Effect of Droplets on the Flame Speed of Laminar Iso-octane and Air Aerosols

It is well established that the laminar burning rate plays an important role in turbulent combustion and previous work at Leeds has shown that the laminar burning velocity of an aerosol mixture is little different from that of a gaseous mixture at similar conditions. However, it has been shown that flames within aerosol mixtures more readily become unstable than for gaseous ones and this increases the practical burning rate. Flame instabilities, characterised by wrinkling and cellular surface structure, increase the burning rate due to the associated increase in surface area. In aerosol combustion, the presence of liquid droplets has been shown to influence instabilities by causing earlier onset of cellularity than for gaseous flames. Thus an understanding of the influence of the presence of liquid droplets in laminar aerosol flames is vital before the behaviour of practical spray combustion can be fully understood. In this paper, spherically expanding flames at near atmospheric pressures are employed to quantify the differences in the burning rates in laminar gaseous and aerosol flames. 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. It is shown that the burning rate of lean mixtures is independent of droplet diameter. However, at higher equivalence ratios, the burning rate becomes a strong function of droplet diameter and equivalence ratio.