Numerical study of the multi-flame structure in spray combustion

We investigate the multi-flame phenomenon often seen in spray combustion. To do so, simulations are made of two cases from the Delft Spray in Hot Coflow (DSHC) database. These two cases have the same fuel (ethanol) but differ in the type of coflow: room temperature air (case A(II)) or products of lean premixed combustion (case H-II), and they exhibit remarkably different flame structure, respectively with a 'double flame' and a 'single flame'. The two-phase flow was handled with an Eulerian-Lagrangian approach. Large Eddy Simulation (LES) with Flamelet Generated Manifolds (FGM) method was used to account for the turbulence and combustion. Validation was done by using available experimental data as well as information on similar systems reported in literature. Major features of these two cases were correctly reproduced. Simulation results revealed that four spatially separated reaction regions exist in the flame A(II) and two in the flame H-II. These regions are of different types, premixed or non-premixed, and are formed by different species, major fuel (ethanol) or intermediate species, e.g. carbon monoxide. The mechanism underlying the multi-flame structure was investigated and explained in terms of the relative magnitude of different time scales, namely of droplet evaporation, dispersion, convection and reaction. Parametric studies on the effect of spray polydispersity and coflowing air temperature were carried out, demonstrating an even wider range of flame structures. An important observation is that the 'single flame' structure usually present in the hot-diluted coflow case (H-II), is also generated in a case of room temperature air coflow, provided the injected droplets are small. This is attributed to the fact that representative droplets in all cases with single flame have similar evaporation time scale. Or stated more generally, by matching important time scales, similar flame structure can be created under considerably different conditions. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.