External group combustion of droplet clouds under two-stage autoignition conditions

Abstract

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 = 2πndRc2 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. Gig = 4πn2/3d2. The physical meaning of Gig is the dimensionless total surface area of droplets within a local layer.