Inertial effects on the interaction of water droplets with turbulent premixed flames: A direct numerical simulation analysis

Abstract

The effects of droplet inertia on the reaction zone structure, overall burning rate, and flame surface area during interaction of water droplets with a statistically planar turbulent premixed stoichiometric n-heptane-air flame based on three-dimensional carrier phase Direct Numerical Simulations have been analysed. Different initially mono-sized droplets are considered for this study in order to analyse the inertial effects for different droplet sizes. Droplet inertia has been demonstrated to have an important influence on the extent of evaporation of water droplets and water vapour concentration arising from evaporation within the flame. It has been found that the residence time of droplets within the flame increases due to droplet inertia and therefore the evaporation and the cooling effect associated with the extraction of latent heat are stronger for inertial droplets than in the case of hypothetical inertialess droplets. The stronger cooling effects for inertial droplets lead to smaller burned gas temperature and thicker flame than the corresponding cases with inertialess droplets. The aforementioned cooling effect induced by latent heat of evaporation of water droplets reduces the burning rate and the likelihood of obtaining high gradient magnitudes of reaction progress variable and temperature within the flame for the cases with water droplets in comparison to the corresponding purely gaseous turbulent premixed flames, and this tendency is stronger for inertial droplets. The stronger cooling effect, along with the reduction of reactant concentrations within the flame due to greater extent of evaporation for inertial droplets, gives rise to a smaller extent of flame area generation and overall burning rate than in the corresponding cases with inertialess droplets. It has been found that water droplet inertia acts to reduce the burning rate and the consumption rate of reactants per unit flame surface area during the interaction of water droplets with turbulent premixed flames within the parameter range analysed in this paper.