Direct numerical simulations of NOx effect on multistage autoignition of DME/air mixture in the negative temperature coefficient regime for stratified HCCI engine conditions
Direct numerical simulations of NOx effect on multistage autoignition of DME/air mixture in the negative temperature coefficient regime for stratified HCCI engine conditions
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DOI：
10.1016/j.combustflame.2013.07.012
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发表时间：
2014
影响因子：
4.4
通讯作者：
H. El-Asrag;Y. Ju
中科院分区：
文献类型：
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作者：
H. El-Asrag;Y. Ju
采用直接数值模拟（DNSs）方法，研究了在类均质压燃（HCCI）发动机分层流动条件下，NOx废气再循环（EGR）和温度/混合气分层对负温度系数（NTC）区二甲醚（DME）自燃的影响。详细的化学二甲醚/空气混合物与NOx添加和解决的混合多时间尺度（HMTS）算法。观察到三个点火阶段。结果表明，添加（1000 ppm）NO提高了低温和中温点火延迟时间的快速OH自由基池的形成（一至两个数量级更高的OH自由基浓度观察）。此外，从EGR NO被发现改变不同的热释放速率在每个点火阶段，其中它主要增加低温点火热释放速率与最小的影响，在第二和第三点火阶段的点火热释放速率。进行了敏感性分析，并指定了低温化学和点火增强NO添加的重要反应途径。分层湍流点火的DNSs表明，混合物和热分层引入的尺度对第二和第三阶段点火有较强的影响。与均质点火相比，分层点火显示出相似的第一自燃延迟时间，但在第二和第三点火延迟时间约19%的减少。分层，然而，在一个较低的平均LTC点火热释放速率和较高的平均热点火热释放速率相比，均匀点火。结果还表明，分子输运在分层低温点火中起着重要作用，标量混合时间尺度受局部点火的强烈影响。观察到两种点火核传播模式：波形，低速，爆燃模式（D-模式）和自发，高速，动力学驱动的点火模式（S-模式）。引入了三个标准来区分这两种模式，通过不同的特征时间尺度和Damkhöler（Da）数使用一个由适当的点火内核指示器（伊基）条件下的进度变量。结果表明，S模式具有低标量耗散率、高位移速度火焰阵面和高混合Damkhöler数的特征; D模式具有高标量耗散率、低位移速度火焰阵面和低于1 Da数的特征。建议的标准适用于不同的点火阶段。
Direct numerical simulations (DNSs), for a stratified flow in HCCI engine-like conditions, are performed to investigate the effects of exhaust gas recirculation (EGR) by NOxand temperature/mixture stratification on autoignition of dimethyl ether (DME) in the negative temperature coefficient (NTC) region. Detailed chemistry for a DME/air mixture with NOxaddition is employed and solved by a hybrid multi-time scale (HMTS) algorithm. Three ignition stages are observed. The results show that adding (1000 ppm) NO enhances both low and intermediate temperature ignition delay times by the rapid OH radical pool formation (one to two orders of magnitude higher OH radicals concentrations are observed). In addition, NO from EGR was found to change the heat release rates differently at each ignition stage, where it mainly increases the low temperature ignition heat release rate with minimal effect on the ignition heat release rates at the second and third ignition stages. Sensitivity analysis is performed and the important reactions pathways for low temperature chemistry and ignition enhancement by NO addition are specified. The DNSs for stratified turbulent ignition show that the scales introduced by the mixture and thermal stratifications have a stronger effect on the second and third stage ignitions. Compared to homogenous ignition, stratified ignition shows a similar first autoignition delay time, but about 19% reduction in the second and third ignition delay times. Stratification, however, results in a lower averaged LTC ignition heat release rate and a higher averaged hot ignition heat release rate compared to homogenous ignition. The results also show that molecular transport plays an important role in stratified low temperature ignition, and that the scalar mixing time scale is strongly affected by local ignition. Two ignition-kernel propagation modes are observed: a wave-like, low-speed, deflagrative mode (the D-mode) and a spontaneous, high-speed, kinetically driven ignition mode (the S-mode). Three criteria are introduced to distinguish the two modes by different characteristic time scales and Damkhöler (Da) number using a progress variable conditioned by a proper ignition kernel indicator (IKI). The results show that the spontaneous ignition S-mode is characterized by low scalar dissipation rate, high displacement speed flame front, and high mixing Damkhöler number, while the D-mode is characterized by high scalar dissipation rate, low displacement speeds in the order of the laminar flame speed and a lower than unity Da number. The proposed criteria are applied at the different ignition stages.