Combustion characteristics of O2/CH4 coaxial jet flames in a model combustor through their visualization and the statistical analysis

To understand the fundamental properties of newly attractive oxygen (O2)/methane (CH4) bipropellants, the combustion characteristics of O2/CH4 coaxial jet flames in a model combustor are experimentally studied for various Reynolds number (Re) and oxygen-to-fuel momentum flux ratio conditions through the flame visuali-zation such as OH* and CH* chemiluminescence and OH planar laser-induced fluorescence. Two different flame behaviors and two extinction limits are observed: stable attached and oscillating near-blowout flames and blowout and blowoff limits. With increasing Re of reactants, the OH layer thickness decreases and the flame wrinkling increases due to the enhanced local strain rate and vorticity. Also, the flame visualization clearly exhibits the effects of a nozzle which is located on the bottom plate of the combustor on the flame structure, e.g., the increased OH layer thickness near the injector lip compared with the fully opened nozzle. A statistical analysis using the normalized root mean square (NRMS) of the OH* and CH* chemiluminescence signal is conducted to investigate the combustion characteristics including the local flame extinction, and a modified OH* NRMS is introduced, which is found to be a useful statistical parameter to represent the occurrence and region of the instability phenomenon.