ABSTRACT
An experimental study is performed with the aim of investigating the effect of the
presence of droplets in flames of very rich iso-octane-air mixture under normal gravity.
Experiments are conducted for initial pressures in the range 100-160 kPa and initial
temperatures 287-303 K at an equivalence ratio of 2.0. Iso-octane-air aerosols are
generated by expansion of the gaseous pre-mixture (condensation technique) to
produce a homogeneously distributed suspension of near mono-disperse 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. Flame propagation behavior is observed in a cylindrical vessel equipped with
optical windows by using schlieren photography. Local flame speeds are measured to
assess the effect of buoyancy in gaseous and aerosol flames. It is found that the
presence of droplets results in a much earlier onset of instabilities, at a rate faster than
that taken for the buoyancy effect to take place. Flame instabilities, characterised by
wrinkling and cellular surface structure, increase the burning rate due to the associated
increase in surface area. Consequently, the presence of droplets results in a faster
flame propagation rate than that displayed by a gaseous flame. The mechanism of
flame instabilities that causes a significant reduction of the buoyancy effect is
discussed.