Double, Triple, and Tetra-Brachial Flame Structures around a Pair of Droplets in Tandem

Abstract

The multiple flame structures for a pair of droplets in tandem in hot convective flows are analyzed numerically, by employing a body-fitted technique in association with a non-orthogonal curvilinear coordinate system. For selected droplet spacing and Reynolds numbers, three flame structures, including the double, triple, and tetra-brachial flames, are identified at specific ambient temperatures and droplet size ratios. As the droplet size ratio is small, the numerical results indicate that a double-flame structure composed of the front fuel-lean premixed flame and the rear diffusion flame develops at lower ambient temperature. Increasing the droplet size ratio results in the two flames experiencing the separation, then merger, and eventually destruction. Hence a triple flame consisting of both fuel-rich and fuel-lean premixed flames, as well as a diffusion flame appears. When the ambient temperature is higher, for the double-flame the front flame departs from the rear one as a result of the increased flame speed. On the other hand, because the flame moves upstream and the blowing effect of the trailing droplet grows, the triple-flame converts into a tetra-brachial flame at the larger droplet size ratio. The formation of these flames is attributed to the coupling of fuel vapor, cooling effect, and oxidizer depletion stemming from the leading droplet, and these observations can provide a more accurate modeling on droplets behaviors in spray.