Combustion stability of gaseous CH4/O2 and H2/O2 coaxial jet flames in a single-element combustor

In order to understand the combustion stability of a methane (CH4)/oxygen (O2) bipropellant as a next-generation rocket liquid propellant, the combustion stability limits and morphology of gaseous CH4/O2 (GCH4/GO2) coaxial jet flames, among various phases, in a single-element combustor are experimentally studied compared with the gaseous hydrogen/O2 (GH2/GO2) coaxial jet flames. Only the stably attached flame and blowoff regimes are observed for both the GCH4/GO2 and GH2/GO2 flames, showing the flame thickness smaller than the injector lip thickness. Although the combustion stability limits of the GCH4/GO2 flames are narrower than the GH2/GO2 flames, practical use of CH4 in rocket engine applications seems to be acceptable since the fuel-rich CH4/O2 flames show very stabilized and intensified burning. For the GCH4/GO2 flames, the outer flame generated by the recirculating O2 is relatively weak and OH∗ is distributed up to the downstream. With increasing O2 injection velocity the length of the GCH4/GO2 flames and the location at the maximum OH∗ intensity increase even under turbulent combustion condition, due to the saturated enhancement of CH4-O2 diffusivity and the strong burning of pure O2 near the injector lip. The present results provide a useful database to model combustion of CH4/O2 bipropellants under various phases.