Three-dimensional compressible Direct Numerical Simulations have been used to investigate the localised forced ignition of statistically planar biogas/air mixing layers for different levels of turbulence intensity and biogas composition. The biogas is represented by a \(\hbox {CH}_4\)/\(\hbox {CO}_2\) mixture and a two-step mechanism capturing the variation of the unstrained laminar flame speed with equivalence ratio and \(\hbox {CO}_2\) dilution was used. The mixture composition was found to significantly affect the flame kernel development which was reflected in the diminished growth rate of the burned gas volume with increasing \(\hbox {CO}_2\) dilution. A successful ignition of \(\hbox {CH}_4\)/\(\hbox {CO}_2\)/air mixing layer gives rise to a tribrachial flame structure involving fuel-rich and lean premixed branches on either side of the diffusion flame stabilised on the stoichiometric mixture fraction iso-surface. The most probable edge flame speed decreases in time and converges to a value that is at most equal to its laminar theoretical limit, and can even locally become negative for large values of the dilution and/or turbulence intensity. The decomposition of the edge flame speed showed a negligible or negative contribution of the mixture fraction surface displacement speed, while the displacement speed of the fuel mass fraction surface appeared as the dominant contributor. Finally, the edge flame speed dependences to the fuel mass fraction and mixture fraction gradients, fuel mass fraction iso-surface curvature and tangential strain rate have been analysed and found, within the dilution values considered, qualitatively similar to those of undiluted mixtures regardless of the amount of \(\hbox {CO}_2\), although quantitative differences were observed.

三维可压缩直接数值模拟已用于研究不同湍流强度和沼气成分的统计平面沼气/空气混合层的局部强制点火。沼气由\（\ hbox {CH} _4 \） / \（\ hbox {CO} _2 \）混合物和两步机制表示，该机制捕获了当量比和\（\使用hbox {CO} _2 \）稀释液。发现混合物组成显着影响火焰核的发展，这反映为随着（\ hbox {CO} _2 \）稀释度的增加，燃烧气体体积的增长率降低。的点火成功\（\ hbox中{CH} _4 \） /\（\ hbox {CO} _2 \）空气混合层产生三臂火焰结构，其在稳定于化学计量混合物分数等值面上的扩散火焰的两侧包括富燃料和贫燃料的预混合分支。最可能的边缘火焰速度随时间降低，并收敛至最大等于其层流理论极限的值，甚至对于较大的稀释和/或湍流强度值，甚至可能局部变为负值。边缘火焰速度的分解显示混合分数表面位移速度的贡献可忽略不计或负的影响，而燃料质量分数表面的位移速度似乎是主要的贡献者。最后，边缘火焰速度取决于燃料质量分数和混合物分数梯度，\（\ hbox {CO} _2 \），尽管观察到数量差异。