Kinetic extinction limit of a spherical diffusion flame attached to a burner surface

Abstract

In this study the kinetic extinction of diffusion flames at low flow rates was identified. A microgravity burner-stabilized spherical diffusion flame was used to identify the smallest Damkohler number, representing the weakest burning intensity, at which a flame exists. An activation energy asymptotics that split the flow domain to four main regions, namely the core, porous burner, reaction, and outer regions, was used to identify the extinction state. Four limiting flames, based on flow direction and inert distribution, were used to study the effects of various controlling parameters. By reducing the mass flow rate from a flame sufficiently away from the burner, the flame radius is decreased until the flame reaches the burner exit. Further reduction of the flow rate changes the burning to behave like a premixed flame. Results show that for a flame with a constant flow rate, there exists a minimum reaction rate, or Damkohler number, below which steady burning is not possible. For a specific flame with a specified reaction rate, extinction occurs at a higher flow rate, meaning that extinction is easier to occur when the mass diffusion rate (represented by a specified Lewis number) of the burner reactant is higher or when the mass diffusion rate of the ambient reactant is lower. Comparison of the four limiting flames with the same fuel consumption rate reveals that a flame with a smaller mass flow rate is easier to extinguish.