Dynamics of Flame Lift-Off in Biogas Swirl Flames

Abstract

Lift-off limits and mechanism of biogas swirl flames were investigated in a gas turbine model combustor using high-repetition-rate OH* chemiluminescence and simultaneous particle image velocimetry (PIV) and OH planar laser induced fluorescence (PLIF). The biogas fuel was represented by 60% methane and 40% carbon dioxide, volumetrically. The test matrix consisted of three preheat temperatures and three target adiabatic flame temperatures, with a total of 9 test cases. Total lift-off was defined as a distinct and complete flame detachment from the burner nozzle, which was approached by increasing both air and fuel flow rates at a fixed equivalence ratio. With the increase of bulk velocity, more intermittent lift-off events were observed, with the flame temporarily detaching from the nozzle before reattaching. By analyzing flow-flame interactions during these events from the temporally-resolved PIV and OH PLIF measurements, the lift-off mechanism was observed. As a precursor of lift-off, a local flame extinction event occurred near the flame base due to a pulse of high strain-rate on the flame. Noticeable flow and flame asymmetry subsequently developed, which led to further thinning and wrapping-up of the flame base. Further local extinction subsequently occurred due to the high strain-rate associated with the asymmetric flow. This eventually caused the entire flame base to quench and the flame to detach. Analysis of the vorticity field indicated that the flow asymmetries were due to the formation of a helical precessing vortex core (PVC) after the first local extinction event as a result of density field change near the nozzle exit.