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/O-2 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 Re-D,Re-O2 caused the flame to change from a stable anchored flame, to a detached flame. At (O/F)(mom)<= 0.9, any further increase in Re-D,Re-O2 resulted in a transition from a stable anchored flame to a detached flame and then to a near-blowout flame.