Effect of switching methane/oxygen reactants in a coaxial injector on the stability of non-premixed flames

An investigation was performed to examine the effect of switching non-premixed gaseous fuel and oxidizer coaxial injector flow configurations on the stability of a CH4/O2 diffusion flame. Over 170 combustion tests with oxidizer-to-fuel mass ratios [(O/F)mass] of 1.3 to 4.0 were conducted at initial reactant temperature of 298K and chamber pressure of 1atm. Stability maps were developed based on Reynolds number associated with a given injector flow configuration. The process was repeated by switching the primary and secondary reactants to develop new stability maps. Results showed that there were pronounced differences in non-premixed flame stability between these two different flow supply conditions. Flames with fuel as primary flow from the center tube showed greater stability and never achieved a near-blowout or extinction state for the (O/F)mass range tested. In addition, these flames remained anchored to the injector exit until (O/F)mass was increased to 2.25. Beyond this condition, the detached flame was observed. When the oxidizer was supplied in the center tube, an increase in the ReD,O2 caused the flame to change from a stable anchored flame, to a detached flame. At (O/F)mom0.9, any further increase in ReD,O2 resulted in a transition from a stable anchored flame to a detached flame and then to a near-blowout flame.