Pilot-ignited dual fuel combustion involves a complex transition between the pilot fuel autoignition and the premixed-like phase of combustion, which is challenging for experimental measurement and numerical modelling, and not sufficiently explored. To further understand the fundamentals of the dual fuel ignition processes, the transient ignition and subsequent flame development in a turbulent dimethyl ether (DME)/methane-air mixing layer under diesel engine-relevant conditions are studied by direct numerical simulations (DNS). Results indicate that combustion is initiated by a two-stage autoignition that involves both low-temperature and high-temperature chemistry. The first stage autoignition is initiated at the stoichiometric mixture, and then the ignition front propagates against the mixture fraction gradient into rich mixtures and eventually forms a diffusively-supported cool flame. The second stage ignition kernels are spatially distributed around the most reactive mixture fraction with a low scalar dissipation rate. Multiple triple flames are established and propagate along the stoichiometric mixture, which is proven to play an essential role in the flame developing process. The edge flames gradually get close to each other with their branches eventually connected. It is the leading lean premixed branch that initiates the steady propagating methane-air flame. The time required for the initiation of steady flame is substantially shorter than the autoignition delay time of the methane-air mixture under the same thermochemical condition. Temporal evolution of the displacement speed at the flame front is also investigated to clarify the propagation characteristics of the combustion waves. Cool flame and propagation of triple flames are also identified in this study, which are novel features of the pilot-ignited dual fuel combustion.

引燃双燃料燃烧涉及引燃燃料自燃和类似燃烧的预混合相之间的复杂过渡，这对于实验测量和数值建模来说是具有挑战性的，因此没有得到充分的探索。为了进一步了解双燃料点火过程的基本原理，通过直接数值模拟（DNS）研究了在柴油机相关条件下的湍流二甲醚（DME）/甲烷-空气混合层中的瞬态点火和随后的火焰发展。结果表明，燃烧是由涉及低温和高温化学物质的两阶段自燃引发的。第一阶段自燃在化学计量混合气下开始，然后点火前沿逆着混合分数梯度传播成浓混合气，并最终形成扩散支撑的冷火焰。第二级点火核在空间上分布在反应性最高的混合气馏分周围，标量耗散率低。建立了多个三重火焰并沿着化学计量混合物传播，事实证明，这在火焰产生过程中起着至关重要的作用。边缘火焰逐渐靠近彼此，并且最终连接了它们的分支。它是领先的贫油预混分支，可引发稳定的甲烷-空气火焰蔓延。在相同的热化学条件下，引发稳定火焰所需的时间明显短于甲烷-空气混合物的自燃延迟时间。还研究了火焰前沿位移速度的时间演变，以阐明燃烧波的传播特性。在这项研究中还确定了冷火焰和三重火焰的传播，这是引燃双燃料燃烧的新颖特征。