A direct numerical simulation analysis of spherically expanding turbulent flames in fuel droplet-mists for an overall equivalence ratio of unity

Abstract

Three-dimensional direct numerical simulations with a modified single-step Arrhenius chemistry have been used to analyze spherically expanding n-heptane flames propagating into mono-sized fuel droplet mists for different droplet diameters and an overall equivalence ratio of unity. The evolutions of flame surface area and burned gas volume for both laminar and turbulent spherically expanding droplet flames have been compared to the corresponding gaseous stoichiometric premixed spherically expanding flames with the same initial burned gas radius. It has been found that the initial droplet diameter significantly affects the burned gas volume and flame area generation, which increase with decreasing droplet diameter for both laminar and turbulent cases. The droplet-flame interaction plays a key role in determining flame wrinkling under laminar conditions, which is reflected in a range of local curvatures for a given reaction progress variable isosurface, whereas each progress variable isosurface in spherically expanding laminar premixed flames exhibits a single value of curvature. The effect of droplet-induced curvature becomes less distinguishable from the flow-induced wrinkling for the turbulent cases considered here, but the reaction progress variable isosurfaces in droplet cases exhibit wider curvature probability density functions than in the corresponding turbulent premixed flame cases. It has been found that the heat release rate arises principally from premixed mode in small droplet cases, whereas the contribution of the non-premixed mode to the overall heat release rate increases with increasing droplet diameter and turbulence intensity.