Burning Rate and Regimes of Spherically Propagating Laminar Flames of Iso-Octane-Air Aerosol Mixtures

Combustion of aerosol or two-phase fuel-air mixture is of technological importance but is not fully understood, hence the need for fundamental aerosol combustion studies. Previous researches qualitatively revealed that the presence of liquid droplets in the combustion of narrowly dispersed aerosol mixture could influence instabilities, characterized by cellular flame structure, which promotes a faster burning rate as compared to the flames of a fully vaporized homogeneous mixture. In addition, while the burning rate of laminar flames is well known to play an important role in turbulent combustion, data on laminar burning velocities, particularly for droplet-vapor-air mixtures, is still very scarce. Hence, the study of the combustion droplets-vapor-air mixture is imperative in order to assess the effect of the presence of droplets to burning rates and flame instabilities. In the present work, spherically expanding laminar flames at near atmospheric pressures are employed to study the burning properties of droplet-vapor-air mixtures at various ranges of equivalence ratio and droplet size, by recording the schlieren images of the flames using a high-speed digital camera. Iso-octane-air aerosols are generated by expansion of the gaseous pre-mixture to produce a homogeneously distributed suspension of fuel droplets. The droplet size varies with time during expansion; hence the effect of droplet size in relation to the cellular structure of the flame is investigated by varying the ignition timing. Comparisons are made with gaseous flames, in which the results suggest that flames of droplet-vapor-air mixtures become unstable earlier and have a higher degree of cellularity than those of gaseous. It is also observed that generally three regimes of combustion modes exist, which are stable, accelerating and oscillating.