Enhanced stability and reduced emissions in an elliptic swirl-stabilized burner

Unstable thermoacoustic modes were studied and controlled passively by changing the geometry of a swirl-stabilized burner to an elliptic shape. The elliptic burner modified the mixing of the fuel and the air, the fresh fuel/air mixture, and the hot combustion products. The flow dynamics induced by the noncircular burner affected the flame stabilization at the central recirculation zone and at the sudden expansion. In addition to the enhanced mixing, the elliptic burner achieved lower emissions while stabilizing the combustion by reducing the coherence of the large-scale vortices at the burner's exit. The combustion tests were performed in a range of normalized equivalence ratios from near lern blowout to 1.2 times the nominal equivalence ratio, at two different power levels. In addition, power variations from 0.54 to 1.15 of the nominal power at nominal equivalence ratios were performed. Pressure and beat-release fluctuations were measured and analyzed for all operating conditions. The elliptic burner showed reductions of more than two orders of magnitude (-24 d B) in pressure and heat-release oscillations in the entire range of equivalence ratios measured. The structures of the different instability modes (axisymmetric at St = 0.6 and helical at 1.7 and 7.1) were studied with an intensified charge-coupled-device camera filtered to monitor OH chemiluminescence using phased-locked visualization. The axisymmetric mode showed substantially reduced variability in heat-release rate during the cycle of instability, compared with the axisymmetric baseline burner. This behavior resulted in a reduced level of instability, as evidenced from the spectral analysis of the pressure oscillations. NO(x) and CO emissions were considerably reduced: NO, by up to four times and CO by a factor of 2-3. Unburned-hydrocarbon emissions were significantly reduced for normalized equivalence ratios below 0.9, indicating an extension of the lean-blowout limit. The strong suppression of thermoacoustic instability and large reductions of emissions persisted in the entire range of power output tested.