Ignition of binary mixture droplets by a propagating laminar flame

Abstract

An experimental study was made of the ignition process of single droplets of binary mixtures subjected to a laminar flat flame propagating through a lean homogeneous propane/air mixture at constant pressure. Binary fuels consisting of n -hexane and n -hexadecane or n -dodecane, and benzene and n -hexadecane were tested. Water-emulsified liquids consisting of n -dodecane, and pure fuels like n -hexane, n -octane, n -dodecane, and n -hexadecane, were used as well. A combustion chamber made of a transparent duct was installed with spark electrodes, fine quartz fibers to suspend fuel droplets, and a shutter to allow the burned gas to escape at the top end. A high-speed video camera was provided for photographic observation of each droplet ignited by the propagating flame. The ignition delay of pure fuel droplets increased monotonically with increase of the initial droplet diameter and decrease of fuel volatility. The influence of the initial droplet diameter on the ignition delay decreased with increasing fuel volatility. Ignition delay of the binary fuel droplets decreased with an increase of the volume concentration of the higher-volatility fuel. Ignition delay of the binary fuels showed a minimum and a maximum as a function of the initial droplet diameter. The diameters at the minimum and the maximum of ignition delay decreased with increasing higher-volatility fuel concentration. For the emulsified fuel, on the other hand, ignition delay increased with increasing initial droplet diameter and increasing water content. Part of the propagating flame behind the droplet became convex toward the unburned gas as it passed a droplet of higher-volatility fuels, while the flame remained flat for the lower-volatility fuels. The deformation length of the propagating flame behind the droplet increased with increasing higher-volatility fuel concentration.