A comprehensive experimental and modeling study of isobutene oxidation

Abstract

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 2 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 (RȮ 2 ) 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 RȮ 2 formation compared to ~35 kcal mol -1 in alkanes, the Ṙ + O 2 ⇌ RȮ 2 equilibrium lies more to the left favoring Ṙ + O 2 rather than RȮ 2 radical stabilization. Based on this work, and related studies of allylic systems, it is apparent that reactivity fo...

Investigations of Chemical Kinetic Mechanisms for Low-to-medium Temperature Ignition of Ethylene

“…超燃发动机中空气在燃烧室内只能停留几个毫秒, 要在这样短的时间内完成点火并燃烧, 点火延时的研究 是关键 [1] . [9] 和最新的 AramcoMech_2.0 [10] 能够描述乙烯宽温度范围燃烧反应动力学过程; Ranzi 等通过加入三类新的低温反应类型, 完善了从低温到高 温的 Creck [11] 集总反应机理; Glarborg 等 [12] 在 600～900 K, 60×10 5 Pa 下, 通过分析乙烯氧化重要反应, 再结合 量化计算得到一套乙烯低温高压燃烧反应机理, 并与平 推流实验测得的物种浓度吻合良好; Williams 等通过研 究乙烷和丙烷低温点火 [13] 更新的 UCSD 机理 [14] 只包含 50 个物种 247 步反应, 是研究小分子碳氢燃料燃烧机理 和实验验证的重要参考 [8,15] [19] 软件激波管模型和封闭均 相模型, 用 AramcoMech_1.3 [9] 机理、 UCSD [14] 核心机理, Glarborg [12] 的机理、Wang [16] 的乙烯氧化反应详细机理和 Exp. [5] Exp.…”

“…[21] 图 1 乙烯点火延时实验数据比较 Figure 1 Comparison of ethylene experimental ignition delay times Figure 4 Comparison of R1 and R2 reactions' rate constants in different mechanisms with calculated results [23] 围内, 随温度降低 τ～1000/T 斜率变小, 即乙烯在中温 段出现负温度效应趋势, 这是新出现的现象, 也是区分 高温和低温机理的关键 [17] . AramcoMech_2.0 机理也采 用 Goldsmith 等 [23] 的数据更新了 R1 和 R2 反应的动力学 参数, 主要用于研究 C4 物种的机理 [10] . 与 AramcoMech Exp.…”

Fuel Pyrolysis at Atmospheric and Elevated Pressure Conditions in Micro-Flow Tube Reactor

“…The system has a good retention time resolution for C1-C3 species, while C4 species quantification suffer from overlapping peaks of as shown in shows ethane conversion at various temperatures at 5 and 15 atm compared to with two models by Wang et al [26] and AramcoMech 2.0 [70]. Aramco Mech 2.0 was chosen for As explained in chapter 3, the fuel and diluent is premixed before entering the heated section.…”

“…In flow species composition are XC 4 H 10 =0.02, with three different models by Wang et al [26], Pyun et al [80] and Curran and coworkers (AramcoMech 2.0 [70]). In flow species composition are XC 4 H 10 =0.02, with three different models by Wang et al [26], Pyun et al [80] and Curran and coworkers (AramcoMech 2.0 [70] …”

“…[26] , Naik and Dean [76] and Curran and co-workers (AramcoMech 2.0 [70]), for inflow species compositions of XC 2 H 6 = 0.02, XN 2 [26] , Naik and Dean [76] and Curran and co-workers (AramcoMech 2.0 [70]), for inflow species compositions of XC 2 H 6 = 0.02, XN 2 with three different models by Wang et al [26], Pyun et al [80]and Curran and coworkers (AramcoMech 2.0 ) [70]. In flow species composition are XC 4 H 10 =0.02, with three different models by Wang et al [26], Pyun et al [80] and Curran and coworkers (AramcoMech 2.0 [70]). In flow species composition are XC 4 H 10 =0.02, with three different models by Wang et al [26], Pyun et al [80] and Curran and coworkers (AramcoMech 2.0 [70] …”