Simultaneous effects of fuel/oxidizer concentrations on the extinction of counterflow diffusion flames

Abstract

The combined influence of the fuel and the oxidizer concentrations and Lewis numbers on the extinction of finite-jet counterflow diffusion flames is experimentally and theoretically investigated. In the experiments, either methane or butane mixed in nitrogen is the fuel jet, while mixtures of oxygen and nitrogen are used as the oxidizer jet. The critical minimum values of the fuel (oxidizer) concentrations at extinction are determined at different values of the oxidizer (fuel) concentration in the opposing jet and various fixed values of the jet velocity. Thus, the domain of “flammability” of the diffusion flames within the complete range of the fuel/oxidizer molar concentration ( X F , X O ) is obtained at various fixed values of the mean nozzle velocities. The concept of the “external” or global versus the “internal” or local equivalence ratio is introduced to describe the simultaneous impact of the fuel and the oxidizer concentrations on the burning characteristics of diffusion flames. In the theoretical study, the hydrodynamics of viscous incompressible flow is used in order to investigate the thermodiffusive structure of the diffusion flames for general Lewis numbers of the fuel and oxidizer. Quantitative comparisons are made between the theoretical calculations and the available experimental data. The relevance of the findings to the modeling of flamelet combustion/extinction in turbulent diffusion flames is discussed.