Direct numerical simulations of ignition of a lean n-heptane/air mixture with temperature inhomogeneities at constant volume: Parametric study
Direct numerical simulations of ignition of a lean n-heptane/air mixture with temperature inhomogeneities at constant volume: Parametric study
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DOI：
10.1016/j.combustflame.2011.01.025
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发表时间：
2011-09
影响因子：
4.4
通讯作者：
C. Yoo;T. Lu;Jacqueline H. Chen;C. Law
中科院分区：
文献类型：
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作者：
C. Yoo;T. Lu;Jacqueline H. Chen;C. Law
用直接数值模拟方法研究了热分层对等体积高压下稀薄均质正庚烷/空气混合物着火的影响，并根据LLNL机理建立了一个新的58组分还原动力学机理(H.J.Curran等人，Burust)。火焰129(2002)253-280)。在初始标量场上叠加了二维各向同性速度谱和温度涨落，在固定的体积内进行了二维数值模拟。研究了通过改变温度的均值和方差以及湍流与点火延迟时间之比而引起的初始温度场的变化对稀正庚烷/空气混合物多级点火的影响。总体而言，无论平均初始温度如何，平均热释放速率随着热层结的增加而增加的速度较慢。相对于负温度系数(NTC)区，平均初始温度较高时，点火延迟期随热分层程度的增加而减小。然而，在平均初始温度相对较低的情况下，由于混合气的总点火延迟时间较长，它随着热波动的增加而增加。置换速度和Damköhler数分析表明，较高的热层化程度导致了反应前沿的爆燃而不是自燃，因此，在区域内最高温度区域发生热失控后，平均热释放速率更平稳。化学爆炸模式分析(CEMA)还证实，对于温度波动较大的情况，混合抵消了反应前沿的化学爆炸。研究还发现，如果湍流与点火延迟时间之比较小，则所产生的标量起伏减弱会导致整体点火以自燃方式发生。然而，与热力层结的影响相比，湍流的整体影响较小。这些结果表明，均质充气压燃(HCCI)发动机平稳运行的临界热分层程度取决于平均温度和初始温度的波动，在控制着火时刻和防止HCCI燃烧中压力过快上升时应考虑这一点。
The effect of thermal stratification on the ignition of a lean homogeneous n-heptane/air mixture at constant volume and high pressure is investigated by direct numerical simulations (DNS) with a new 58-species reduced kinetic mechanism developed for very lean mixtures from the detailed LLNL mechanism (H.J. Curran et al., Combust. Flame 129 (2002) 253–280). Two-dimensional DNS are performed in a fixed volume with a two-dimensional isotropic velocity spectrum and temperature fluctuations superimposed on the initial scalar fields. The influence of variations in the initial temperature field, imposed by changing the mean and variance of temperature, and the ratio of turbulence to ignition delay timescale on multi-stage ignition of a lean n-heptane/air mixture is studied. In general, the mean heat release rate increases more slowly with increasing thermal stratification regardless of the mean initial temperature. Ignition delay decreases with increasing thermal stratification for high mean initial temperature relative to the negative temperature coefficient (NTC) regime. It is, however, increased with increasing thermal fluctuations for relatively low mean initial temperature resulting from a longer overall ignition delay of the mixture. Displacement speed and Damköhler number analyses reveal that the high degree of thermal stratification induces deflagration rather than spontaneous ignition at the reaction fronts, and hence, the mean heat release rate is smoother subsequent to thermal runaway occurring at the highest temperature regions in the domain. Chemical explosive mode analysis (CEMA) also verifies that mixing counterbalances chemical explosion at the reaction fronts for cases with large temperature fluctuation. It is also found that if the ratio of turbulence to ignition delay timescale is short, resultant diminished scalar fluctuations cause the overall ignition to occur by spontaneous ignition. However, the overall effect of turbulence is small compared to the effect of thermal stratification. These results suggest that the critical degree of thermal stratification for smooth operation of homogeneous charge compression-ignition (HCCI) engines depends on both the mean and fluctuations in initial temperature which should be considered in controlling ignition timing and preventing an overly rapid increase in pressure in HCCI combustion.