Skeletal Kinetic Mechanism Generation and Uncertainty Analysis for Combustion of Iso-octane at High Temperatures

A detailed mechanism for combustion of iso-octane with 116 species and 754 reactions has been reduced using a directed relation graph with error propagation (DRGEP) and DRGEP with sensitivity analysis (DRGEPSA) methods under high-temperature conditions. Two skeletal mechanisms, i.e., a 63-species mechanism with a maximum error of 7.2% and a 51-species mechanism with a maximum error of 28.5% on autoignition delay times have been generated. These two skeletal mechanisms are shown to reproduce ignition delays, laminar flame speeds, species and temperature profiles in good agreement with those of the detailed mechanism. Uncertainty in the ignition predictions by detailed and two skeletal mechanisms induced by the uncertainties in reaction rate coefficients has been studied. Probability distribution of autoignition predictions demonstrated that the 63-species mechanism can still keep the uncertainty characteristics, while the 51-species mechanism has significant discrepancy compared with the detailed one. Further analysis of autoignition shows that the structure and integrality of the reaction system in the 51-species mechanism has changed. Global sensitivities of 63-species and detailed mechanisms on ignition have been investigated using the high-dimensional model representation (HDMR) method. The highly important reactions for ignition in the detailed mechanism are the same as those in the 63-species mechanism, and sensitivity coefficients of the listed reactions agree well with each other. The most important reactions in the first-order sensitivity on autoignition in the detailed mechanism are the same as those in the 63-species mechanism, especially for the five most important reactions. The most important 10 reactions contribute almost 75% to the overall variance in ignition delay under the present conditions, while the second-order effects are quite small and almost negligible. The top ranked reactions show that small-molecule chemistry (C0–C4) contributes significantly to uncertainties in the ignition predictions at high temperatures.针对异辛烷（iso-octane）的详细燃烧机理（包含116种组分与754个基元反应），在高温工况下采用带误差传播的定向关系图（directed relation graph with error propagation, DRGEP）以及带敏感性分析的DRGEP（DRGEP with sensitivity analysis, DRGEPSA）方法完成了简化。最终得到两款骨架燃烧机理：其一为含63种组分的骨架机理，其自燃延迟时间的最大误差为7.2%；其二为含51种组分的骨架机理，对应最大误差达28.5%。经检验，这两款骨架机理均可较好地复现详细机理的点火延迟时间、层流火焰速度、组分分布与温度分布特征。针对详细机理与两款骨架机理，研究了由反应速率系数不确定性所引发的点火预测不确定性。自燃预测的概率分布结果表明：63组分骨架机理仍可保留详细机理的不确定性特征，而51组分骨架机理则与详细机理存在显著偏差。进一步的自燃特性分析显示，51组分骨架机理的反应系统结构与完整性已发生改变。采用高维模型表示（high-dimensional model representation, HDMR）方法，研究了63组分骨架机理与详细机理的点火全局敏感性。详细机理与63组分骨架机理中，对点火至关重要的反应种类完全一致，且对应反应的敏感性系数吻合度较高。在详细机理与63组分骨架机理中，针对自燃的一阶敏感性分析显示，其最重要的反应类别完全相同，尤其在前五大重要反应中表现一致。在当前工况下，排名前十的重要反应几乎贡献了点火延迟时间总方差的75%，而二阶效应极小，几乎可以忽略。对排名靠前的反应分析表明，小分子化学反应（C0–C4）对高温下点火预测的不确定性具有显著影响。