Droplet/turbulence interaction and early flame kernel development in an autoigniting realistic dense spray

Abstract

A very detailed evaporating/reacting spray simulation has been conducted extending our previous liquid atomization data in order to understand the physical processes leading to group combustion. The head region of a realistic dense Diesel jet spray is considered, and the droplets and diffusive layers are resolved. At this stage, the droplets have the relative velocity to the high-temperature air flow and this generates comparable-size turbulent eddies around the droplets. By these eddies, mixing is enhanced more strongly than molecular diffusion and merged vapor clouds are formed especially in the area of large droplet number density. This suggests that external group combustion is likely to occur and this is also confirmed by the estimation of group combustion number. This finding requires a modification to the point-source model so that the droplet-generated mixing enhancement effect should be more accurately included. A simple time scale analysis shows that final ignition occurs after the liquid jet head disappears and the resident time is sufficiently large enough.