Kinetic Modeling Study of the Effect of Iron on Ignition and Combustion of <i>n</i>‑Heptane in Counter-flow Diffusion Flames

A kinetic modeling study of the effect of iron on the ignition and combustion characteristics of diesel, modeled as n-heptane, in compression ignition engines was carried out using CHEMKIN PRO. The ignition was simulated using the SENKIN code, and combustion was modeled using the OPPDIF code. The kinetic models incorporated n-heptane mechanisms involving 159 species and 1540 reactions and iron reaction mechanisms of 7 iron species and 46 reactions. It was found that small amounts of iron in the fuel significantly reduced the ignition delay time. The ignition delay time decreased with an increasing iron concentration. A reaction pathway analysis showed that the ignition was promoted as a result of an early injection of the OH radicals. It was also showed that the addition of iron increased the peak flame temperature of n-heptane in the counter-flow diffusion flame and reduced the maximum mole fractions of H and O in the peak flame region as a result of the catalytic recombination cycles involving FeO, Fe­(OH)2, and FeOH. The reaction rates of H + O2 ⇔ O + OH and CO + OH ⇔ CO2 + H in the peak flame region were found to increase, which is considered to be responsible for the increased peak flame temperature.本研究采用CHEMKIN PRO软件，针对以正庚烷（n-heptane）作为柴油替代燃料的压燃发动机，开展铁对柴油着火与燃烧特性影响的动力学建模研究。其中，着火过程通过SENKIN代码进行数值模拟，燃烧过程则借助OPPDIF代码构建动力学模型。本研究所用的动力学模型包含两类反应机理：一是涵盖159种组分与1540个基元反应的正庚烷反应机理，二是包含7种铁基组分与46个基反应机理的铁反应机理。研究结果表明，燃料中添加微量铁可显著缩短着火延迟期，且着火延迟期随铁浓度升高进一步降低。反应路径分析显示，OH自由基的早期生成是促进着火过程的关键原因。此外，研究还发现，在逆流扩散火焰中，铁的添加可提升正庚烷的火焰峰值温度；而由于涉及FeO、Fe(OH)₂与FeOH的催化复合循环反应，火焰峰值区域内H与O的最大摩尔分数有所降低。研究同时观测到，火焰峰值区域内H + O₂ ⇔ O + OH与CO + OH ⇔ CO₂ + H这两个基元反应的反应速率显著提升，该现象被认为是火焰峰值温度升高的核心诱因。