LES study of deflagration to detonation mechanisms in a downsized spark ignition engine
LES study of deflagration to detonation mechanisms in a downsized spark ignition engine
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DOI：
10.1016/j.combustflame.2015.04.010
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发表时间：
2015-07
影响因子：
4.4
通讯作者：
A. Robert;S. Richard;O. Colin;T. Poinsot
中科院分区：
文献类型：
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作者：
A. Robert;S. Richard;O. Colin;T. Poinsot
使用15个LES循环的高负荷/低速火花点火发动机的工作点，两个不同的新鲜气体自燃制度称为爆震和超级爆震进行了分析。提出了一种直接的“后验”分析的压力波和自燃放热观察LES。它揭示了低到中等爆震强度，对应于晚火花正时（ST）的特征在于一个或几个随机自燃（AI）点消耗周围的新鲜气体，而不耦合与AI热释放。相反，最高爆震强度对应于通常称为超级爆震的爆震，即在预点火条件下或对于非常早期的ST观察到的非常强烈的爆震，如在本研究中所做的。LES表明，由一个或几个AI点产生的压力波是足够强大的，以诱导本地一个强大的新鲜气体的温度增加，导致本身的AI延迟大幅减少。这使得压力波和AI反应速率之间产生相互增强的耦合，从而导致最大压力和传播速度接近爆炸的压力和传播速度。因此，这些结果有力地支持了文献中提出的假设，即超级爆震的特征在于爆燃到爆震转变（DDT）。由于使用了基于布拉德利DDT图的局部爆震指示器，还进行了“先验”分析。结果表明，该工具不仅预测了燃烧状态的变化作为ST的函数，但它也大致成功地预测了DDT在室内出现的位置和时间。不幸的是，第一个AI点并不总是对DDT负责，这意味着使用冷流LES来计算爆震指示器而不是这里提出的反应LES，在许多情况下会导致指示器失效。
Using 15 LES cycles of a high load/low speed spark ignition engine operating point, two different fresh gases autoignition regimes called knock and super-knock are analyzed. A direct “a posteriori” analysis of pressure waves and autoignition heat release observed in LES is proposed. It reveals that low to moderate knock intensity, corresponding to late spark timings (ST) is characterized by one or several random autoignition (AI) spots which consume the surrounding fresh gases without coupling with the AI heat release. On the contrary, the highest knock intensities correspond to what is usually called super-knock, a very intense knock observed under pre-ignition conditions or for very early ST, as done in this study. LES shows that the pressure waves generated by one or a couple of AI spots are strong enough to induce locally a strong fresh gases temperature increase leading itself to a substantial decrease of the AI delay. This allows to generate a coupling between the pressure wave and the AI reaction rate which reinforce each other, leading to maximum pressures and propagation speeds close to those of a detonation. These results therefore strongly support the hypothesis proposed in the literature that super-knock is characterized by a deflagration to detonation transition (DDT). An “a priori” analysis is also performed thanks to the use of a local detonation indicator based on Bradley’s DDT diagram. It is shown that this tool not only predicts the change of combustion regime as a function of the ST, but it also roughly succeeds in predicting the location and time of appearance of the DDT in the chamber. Unfortunately, the first AI spot is not always responsible for the DDT, implying that using cold flow LES to calculate the detonation indicator instead of a reacting LES as proposed here, would lead to a failure of the indicator in many cases.