Opposed flow polydisperse spray diffusion flames: Steady state and extinction analysis

Abstract

A new analysis of polydisperse opposed flow spray diffusion flames is presented. The model permits non-unity Lewis number effects for the gaseous components, as well as a finite rate of evaporation for the droplets. Combined effects of droplet loading and spray polydispersity on the characteristics of spray flames are investigated using outer region analytic solutions. Under the assumption that the droplets are in dynamic and thermal equilibrium with their surroundings it is demonstrated that spray polydispersity alone has a profound effect on flame location, temperature and the mode of combustion (homogeneous, or homogeneous and heterogeneous combustion). Conditions for flame extinction are also examined. Frequently, polydisperse sprays are described in a global fashion by relating to them as if they were comprised of single size droplets having some average diameter, such as the Sauter mean diameter or a mean diameter based on the total surface area of the spray. It is shown that this approach can be rather erroneous when flame extinction is under consideration. By comparing strain rates for extinguishment it is demonstrated that critical extinction conditions must be based on a true representation of the spray's polydispersity, otherwise errors of as much as almost 60% can ensue.