ABSTRACT

Exhaust gas recirculation (EGR) is an effective technique used widely to meet current emission regulations. In this work, the mechanism of H₂O and CO₂ addition (a main component of exhaust gas) on polycyclic aromatic hydrocarbons (PAHs, the precursor of soot) formation was investigated numerically in ethanol diffusion flames, motivated by current concerns on particulate matter (PM) emissions when ethanol is applied to modern gasoline engines, especially under high load and temperature. To understanding the underlying mechanisms of H₂O and CO₂ on ethanol flame, a comprehensive chemical kinetic analysis was performed by modeling gas-phase chemistry with PAHs formation up to pyrene. In order to distinguish the important chemical role of H₂O and CO₂ addition through partial replacement of N₂ from their thermal effects, additional calculations were performed with two fictitious species. The numerical results show that the thermal effect of H₂O and CO₂ reduces the peak flame temperature, but their chemical effect influences the peak flame temperature differently. The results also showed that the addition of either H₂O or CO₂ led to a significant reduction of PAHs via thermal and chemical effects. The chemical suppressing effect of H₂O and CO₂ addition on PAHs formation was identified and attributed to reduced PAHs precursor species and slow of PAH growth process. However, reaction pathway analysis showed that the chemical effects of H₂O and CO₂ influence the PAHs chemistry differently.

摘要

废气再循环 (EGR) 是一种广泛用于满足当前排放法规的有效技术。在这项工作中，受当前对颗粒物的关注的推动，在乙醇扩散火焰中对多环芳烃（PAHs，烟灰的前体）形成的 H ₂ O 和 CO ₂添加（废气的主要成分）的机理进行了数值研究。将乙醇应用于现代汽油发动机时的 (PM) 排放，尤其是在高负荷和高温下。了解 H ₂ O 和 CO _{2的潜在机制}在乙醇火焰上，通过模拟气相化学（PAHs 形成直至芘）进行了全面的化学动力学分析。为了区分通过部分替换 N _{2添加 H 2} O 和 CO ₂的重要化学作用与它们的热效应，对两个虚构的物种进行了额外的计算。数值结果表明，H ₂ O和CO ₂的热效应降低了火焰峰值温度，但它们的化学效应对火焰峰值温度的影响不同。结果还表明，添加 H ₂ O 或 CO ₂通过热和化学作用导致 PAHs 的显着减少。_{H 2} O 和 CO ₂添加对 PAHs 形成的化学抑制作用已被确定并归因于 PAHs 前体种类的减少和 PAH 生长过程的缓慢。然而，反应途径分析表明，H ₂ O 和 CO ₂的化学效应对 PAHs 化学的影响不同。