Impact of CO2/N2/Ar Addition on the Internal Structure and Stability of Nonpremixed CH4/Air Flames at Lifting

Abstract : The authors focused on how adding CO2 to the air influences the transition from an attached flame to a lifted flame issued from a coaxial nonpremixed methane-air jet. To discriminate between effects due to a diluent (dilution, thermal, or chemical impacts), chemically and thermally inert N2 and chemically inert Ar were also investigated. Flame lifting always occurs, essentially controlled by the critical flow-rate ratio, (Qdiluent/Qair)lifting. CO2 has the strongest ability to break flame stability, followed by N2, then by Ar. A unique attachment height and OH thickness characterize lifting for all the diluents; lifting is attained once the same critical flame edge propagation speed is reached. (Qdiluent/Qair)/(Qdiluent/Qair)lifting is the affine parameter of similarity laws describing Ha and EpOH evolutions with dilution. Aerodynamics competes with dilution to impose lifting and boundary effects cannot be ignored in a fine analysis. The flame behaves differently according to whether lifting results from aerodynamics or dilution.