Early flame development in the combustion of droplet and vapour mixtures

Abstract

Experimental investigations of combustion of droplet and vapour mixtures under quiescent condition have been conducted in a closed combustion vessel. Droplet and vapour mixtures or aerosol mixtures were generated by expansion of iso-octane gaseous pre-mixture to produce a homogeneously distributed suspension of fuel droplets. The aerosol mixtures were ignited centrally in the combustion vessel and the flame development was recorded by schlieren photography. The effect of fuel droplets in the early stage of flame propagation was investigated by comparing the flame structure and flame speed of gaseous mixtures at identical conditions. Comparison between gaseous and aerosol flame have shown quantitatively that the presence of fuel droplets causes earlier onset of instabilities and cellularity than for gaseous flames, particularly at rich conditions. This difference was shown to be a function of droplet size and overall equivalence ratio. It is suggested that these factors lead to vary the local equivalence ratio which increases the initial burning rate of lean aerosols, but decreases that of rich ones.