Effect of Annular Flow Area on Flame Stability in a Methane/Oxygen Diffusion Flame Burner with a 15 Degree Impingement Angle

The impact of secondary annular flow area on diffusion flame stability was examined in a horizontally mounted experimental burner. Three inverse single coaxial injectors were created with a fixed 15-degree secondary flow exit impingement angle with respect to the primary flow axis. Each injector had a different secondary methane annular flow area while maintaining a set primary oxygen flow area. Diffusion flame operation and standoff distance were observed using non-intrusive optical windows parallel to the axis of the flame. Flame stability maps were generated based upon gaseous reactant equivalence ratio, Reynolds number, and injector secondary flow area as well as nondimensional detached flame standoff distances. It was observed that the largest secondary annular flow area provided the greatest chance of anchored, stable fuel-rich diffusion flames for equivalence ratios below 1.4 and oxygen Reynolds numbers up 30,000. For equivalence ratios above 2, anchored flames transitioned to near blow-off flames above oxygen Reynolds numbers of 7,500. As secondary annular flow area decreased, the 15-degree impingement angle managed to cluster the detached flame standoff distances closer together over a wide range of fuel-rich operating conditions.