The present study aims to extend the understanding of the external group combustion for droplet clouds under two-stage autoignition conditions. First, an exemplar case has been chosen to analyze the two-stage ignition behavior and flame structure. It was found that the fuel vapor transport of the droplet cloud is mainly supported by the thin vaporization layer near the edge of the cloud, which is the dominant factor of the ignition process. The case shows that a typical two-stage ignition includes the following detailed processes: first stage ignition, quasi-steady non-premixed cool flame, second stage ignition, and premixed hot flame. Three possible ignition modes for droplet clouds were identified through parametric studies, i.e. no ignition, single-stage ignition, and two-stage ignition. The transition between these modes is mainly controlled by the competition between chemical reaction and the vaporization of a thin layer of fuel droplets. Phase diagrams of different ignition modes at different ambient temperatures and sub parameters of the group number were presented. The previous group number G = 2πndR_c² demonstrated unsatisfactory prediction in the ignition modes of the droplet clouds. Basing on the vaporization layer feature of the droplet clouds, a new dimensionless group ignition number with much better predictability is first proposed in this work, i.e. G_ig = 4πn^2/3d². The physical meaning of G_ig is the dimensionless total surface area of droplets within a local layer.

本研究旨在扩展对两级自燃条件下液滴云外部群燃烧的理解。首先，选择了一个示例案例来分析两阶段点火行为和火焰结构。研究发现，液滴云的燃料蒸汽传输主要由云边缘附近的薄汽化层支持，这是点火过程的主导因素。案例表明，典型的二级点火包括以下详细过程：一级点火、准稳态非预混冷焰、二级点火、预混热焰。通过参数研究确定了三种可能的液滴云点火模式，即不点火、单级点火和两级点火。这些模式之间的转变主要受化学反应和燃料液滴薄层汽化之间的竞争控制。给出了不同环境温度下不同点火方式的相图和组数子参数。上一组号G  = 2π ndR _c ²在液滴云的点火模式中表现出不令人满意的预测。基于液滴云的汽化层特征，本文首次提出了一种新的具有更好预测性的无量纲群点火数，即G _ig  = 4π n ^2/3 d ²。G _ig的物理意义是局部层内液滴的无量纲总表面积。