High-speed simultaneous PLIF/PIV imaging of a lift-off swirling flame under acoustic forcing

The detailed understanding of the flow-flame interaction in a swirling flame is crucial for the study of turbine engine combustion. The phase and amplitude of the heat release rate as a function of the frequency and amplitude of the flow-field oscillations determine the thermo-acoustic instability of the combustion system. The responses of a lift-off swirling flame under acoustic forcing, which operates near its blow-off limit and often shows flame shape bifurcation between attached and lift-off configurations, have not been experimentally investigated. Here, we present simultaneous 20 kHz particle image velocimetry (NV), fuel tracer and CH2O planar laser-induced fluorescence (PLIF) measurement of a lean-premixed swirling flame enclosed in a model gas turbine burner in order to study the effect of equivalence ratio and forcing on the flame structure and dynamics. We studied the bistable states that have intermittent transitions between the anchored V-shape flame and the detached lift-off flame in which the precessing vortex core is formed. The time-resolved lift-off and re-attachment transitions between the V-flame and lifted flame configuration of the bistable flames are captured by the high repetition rate burst mode imaging. The high-repetition PLIF/PIV results demonstrate that the lifted flame can be re-attached on increasing the equivalence ratio or the forcing frequencies and amplitudes.