STOICHIOMETRY AND POLYDISPERSE EFFECTS IN PREMIXED SPRAY FLAMES

An analysis of a one-dimensional laminar premixed polydisperse spray flame is presented, for the whole range of fuel rich/lean mixtures. The spray of droplets is modeled using a sectional approach, thus making it possible to account for droplet-droplet interaction within the spray. A dimensional analysis of the mathematical model for the spray flame reveals its structure as consisting of a primary evaporation zone, a preheat zone, a homogeneous reaction zone, and a postreaction zone. If droplets survive the flame they may continue to burn or evaporate in the post reaction zone, depending on the fuel vapor-to-air ratio in the homogeneous reaction zone. Detailed analytic solutions are developed and matched for each of the aforementioned zones. Computed results show that the initial size distribution of the droplets in the spray has a profound influence on the flame's characteristics. Although the use of the Sauter mean diameter (SMD) may be sufficient for the treatment of monodisperse sprays it is demonstrated here that its use to characterize the average evaporation rate of polydisperse sprays may lead to erroneous results, since the local SMD may actually decrease or increase depending on the initial droplet size distribution.