Simulations of transient n-heptane and n-dodecane spray flames under engine-relevant conditions using a transported PDF method
Simulations of transient n-heptane and n-dodecane spray flames under engine-relevant conditions using a transported PDF method
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DOI：
10.1016/j.combustflame.2013.05.003
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发表时间：
2013-10
影响因子：
4.4
通讯作者：
S. Bhattacharjee;D. Haworth
中科院分区：
文献类型：
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作者：
S. Bhattacharjee;D. Haworth
采用骨架化学机理对两种单组分燃料在高压定容腔中的瞬态湍流喷雾火焰进行了时间相关的CFD计算。模拟跨越了对应于常规和先进（例如，低温）压缩点火发动机燃烧。其目的是建立在何种程度上的湍流波动的成分和温度的影响点火延迟和升空长度和湍流火焰结构下发动机相关的条件下，并提供洞察力的化学相互作用。这是通过比较的结果，从一个模型，占湍流波动，使用传输组成概率密度函数（PDF）方法与那些从一个模型，忽略了湍流波动的影响，局部平均反应速率（局部搅拌良好的反应器- WSR模型）。对于稳定的柴油机燃烧工况，WSR和PDF计算的着火延迟期和提离长度彼此接近，并且两者都与实验吻合得很好。对于较低的初始温度，两种模型的点火延迟和提离长度有显著差异，PDF模型的计算结果与实验结果吻合较好。正十二烷的差异尤其显著。对于初始温度为900 K或更高（密度为22.8 kg/m3，氧含量为15%），PDF模型计算的点火延迟和提离长度与测量值相差在10%以内，而WSR模型预测的点火延迟是900 K时测量值的三倍。在初始温度为800 K时，WSR模型不能点火，而PDF模型计算的点火延迟和提离长度在测量值的30%以内。在所有的情况下，WSR和PDF模型产生显着不同的湍流火焰结构，并与初始温度和氧气水平的差异增加。WSR模型产生一个薄的层状火焰，而PDF模型给出了一个扩大的湍流火焰刷，这是定性更符合预期的这些高度湍流火焰和实验观察到的。因此，虽然它可能是可能的，以重现一些全球点火特性使用WSR模型（取决于选择的化学机制），湍流波动在较低的初始温度和氧气水平发挥越来越重要的作用。
Time-dependent Reynolds-averaged CFD is performed for transient turbulent spray flames in a high-pressure, constant-volume chamber for two single-component fuels using skeletal chemical mechanisms. The simulations span a range of initial pressures, temperatures and compositions that correspond to conventional and advanced (e.g., low-temperature) compression-ignition engine combustion. The objectives are to establish the extent to which turbulent fluctuations in composition and temperature influence ignition delays and lift-off lengths and turbulent flame structure under engine-relevant conditions, and to provide insight into turbulence-chemistry interactions. This is done by comparing results from a model that accounts for turbulent fluctuations using a transported composition probability density function (PDF) method with those from a model that ignores the influence of turbulent fluctuations on local mean reaction rates (a locally well-stirred reactor – WSR – model). For robust diesel combustion conditions, the WSR and PDF computed ignition delays and lift-off lengths are close to each other, and both are in good agreement with experiment. For lower initial temperatures, ignition delays and lift-off lengths from the two models are significantly different, and the results from the PDF model are in better agreement with experiment. The differences are especially striking for n-dodecane. There the PDF-model computed ignition delays and lift-off lengths are within 10% of measured values for initial temperatures of 900 K and higher (for 22.8 kg/m3density, 15% oxygen), while the WSR model predicts an ignition delay that is three times the measured value at 900 K. At an initial temperature of 800 K, the WSR model fails to ignite, whereas the PDF model computed ignition delay and lift-off length are within 30% of the measured values. In all cases, the WSR and PDF models produce significantly different turbulent flame structures, and the differences increase with decreasing initial temperature and oxygen level. The WSR model produces a thin laminar-like flame, while the PDF model gives a broadened turbulent flame brush that is qualitatively more consistent with what is expected for these highly turbulent flames and what is observed experimentally. Thus, while it may be possible to reproduce some global ignition characteristics using a WSR model (depending on the choice of chemical mechanism), turbulent fluctuations play an increasingly important role at lower initial temperatures and oxygen levels.