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, (Q_diluent/Q_air)_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. (Q_diluent/Q_air)/(Q_diluent/Q_air)_lifting is the affine parameter of similarity laws describing H_a and Ep_OH 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.

Keywords:

• Ar
• CO₂
• Dilution
• Flame stability
• Lifting
• N₂
• Nonpremixed
• PLIF-OH

ACKNOWLEDGMENT

This work was supported by the French National Research Agency (Program STREAM) and the Région Haute-Normandie, France.