Flame-spray interaction and combustion features in split-injection spray flames under diesel engine-like conditions
Flame-spray interaction and combustion features in split-injection spray flames under diesel engine-like conditions
复制标题
柴油机条件下分流喷射喷雾火焰的火焰喷雾相互作用和燃烧特征
DOI：
10.1016/j.combustflame.2019.08.031
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发表时间：
2019
影响因子：
4.4
通讯作者：
Zhou Lei
中科院分区：
文献类型：
--
作者：
Zhao Wanhui;Wei Haiqiao;Jia Ming;Lu Zhen;Luo Kai H.;Chen Rui;Zhou Lei
在压燃式发动机上，分流喷射策略在降低污染物排放和提高燃烧效率方面显示出了巨大的效益。分流喷射中的喷雾-火焰相互作用非常复杂，影响着火过程，甚至影响污染物的排放。因此，本研究的目的是研究在类似柴油机的条件下，火焰-喷雾相互作用如何影响分流喷射的后续着火过程和燃烧特性。本文采用大涡模拟方法，结合54组分正十二烷分流喷射机理，研究了喷射持续时间和停留时间对喷雾-火焰相互作用的影响及点火机理。数值模拟结果表明，汽体穿透长度、着火延迟时间、混合气分数分布和火焰结构与实验结果吻合较好。研究表明，分流燃烧是一个多阶段的过程，第一次和第二次喷射的着火过程受不同的机理控制，即第一次喷射的自燃和第二次喷射由于第一次喷射形成的中间物质和加热效应而加速着火。此外，两次喷射之间的停留时间的增加降低了随后对第二次喷射的点火促进作用，从而削弱了两次喷射之间的相互作用过程。第一次喷射燃料的消耗导致温度升高和产生不同的物种，这反过来又加速了第二次喷射的点火。最后，基于化学爆炸模式分析(CEMA)方法研究了局部流动时间尺度与化学时间尺度之间的竞争关系。在DT较短的二次喷射过程中，反应过程和混合过程之间的平衡控制着准稳态喷雾的燃烧。然而，当使用较长的DT时，火焰由自动点火控制。
In compression ignition engines, split-injection strategy has shown great benefits in reducing pollutant emissions and improving combustion efficiency. Spray–flame interaction involving in split injections is significantly complex, which affects the ignition process and even pollutant emissions. Therefore, the objective of this study is to investigate how the flame–spray interaction affects the subsequent ignition process and combustion features in split injections under diesel engine-like conditions. In this work, large eddy simulation coupled with a 54-species mechanism for split injections of n-dodecane is performed to study the effect of injection duration and dwell times (DTs) on spray–flame interactions and the ignition mechanism. The numerical model gives a reasonable agreement with the experiments in terms of the vapor penetration length, ignition delay times, mixture fraction distributions and the flame structures. The present study revealed that combustion for split injections is a multi-stage process and the ignition processes for the first and second injections are controlled by different mechanisms, namely autoignition for the first injection, and the accelerating ignition for the second injection due to the intermediate species and heating effect formed in the first injection. Moreover, the increase in dwell time between individual injections reduces the subsequently promoting ignition effect for the second injection and thus weakens the interacting process between the two injections. Consumption of the fuel in the first injection leads to a temperature increase and production of different species, which in turn accelerates the ignition of the second injection. Finally, the competition between the local flow timescale and chemical timescale is investigated based on the chemical explosive mode analysis (CEMA) methods. A balance between reaction and mixing processes dominates the combustion of the quasi-steady spray in the second injection with a short DT. However, the flame is controlled by autoignition when a longer DT is used.