Analysis of Unburned Methane Emission Mechanisms in Large-Bore Natural Gas Engines with Prechamber Ignition

Mark Patterson, Nelson Xie, Kyle Beurlot, Timothy J. Jacobs, Daniel B. Olsen
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Abstract

Although pre-combustion chambers, or prechambers, have long been employed for improving large-bore two-stroke natural gas engine ignition and combustion stability, their design predates modern analysis techniques. Employing the latest CFD modeling techniques, this study investigates the importance of temperature and chemistry for ignition of the main chamber, with an emphasis on eliminating unburned methane. The sensitivity of the ignition and complete combustion to main chamber air/fuel mixture homogeneity was also explored. This study compares the effect of purely thermal ignition, purely chemical ignition, and how their interplay can influence the complete combustion of methane in typical mixtures and in homogeneous distributions of fuel in the combustion chamber. The CFD results demonstrated that temperature and chemistry are equally important in the ignition mechanism, and combining the two phenomena is effective at igniting the main chamber. Reduction of residual methane in the main combustion chamber is most effective when chemical intermediates and thermal ignition are combined. A rudimentary analysis of the effect of fuel/air stratification was also conducted, and it demonstrated that a dramatic reduction in methane emissions is observed for homogeneous mixtures. The flow field in the main combustion chamber was shown to create detrimental stratification of the fuel/air mixture, which inhibited complete combustion of the methane in the main chamber. By contrast, in the extreme case of a perfectly homogeneous distribution of both chemical intermediates and fuel in the combustion chamber, it is possible to completely eliminate unburned methane in the main combustion chamber.
带预室点火装置的大口径天然气发动机未燃烧甲烷排放机理分析
尽管预燃烧室(或称预室)长期以来一直被用于改善大口径二冲程天然气发动机的点火和燃烧稳定性,但其设计却早于现代分析技术。本研究采用最新的 CFD 建模技术,研究了温度和化学成分对主燃烧室点火的重要性,重点是消除未燃烧的甲烷。此外,还探讨了点火和完全燃烧对主燃烧室空气/燃料混合物均匀性的敏感性。这项研究比较了纯热点火和纯化学点火的效果,以及它们之间的相互作用如何影响甲烷在典型混合物和燃料在燃烧室中均匀分布的情况下的完全燃烧。CFD 结果表明,温度和化学在点火机制中同等重要,将这两种现象结合起来可有效点燃主燃烧室。当化学中间产物和热点火相结合时,减少主燃烧室中的残余甲烷最为有效。此外,还对燃料/空气分层的影响进行了初步分析,结果表明,在混合均匀的情况下,甲烷排放量会大幅减少。结果表明,主燃烧室中的流场会造成燃料/空气混合物的有害分层,从而抑制甲烷在主燃烧室中的完全燃烧。相比之下,在燃烧室中化学中间体和燃料分布完全均匀的极端情况下,主燃烧室中未燃烧的甲烷有可能完全消失。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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