Characteristics of oxy-methane flame in an axial/tangential swirl jet burner

•Steady and complete CH4/O2/CO2 combustion is achieved in an axial/tangential swirl jet burner.•Regimes for attached and lifted flames are determined in a wide operating range.•Detailed evolution from flame attachment to lift-off has been carefully examined.•Two linear fitting lines are obtained to predict the flame lift-off height. This work designs an axial/tangential swirl jet burner to establish stable CH4/O2/CO2 combustion under a wide range of operating conditions. As the flow rate of the axial fuel stream increases, the flame is lifted and stabilized near the downstream wall. The lifted flame is robust and shows good combustion completeness. The critical fuel flow rate for flame lift-off is affected by the oxygen mole fraction of the oxidant flow. The diagram controlling flame attachment and lift-off is investigated experimentally to derive the critical Schmidt number. In addition, high-speed photography and planar laser-induced fluorescence (PLIF) techniques are employed to resolve instantaneous flame structures near lift-off. It is visualized that as the flame height increases at larger fuel flow rates, the flame layer encounters local extinction due to the stretching of the surrounding high-speed oxidant flow. The local stretch rate measured in cold flow agrees well with the theoretic extinction strain rate. Further, a fitting line is obtained to summarize the effect of flow conditions and burner geometry on flame lift-off height. Particularly, the normalized lift-off height shows a good linear relationship with the Reynolds number of the axial fuel jet.