Abstract We present a direct numerical simulation of a temporal jet between n-dodecane and diluted air undergoing spontaneous ignition at conditions relevant to low-temperature diesel combustion. The jet thermochemical conditions were selected to result in two-stage ignition. Reaction rates were computed using a 35-species reduced mechanism which included both the low- and high-temperature reaction pathways. The aim of this study is to elucidate the mechanisms by which low-temperature reactions promote high-temperature ignition under turbulent, non-premixed conditions. We show that low-temperature heat release in slightly rich fuel regions initiates multiple cool flame kernels that propagate towards very rich fuel regions through a reaction-diffusion mechanism. Although low-temperature ignition is delayed by imperfect mixing, the propagation speed of the cool flames is high: as a consequence, high-temperature reactions in fuel-rich regions become active early during the ignition transient. Because of this early start, high-temperature ignition, which occurs in fuel-rich regions, is faster than homogeneous ignition. Following ignition, the high-temperature kernels expand and engulf the stoichiometric mixture-fraction iso-surface which in turn establish edge flames which propagate along the iso-surface. The present results indicate the preponderance of flame folding of existing burning surfaces, and that ignition due to edge-flame propagation is of lesser importance.. Finally, a combustion mode analysis that extends an earlier classification [1] is proposed to conceptualize the multi-stage and multi-mode nature of diesel combustion and to provide a framework for reasoning about the effects of different ambient conditions on diesel combustion.

中文翻译：

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与柴油相关的低温条件下随时间变化的空气/正十二烷射流的直接数值模拟

摘要 我们提出了在与低温柴油燃烧相关的条件下经历自燃的正十二烷和稀释空气之间的时间射流的直接数值模拟。选择射流热化学条件以产生两级点火。使用包括低温和高温反应途径的 35 种简化机制计算反应速率。本研究的目的是阐明在湍流、非预混条件下低温反应促进高温点火的机制。我们表明，在略富燃料区域的低温热释放引发了多个冷火焰内核，这些火焰内核通过反应扩散机制向非常富燃料区域传播。虽然低温点火会因混合不完全而延迟，冷火焰的传播速度很高：因此，在点火瞬变期间，富燃料区域的高温反应很早就开始活跃。由于这种早期启动，在燃料丰富的地区发生的高温点火比均质点火更快。点燃后，高温内核膨胀并吞没化学计量混合分数等值面，这反过来又建立了沿等值面传播的边缘火焰。目前的结果表明，现有燃烧表面的火焰折叠占优势，边缘火焰传播引起的点火不太重要。 最后，