High-Speed Schlieren Imaging 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 its flame behaviour is not fully understood. Furthermore, data on laminar burning velocities, which is well known to play an important role in turbulent combustion, is still very scarce particularly for droplet-vapour-air mixtures. Hence, there is a need for fundamental aerosol combustion studies in order to understand practical combustion of aerosols and sprays. Previous researches revealed, through schlieren cinematography, that the presence of liquid droplets in the combustion of aerosol mixture could influence flame instabilities. Such instabilities, which are characterized by cellular and wrinkling flame structure, promote a faster burning rate. However, until recently the properties related to the structure of the flame surface, such as the point of the onset of cellularity and the shape of the cells, were remained unclear due to the technique and condition of the imaging system available at that time. In the present work, spherically expanding laminar flames at near atmospheric pressures were employed to study the flame structure and the burning properties of droplet-vapour-air aerosol mixtures at various ranges of equivalence ratio and droplet size. The schlieren images of the flames were recorded using a high-speed digital camera, which was synchronised with the ignition system. The droplet-vapour-air aerosol mixtures were generated by expansion of the gaseous pre-mixture to produce a homogeneously distributed suspension of near monodisperse fuel droplets. The resulting schlieren images captured with the present recording system are shown to be of improved quality, which enabled clear distinction between cellular and smooth flame surface.