Instability of a flame front propagating through a fuel-rich droplet-vapour-air cloud

A simple model of a flame front propagating through a fuel-rich droplet-vap our-air mixture is presented in which the fuel droplets are assumed to evap orate in a sharp front ahead of the reaction front. By performing a linear stability analysis neutral stability boundaries are determined. It is shown that the presence of the spray of droplets in the fresh mixture can have a profound effect by causing cellularization of the flame front. Specificall y, we demonstrate that under certain circumstances a spray flame can be cel lular when its equivalent non-spray flame is completely stable. Furthermore , it is shown that even when the non-spray flame is itself cellular the equ ivalent spray flame will have a finer cellular structure. These theoretical predictions verify qualitatively for the first time independent experiment al observations from the literature. It is thus shown that the primary effe ct of the spray on the stability of these flames is due to heat loss from t he absorption of heat by the droplets for vaporization. The influence of th e initial liquid fuel loading and the latent heat of vaporization on the cr itical wavenumber associated with cellularity provide further evidence of t he responsibility of the heat loss mechanism for these spray-related phenom ena. Finally, the cellularity of the spray flames with their attendant incr ease in flame front area suggest a plausible rationale for the experimental ly observed burning velocity enhancement induced by the use of a spray of f uel droplets.