Inertial effects on globally stoichiometric spherically expanding turbulent flames propagating in droplet-laden mixtures

Abstract

Influences of droplet inertia on the reaction zone structure, burning rate and flame surface area in spherically expanding flames propagating into mono-sized droplet mists with an overall (liquid+ droplet phases) equivalence ratio of unity have been analysed based on three-dimensional carrier phase Direct Numerical Simulations for different droplet diameters and turbulence intensities. Droplet inertia has been demonstrated to have an important influence on gaseous equivalence ratio within the flame. The gaseous phase combustion in the cases with inertialess droplets has been shown to take place predominantly under leaner conditions and this trend strengthens further for higher turbulence intensities. Consequently, the flames in the case of inertialess droplets have been found to be thicker than the corresponding cases with inertial droplets. The number density of droplets within the flame remains smaller in the inertialess droplet cases than in the corresponding cases with inertial droplets, which leads to smaller extents of droplet-induced flame wrinkling. As a result, the probability density functions of flame curvature for the inertial droplet cases have been found to be wider than those in the corresponding inertialess droplet cases. The reduced probability of obtaining stoichiometric gaseous mixture within the flame for inertialess droplets leads to a smaller percentage share of total heat release rate from non-premixed mode of combustion than those with inertial droplets. This, along with the predominance of fuel-lean gaseous mixture composition within the flame, gives rise to smaller extent of flame area generation and burned gas volume in the inertialess droplet cases than in the corresponding cases with inertial droplets. This tendency strengthens with increasing turbulence intensity and the cases with large inertialess droplets under high turbulence intensity show indications of eventual flame extinction.