 2-methylallyl radicals with hydroperoxyl radicals; and the recombination of 2-methylallyl radicals with methyl radicals; (c) addition reactions, including hydrogen atom and hydroxyl radical addition to isobutene; and (d) 2-methylallyl radical decomposition reactions. The current mechanism accurately predicts the IDT and LFS measurements presented in this study, as well as the JSR and flow reactor speciation data already available in the literature.
The differences in low-temperature chemistry between alkanes and alkenes are also highlighted in this work. In normal alkanes, the fuel radical Ṙ adds to molecular oxygen forming alkylperoxyl (RȮ2) radicals followed by isomerization and chain branching reactions which promote low-temperature fuel reactivity. However, in alkenes, because of the relatively shallow well (∼20 kcal mol–1) for RȮ2 formation compared to ∼35 kcal mol–1 in alkanes, the Ṙ + O2 ⇌ RȮ2 equilibrium lies more to the left favoring Ṙ + O2 rather than RȮ2 radical stabilization. Based on this work, and related studies of allylic systems, it is apparent that reactivity for alkene components at very low temperatures (1300 K), the reactivity is mainly governed by the competition between hydrogen abstractions by molecular oxygen and ȮH radicals.
KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: Chong-Wen Zhou thanks the entire group members at Combustion Chemistry Centre for helpful discussions. The work at NUI Galway was supported by Saudi Aramco under the FUELCOM program. The TAMU effort was supported by the Texas A&M Engineering Experiment Station and by the TEES Turbomachinery Laboratory. The Rensselaer group was supported by the U.S. Air Force Office of Scientific Research (Grant no. FA9550-11-1-0261). The work at UConn was supported by the National Science Foundation under Grant no. CBET-1402231. The work of KAUST authors was supported by Saudi Aramco under the FUELCOM program and by King Abdullah University of Science and Technology (KAUST). The work at LRGP was supported by the European Commission through the “Clean ICE” Advanced Research Grant of the European Research Council (Grant no. 227669). Collaboration between NUI Galway and LRGP enters in the frame the COST Action CM1404. Y.J. would like to thank the Grant support from NETL DE-FE0011822 and NSF CBET-1507358. F.L.D. acknowledges support of this at Princeton by the University Turbine Systems Research (UTSR) program, administered by the National Energy Technology Laboratory of the US Department of Energy (DOE), under award DE-FE0012005.
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