Laminar mass burning and entrainment velocities and flame instabilities of i-octane, ethanol and hydrous ethanol/air aerosols

Abstract

The paper reports experiments employing the cloud chamber technique for creating fuel aerosols, in studies of premixed laminar flames. Spherical explosion flames were initiated at different times after the start of expansion of the original gaseous mixture to lower pressure. Flame speeds were measured close to atmospheric pressure, over a range of equivalence ratios of iso-octane, ethanol and hydrous ethanol with air. A methodology was developed for deriving mass burning velocities and entrainment velocities, as well as mass burning fluxes, from the measurements of aerosol number densities, droplet sizes and flame speeds. It was vital to estimate whether droplet evaporation was completed in the flame preheat zone. This was done by calculating the spatial progress of droplet evaporation for the different aerosols from values of the evaporation rate constants of the different fuels.With predominantly the leaner mixtures and smaller droplet diameters, evaporation was close to completion, but the mass burning velocities of the aerosols were somewhat lower than those of the corresponding gaseous phases, because of the lower final temperatures due to the required evaporation enthalpies. However, the mass burning fluxes were higher than those for the purely gaseous flames, due to the higher two-phase reactant densities. At the higher values of the liquid phase equivalence ratio, in overall lean mixtures, the mass burning velocity could exceed that in the purely gaseous phase due to localised enrichment around the droplets.The presence of fuel droplets is shown to enhance the generation of Darrieus–Landau, thermo-diffusive instabilities and the associated flame wrinkling. With richer mixtures and larger droplets, it is possible for droplets to enter the reaction zone and further enhance existing gaseous phase instabilities through the creation of yet further flame wrinkling. This leads to the maximum entrained fuel mass flux, in the richest mixture, being significantly higher than that occurring at the maximum burning velocity of a premixed gaseous flame.