Kinetic and radiative extinctions of spherical diffusion flames

Abstract

In this thesis, the effect of radiative heat loss on extinction of spherical diffusion flame stabilized by a spherical porous burner was investigated by activation energy asymptotics. The flow field was developed by issuing a reactant flow from the burner into a quiescent ambient filled with the other reactant. A one-step, overall and irreversible reaction that follows an Arrhenius kinetics with high activation energy was adopted to model the combustion reaction. The radiative heat loss rate was described by an optically thin model. Based on which reactant is supplied from the burner and how the inert gas is distributed, four model flames, namely the flames with fuel issuing into air, diluted fuel issuing into oxygen, air issuing into fuel, and oxygen issuing into diluted fuel were studied to understand the effects of stoichiometric mixture fraction and flow direction. Results show that when the flow rates fixed, only the conventional kinetic extinction limit at low Damköhler number (low residence times) was observed. The effect of radiative heat loss was to promote extinction such that it is easier to occur. By keeping the radiation intensity constant while varying the flow rate, both the kinetic and radiative extinction limits, representing the smallest and largest flow rates between which steady burning is possible, were exhibited. For flames with low radiation intensity, extinction was primarily characterized by the residence time such that the high-flow rate flames were easier to be quenched. As to the flames suffering strong radiative heat loss, extinction was dominated by the energy loss so the flame with larger size is weaker and easier to extinguish.