Abstract Cool flames propagating in autoignitive mixtures may be characterized as deflagrations or spontaneous ignition fronts depending on the relative importance of diffusion. In this paper, it is shown that, for certain thermochemical conditions in the negative temperature coefficient (NTC) regime, including at the Engine Combustion Network’s (ECN’s) baseline Spray A conditions, the cool flame products’ reactivity and hot flame speed can vary significantly from one cool flame propagation regime to the other. Simulations of steady one-dimensional cool flames in fixed-length inflow-outflow domains are performed. By varying the inflow velocity and controlling accordingly the domain length, isolated cool flames ranging from spontaneous ignition fronts to deflagrations are obtained. Towards the deflagration regime, the increasingly important contribution from diffusion (mostly heat conduction) promotes intermediate or high temperature fuel oxidation channels at the expense of low-temperature chain branching. As a result, the cool-flame products’ composition and temperature are significantly affected, with the product-side’s mixture reactivity and hot flame speed significantly reduced. Qualitatively similar results obtained with four chemical kinetics mechanisms and two transport models (mixture-averaged and unity Lewis number) are presented. More specifically, in a lean n-heptane–air mixture (equivalence ratio of 0.7) at 650 K and 1 atm, the time to second stage ignition is increased by a factor of up to 6 following a cool flame deflagration as opposed to a spontaneous ignition. The peak heat release rate is also reduced by a factor of more than five. With n-dodecane–oxidizer mixtures (equivalence ratios of 0.7 to 1.3) at the ECN’s baseline Spray A conditions, the role of diffusion on cool-flame products is observed to increase the remaining time to second stage ignition by a factor of up to 2.5, reduce hot flame speed by up to 30% and decrease peak heat release rate by a factor of up to five. These effects are shown to lead to a significant alteration of the double cool-hot flame ignition and stabilization. This is in part due to the fact that the cool flames are found to be as fast, and faster than hot flames at these conditions, such that a deflagrative cool flame can play a significant role on both ignition and flame stabilization. Finally, it is found that the effect of diffusion on chemical pathways and peak heat release rate can be even more significant at the baseline Spray A conditions in rich mixtures beyond the NTC regime.

中文翻译：

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在 NTC 状态下受扩散影响的自燃冷火焰产物中的混合物反应性和热火焰速度降低

摘要 根据扩散的相对重要性，在自燃混合物中传播的冷火焰可以表征为爆燃或自燃前沿。本文表明，对于负温度系数 (NTC) 状态下的某些热化学条件，包括发动机燃烧网络 (ECN) 的基线喷涂 A 条件，冷焰产物的反应性和热焰速度可能会发生显着变化从一种冷火焰传播方式到另一种。对固定长度的流入-流出域中的稳定一维冷火焰进行了模拟。通过改变流入速度并相应地控制域长度，可以获得从自发点火前沿到爆燃的孤立冷火焰。走向爆燃制度，扩散（主要是热传导）的日益重要的贡献以低温链支化为代价促进了中温或高温燃料氧化通道。结果，冷焰产品的成分和温度受到显着影响，产品侧的混合物反应性和热焰速度显着降低。使用四种化学动力学机制和两种传输模型（混合平均和统一路易斯数）获得了定性相似的结果。更具体地说，在 650 K 和 1 atm 的稀正庚烷 - 空气混合物（当量比为 0.7）中，与自发爆燃相比，在冷火焰爆燃后到第二阶段点火的时间增加了 6 倍点火。峰值放热率也降低了五倍以上。在 ECN 的基线喷涂 A 条件下，使用正十二烷-氧化剂混合物（当量比为 0.7 到 1.3）时，观察到扩散对冷焰产物的作用将第二阶段点火的剩余时间增加了 2.5 倍, 将热火焰速度降低多达 30%，并将峰值放热率降低多达五倍。这些影响被证明会导致双冷热火焰点火和稳定性的显着改变。这部分是由于在这些条件下发现冷火焰与热火焰一样快，而且比热火焰快，因此爆燃的冷火焰可以在点火和火焰稳定方面发挥重要作用。最后，