Structure and extinction of a counterflow partially premixed, diffusion flame

Abstract

The fundamental heat and mass transport processes in a partially premixed, diffusion flame stabilized between counterflowing streams of fuel A, and fuel B premixed with an oxidizer C and an inert gas are analysed. The gas phase chemical reaction between fuel A and the oxidizer C and between fuel B and the oxidizer C is approximated as a one step process. Asymptotic analysis is performed in the limit of a large value for the ratio of the activation energy characterizing the chemical reactions to the thermal energy in the flame. Guided by experimental results it is presumed that two distinct, thin reaction zones are present, a premixed flame and a diffusion flame. The outer structure and the inner structure of the reaction zones are analysed. It is shown that for the flame to extinguish the reaction zones must merge. The outer structure and inner structure of the merged reaction zone is analysed. An explicit algebraic relation is obtained relating the Damköhler number at extinction to the ambient conditions in the counterflowing streams, thermophysical properties of the reactants, and the overall chemical kinetic rate parameters characterizing the gas phase oxidation of the fuels. The results of the analysis are then extended to a merged flame that is stabilized in a stagnation point boundary layer over the surface of a liquid fuel when a premixed stream of gaseous fuel and oxidizer flows over its surface. To test the predictions of the theory, extinction experiments are performed on a partially premixed, diffusion flame stabilized between a vaporizing surface of heptane and a gaseous stream consisting of methane, oxygen and nitrogen. The results are used to deduce the overall chemical kinetic rate parameters characterizing the gas phase oxidation of methane in a premixed flame.