The minimum ignition energy (MIE) requirements for ensuring successful thermal runaway and self-sustained flame propagation have been analysed for forced ignition of homogeneous stoichiometric biogas-air mixtures for a wide range of initial turbulence intensities and CO2 dilutions using three-dimensional Direct Numerical Simulations under decaying turbulence. The biogas is represented by a CH4 + CO2 mixture and a two-step chemical mechanism involving incomplete oxidation of CH4 to CO and H2O and an equilibrium between the CO oxidation and the CO2 dissociation has been used for simulating biogas-air combustion. It has been found that the MIE increases with increasing CO2 content in the biogas due to the detrimental effect of the CO2 dilution on the burning and heat release rates. The MIE for ensuring self-sustained flame propagation has been found to be greater than the MIE for ensuring only thermal runaway irrespective of its outcome for large root-mean-square (rms) values of turbulent velocity fluctuation, and the MIE values increase with increasing rms turbulent velocity for both cases. It has been found that the MIE values increase more steeply with increasing rms turbulent velocity beyond a critical turbulence intensity than in the case of smaller turbulence intensities. The variations of the normalised MIE (MIE normalised by the value for the quiescent laminar condition) with normalised turbulence intensity for biogas-air mixtures are found to be qualitatively similar to those obtained for the undiluted mixture. However, the critical turbulence intensity has been found to decrease with increasing CO2 dilution. It has been found that the normalised MIE for self-sustained flame propagation increases with increasing rms turbulent velocity following a power-law and the power-law exponent has been found not to vary much with the level of CO2 dilution. This behaviour has been explained using a scaling analysis and flame wrinkling statistics. The stochasticity of the ignition event has been analysed by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and it has been demonstrated that successful outcomes are obtained in most of the instances, justifying the accuracy of the MIE values identified by this analysis.

中文翻译：

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燃料成分对均质沼气-空气混合物最小点火能影响的数值研究

已使用三维直接数值模拟分析了确保成功热失控和自持火焰传播的最小点火能量 (MIE) 要求，以针对各种初始湍流强度和 CO2 稀释的均匀化学计量的沼气 - 空气混合物进行强制点火在衰败的湍流下。沼气由 CH4 + CO2 混合物和两步化学机制表示，包括 CH4 不完全氧化成 CO 和 H2O，并且 CO 氧化和 CO2 解离之间的平衡已用于模拟沼气-空气燃烧。已经发现，由于 CO2 稀释对燃烧和热释放速率的不利影响，MIE 随着沼气中 CO2 含量的增加而增加。已发现用于确保自持火焰传播的 MIE 大于用于确保仅热失控的 MIE，而不管其对于湍流速度波动的大均方根 (rms) 值的结果如何，并且 MIE 值随着增加而增加两种情况下的均方根湍流速度。已经发现，与湍流强度较小的情况相比，MIE 值随着超过临界湍流强度的均方根湍流速度的增加而增加得更陡峭。发现归一化的 MIE（由静止层流条件的值归一化的 MIE）与归一化的沼气-空气混合物的湍流强度的变化在性质上与未稀释混合物的变化相似。然而，已经发现临界湍流强度随着 CO2 稀释度的增加而降低。已经发现，自持火焰传播的归一化 MIE 随着 rms 湍流速度的增加而增加，遵循幂律，并且发现幂律指数随 CO2 稀释水平变化不大。已使用缩放分析和火焰起皱统计来解释这种行为。通过对对应于 MIE 的能量输入使用统计上相似的湍流场的不同实现来分析点火事件的随机性，并且已经证明在大多数情况下获得了成功的结果，证明了 MIE 值的准确性通过该分析确定。