高碳烃宽温度范围燃烧机理构建及动力学模拟

发动机中燃料点火特性以及燃烧能量的释放对于发动机设计具有非常重要的作用,为了提高燃料的燃烧效率以及减少燃料在燃烧过程中污染物的排放,基于反应动力学机理对燃料燃烧过程的模拟就显得十分必要。因此需要更加深入的认识碳氢燃料的燃烧机理,探索其在燃烧过程中十分复杂的化学反应网络。为了发展能够适用于实际燃料多工况条件(宽温度范围、宽压力范围和不同当量比)燃烧的燃烧机理,基于碳氢燃料机理自动生成程序ReaxGen构建了正癸烷燃烧详细机理(包含1499个物种,5713步反应)和正十一烷燃烧详细机理(包含1843个物种,6993步反应)。详细机理主要由小分子核心机理和高碳烃类(C5以上)机理两部分组成。为了验证机理的合理性与可靠性,本文对于高碳烃燃烧新机理在点火延时时间以及物种浓度曲线进行了动力学分析,并与实验数据及国内外同类机理进行了对比,结果表明本文提出的正癸烷和正十一烷燃烧新机理在比较宽泛的温度、压力和当量比条件下都具有较高的模拟精度,为发展精确航空煤油燃烧模型提供了基础数据。同时考虑到详细机理的复杂性以及机理分析的计算量大和时耗长,本文基于误差传播的直接关系图形(Directed Relation Graph with Error Propagation,DRGEP)方法简化得到的包含709组分2793反应的正癸烷和包含820组分3115反应的正十一烷简化机理,使用DRGEP方法时所采用的数据点选自压力范围从1.0×10~5 Pa到1.0×10~6Pa,当量比范围从0.5到2.0,初始温度范围从600到1400时恒压点火的模拟结果在点火延迟时间附近区域的抽样,同时在正癸烷机理简化中选取正癸烷、O_2和N_2作为初始预选组分,正十一烷的机理简化中主要选取正十一烷、O_2和N_2作为初始预选组分,得到的简化机理在比较宽泛的条件下的预测结果与详细机理吻合很好。最后结合敏感度分析方法分析了正癸烷和正十一烷的点火延迟敏感性,考察了机理中影响点火的关键反应。结果表明:这些机理能够合理描述正癸烷和正十一烷的自点火特性,在工程计算流体力学仿真设计中有很好的应用前景。

Mechanism Construction and Simulation for Combustion of Large Hydrocarbon Fuels Applied in Wide Temperature Range

The ignition characteristics of fuels and the release of energy in combustion engines are of crucial importance to engine design and improvement. To improve the fuel combustion efficiency and to reduce the associated pollutant emission, it is necessary to develop reliable high-precision reaction mechanisms for simulating combustion. Consequently, we need to comprehensively understand the combustion mechanisms of hydrocarbon fuels, and to explore their complicated chemical reaction networks. In order to construct combustion mechanisms that can be applied to conditions over a wide temperature range, wide pressure range, and for different equivalent ratios, two detailed mechanisms for the combustion of large hydrocarbons were developed based on ReaxGen, an automatic generation program for combustion and pyrolysis mechanisms developed by LI Xiangyuan et al. Using this program, one mechanism for n-decane combustion was developed, containing 1499 species and 5713 reactions, and another was developed for n-undecane combustion, containing 1843 species and 6993 reactions. All the detailed mechanisms of the alkanes consisted of two parts, a validated core mechanism and a sub-mechanism produced by ReaxGen which worked mainly based on the rules of the reaction class. The major classes of elementary reactions considered in our detailed mechanisms for n-decane and n-undecane combustion included 10 kinds of high-temperature combustion reactions and 19 kinds of low-temperature combustion reactions. To verify the rationality and reliability of the mechanisms, ignition delay times in shock tubes and the concentration profiles of important species in a jet-stirred reactor were obtained using CHEMKIN software. The obtained calculated data were compared with the experimental data and the results of similar mechanisms at home and abroad. It was shown that the numerically predicted results of our new mechanisms were in good agreement with available experimental data in the literature. Our newly developed n-decane and n-undecane combustion mechanisms are useful for completing the combustion model of aviation kerosene. Furthermore, considering the complexity of the detailed mechanisms, the large amount of calculation and the long time required for mechanism analysis, mechanism simplification was carried out. The sampling points required for mechanism reduction were taken from simulation results near the ignition delay time with pressures ranging from 1.0 × 10⁵ Pa to 1.0 × 10⁶ Pa, equivalence ratios ranging from 0.5 to 2.0, and initial temperatures ranging from 600 K to 1400 K. The species n-C₁₀H₂₂, N₂, and O₂ were selected as the initial important species for the n-decane combustion mechanism and the species n-C₁₁H₂₄, N₂, and O₂ were selected as the initial important species for the n-undecane combustion mechanism. The predicted results of ignition delay time from the simplified mechanism for n-decane combustion (including 709 species and 2793 reactions) and simplified mechanism for n-undecane combustion (including 820 species and 3115 reactions) generated by the reduction method of Directed Relation Graph with Error Propagation (DRGEP) agreed well with the detailed mechanisms. Finally, sensitivity analysis for the ignition delay time was carried out to identify reactions that affected ignition delay times at specific temperatures, pressures and equivalence ratios. The results indicate that these mechanisms are reliable for describing the auto-ignition characteristics of n-decane and n-undecane. These mechanisms would also be helpful in computational fluid dynamics (CFD) for engine design.