Internal group combustion of droplet clouds and its transition to external group combustion under two-stage autoignition conditions

Abstract

The present study is an extension of our prior study on the external group combustion (EGC) of droplet clouds under two-stage autoignition conditions [Zhou and Liu, Combust. Flame 234 (2021) 111689]. Effects of droplet heating and different n-alkane fuel candidates have been further considered. By comparing numerical results from n-heptane and n-dodecane, it was found that the fuel volatility has little effect on the combustion modes of droplet clouds. Among all the computed cases, internal group combustion (IGC) occurs for relatively low value of group ignition number Gig (= 4πn2/3d2, n and d are the droplet number density and diameter, respectively). When Gig further increases, the combustion mode can switch to EGC. IGC modes can be sustained by cool or hot flame. IGC of cool flame exhibits a partial burning structure, while IGC of hot flame is firstly premixed, followed by non-premixed structure. These flame structures were analyzed in details. A regime diagram based on Gig and ambient temperature Ta was developed for all the IGC modes. It was found that for high Ta (1500 K), Gig can be used to predict ICG modes, but for low Ta (900 K) and intermediate Ta (1200 K), additional parameters might be needed for the prediction, especially for the cases when Gig is around 0.03. The inapplicability is due to the strong interaction between droplet behavior (droplet heating and vaporization) and chemical kinetics (low and high temperature reaction) that were not inherent to description of Gig. To clarify the interaction, time scale analysis of these processes was performed for better complementation of the regime diagram characterized by Gig and Ta for various IGC modes.