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科学通报  2014 

基于碳烟表面活性的多步碳烟生成机理研究柴油机碳烟生成的主要影响因素

DOI: 10.1360/csb2014-59-14-1348, PP. 1348-1357

Keywords: 混合气不均匀程度,乙炔生成历程,碳烟表面活化程度,碳烟表面生长,碳烟排放

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Abstract:

随着排放法规对发动机碳烟排放量以及碳烟颗粒尺寸的要求日益严格,柴油机部分预混燃烧技术通过协同控制燃烧过程中混合与化学反应参数,可以实现高热效率与超低的碳烟排放.这样使得燃烧边界条件对碳烟生成过程的影响成为研究的关键问题.本研究利用三维CFD数值模拟方法,耦合正庚烷化学动力学简化模型以及改进的多步现象学碳烟模型,来预报以正庚烷为替代燃料的柴油机部分预混燃烧过程及其排放特性.乙炔作为碳烟前驱物形成以及碳烟表面生长的主要组分,其生成历程的准确预报为模拟碳烟形成奠定了基础.碳烟表面生长过程是碳烟质量积累的重要过程,本研究引入碳烟颗粒活化表面比例分数αCH作为评价碳烟表面活化程度的标准,探讨了不同燃烧边界条件对碳烟表面活化程度的影响.研究结果表明,对于混合气均匀的燃烧工况,氧浓度降低导致燃烧温度降低,引起单位面积上的碳烟生长速率大幅降低是表面生长速率减慢的主要原因,最终导致碳烟的生成量减少;随着混合气分层程度增加,燃烧过程中积累了大量未燃碳氢化合物,促进了乙炔的生成和碳烟表面活化程度升高,加速了碳烟表面生长过程,导致最终碳烟排放恶化.此外,燃烧过程中残留的CO对碳烟后期氧化产生一定的抑制作用.

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